#  Next Generation Observatories
Report from the Dawn VI workshop; October 5-7 2021

Dawn VI SOC and Presenters

###### Contents

  1. 1 Preface
  2. 2 Executive summary
    1. 2.1 Recommendations
  3. 3 Survey of progress
    1. 3.1 Progress in the field between Dawn IV and Dawn VI
    2. 3.2 Status of Recommendations from Dawn IV
    3. 3.3 Gravitational-Wave International Committee Perspective
      1. 3.3.1 Ground-based Gravitational-wave Detectors
      2. 3.3.2 Space-based Gravitational-wave Detectors
      3. 3.3.3 Pulsar Timing Array Gravitational-wave Detectors
    4. 3.4 GW and MMA Community Planning Activities
    5. 3.5 Communication with, and between, funding and advisory agencies
  4. 4 Observational Science
    1. 4.1 Gravitational Waves
    2. 4.2 Cosmology
    3. 4.3 Radio synergy with the Next Generation Gravitational Wave Observatories
    4. 4.4 Gamma/X-ray Observations
    5. 4.5 Nuclear Physics
    6. 4.6 Prospects and Challenges for Optical/Infrared Counterparts in the CE Era
    7. 4.7 Cosmic Neutrinos
    8. 4.8 Panel: GW-enabled Observational Science between today and 2035
    9. 4.9 Next-generation Observatory science goals
    10. 4.10 Observational Science Recommendations
  5. 5 Current and future Observatories
    1. 5.1 Introduction
    2. 5.2 The roadmap between current observatories and next-generation detectors
    3. 5.3 Einstein Telescope Project
      1. 5.3.1 ET Concept
      2. 5.3.2 Project Status
    4. 5.4 NEMO
    5. 5.5 The Cosmic Explorer Project
      1. 5.5.1 CE Concept and Status
      2. 5.5.2 Cosmic Explorer Reference Design
      3. 5.5.3  CE Path Forward
    6. 5.6 Round Table on Data Access Models
    7. 5.7 LIGO Laboratory Perspective
    8. 5.8 LIGO Scientific Collaboration perspective
    9. 5.9 National Science Foundation perspective
    10. 5.10 International Collaboration
  6. 6 Realizing Designs
    1. 6.1 Megaprojects Round Table
    2. 6.2 Recommendations and Conclusions
  7. A Contributors to and endorsers of this report

##  1 Preface

The workshop Dawn VI: Next Generation Observatories took place online over
three days, 5-7 October, 2021. More than 200 physicists and astronomers
attended to contribute to, and learn from, a discussion of next-generation
ground-based gravitational-wave detectors. The Dawn meetings111The previous
Dawn meeting presentations and reports can be found at
https://gwic.ligo.org/conferences.html were suggested in 2015 by the National
Science Foundation (NSF) to provide a forum for the community interested in
gravitational wave observational and instrument science, and have been vital
in establishing a consensus view of the path forward for US work in the field,
and more broadly for work on next-generation science. The Dawn VI focus was
the US plans for a next-generation observatory, Cosmic Explorer.

The LIGO and Virgo Collaborations had completed the Third Observing Run (O3)
in March 2021 with public alerts indicating numerous black hole binaries,
several neutron star binaries, and two neutron-star – black-hole binaries.
Within weeks of the Dawn VI workshop, the Catalog
GWTC-3222https://arxiv.org/abs/2111.03606 was published, detailing the
detection of gravitational waves from a total of 90 astrophysical sources. The
inferences from the multi-messenger observation of GW170817 have continued to
grow. KAGRA has joined with Virgo and NSF’s LIGO detectors to share observing
data. Incremental improvements in the LIGO, named
‘A+’333https://arxiv.org/abs/1410.5882, https://arxiv.org/abs/1304.0670 and
the Virgo ‘AdV+’ project444Proc. SPIE 11445, 1144511 (13 December 2020);
https://doi.org/10.1117/12.2565418 instruments in the current 3 km and 4 km
observatories are well underway, with the start of the next observing run O4
of LIGO, Virgo, and KAGRA planned for late 2022. LIGO and Virgo are starting
to consider possible further improvements in the currently operating
observatories, but there are practical limits to sensitivity given the 4 and 3
km length of the arms.

Excitement from scientists and the general public world-wide has further
fueled the exploration of ideas and plans for the next generation of
terrestrial gravitational-wave observatories. The 2020 Astro Decadal
Survey555https://www.nap.edu/catalog/26141/pathways-to-discovery-in-astronomy-
and-astrophysics-for-the-2020s spoke highly of the scientific value of
improved instruments, and the potential of Multi-Messenger Astrophysics (MMA)
including gravitational waves was recognized as a priority by a very wide
range of scientists. Both Einstein Telescope, a European concept, and Cosmic
Explorer, a US concept, have made significant progress toward realization in
the last few years.

During the three-session workshop, sessions and discussions explored the
context and the concepts around these next-generation observatories, organized
around the following topics:

  * •

Observational Science opportunities (Section 3)

  * •

Detector and Observatory designs (Section 5)

  * •

Realizing designs and evolving practice (Section 6)

This report represents the views of those participants at the workshop and
other interested colleagues who reviewed the document and endorsed it.

David Shoemaker, on behalf of the Scientific Organizing Committee:

Stefan W. Ballmer | Syracuse University
---|---
Matteo Barsuglia | CNRS - APC, Paris
Josh Frieman | Fermilab
Mansi Kasliwal | Caltech
Jackie Hewitt | MIT
Chuck Horowitz | Indiana University
Brian Lantz | Stanford
Albert Lazzarini | Caltech
David Shoemaker, Chair | MIT

Dawn IV report section presenters and round-table organizers:

Stefan W. Ballmer | Syracuse University
---|---
Emanuele Berti | Johns Hopkins University
Patrick Brady | University of Wisconsin-Milwaukee
Duncan Brown | Syracuse University
Poonam Chandra | National Centre for Radio Astrophysics, TIFR, India
Matt Evans | MIT
Ke Fang | University of Wisconsin-Milwaukee
Wen-fai Fong | CIERA & Northwestern University
Peter Fritschel | MIT
Harald Lück | Leibniz Universität Hannover
Hiranya Peiris | University College London / Stockholm University
Michele Punturo | INFN Perugia
Dave Reitze | Caltech
Gary H. Sanders | Simons Observatory, Center for Astrophysics and Space Sciences, UC San Diego
Bangalore Sathyaprakash | Penn State and Cardiff University
Bram J. J. Slagmolen | Australian National University and OzGrav
Joshua Smith | California State University, Fullerton
Andrew Steiner | University of Tennessee, Knoxville and Oak Ridge National Laboratory
Eleonora Troja | University of Rome Tor Vergata
V. Ashley Villar | Penn State
Rainer Weiss | MIT

##  2 Executive summary

The workshop Dawn VI: Next Generation Observatories took place online over
three days, 5-7 October, 2021. More than 200 physicists and astronomers
attended to contribute to, and learn from, a discussion of next-generation
ground-based gravitational-wave detectors.

The program was centered on the next generation of ground-based gravitational-
wave observatories and their synergy with the greater landscape of scientific
observatories of the 2030s. Cosmic Explorer (CE), a concept developed with US
National Science Foundation support, was a particular focus; Einstein
Telescope (ET), the European next generation concept, is an important
complement and partner in forming a network.

The first day, the organizers invited speakers from a range of astrophysics
disciplines, with the objective of understanding how gravitational wave
observations can complement and enable progress in those non-GW disciplines.
Presentations were given on the synergy of GWs with radio astronomy,
gamma/x-ray observations, nuclear physics, visible and near IR observations,
and particle physics. The possibilities for cosmology, and for multi-band and
isolated gravitational-wave observations, were also discussed, along with the
transition from the current state of the detectors (in preparation for a
late-2022 start for the O4 observing run) to the time when the next-generation
observatories of CE and ET can commence operations. There was a clear sense of
opportunity and potential for combining GW and non-GW messengers to realize
otherwise inaccessible science. The example of GW170817, the binary neutron-
star merger whose excellent sky localization, provided by LIGO and Virgo,
allowed it to be studied in GWs, gamma rays, and multiple additional photon
wavelengths, was repeatedly invoked to stress the importance of a 3 (or more)
GW detector network.

The second day, the instruments were the focus. The work in the LIGO and Virgo
collaborations to explore upgrades between the end of the current instrument
plan (end of O5, $\sim$2028) to the start of the CE/ET epoch ($\sim 2035$) was
discussed. The ET and CE Projects were described, with CE as the focus of the
meeting in more detail. The design for CE stresses low initial technical risk,
with an instrument design closely based on NSF’s Advanced LIGO detector, and
increasing the length of the interferometer arms to increase sensitivity; it
trades away technical risk for a larger infrastructure, and value engineering
of the vacuum system is a strong focus of the team. The perspectives of
organizational entities that played key roles in the success of Advanced LIGO
were provided: LIGO Laboratory, LIGO Scientific Collaboration, and the NSF.

The final day featured a discussion of very large science projects, of the
scale of CE ($1 - $2 billion USD). Experienced scientists who have managed
‘megaprojects’ for NASA, DOE, and NSF offered insights for CE. Assembling an
approach to funding may call on multiple funding agencies, potentially on
private foundations, and international collaboration will be an early
challenge. Finding an organizational structure which can credibly manage the
Project must be an early goal of the CE team. Site selection is complex, time
consuming, and key. Establishing a real integration of the CE team with the
Indigenous Peoples on whose land the sites will lie is imperative. Given the
duration of the Project, a pipeline of early career scientists is crucial;
related is the importance of forming a core team from a broad range of
institutions. The Cosmic Explorer Project must create a diverse, inclusive,
and equitable environment to both engender and profit from a full spectrum of
participation.The bottom line is that a project of this scale will be rife
with challenges, but the goal is clearly of sufficient value to merit the
effort to make it happen.

A round table on data processing and access models for the CE epoch followed.
CE plans to keep within the scope of the Project the effort to characterize
the detectors, to calibrate the data, to ‘clean’ the data of defects, and to
issue low-latency alerts to the scientific public. The alerts will carry all
available information to aid in prioritization by non-GW observers. CE will
require greater automation of data conditioning than is currently taking place
with the second generation detectors: however both LIGO and Virgo will be
focusing on this during the coming years, so it is expected that they will
provide a legacy upon which to build by the time of CE observations. The full
CE data stream is planned to be released as soon as it is in a suitable form
for analysis, without a proprietary period. This is a different approach than
the current LIGO-Virgo-KAGRA practice, where the data are also analyzed deeply
by the Collaborations before release, but feels to the CE team to be forward-
looking and the best assurance of the broadest possible participation in
Cosmic Explorer’s science. To properly analyze the data, progress will be
needed in waveform generation and parameter estimation methods. The computing
per se does not look excessively difficult, with few overlapping signals
anticipated and with anticipated progress in computing speed in the coming 15
years.

The concluding summary of the meeting expressed the sentiment that the
observational science accessible to CE and ET, also in combination with data
from other non-GW observatories, will stimulate a very broad community of
analysts and yield insights which are exciting given the access to GWs from
the entire universe. The need, and desire, for closer collaboration between ET
and CE was expressed; a three-detector network is optimal for delivering much
of the science.

The science opportunities afforded by CE and ET are broad and compelling,
impacting a wide range of disciplines in physics and high energy astrophysics.
There was a consensus that CE is a concept that can deliver the promised
science. A strong endorsement of Cosmic Explorer, as described in the CE
Horizon Study, is a primary outcome of DAWN VI.

###  2.1 Recommendations

Observational Science

  * •

Coordination between the current generation of detectors is crucial. Once
KAGRA and LIGO-India are at a sensitivity level comparable to LIGO and Virgo,
phased upgrades while maintaining a 3-detector network will be possible. The
planning should have input from the non-GW observing community and be take
into account limited-lifetime space missions.

  * •

A network of next-generation detectors is important to realizing the science
goals of the community since sky localization is crucial for MMA. This point
came up throughout the meeting, and the strong consensus recommendation is
that three (or more) detectors, closely coordinated in observing and
upgrading, must be the goal.

  * •

Discussion is needed between the U.S. and E.U. projects on the path forward
for data access in the epoch of CE and ET. Realizing the network will require
a common policy.

  * •

A number of topics requiring one-time or sustained communication between GW
and non-GW observers led to recommendations for meetings:

    * –

Explicit coordination of GW observation with (in particular) space missions
worldwide, due to their limited lifetime, is needed to ensure that MMA can be
best realized.

    * –

A regular interaction is needed between the GW and EM/particle/nuclear domains
to ensure that scientific and strategic insights which can impact any of the
domains are communicated clearly and in a timely way. A small group that has
visibility and credibility with all communities could be effective.

    * –

The scientific benefit of precise localization by a GW network, and the need
for low latency vs. high latency with improved precision, should be discussed
in a Town Hall to develop a common understanding and consensus.

    * –

A discussion is needed on how (or if) additional low latency GW information
can be used to support triage by EM/particle observers to manage the higher
event rates in O4 and beyond.

    * –

The long-standing tension between the duty-cycle of observing vs. improving
the sensitivity with its concomitant downtime continues, covering the upgrade
arguments (net event count and signal-to-noise ratio (SNR)) vs. the value of
extended observation at a fixed sensitivity. Post-O5 upgrades should be
included in the scope of discussion.

Instrument and Project Recommendations

  * •

We recommend the establishment of a small group charged to explore and launch
efforts to initiate specific limited coordinated activities between CE and ET
such as model code and parameters, and vacuum equipment value engineering.

  * •

To foster the engagement of the LIGO Lab in CE, discussions should begin soon
among the CE team, the LIGO Laboratory management team, and the NSF.

  * •

It is urgent that the CE project design phase take place within an
organizational structure developed as a precursor that can evolve to become
the eventual, robust facility organization.

  * •

The CE design stage should move early to a baseline configuration and fully
develop that design rather than continuing the trade studies of potential
design option unless consideration of options is found to be required to
manage uncertainty and risk.

  * •

A group should be identified or established to coordinate GW and EM/particle
strategic planning, to ensure the best use of time-constrained facilities.

  * •

CE should establish now a Project Advisory Committee with membership including
expertise in similar-scale projects.

  * •

GW facility leadership (both present and planned) should develop a long-term
“mission statement” relevant to lifetimes of space missions dedicated to GW
follow-up. This requires a decade-scale horizon for the planning of instrument
operation, to match the time scale for space missions.

  * •

The observational science value to having a network node in the southern
hemisphere is significant . The community should continue to explore means to
realize a next-generation observatory there.

##  3 Survey of progress

###  3.1 Progress in the field between Dawn IV and Dawn VI

Dawn IV was held in August
2018666https://indico.nikhef.nl/event/1174/;https://gwic-documents.s3.us-
west-2.amazonaws.com/dawn/Dawn-IV-report.pdf, and Dawn V777https://indico.ego-
gw.it/event/20/ in May 2019 (this Dawn VI report also covers Dawn V).
Following the O2 Observing run which ended in August 2017, NSF’s LIGO and the
European Virgo detectors were brought to an improved sensitivity, through the
use of squeezed states of light (improving the high-frequency shot-noise
limited sensitivity for a given laser power) and many technical advances. The
joint Virgo-LIGO O3 Observing run, April 2019 to March 2020, yielded some 79
new events. Very massive black holes, compact objects straddling the mass
range between neutron stars and black holes, and mixed black-hole – neutron-
star mergers were among the events. The catalog
GWTC-3888https://arxiv.org/abs/2111.03606 provides an overview and pointers to
further papers. KAGRA came on line with a limited sensitivity toward the end
of this period and jointly observed with GEO-600.

In parallel, improvements to the sensitivity of LIGO and Virgo were planned,
funded, engineered, and are being implemented at the end of 2021. The LIGO
‘A+’ and Virgo ‘AdV+’ upgrades add frequency-dependent squeezing (to allow
broadband quantum noise improvement) as well as a range of other changes
(higher laser power, additional baffling, replacement of some mirrors, etc.)
intended to improve the reach of the instruments. The next run, O4, is
expected to start in late 2022 once the upgraded detectors have been initially
commissioned.

The GWIC 3G Committee reports, which were in draft form for Dawn IV, have been
refined, and are available on the GWIC web
pages999https://gwic.ligo.org/3Gsubcomm/documents.html or the
arXiv101010https://arxiv.org/abs/2111.06991,https://arxiv.org/abs/2111.06990,https://arxiv.org/abs/2111.06989,
https://arxiv.org/abs/2111.06988,https://arxiv.org/abs/2111.06987,https://arxiv.org/abs/2111.06986.
These reports aided greatly in the preparation of the proposal for the
European Einstein Telescope (ET) proposal to the ESFRI process, as well as for
the preparation of the Cosmic Explorer Horizon Study (CEHS)111111ibid. ET was
placed on the ESFRI (European Strategic Forum for Research Infrastructures)
roadmap; the CEHS was completed and submitted to the NSF.

The US Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020
Decadal121212https://www.nap.edu/catalog/26141/pathways-to-discovery-in-
astronomy-and-astrophysics-for-the-2020s) Study took place in this interval,
and produced its draft report shortly after the Dawn VI meeting. It was
informed by many white papers from the astronomical community including the
gravitational-wave field – both for instrumentation and for observational
science. The report speaks highly of the value of gravitational-waves as an
astronomical messenger both stand-alone and in multi-messenger astrophysics.
The report “strongly endorses investments in technology development for
advanced gravitational wave interferometers, both to upgrade NSF’s Laser
Interferometer Gravitational-Wave Observatory (LIGO), and to prepare for the
next large facility.” Further, “The survey committee strongly endorses
gravitational wave observations as central to many crucial science
objectives”.

###  3.2 Status of Recommendations from Dawn IV

  * •

GWIC should found an international Umbrella Organization by the Dawn V meeting
in Spring 2019 to coordinate international research and development for 3G and
detector upgrade plans. This topic was discussed in some depth at Dawn V, and
the conclusion was that the two primary next generation observatory projects
(CE and ET) were in a sufficiently different state of maturity that it might
not be productive to couple the projects closely at that time. The current
observatories and collaborations are starting to share more computing and
communications infrastructure through IGWN (International Gravitational-Wave
Network), and informal technical exchanges are growing between ET and CE.

  * •

The ground-based GW community should prepare to respond to calls for input to
roadmaps. Specifically for the US Astro2020 Decadal Survey, we should respond
through the submission of i) a coordinated set of science white papers, and if
required by the Astro2020 charter ii) roadmaps for development of mid-scale
technologies and programs to enable 2.5 and 3G detectors. A proposal should
also be included in the 2021 European Strategic Forum for Research
Infrastructures (ESFRI) roadmap. White Papers were indeed prepared for the
Astro2020 Decadal survey, both on the general topic of gravitaional waves as a
meessenger, and more specifically to inform the community about CE; the Report
(released in early November 2021) indicates that those White Papers were quite
influential in the preparation of the report. ET did propose to the ESFRI
program and was adopted in Summer 2021 to the ESFRI Roadmap.

  * •

A Dawn V meeting should be held when the GWIC-led 3G subcommittee report is
expected to be released to the community. The focus should be the 3G
subcommittee report and its use for informing international funding agencies,
including the the US Astro2020 Decadal Survey and the 2021 ESFRI roadmap in
Europe. Dawn V was held 26-26 May 2019131313https://indico.ego-
gw.it/event/20/. The 3G reports were in good draft condition at the time of
the meeting, and the approach of the community for the US Astro2020 Decadal
Survey and the 2021 ESFRI roadmap in Europe was discussed. A range of papers
and proposals were created using the GWIC 3G reports, and this was directly
helpful in the success of ESFRI adoption of ET and in the preparation of the
CE Horizon Study with recognition by the Decadal Committee.

  * •

Exploring the astrophysical science gain of a third 3G facility and placement
in the southern hemisphere should be a top priority for GWIC. The GWIC 3G
Science Book carried out an analysis of the advantages of a southern-
hemisphere detector, and the CE Horizon Study included evaluation of an
Australian site. An Australian study for an observatory with a focus on the
end-phase of neutron-star mergers (NEMO141414https://arxiv.org/abs/2007.03128)
has made significant progress. Further work on the value of and means to
realize a next-generation observatory in the southern hemisphere is
recommended.

  * •

We recommend that the 3G science case evaluate the science contribution from
below 10 Hz versus above 10 Hz for each item, to help inform detector
requirements, and for 500 Hz to 4 kHz, potentially from 2G and 2.5G detectors
with shorter baselines of 3-4 km. This study was pursued first for the GWIC 3G
documents, and then in more detail for the CE Horizon Study. The outcomes were
applied to the trades for the CE baseline design, and led to the conclusion
that CE should consist of two widely separated observatories of 40km and 20km
length to span the anticipated sources. The specific value as a function of
lower frequency cutoff was explored in the ET ESFRI submission materials. The
field has a clearer vision of the observational science as a function of
frequency span.

  * •

The GW community should address the development of software and computing
hardware in parallel with the instrumentation and science development. The
preparation of both the ET ESFRI proposal and the CE Horizon Study provided a
motivation and opportunity to work on the requirements for next-generation
observatory observational science needs for computation and for refinement of
the waveform models required to exploit the very high signal-to-noise ratio
one can expect for some events. In addition, Astro2020 Decadal White Papers
brought home the need for continued work (and funding!) for this domain. The
continuum of improvement in the current Virgo, LIGO, and KAGRA detectors is
also stimulating progress there.

  * •

The 3G community should adopt the following common strategies:

    * –

Establishment of a common research and development program within the U.S. and
Europe to facilitate the exchange of information and optimize the global
expenditure of efforts. This has not happened. There has been some exchange of
thinking at meetings, but between the tight focus on the ET ESFRI proposal and
CE Horizon Study, and past practice, this remains an open action item.

    * –

As part of a broader global research and development effort, investment in
more global resources devoted to characterization of coatings at cryogenic
temperatures, such as a dedicated, internationally resourced coatings center.
No central research center has been developed. However, there has been greater
collaborative effort between the European and US efforts, spanning the
applications from current to CE and ET. Yet closer integration of the efforts
would be advantageous.

    * –

Global coordination of prototype engineering and scaled tests for 3G
detectors, including beam tube construction, vacuum technologies, and
excavation and construction methods. Some progress has been made here. The ET
Pathfinder Laboratory, in Maastricht, Netherlands, was funded and has just
been inaugurated in late 2021. It is intended to provide an environment that
can allow full-scale testing of components of next-generation observatory
detectors, and explicitly wishes to serve both ET and CE. Joint discussions on
approaches to lower-cost vacuum systems have been undertaken, with one in-
person meeting including CERN and US vacuum experts (the next joint meeting is
tentatively scheduled to be hosted by CERN in the fall of 2022), and some
initial research on surface treatments underway. There has been little global
discussion on excavation, tunneling, and general civil construction to date,
and there are fewer evident commonalities with the present conceptual designs.
More effort in all these areas would be fruitful.

    * –

Development of a long term plan that balances observing with installation and
commissioning breaks to make use of current generation facilities as a testbed
for 3G technologies. Both Virgo and LIGO have launched studies of the use of
the present 3- and 4-km infrastructures to help explore next-generation
detector designs. There is good communication but no formal coordination
between the technical studies, and this should be encouraged.

  * •

The 3G community should continue to explore paths from the current
organization of largely independent projects toward a global unified endeavor,
with the objective to optimize the use of financial and human resources, and
to maximize the science from a 3G network. Dawn V attempted to stimulate
discussion for the form that a common organization could take, but at that
time there was little enthusiasm to adopt any overarching structure. The need
for each of ET and CE to focus on important deadlines (ESFRI, CE Horizon
Study) played a role, combined with a continuing desire for the lack of
constraints in the current approach. Dawn VI chose to focus on the US Cosmic
Explorer concept, and did not confront this question. In parallel, there is
some effort between the current LIGO Scientific Collaboration, Virgo
Collaboration, and KAGRA Collaboration to bring some of the common computing
effort under an informal organization of ‘IGWN’ (International Gravitational-
Wave Network). This may be a kernel which can grow to cover a wider scope, but
in any event serves to show the advantages of shared resources and common
management in a limited domain.

  * •

The GW, EM, and neutrino communities should coordinate to identify key joint
science targets for multi-messenger studies. The O3 Observing run, April 2019
to March 2020, resulted in many triggers to the EM and neutrino communities.
While a number of events were followed up, no new ‘MMA’ (multi-messenger
astrophysics involving gravitational waves) events were identified. Through
followups, joint papers involving all partners, and White Papers for the
Astro2020 Decadal, more planning and proposing along the lines of coordination
across the MMA field has taken place.

  * •

Lastly, we recognize that the GW community is currently dependent on a single
optic coating facility (LMA) for detectors present and future, and a single
source for any critical component is viewed as a potential risk to the whole
community. In addition to the recommendations for enabling a next-generation
global detector network above, we also highly recommend the re-establishment
of the coating capabilities in hardware and staffing once offered by CSIRO in
Australia to mitigate this present risk and as a significant contributor to
broader ongoing research efforts. The Australian National University has
funded the creation of a laboratory for coating and the CSIRO equipment has
been installed there and is undergoing commissioning at the end of 2021. In
addition, LMA has established an oversight committee including a US member and
has worked in close collaboration with both US and European researchers in the
evolution of coatings targeted for detectors in the current observatories.

###  3.3 Gravitational-Wave International Committee Perspective

The Gravitational Wave International Committee (GWIC) was formed in 1997 to
facilitate international collaboration and cooperation in the construction,
operation and use of the major gravitational wave detection facilities world-
wide. It is associated with the International Union of Pure and Applied
Physics as its Working Group WG.11. Through this association, GWIC is
connected with the International Society on General Relativity and Gravitation
(IUPAP’s Affiliated Commission AC.2), its Commission C19 (Astrophysics), and
another Working Group, the AstroParticle Physics International Committee
(APPIC).

GWIC meets yearly to assess the state of the field, and to launch initiatives
intended to help facilitate the growth of the field. The ‘3G’ reports which
helped ET and CE are a good example; another is a recent publication in Nature
Reviews Physics of “Gravitational-wave physics and astronomy in the 2020s and
2030s151515https://www.nature.com/articles/s42254-021-00303-8”, a roadmap for
the future of the field.

Here we very briefly summarize the activities in the field to provide context
for the Dawn VI notes and recommendations.

####  3.3.1 Ground-based Gravitational-wave Detectors

Current ground-based gravitational-wave detectors include KAGRA, Virgo,
GEO-600, and LIGO. All of these instruments use variations on the idea of a
Michelson interferometer, illuminated with a laser, and with test masses well
isolated from external forces. A quadrupolar gravitational wave induces a
differential phase shift in the light propagating from the beamsplitter to the
end test mass which serves as a mirror and upon recombining at the
beamsplitter acts as a transducer, converting gravitational-wave strain into
changes in light intensity which then can be converted into an electrical
signal. As the gravitational wave is a strain in space, $h=\Delta L/L$, longer
arms $L$ lead to larger signals for gravitational wavelengths shorter than the
arm lengths. KAGRA and Virgo have 3 km-long arms, and LIGO 4 km. Isolating the
test masses from external forces leads to a range of approaches. KAGRA is
underground, reducing the effect of surface seismic excitation. Virgo and
KAGRA use many pendulums in series to act as a mechanical low-pass filter,
attenuating seismic motion. LIGO uses sensors and high-gain servo systems in
series with pendulum stages to attenuate mechanically-coupled seismic motion.
Thermal motion of the test mass can also mask gravitational waves, and low-
loss materials for the test mass and optical coatings are used to concentrate
the motion in a limited frequency range. KAGRA also plans to reduce the
temperature to tens of Kelvin to reduce the thermally-driven motion.

Larger laser powers improve the sensitivity, by improving the Poisson counting
statistics. 100W-class lasers are used, and recently prepared states of light
– squeezed light – have been introduced to manage the quantum noise. The
present optical systems, while based on a Michelson interferometer, are quite
complex to make the best use of the light and to match the constraints of the
observatory infrastructures. The light must travel in an ultra-high vacuum to
avoid scintillation in the path, and much of the cost of the observatories is
in the providing the vacuum system, its support, and its protection.

Thus far Virgo and LIGO have achieved a sufficient sensitivity to detect
gravitational waves, and some 90 candidates have been observed to date.
Programs to further refine the sensitivity are underway, and the next
observing run is anticipated for late 2022. The observatory infrastructures,
originally built in the 1990’s, can continue to be used for some time yet if
properly maintained, and there are presently concepts for making incremental
increases in the sensitivity of the detectors.

However, making a significant step forward in sensitivity will require moving
to larger arm lengths $L$ to make larger phase shifts in the light for a given
gravitational-wave strain $h$. The Dawn VI meeting focused on the US plans for
such a revised observatory infrastructure, Cosmic Explorer, where 20km and
40km long arms are proposed.

The target frequency range for ground-based detectors is from several Hz
(limited by the Newtonian background) to several kHz (limited by the current
and likely future practical constraints of laser interferometry).

####  3.3.2 Space-based Gravitational-wave Detectors

Space-based interferometer gravitational-wave detectors have a significant
attraction in avoiding the stray forces that come from seismic motion, and the
Newtonian background due to time-varying density changes near the test masses.
In addition, the vacuum path for the light is ‘free’ once the complexity and
expense of putting the detector in space is made.

The LISA (Laser Interferometer Space
Antenna161616https://www.cosmos.esa.int/web/lisa/lisa-documents) is the most
advanced of the concepts for this approach. It is an ESA-led mission, with
NASA as a junior partner, to place three satellites in a triangular
configuration, and to sense the motion of shielded test masses using the
timing of laser beams passed between pairs of satellites. The detector targets
inspirals of $10^{6}$-$10^{9}$ solar mass binaries, with some sensitivity for
less massive systems. The planned armlength is $2.5\times 10^{9}$ meters,
which is comparable to the gravitational-wave wavelengths in the mid-band of
sensitivity. At low frequencies (around $10^{-7}$ Hz) it is limited by
residual forces on the imperfectly-shielded test mass, and at high frequencies
(around $0.1$ Hz) by the smaller net path-length change when multiple
wavelengths of the gravitational-wave fit in the arm length. It is planned for
a mid-2030’s launch.

In Japan, the gravitational wave community agrees that DECIGO will be the next
main project after KAGRA. DECi-hertz Interferometer Gravitational-wave
Observatory (DECIGO) is a future Japanese space-based gravitational-wave
antenna that will target the detection of an early-universe stochastic
gravitational wave signature imprinted by inflation.

####  3.3.3 Pulsar Timing Array Gravitational-wave Detectors

A Pulsar Timing Array (PTA171717Hellings, R. W. & Downs, G. S. Upper limits on
the isotropic gravitational radiation background from pulsar timing analysis.
Astrophys. J. Lett. 265, L39–L42 (1983).) measures variations in the radio
frequency pulse arrival times at the Earth from an array of millisecond
pulsars. Pulsars are highly-magnetized neutron stars, spinning on an axis due
to angular momentum acquired from the original pre-collapse star. Many pulsars
are observed to be extraordinarily accurate clocks. As a gravitational-wave
passes between the Earth and a pulsar, the time-of-flight of radio waves is
modulated, and this then can be interpreted as an observation of gravitational
waves. In practice, many pulsars are observed, and one seeks to find the
quadrupolar signature of GWs in the spatial distribution of phase shifts. The
distances to pulsars (the closest is some 400 light years distant) makes this
approach best suited to signals around $10^{-9}$ Hz, and so mergers of the
largest BH at centers of galaxies. Data have been accumulated with this goal
in mind over several decades, and first indications of a signal may be
appearing181818See, for example, https://arxiv.org/abs/2005.06495.

###  3.4 GW and MMA Community Planning Activities

There is a significant level of activity in the broader community to look for
synergies and to plan for future facilities which supports both the GW
detectors and the multi-messenger astrophysics that can be achieved. We list
here some of these activities:

  * •

Snowmass191919https://snowmass21.org/: The Snowmass/P5 process is a two-step
process used by the particle physics community and its U.S. funding agencies,
Department of Energy (DOE) and National Science Foundation (NSF) to formulate
the strategic plan for U.S. particle physics. Snowmass is the first step in
the planning process. It is a community-driven study of the scientific
opportunities organised by a number of the American Physical Society’s
Divisions: DPF, DPB, DAP, DNP, and DGRAV. The 2022 Snowmass Community Summer
Study will be held at the University of Washington from 17-22 July 2022.

  * •

Aspen202020https://www.aspenphys.org/physicists/summer/program/currentworkshops.html:
The Aspen Center for Physics is hosting a meeting 5 June-3 July, 2022, on
‘Fundamental Physics and Astrophysics with the Next Generation of
Gravitational-Wave Detectors’. The chief goal of this Aspen workshop is a
cross-disciplinary extensive debate and careful planning to prepare for the
data from the next generation GW observatories.

  * •

GWADW212121Queries to<EMAIL_ADDRESS>The Gravitational-Wave
Advanced Detector Workshop series is an annual meeting, typically held in a
relatively isolated locale, where a true workshop approach is taken to sharing
ideas on technologies and instrument science for future detectors. The next
meeting is planned for Hokkaido, Japan, from 30 May-6 June 2022.

  * •

GWPAW222222https://gwpaw2021.aei.mpg.de: The Gravitational-Wave Physics and
Astronomy Workshop is an annual meeting, bringing together GW and other
interested domains. The setting is informal, with no parallel sessions and
adequate time for discussion. This year the meeting is held December 14-17,
2021.

  * •

PAX232323Summer 2022 meeting will be announced at
https://indico.mit.edu/e/PAX2022; Contact S. Vitale, MIT<EMAIL_ADDRESS>Physics and Astrophysics at the eXtreme (PAX) is a discussion-based workshop
with very strong involvement of participants. The most recent PAX-
VII242424https://sites.psu.edu/paxvii/, held 23-27 August 2021, was to better
understand what the requirements are for the 3G observatory network from
different science perspectives, providing useful input to this Dawn VI
meeting. The next PAX is planned to be held at MIT in August, 2022.

There is also a broad and deep interest in the MMA community to strive for a
diverse, equitable, and inclusive work environment. Given the reality of
scientific work, where the boundaries between work per se and social life
tends to blur, it is all the more important to ensure that social issues are
treated as a central element in project and operations planning. To this end,
a number of organizations are active in promoting this approach, developing
materials and processes to further it, and providing gathering places for all
who want to put some time into these issues to contribute. We list here some
which are familiar to the MMA domain, and encourage readers to participate in
this evolution of the field:

  * •

The Multi-messenger Diversity Network252525https://astromdn.github.io

  * •

APS Inclusion, Diversity, and Equity
Alliance262626https://www.aps.org/programs/innovation/fund/idea.cfm

  * •

IAU subgroup on Management of Diversity and Inclusion in Large International
Collaborations272727https://iau-oao.nao.ac.jp/iau-inclusion/management-of-
diversity-and-inclusion-in-large-international-collaborations/

  * •

IGrav282828https://www.igrav.org to share best practices among GWIC’s Projects

###  3.5 Communication with, and between, funding and advisory agencies

There is value for GWIC and the working groups in the field to identify and
establish communication channels with funding and advisory agencies who
currently or may in the future support GW detectors. This provides GWIC a
recognized approach to communicate as needed with those agencies on the
scientific needs, desires, and constraints from the communities and provides a
coherent framework to serve as an advocacy group for the communities.

Groups that GWIC and the community have identified as directly interested in
the field include:

  1. 1.

The ‘Gravitational Waves Agency Correspondents’
(GWAC292929https://www.nsf.gov/mps/phy/gwac.jsp). GWAC is an informal body of
representatives of funding agencies covering ground and space-based
gravitational wave projects and science, initially formed in 2015 at the
initiative of the US National Science Foundation. Many of the organizations
currently supporting the field are represented in GWAC. GWIC has received
formal invitations from APPEC and GWAC to present the status and planning
activities of the broad gravitational wave community that it represents. GWIC
representatives attended telecons with presentations to GWAC (April 2019,
March 2020, April 2021). In those presentations, GWIC communicated the status
of 3G subcommittee reports and more generally activities in the field. GWAC
also, at the request of GWIC, provided a very significant review of the ‘3G
Subcommittee’ documents. The authors implemented the recommendations in the
thoughtful and thorough review, and the finished
reports303030https://gwic.ligo.org/3Gsubcomm/documents.html reflect the
improvements made.

  2. 2.

The ‘Astro-particle Physics European Consortium’
(APPEC313131https://www.appec.org). This is a consortium of European agencies
with responsibility for funding particle astrophysics. APPEC appoints a
Scientific Advisory Committee drawn from the European scientific community to
maintain a scientific roadmap for particle astrophysics, may choose when
appropriate to create financial instruments to support strategic areas, and
can act via influence to support projects in non-financial actions. The “Mid-
term review of the European Astroparticle Physics Strategy
2017-2026”323232https://www.appec.org/mid-term-review contains a chapter on
gravitational waves covering current and future ground- and space-born
instruments, and the 2022 APPEC Town Meeting is expected to devote significant
time to GWs.

  3. 3.

The European Consortium for Astroparticle Theory,
EuCAPT333333https://eucapt.org/, was established as a centre of excellence
hosted at CERN. EuCAPT organizes many events related to GWs such as virtual
colloquia or the EuCAPT annual symposium343434https://www.eucapt.org/events,
and also a white paper (containing GWs) was
written353535https://www.eucapt.org/white-paper.

  4. 4.

The Joint ECFA-NuPECC-APPEC Activities.
(JENAA363636http://www.nupecc.org/jenaa/) exists to address common questions
in particle, astroparticle and nuclear physics and to better exploit synergies
between these fields. It already led to five Expressions of Interest for
interdisciplinary cooperation, including one for Gravitational Waves for
fundamental physics373737http://www.nupecc.org/jenaa/?display=eois

  5. 5.

The ‘Group of Senior Officials’ (GSO383838https://www.gsogri.org), on Global
Research Infrastructures. The GSO was established and active from 2011 onward
to informally explore cooperation opportunities in Global Research
Infrastructures (GRIs). Participating countries are represented on the GSO by
“government officials and experts in the areas of international research
facilities and international relation”. ET has been presented to the GSO at
their request.

  6. 6.

ESFRI393939https://www.esfri.eu/forum, the European Strategy Forum on Research
Infrastructures, is a strategic instrument to develop the scientific
integration of Europe and to strengthen its international outreach. It
supports and benchmarks the quality of the activities of European scientists.
ESFRI has the objective of translating political objectives into concrete
advice for RI in Europe. ESFRI placed the European Einstein Telescope Project
on its roadmap in 2021.

##  4 Observational Science

The organizers invited speakers from a range of astrophysics disciplines, with
the objective of understanding how gravitational wave observations can
complement and enable progress in those non-GW disciplines. The speakers
collected their main points, and comments and questions for the GW field,
below.

###  4.1 Gravitational Waves

Presenter: Emaunele Berti

The Cosmic Explorer Horizon Study, the inclusion of Einstein Telescope in the
2021 ESFRI roadmap and the strong endorsement that LISA received from the
Astro2020 Decadal show that the future of gravitational wave astronomy is
bright, and we must think about ways in which these observatories will
complement each other.

At PAX-VII404040https://sites.psu.edu/paxvii/ we tried to understand the
requirements for the next-generation observatory network from different
science perspectives: astrophysical populations of compact objects, black hole
binaries from population III stars and beyond, cosmological parameters and
backgrounds, waveform and data analysis requirements, tests of gravity and
fundamental physics, dark matter candidates, dense matter equation of state
and the QCD phase diagram, multimessenger astronomy. This presentation focused
on gravitational waves alone, but even in this more limited context, the
synergy between multiple GW detectors is crucial.

This is indeed the lesson we learned from the O1, O2 and O3 runs. The
detections from August 2017 proved that adding the third detector to the
network (even if it is somewhat less sensitive than existing detectors) does
not just make incremental changes, it can mean a phase transition in the
science capabilities of the network: the rate of detections increased, we were
finally able to localize black hole binaries, and the localization of GW170817
marked the beginning of GW multimessenger astronomy.

A comparison of GWTC-2 and GWTC-1 shows that we learn the most by looking at
the corners of the parameter space: events with large mass ratios like
GW190412 and GW190814, the mass-gap event GW190521, or the heaviest binary
neutron star GW190425 challenged astrophysical formation scenarios. The most
informative events also challenge our understanding of waveform models and
they can cause controversial interpretations: What is the nature of the
2.6$\rm M_{\odot}$ compact object? Have we really observed overtones and
higher harmonics of the ringdown? Note that “rare” events will be much more
common when we get into the CE-ET era of big data. We will mostly learn new
physics and astrophysics not by consolidating our understanding of the bulk of
the population, but by looking at the unexplored corners of the parameter
space.

The large-redshift reach of next-generation ground-based observatories and
LISA also means that we will probe populations that are not accessible now.
This will allow us to:

  * •

probe compact objects that may have primordial or Population III origin; CE
and ET will be probing the population before the peak of star formation. This
gives lots of insight into metallicity, generations of stars (pop III),
formation channels, etc. It teaches us about stellar formation and evolution
in a unique and powerful way.

  * •

do cosmology with standard sirens at higher redshifts, where we could get
hints about the resolution of the Hubble tension (if it still holds by the
mid-2030s).

Finally, the synergy of detectors operating in different frequency bands will
probe different possible modifications of general relativity (or different
“new physics”). Multiband sources are best at probing modifications that enter
at negative post-Newtonian (PN) orders in the gravitational wave phasing.
Massive black holes observed by LISA give better bounds than stellar-origin
black holes at negative PN orders, but ground-based observations do slightly
better than massive black holes observed by LISA for modifications that enter
at positive PN orders. Also, sensitivity improvements in ground-based
detectors make a big difference for constraints that appear at negative PN
orders. In summary: constraining a variety of possible modifications of GR
will also require a synergy between the whole network of Earth and space-based
detectors.

Note that much of the science we can do is unlikely to be within reach of
other observatories. PTAs or EHT will not detect mergers or probe high-
curvature regions. However PTAs, and missions looking specifically for the
primordial cosmic gravitational-wave background414141https://cmb-s4.org may
discover backgrounds that may not be detectable with ground-based or space-
based interferometers.

Also to note is that there will be significant progress needed in waveform
theory and production, and potentially in analysis coding, to profit from the
much greater signal-to-noise ratio and rate expected with the detectors of the
future, ground- and space-based. A sustained effort on that front will be well
rewarded.

While the search for B-modes in the CMB is not a topic for this meeting, it is
clearly a very exciting avenue for tests of GR and for cosmology as informed
by GWs. A direct detection of the time-varying GW strain from e.g.,
inflationary cosmic gravitational-wave background is inaccessible to the next-
generation observatories on the ground and LISA. Space-based detectors
targeting this source, perhaps around 0.1-1 Hz, should be the focus of an R&D
effort. In addition, the finite lifetime of space missions invites a series of
space-based GW detectors.

###  4.2 Cosmology

Presenter: Hiranya Peiris

In this session was presented an overview of what the landscape of cosmology
is likely to be by the end of the Stage IV “dark energy surveys” in 2030s, and
encouraged the focus of Cosmic Explorer to look ahead to the likely
breakthrough questions in that era, which move beyond the questions that are
the focus of cosmological research today (such as measuring cosmological
parameters). There are several avenues with breakthrough potential, including
novel signals of phase transitions in the early universe, signatures of the
physical nature of dark matter and tests of general relativity that are not
feasible using EM data. Finally, one can make a compelling case that the most
exciting breakthrough potential of Cosmic Explorer for cosmology comes from
its discovery potential, particularly through its unprecedented redshift
reach. However, the discovery potential for cosmology needs quantification and
clear articulation (within the scientific community and to funding agencies),
both to sharpen considerations that affect design decisions and to gather
support for funding decisions. While CE has the potential to discover new
physics beyond both the standard models of Particle Physics and Cosmology, a
major challenge is apparent in quantifying this discovery potential. Beyond-
Standard-Model (BSM) signal predictions carry substantial model-dependence,
and theoretical uncertainties are significant. Several issues were put forward
for reflection and discussion:

  * •

How to better define the discovery potential for BSM signals as a function of
theoretical uncertainty? e.g., start by defining what broad classes of models
can be ruled out?

  * •

How to ensure experimental design and data reduction does not inadvertently
diminish discovery space while taking into account theoretical and
observational advances in the coming decade?

  * •

How can CE help the theory/modelling community to make forecasts that
accurately reflect CE capabilities?

  * •

How to ensure discovery space viability if coincident EM signals (esp at high
z) rare and/or non-existent?

  * •

Does increased temporal resolution in detector frame for high-z signals enable
any novel probes?

The subsequent discussion led off with the question of how the cosmology
community currently justifies projects where the dominant breakthrough comes
from discovery potential. This is typically done by clearly articulating what
broad classes of physical models (as opposed to individual models with tuned
parameters) will be ruled out if an experiment which can reach a particular
signal-to-noise detects no signal. Then a discussion followed which clarified
that the discovery potential at high redshift may be in a regime where EM
counterparts are not available or common, and hence a detector network is
crucial to preserving discovery potential in this regime.

Once again the importance of a network that can determine the source position
in the sky was stressed.

###  4.3 Radio synergy with the Next Generation Gravitational Wave
Observatories

Presenter: Poonam Chandra

A true watershed event occurred on 17 Aug 2017, when LIGO and VIRGO registered
GWs arising from the merger of two neutron stars in GW170817. This was the
first and the only event from which electromagnetic (EM) radiation was seen
from a GW source to date. GWs give information about the mass, spin, and
geometric properties like inclination angle, source position and luminosity
distance; the EM provides information about the energetics, redshift, host,
beaming, environment, ejecta mass and velocity. While the GW 170817 revealed
itself in all bands of EM spectrum, it was radio observations which proved
quite critical. GW170817 event was instrumental in proving that GW moves with
the speed of light, that the merger of neutron stars produce SGRBs and that
the heavier elements like gold etc. are synthesized inside during the binary
neutron star (BNS) mergers.

Radio observations are critical in deciphering energetics, environment and
evolution. There is a speculation of ‘prompt’ radio emission during the
merger; however, it hasn’t been seen yet. If confirmed, it could be a very
critical input to fast radio burst (FRB) origin. On the other hand, the radio
‘afterglow’ emission from compact object mergers involving neutron stars can
come via the ejecta interaction with the surrounding medium. During merger a
substantial amount of neutron-rich material is ejected. The r-process
nucleosynthesis produces a kilonova. In addition, a jet is launched due to the
rapid accretion of ejected matter onto a compact remnant, which will propagate
through the merger ejecta medium. This will be the source of non-thermal
emission. The interaction of merger ejecta with the surrounding interstellar
medium (ISM) produces a long-lasting synchrotron emission observable in multi-
wavelength bands; the radio emission is visible for a longer timescale (months
and years) and uniquely probes the environment, geometry and the energetics of
these systems. An important element in GW170817 was the jet, which transfers a
large fraction of its energy into the surrounding ejecta, forming a hot cocoon
expanding over a wide angle while traveling at mildly relativistic velocities.
As the cocoon propagates into the ISM, it will also produce a radio signal
(and other EM signals too).

The clearest evidence of cocoon was seen in GW170817. The rise of the radio
light curve at early time was consistent with a model where the radio emission
was arising from the subrelativistic cocoon4242422017, Science, 358, 1579,
2018, Nature, 554, 207. This model could also explain the EM emission in other
bands also. The microarcsecond resolution radio observations, i.e., very long
baseline interferometry (VLBI) observations, clearly revealed the presence of
a jet moving with relativistic speeds4343432018, Nature, 561, 355. In
addition, the modeling of radio observations obtained via worldwide campaign
involving multiple telescopes revealing that after the slow rise due to
cocoon, the jet did manage to pass through cocoon and revealed itself at late
time444444arXiv:2006.02382.

As is clear from above that the radio observations in GW170817 provided a
plethora of information. While it answered many mysteries, many more questions
arose. One needs to create a statistically significant sample to understand
the diversity of these mergers. For this it is important to focus on next
generation GW and EM observatories: The proposed Cosmic Explorer and Einstein
Telescope in the GW front and the next generation Very Large Array (ngVLA) and
Square Kilometre Array (SKA), are critical towards this venture. Next-
generation GW instruments will be able to detect the BNS mergers up to a
redshift of $\geq 1$ and uncover the nature of vast majority of BNS mergers at
large distances. Radio observations will provide information on the energy of
these mergers and unravel the underlying relation between NS-NS mergers and
SGRBs, and provide critical insights into how many such mergers launch a jet.
In NS-BH mergers, which may lack the bright optical emission, an optimized
radio search strategy is critical to see EM emission from them.

Radio observations provide additional constraints as well. The early time
radio observations are scintillated as the inhomogeneities in the local
interstellar medium cause modulations in the radio flux density for a source
whose angular size is smaller than the characteristic angular size for
scintillations4545451997, New Astronomy, 2, 449, 1997, Nature, 389, 261, 2008,
ApJ, 683, 924. The VLBI observations put strong size constraints as well as
directly measure the speeds of the jet. Catching the early radio reverse shock
(RS) emission (1-3 days) is critical in constraining the Lorentz factor. In
addition, the radio observations are the most sensitive probe of the immediate
environments of these mergers464646E.g., 2010, ApJL, 712, L31.

The late time optically thin spectra gives information on the particle
acceleration mechanisms. Due to the slow evolution of the synchrotron in radio
bands, the radio emission is detectable even when the GRB jet is in the non-
relativistic regime. However, by this time, the jet geometry has become quasi-
spherical. Thus, one can constrain the energetics irrespective of the
information on the jet geometry. Even in cases where the jet is off-axis, only
possibility of EM detection is the late time measurements at radio bands. That
is because the slowing down of the jet may reveal radiation towards earth,
detectable only in radio bands at late times. In addition, the late-time radio
observations of the sub-relativistic kilonova afterglow can complement post-
merger GW searches aimed at identifying the nature of the leftover merger
remnant and provide indirect constraints on the EoS. X-ray emission from this
component has already been seen in case of GW 170817474747arXiv:2104.02070,
and it may be revealing itself in radio bands quite soon4848482021, ApJL 914,
L20. Another advantage of radio observations in searching for EM counterparts
is the quieter transient sky than that in optical bands, hence fewer false
positives.

Radio observations are also capable of constraining some fundamental
constants. For example, the joint GW plus EM analysis led to Hubble constant
of $H_{0}=74^{+16}_{-8}\rm kms^{-1}Mpc^{-1}$. The uncertainly in $H_{0}$ was
dominated due to the degeneracy between the source distance and the observing
angle. The radio VLABI observations put strong constraints on the observing
angle which allowed to improve Hubble constant significantly, i.e.,
$H_{0}=70.3^{+5.3}_{-5.0}\rm kms^{-1}Mpc^{-1}$ 494949E.g.,
https://arxiv.org/abs/1806.10596. It is to be noted that while 50-100 GW
events needed to resolve the tension between the Planck and Cepheid–supernova
measurements, only 15 GW170817-like events, having radio images and light
curve data, suffice.

With the improved sensitivity of the ngVLA working in tandem with GW
observatories, one can put constraints on the radio polarization of the BNS
afterglow which, together with very long baseline interferometry (VLBI)
efforts aimed at directly characterizing the structure of the ejecta, can
reveal information (which so far remains inaccessible) regarding the structure
of the magnetic fields. The GW-triggered events are ideal to do this because
they are easier to see off-axis, hence potentially constraining the
polarization signal with enough sensitivity.

GW plus radio synergy will also reveal critical information on the supernova
(SN) explosion mechanism. CE will be sensitive to SNe explosions within Milky
Way or at most its satellite galaxies. This will be complemented by the
proposed Deep Underground Neutrino Experiment (DUNE) which will also be
sensitive to detect such SNe. While radio measurements will trace the mass
loss history of the progenitor and hence constrain the progenitor model, the
neutrinos and GWs will unveil the heart of the star, where EM cannot reach.

Another potential area for GW plus radio synergy is the origin of fast radio
bursts (FRBs). There are currently two most favourable origins of FRBs:
magnetar origin (for repeating FRBs) and the BNS merger origin (non-repeating
ones). The discovery of faint pulses from the Galactic SGR 1935+2154 has
confirmed that at least some FRBs may originate via the first channel. Radio
observations are most sensitive to density of the environment and can
potentially identify between the two possible progenitors. However, there are
two fundamentally different strategies to discover FRBs (or flares) in BNS
mergers. i) Search for FRBs after GW is detected from mergers involving at
least one NS. In such a case, fast response of radio telescopes is
critical5050502021, MNRAS, 505, 2647. One can also rely on radio surveys. In
such a case building an automated FRB detection pipeline will be most
critical; and ii) to search for GW if a FRB is detected. For such cases it is
important to go back to the GW database and search for an event. In both
cases, accurate localization of the signal is necessary.

In conclusion, I would like to emphasize that the GW plus radio synergy has
potential to bring revolution in the field of BNS and NS-BH mergers, local
supernovae, probe of compact remnants of first stars to current universe and
many new phenomena! The GW Astronomy is an international effort!!! People from
various backgrounds, such as physics and astronomy, engineering, computer
science, data science, and statistics etc. need to come together. The most
critical information needed from GW observatories is the quick access to data
and the smaller localization boxes. The next generation GW observatories are
capable of meeting these challenges. The upcoming radio facilities such as
ngVLA and SKA are well placed to keep up with the GW efforts in this field.
The radio plus GW synergy is likely to open up a new parameter space. The
scope is only limited by our imagination!

###  4.4 Gamma/X-ray Observations

Presenter: Eleonora Troja

The most obvious high-energy counterparts of GW sources are gamma-ray bursts,
short-lived and bright flashes of gamma-ray emission produced by a
relativistic outflow launched by the merger remnant. Joint GW/GRB detections
probe the critical few seconds following the merger, allowing us to explore
the formation of rapidly rotating massive NSs, the birth of BHs and how they
affect their surroundings by launching beams of highly energetic radiation.
There are a number of questions left open by GW170817 whose gamma-ray
counterpart did not resemble a standard GRB in many aspects. For example, the
mechanism driving the gamma-ray emission (typical internal shocks or cocoon
break-out?) and the origin of the delay between the GRB and GW signals
(viewing effects, formation of an intermediate NS or jet launching?) remain
unknown. It is likely that we will be able to address these questions not
through sporadic joint detections but through a population study. Similarly,
the elusive BBH mergers might produce weak EM counterparts, but only in rare
circumstances and only a statistical study of joint detections would allow us
to confirm it or rule it out. There are a number of approved pathfinders (e.g.
HERMES), planned for launch in the next few years, to demonstrate the
feasibility of a constellation of nano-satellites offering nearly complete
coverage of the gamma-ray sky at a sensitivity a few times better than the
Fermi GBM. Given their modest costs and technological readiness, it is
conceivable to think that a similar fleet of gamma-ray satellites will be
operative at the time of CE.

An even more intriguing experiment would be to extend these prompt studies to
lower energies (e.g. soft X-rays), where different components of emissions
such as precursors, extended emission and magnetar plateaus are better
visible. These X-ray features, although present only in a fraction of SGRBs (
8-20%), could be directly linked to the progenitor properties, such as its
mass and spin, and provide us with novel constraints on the NS EOS before and
after the merger. To enable these observations, early pre-merger warnings are
a critical component. Several wide-field ( 1000 sq deg) X-ray monitors with
rapid slewing capabilities (like Swift) are under study and will be proposed
to forthcoming NASA calls (e.g. SIBEX). Luckily the AstroDecadal 2020 strongly
endorsed the launch of small- and medium-size missions with time-domain
capabilities in the X-ray, IR and UV. These could be launched in the late
2020s - early 2030s and have a good chance to be operative at the time of CE.
An early ( 5 min) alert – even with a crude localization – would allow these
X-ray imagers to repoint the GW location and stare at it right at the time of
the merger, catching all the features not accessible through gamma-ray
observations.

Athena, the next flagship X-ray mission, is one assured partner for both LISA
and CE-ET. Athena is planned for launch in the early 30s, with a mission
lifetime of 4-5 years plus possible extensions. Given that the Decadal
recommended to postpone Lynx, we consider these extensions likely as there
will be no other observatory to serve the X-ray community. Athena combines
rapid ToO capabilities ($\leq$ 4 hours in 50% of the cases) with high
sensitivity (x10 Chandra) and will allow us to open new space for discoveries
by looking at the first few hours after the merger with unprecedented
sensitivity, achieving constraints that neither Swift nor Chandra can place
due to their limited sensitivity and longer response time, respectively. For
additional information on Athena and its multi-messenger capabilities, read
the Multi-Messenger White
Paper515151https://ui.adsabs.harvard.edu/abs/2021arXiv211015677P.

There is a wealth of science available by combining gamma and X-ray
observations with GWs, but the availability, timing, and lifetime of space
missions are critical ingredients to realizing the synergy.

###  4.5 Nuclear Physics

Presenter: Andrew W. Steiner

It is clear that gravitational waves can provide insights into nuclear physics
which are probably inaccessible otherwise. Let’s look first at some key
questions which could be pursued with high-SNR GW BNS waveforms alone,
complemented with advances in nuclear physics theory.

Determining the relationship between pressure and energy density at zero
temperature, $P(\varepsilon)$ over the entire range of densities probed in
neutron star interiors is possible if three conditions are met: (i) nuclear
theory can determine $P(\varepsilon)$ at low densities (ii) gravitational wave
observations can determine the relationship between tidal deformability and
gravitational mass, $\Lambda(M)$ over all physically realizable neutron star
masses, and (iii) there is sufficient overlap between the low densities
described by theory and the high-density region constrained by $\Lambda(M)$.
In addition, future work on understanding the role which
$\delta\tilde{\Lambda}$ plays in the post-Newtonian expansion may be required.
The recent detection of gravitational wave signals from several neutron star-
black hole binaries with large mass ratios provides some evidence that
$\Lambda(M)$ could be measured across the full mass range in the near future.

Going a step further, a complete specification of the equation of state (EOS)
would, for example, entail a calculation of the Helmholtz free energy density
as a function of density, temperature, and all of the densities for the
relevant degrees of freedom. While the zero-temperature pressure-energy
density relation is likely to be well-constrained by the pre-merger
gravitational wave signal, determining the full equation of state (and all of
the associated derivatives like the specific heat, the compressibility, and
the speed of sound) from gravitational waves will require information from the
post-merger signal and merger simulations to connect the post-merger signal to
the EOS. This would argue that the 20 km Cosmic Explorer with its targeted
sensitivity in the post-merger frequency range will be important to
determining the nuclear EOS.

While electromagnetic observations of neutron stars may, in general, have
larger systematic uncertainties, constraining the full EOS will likely require
combining gravitational wave information with photon-based observations of
neutron stars which are not merging.

At lower densities where the ground state of quantum chromodynamics consists
principally of neutron and proton degrees of freedom, the equation of state is
only one of a number of quantities which are interesting for nuclear physics.
In addition, it will be useful to use mergers to constrain neutrino opacities,
viscosities, the nucleon dispersion relations and single-particle potentials,
the extent of correlations between nucleons, and the parameters of the
nucleon-nucleon interaction. Additionally, all of these quantities are of
interest in both the core and the crust, where exotic nuclei dramatically
modify transport properties. Finally, the possible presence of non-nucleon
degrees of freedom such as hyperons or quarks is a critical question for
nuclear physics and QCD – addressing, for example, the relationship between
the dynamical breaking of chiral symmetry and deconfinement. All of this
nuclear science is potentially accessible in the post-merger gravitational
wave signal. Again, merger simulations are required to recast the
gravitational wave signal into answers to these nuclear physics questions.
Thus, all of the nuclear physics enumerated above is also required to perform
an accurate merger simulation.

Determining the abundances of r-process nuclei created in each neutron star
merger is at the center of several nuclear astrophysics questions. What
fraction of r-process elements come from mergers? Do mergers produce all
r-process elements up to the actinides? How much do r-process abundances
change depending on the nature of the merger? Answering these questions from
gravitational wave and the concomitant electromagnetic light curves
additionally requires a description of nuclear masses and nuclear reaction
rates. Some of these masses and reaction rates are measurable in new
facilities like the Facility for Rare Isotope Beams (FRIB), and some are
comparatively inaccessible.

Related frontiers in nuclear physics: There are several important related
nuclear physics questions which will need to show progress in order to fully
profit from the GW and EM neutron-star observations:

  * •

Can we improve the accuracy of results obtained from chiral effective theories
above the saturation density, e.g., by calibrating the interaction to heavier
nuclei or adding new degrees of freedom?

  * •

Can we accurately compute the density and energy associated with nuclear
saturation?

  * •

What is the spin response of nucleonic matter?

  * •

How can we improve energy density functionals to describe exotic nuclei and
their associated nuclear reactions?

  * •

Can new quantum many-body methods or quantum computing help address the
fermion sign problem?

  * •

In what density and temperature regimes is the random phase approximation an
accurate method for computing neutrino opacities? What many body methods are
most effective at higher densities?

  * •

How do we combine our models of matter constructed with nucleon degrees of
freedom with models of quarks and gluons at the quark-hadron phase transition?

  * •

How do we create usable nuclear physics input for merger simulations? We will
likely, in the near future, want to go beyond the traditional inputs of the
equation of state and neutrino opacities to include quantities like the
nucleon single particle potential. However, these potentials are only uniquely
defined in the context of a particular many-body method. Do we need to think
about other ways of characterizing the nuclear physics input in terms of
observable quantities?

###  4.6 Prospects and Challenges for Optical/Infrared Counterparts in the CE
Era

Presenter: Wen-fai Fong

The detection of optical and infrared (optical/IR) light from a binary neutron
star (BNS) or neutron star-black hole (NSBH) merger gives a wealth of
information on the astrophysical outflows from mergers. The primary sources of
optical/IR light are thermal transients powered by the radioactive decay of
neutron-rich heavy element isotopes created in the merger (“kilonovae”) and
non-thermal synchrotron emission powered by the relativistic (often jetted)
outflows from the poles of the remnant (“afterglows”).

The main science drivers for studying the opt/IR counterparts are:

  * •

Deciphering the origin of heavy (r-process) elements, and the relative
contributions of mergers versus other explosive phenomena (such as Type II
supernovae)

  * •

Quantifying the impact of chemical enrichment from mergers on the evolution of
galaxies.

  * •

Building an observational picture for the interaction between the mildly
relativistic kilonova ejecta and the relativistic afterglow, and the effect of
that interaction on observational signatures (e.g., jet structure)

  * •

In support of GW observations, placing complementary constraints on the
maximum mass of NSs by inferring the nature of the central remnant left after
the merger, thereby placing indirect constraints on the NS Equation of State.

Cosmic Explorer (CE) provides an exciting opportunity to detect NSNS mergers
to z$\sim$5 and beyond, allowing us to explore the optical/IR counterpart
science drivers as a function of cosmic time. This is an extremely promising
avenue of research. We can also leverage the population of cosmological short-
duration gamma-ray bursts (SGRBs), which are routinely localized and detected
over z$\sim$0.1-2 which provide an automatic baseline for comparison to a
population of CE-detected events.

Clear Challenges as we Move Forward in Time and Outward in Distance

The Triumph of GW170817: The discovery of light across many orders of
magnitude in wavelength from the first BNS merger, GW170817, will forever be a
landmark event. In particular, the rapid “blue-to-red” color evolution of its
kilonova in the first 20 days, coupled with broad features in contemporaneous
optical/IR spectra, were clear-cut signatures of r-process nucleosynthesis.
The synchrotron afterglow remained visible for much longer, thanks to Hubble
Space Telescope Observations, through 400-days post-merger. Coupled with X-ray
and radio observations of the afterglow, the afterglow spectrum and its
evolution over time was used to constrain key properties of the jet
(geometry/opening angle, kinetic energy, microphysics, viewing angle).

Challenges highlighted in the status quo: We use the example of GW190814, with
component masses of $\sim$23.1$\rm M_{\odot}$ and 2.59$\rm M_{\odot}$ at a
distance of 241 Mpc to highlight current challenges with optical/IR
counterpart searches. GW190814 had the key ingredients necessary for follow-
up: (1) a fairly small final localization map of 19 $\rm deg^{2}$, palatable
for many optical/IR observatories, (2) prospects for an EM counterpart given
the ambiguity of the mass of the second object (and potentially the first
announced NSBH merger in the circulars), and (3) not extremely distant, within
the 250 Mpc horizon. Thus, the way that the community followed up this event
is a fantastic proof-of-concept for what we are (in)capable of at present.
Despite many observatories pointed at this event, and employing different
search strategies (galaxy-targeted versus wide-field mosaic approaches), only
a couple of searches actually reached the adequate depths to rule out a
kilonova of similar luminosity to GW170817 and covered the entire localization
region. Although it may be unsurprising to have not seen an optical
counterpart (given the likely BBH merger classification), this highlights that
even with our most capable resources at present-day, we are definitely
resource- and instrumentation-limited at the fairly “local” distances of
LIGO/Virgo events, compared to those which will be detected by CE.

Moreover, across all O3 NS-involved events, the number of viable candidates
that could still be consistent with the behavior of kilonovae (and have not
yet been ruled out to date) still remains high. The advent of real-time tools,
such as services which offer photometric or spectroscopic redshifts, or pre-
merger detections, are fantastic resources that should be leveraged in O4, A+
and the CE era.

Priorities for CE

The light is a bit brighter at the end of the tunnel, though. The pursuit of
SGRB counterparts and afterglows have demonstrated that precise localizations
are key to ever finding a counterpart. Small-field instruments that can
feasibly detect afterglows or kilonovae out to z$\sim$2 are abundant, and will
become even more numerous with the advent of the ELTs during the CE era. Thus,
localization is the number one priority to mitigate the need for wide-field
follow-up and reduce the overall optical/IR candidate/false positive number.
For reference, the average SGRB localization from Swift is a few arcseconds,
and in almost all cases, a host galaxy can be identified.

A second recommendation is to provide component masses or a firm idea of the
probability of ejected material, as soon as the collaboration is able. We will
become increasingly limited by the resources capable of pursuing optical/IR
counterparts at these redshifts. Thus, in the era of multiple events per week,
this is crucial to inform the observational community on how we can devote our
limited resources to follow the most interesting events (or more specifically,
those events with high probability of having ejected material).

One other frontier to look forward to in the CE era is the advent of redshift
surveys, such as DESI (spectroscopic) and Rubin Observatory’s Legacy Survey of
Space and Time (LSST) (photometric) which will enable us to provide informed
host galaxy associations based on a priori information. By cross-matching the
GW luminosity distance with available hosts in the localization with known
redshifts, one could hope to better direct follow-up resources.

Finally, optical observatories should be making themselves “time-domain ready”
in terms of rapid response, rapid data availability, and data reduction
pipelines. This will streamline the ability for observatories to respond in
real time. The Rubin Observatory is perfectly suited to carry out this type of
science, perhaps as a more “full-time job” once the LSST survey is concluded.

###  4.7 Cosmic Neutrinos

Presenter: Ke Fang

Cosmic particles are a unique type of messengers that are complementary to
waves in probing the universe. They include neutrinos, which are weakly
interacting, neutral particles that can travel over cosmological distances,
and cosmic rays, which are charged, strongly interacting particles that are
confined by magnetic fields. While high-energy photons trace the footprints of
both relativistic leptons and hadrons, high-energy neutrinos can only be
produced by interactions of relativistic hadrons. High-energy neutrinos thus
open up a clean window on nature’s hadron accelerators.

A population of high-energy neutrinos has been detected by the IceCube
Neutrino Observatory. They present a flux that cannot be explained by known
models of atmospheric neutrino and muon backgrounds and therefore must be of
astrophysical origin. This year, the first cosmic tau neutrino candidate and
the first hint of an electron anti-neutrino were identified in the IceCube
data. Several potential neutrino sources have been found, including a flaring
blazar, TXS 0506+056, and an active galaxy, NGC 1068. The sources of the bulk
of the astrophysical neutrinos remain unknown.

Neutrinos and gravitational waves (GW) may be combined as a powerful tool to
study compact objects. GW and thermal neutrino emission reveal the dynamics
and thermodynamics conditions of a merger remnant. The extreme environment
also provides promising sites for particle acceleration and production of
high-energy neutrinos. For example, the merger of a neutron star (NS) binary
may result in the formation of a long-lived, or indefinitely stable,
millisecond magnetar remnant surrounded by a low-mass ejecta shell. Ions in
the pulsar wind may be accelerated to ultra-high energies, providing a
coincident source of high-energy cosmic rays and neutrinos. The high-energy
neutrino signal may be detectable for individual mergers out to 100 Mpc by
next-generation neutrino telescopes such as IceCube-
Gen2525252https://www.icecube-gen2.de/index_eng.html and
KM3Net535353https://www.km3net.org, providing clear evidence for a long-lived
NS remnant, the presence of which may otherwise be challenging to identify
from the gravitational waves alone.

Another promising source of combined neutrino and gravitational waves are
core-collapse supernovae. The reach of (even) CE and ET is such that events
are only anticipated at a frequency of one or several per century, but the
results would be rich from an observation. A GW signal from a galactic core-
collapse supernova would encode these inner dynamics, and offer vital new
information. Furthermore, since thermal neutrinos emitted from the hot and
dense proto-NS are also detectable, GWs provide the complementarity necessary
to unlocking longstanding mysteries about the explosion mechanism. Together,
GWs and neutrinos would also provide detailed information about fundamental
processes that can reveal nuclear and particle physics inaccessible in the
laboratory.545454Astrophys. J. Lett. 876 (2019) L9 [1812.07703];
https://arxiv.org/abs/2111.06990

Neutrino follow-up searches have been carried out by the IceCube and ANTARES
collaborations for sources in the GW transient catalogs and events observed by
the LIGO/Virgo O1-O3 runs. No significant association has been identified,
posing limits on the isotropic high-energy neutrino energy released by the GW
events. The next-generation neutrino observatories, and CE and ET, have the
potential to detect signals from a common source. Together they provide
important synergistic capabilities to reveal the physics of compact cosmos.

###  4.8 Panel: GW-enabled Observational Science between today and 2035

Sathyaprakash, Discussion leader/coordinator. Participants: Jenny Greene, Cole
Miller, Stephen Smartt, Ashley Villar

We expect the current facilities to continue interleave observation with
incremental upgrades between now and 2035. The A+, and AdV+ upgrades and LIGO-
India are already funded and in the implementation phase, and others are in a
conceptual phase (see 5.2). The panel focused on what can be accomplished with
the existing facilities with regard to some of the major science questions in
different fields. The panel consisted of Jenny Greene who spoke on
supermassive black holes, their seeds and their growth, Cole Miller on the
population of neutron stars and black holes and Stephen Smartt and Ashley
Villar talking about electromagnetic follow-up of gravitational-wave
transients. The panel was asked the following questions:

Questions posed to the group include

  * •

What are the major questions and challenges in your field?

  * •

What are the requirements from GW observations to make a concrete impact in
your field between now and 2035?

  * •

What non-GW opportunities are there to make progress in your field and how can
GW observatories complement those measurements between now and 2035?

  * •

What challenges do you think will remain to be resolved beyond 2035?

  * •

What non-GW opportunities are there to make progress in your field and how can
GW observatories complement those measurements between now and 2035?

Seeds of supermassive black holes and their growth: A major question in this
field is if there are black holes in the mass range [$10^{2}$, $10^{5}$] solar
masses. This is the mass range in which we have no idea but they are also
interesting because their presence could hint us at the physical mechanism in
the formation of seed black holes that grow to become supermassive black
holes. There are three pathways: (1) direct collapse models of rare, heavy
seeds of $10^{5}$ solar masses, (2) 100 solar mass seeds, or (3) hierarchical
black-hole growth at the centers of dense stellar clusters. We do not know
which of these processes led to the supermassive black holes that we see in
galactic cores.

In the next decade, we will learn exactly how black holes form and evolve with
the help of gravitational and electromagnetic observations. LISA should
observe some $10^{4}$ solar-mass mergers and the mass distribution will depend
on the seeding mechanism. In the 100-solar mass range LIGO and its upgrades
will provide the evidence for gravitational run-away processes. LISA will also
observe mergers involving extreme mass ratios providing further evidence for
the growth of seeds. Heavier seeds will lead to smaller merger rates (about 10
per year) and will occur at redshifts of a few to z 15. Lighter seeds will
lead to merger rates that are an order of magnitude larger.

The Extremely Large Telescopes including the the European Southern
Observatory’s ELT, and the two proposed US-led Thirty Meter and Giant Magellan
Telescopes will allow us to get number densities and the dynamics, while
gravitational-wave observations will help us get a combination of the number
of densities and merger rates of black holes up to redshifts of z=10. We don’t
know how often intermediate mass black holes merge due to the final parsec
problems. Next generation x-ray telescopes will reveal the feeding mechanisms
and the growth of massive black holes. Tidal disruption events will provide
additional information about the growth of black holes.

Neutron Star and Black Hole Populations: The major questions and challenges
are understanding of the equilibrium properties of the cold dense matter in
neutron star cores and the origin and diversity of the masses, mass ratios,
spins (including inclinations), and eccentricities of binary neutron stars and
black holes. In this regard, GW observations will be helpful in the
accumulation of data; trends and exceptions in the observed population. For
nuclear physics, greater precision at high frequency will be extremely useful
in observing the post-merger signal from binary neutron stars and binary black
holes as that will be critical to inferring the dense matter equation of state
and testing general relativity, respectively. For binary populations, greater
sensitivity at low frequencies will help in greater accuracy in the
measurement of parameters as also discovering new populations of intermediate
mass black holes. Further progress in the field would need theoretical
progress in waveform models, especially at high frequencies to understand what
signatures of the dense matter equation-of-state and quark-deconfining phase
transitions might be present in the post-merger signal. Furthermore, continued
development of models of EM afterglow in the case of double neutron star and
neutron star-black hole binaries would be necessary to interpret the data to
learn about the central engines of gamma-ray bursts and the associated
mechanism behind relativistic jets as also to interpret the kilonova light
curves. Transition to genuinely predictive models of binaries, which means a
clear indication of what outcomes are impossible in each model would be
desirable. Challenges that will remain past 2035 include low tidal
deformability of neutron stars of masses greater than about 2 solar masses, so
the NICER radius measurement of a 2.1$\rm M_{\odot}$ star may still be state
of the art. 2. For populations, M$>$upper limit of upper mass gap would be
wonderful but Nature may not be that kind. Improvement in low-frequency
sensitivity would prepare us to receive Nature’s kindness.

EM Follow-up of GW Events: Two major challenges in the electromagnetic follow-
up of gravitational-wave events are in the development of optimal search
algorithms for the detection of post-merger prompt emissions with minimal
survey interruption. First, survey times available in large telescopes will be
limited and hence it is important to develop algorithms that make the best use
of the allocated time. In this regard, it is important to compile complete
galaxy catalogs that are relevant to the sensitivities of GW detector upgrades
and new observatories. Secondly, truly multimessenger astronomy analysis
methods are currently lacking as they don’t address more than two data sets at
any one time. Full forward modeling of all EM signatures would be necessary to
realize the full potential of multimessenger astronomy. EM follow-up would
greatly benefit from smaller, accurate and rapidly available localization
maps. However, it is particularly important to ensure that the smaller maps
are not systematically offset and are accurate when first released keeping in
mind that observing times on large telescopes are too expensive. It would also
be helpful to have rapid classification (e.g., ‘EM bright’) of detected events
as well as quick updates to the classifications of the GW sources when they
change.

Currently, there are early models for kilonova and afterglows but with
improved observations and greater number and variety of events it is necessary
to have better EM models for kilonova light curves and afterglows as well as
tools ready to describe exotic phenomena. For example, kilonova would need
nebular spectral models, especially to be used as calibration for early models
as well as extended grids of numerical simulations. Additionally, we would
need better treatment of off-axis jets to observe afterglows as there will be
significant increase in the number of off-axis systems that will be observed
in the coming years. Continued monitoring of SGRBs and their hosts will be
necessary to build a comprehensive understanding of the physics of these
relativistic sources. By 2035, gravitational-wave detectors will begin to
observe binary neutron star events from redshifts of 0.5 and beyond. We need
EM follow-up strategies for such high-z EM counterparts and none exists today.
Ensuring that Rubin Observatory is operational and running, after the 10 yr
survey is complete would be critical for following up GW events both in high
numbers but also at great distances. It is also crucial to have X-ray and
gamma-ray detectors available on the timescale of the next two decades.
Specifically, planning would need to start now to have space detectors like
Swift in place by 2035.

Open Discussion: Many EM observers are more interested in the continuous
operation of the near-term GW network than upgrades in sensitivity, once GW
detectors are sensitive to sources at z$\sim$0.2. There aren’t any telescopes
that can observe EM counterparts at greater distances and so upgrades will not
help in following-up more events. At O3 sensitivities, sky localization is
pretty poor except for the closest events and the redshift reach is about 120
Mpc for binary neutron stars. This will improve to 160-190 Mpc for O4; still,
only a handful of events will be localized to within 20 $\rm deg^{2}$. In O5
the distance reach for BNS will be about 300 Mpc, z$\sim$0.07 and $\sim$10
events per year with localization typically of 20 $\rm deg^{2}$. In order to
reach better localization than 20 $\rm deg^{2}$ for about 50 events per year
would require sensitivities that are better than what’s expected for O5. The
continuous incremental improvement will be welcome for event rates (with the
gain as the cube of the sensitivity), and the approximate factor of 2 gain in
SNR for a given source will also help in extracting additional information
from the waveforms. It is important to have the detectors running continuously
in order to catch once-in-a-lifetime opportunities such as a supernova in our
own galaxy. The answer is to have an operating network so that it will be
possible to have at least some of the detectors operating at any one time. We
are not there yet but could get there by the turn of the decade. GW detectors
can be tuned to work better at higher frequencies at the cost of worsened
sensitivity at lower frequencies. Should one go after guaranteed science or
take a leap of faith and look for new sources? At low frequencies one is
making a bet that there are sources at those frequencies but this is not
guaranteed since the mass range will be very large. 100 solar-mass events
might be the top end of the mass range from stellar processes but there might
be nothing at 1000 solar masses. At high frequencies, $\sim$1 kHz matters more
than 3 kHz. At 1 kHz neutron stars will be simple: relatively cold compared to
their Fermi energy, not rotating much let alone differentially rotating and
information you get can be interpreted in well-understood theoretical
framework modulo improvement in waveforms. At higher frequencies neutron stars
will be far more complex: hot neutron stars with temperatures comparable to
Fermi energy, differential rotating, neutrino viscosity. It will be a greater
challenge to interpret the data and waveforms are difficult to model. However,
stellar mass black holes at high redshifts will appear as intermediate-mass
black holes and hence it is still important to have good low-frequency
sensitivity. For early warning triggers, how early should the alerts be? For
successful EM follow-up what is important is to provide accurate information
about masses in addition to sky position; early warning is not important for
IR as there is no rapid evolution in the IR spectrum in the first few minutes.
However, optical flash and gamma rays are important and they would change on
the time-scale of seconds and radio emissions could occur even before the
merger.

###  4.9 Next-generation Observatory science goals

Presenter: Joshua Smith

The GWIC ‘3G’ Science
Book555555https://gwic.ligo.org/3Gsubcomm/docs/GWIC_3G_Science_Book.pdf,
prepared by a community-wide team of interested scientists, has formed the
basis for the current models of science objectives for CE and ET. Each of
those projects then considered the trades between the observational science
goals and the technologies that would be needed to realize them.

In the case of CE, a new trade study was undertaken to help focus the design.
A top-level set of science goals was identified, informed by the GWIC Science
Book and many discussions with the scientific community. Networks were studied
consisting of one or two CE detectors, with or without ET and also considering
a detector in the southern hemisphere. Parameters of the designs were adjusted
to find the best combination of features to meet the observational science
goals for different networks, leading to the design laid out in the Horizon
Study.

The CE team chose to identify a limited number of key objectives565656Please
see the Horizon Study for more detail,
https://dcc.cosmicexplorer.org/CE-P2100003/public:

  * •

Black Holes and Neutron Stars Throughout Cosmic Time: Cosmic Explorer is
designed to detect gravitational waves from black holes and neutron stars in
binaries to redshifts of $\sim$10 and above. The goal is to shed light on
Population III stars through the black holes they might have left behind, to
measure the properties of the first black holes and their role in forming
supermassive black holes and galaxies, and to characterize the populations of
compact objects and their evolution.

  * •

Dynamics of Dense Matter: The Cosmic Explorer design has the capability to
measure gravitational radiation from binary neutron star coalescences and
provide the precise source localizations required for multimessenger
astronomy. Here the goal is to determine the internal structure and
composition of neutron stars, explore new regions in the phase diagram of
quantum chromodynamics, map heavy element nucleosynthesis in the universe
through counterpart kilonovae and distant mergers, and reveal the central
engine for the highly relativistic jets that power short gamma-ray bursts.
Observations of binary coalescence, and searches for continuous-wave signals
from pulsars, will inform this science.

  * •

Extreme Gravity and Fundamental Physics: Cosmic Explorer’s significant
increase in sensitivity is intended to deliver exceptionally high resolution
of strong gravitational waves and to increase the reach to see weak and
distant sources. This can reveal the (potentially new) physics of the most
extreme gravity in the universe, enabling investigation of the nature of
strong gravity with unprecedented fidelity, discovery of unanticipated compact
objects beyond the reach of current detectors, and a path independent of
electromagnetic and particle data to pursue the nature of dark matter and dark
energy.

  * •

Discovery potential: This is both the least-well defined goal of observations
in this new domain, and perhaps also the most exciting. For the previous
targets, modeling can lead to specific measures of the ability of a design to
deliver the desired science. For discovery science, pushing the key top-level
performance measures of best sensitivity, bandwidth, and numbers and placement
of the nodes of the network is indicated. Cosmic Explorer and Einstein
Telescope have pursued this goal in their designs, and possible discoveries in
quantum gravity, new particles and fields, and any one or more of sources of
stochastic backgrounds are some outcomes currently imagined. Nature may have
others.

The observational science objectives of these next-generation observatories
will continue to evolve in the coming years, informed by input from the non-GW
community, theory, and the growing body of gravitational-wave signals from the
current detectors. The Observational Science presentations and discussions on
the first day of Dawn VI – and the discussions to follow – will be significant
input for the next round of design refinement for CE and ET, and the growing
level of activity in planning the multi-messenger activities in
instrumentation and observational science.

###  4.10 Observational Science Recommendations

Recommendations and key observations from the presentations and discussions:

  * •

A network of detectors is important to realizing the science goals of the
community. This point came up throughout the meeting, and the strong consensus
is that three (or more) detectors must be the goal.

  * •

Explicit coordination of GW observation with (in particular) space missions
(worldwide) due to their limited lifetime is needed to ensure that MMA can be
best realized.

  * •

In general, there would be value in establishing a structured regular
interaction between the GW and EM/particle/nuclear domains to ensure insights
which can impact any of the domains are communicated clearly and in a timely
way. A small group that has visibility and credibility with all communities
would be best.

  * •

The desired precision of localization by a GW network, and the need for low
latency vs. high precision, may merit a focused meeting or exchange. This
could help drive detector designs and observatory placement (which have of
course also other constraints: Technical capability and cost).

  * •

‘Followup Fatigue’ is starting to appear. GW alerts from this time forward may
have to carry additional information to aid in triage.

  * •

The tension between observing and improving the sensitivity with its
concomitant downtime continues. This would be a valuable topic for a Community
Town Meeting, both to look carefully at the upgrade argument and to hear more
about the value of extended observation at a fixed sensitivity.

  * •

Related is the need for proper coordination between the detectors. Once KAGRA
and LIGO-India at a sensitivity level comparable to LIGO and Virgo, phased
upgrades will be possible. These should have input from the non-GW observing
community and be aware of limited-lifetime space missions.

  * •

Every effort should be made to avoid a long ‘upgrade’ gap. Both LIGO and Virgo
intend, if funding allows it, to continue to observe until CE and ET are ready
to start observations.

##  5 Current and future Observatories

###  5.1 Introduction

There has been very significant progress fulfilling many of the
recommendations for the ground-based detectors developed in Dawn IV and Dawn
V. The LIGO A+ and Virgo AdV+ upgrades have been designed, funded, and are
largely implemented. Planning exercises in LIGO and Virgo are underway to
consider the interval from between the A+/AdV+ epoch to the CE/ET epoch. LIGO-
India has a site and significant R&D infrastructure has been created. KAGRA
has undertaken an initial observational run. The ‘3G Reports’ were completed,
and a more general GWIC Roadmap published. White Papers for the US Astro2020
Decadal were written and submitted. From the perspective of the next-
generation observatories, this culminated in the successful proposal to ESFRI
from ET, and the completion of the CE Horizon Study. This session of Dawn
looked at the state of the current and planned Observatories, with a focus on
the US Program.

###  5.2 The roadmap between current observatories and next-generation
detectors

Peter Fritschel, discussion leader/coordinator; Participants: Daniel Sigg,
Brian Lantz, David Ottaway, Paul Lasky, Matteo Barsuglia, Rana Adhikari, Ryan
Wollaeger

The ‘A+’ project575757https://arxiv.org/abs/1410.5882,
https://arxiv.org/abs/1304.0670 and the Virgo ‘AdV+’ project585858Proc. SPIE
11445, Ground-based and Airborne Telescopes VIII, 1144511 (13 December 2020);
https://doi.org/10.1117/12.2565418 upgrades for 2G detectors, now in progress,
follow an attractive incremental model: rapidly and opportunistically applying
select new technology, as it becomes available, to escalate the observing
horizon in stages. The O4 observing run is expected to start in late 2022 of
about one year duration, with a significant fraction of the ‘plus’
technologies installed and commissioned. The O5 observing run, with the LIGO
detectors ultimately operating at the design sensitivity targeted for A+, is
currently expected to run through mid-2028.

Beyond 2028, the LIGO Laboratory is firmly committed to continued observations
of the gravitational-wave sky. Plans are currently being developed by LIGO and
the LSC (and also in Virgo) to chart a path for interferometer upgrades and
observing periods beyond O5 and improve the detectors’ strain sensitivity
across the frequency band (from $\sim$10 Hz to $\sim$3 kHz employing some of
the technical advances being planned for Cosmic Explorer. These enhancements
can lay firm groundwork for future 3G designs, vetting aspects of the new
technology in the most stringent, realistic context currently possible. Key
questions being pursued are

  * •

What dates bracket when O5 observing will be brought to a close, and the start
of a post-O5 observing run? As one observational-science criterion, O5 should
continue long enough to exceed the event count had the instrument simply
continued with O4 sensitivity. There are also observational-science motivation
for a continuous observation. An upgrade should be ‘ready’ before ending
observation, in order to minimize downtime; the durations of downtime and the
post-O5 run should be such that the observational science goals of the greater
community are best satisfied.

  * •

What observational science drivers are there for both the kind of upgrade and
the timing of the upgrade? As we learn more from observing we can move more
from ‘the best instrument we can build’ to ‘the instrument that best delivers
the science objectives’. We also will want to try to synchronize with
constrained observing programs in the electromagnetic and particle fields.

  * •

What technologies suitable for upgrades have the greatest impact on informing
CE designs? Cryogenics and crystalline coatings are two examples of
technologies with great potential for next-generation observatories; both come
with capital costs and additional schedule, cost, and technical risks.

  * •

The current detectors have significant excess technical noise at low
frequencies, impacting current and future observational science goals. How can
the designs for post-O5 detectors best address this excess (or at least
facilitate the effort to identify and mitigate the excess)?

At low frequencies, the kinds of improvements under consideration for the 3-
and 4km infrastructures include larger test masses and improved suspensions
for them, and means to reduce the motion of the seismic isolation platforms.
At low-to-mid frequencies, we can employ incremental or qualitatively
different coatings with lower thermal noise. At all frequencies, there is room
to reduce quantum noise through higher levels of vacuum squeezing and higher
laser power, again as envisaged for Cosmic Explorer. These plans will be
developed and refined in 2022 and 2023 and may further evolve in response to
inputs from the experience of commissioning and operating the upgraded
instruments, from the LIGO, Virgo, and KAGRA Collaborations, and from the
greater observing community.

The teams looking at the upgrade path for LIGO are considering a range of
options, in terms of magnitude of change and magnitude of sensitivity
improvements. Changes in the system which leave the wavelength at the present
1064 nm (Nd:YAG lasers), the main optics of fused silica, and the system at
room temperature form one ‘branch’ of possible improvements.595959Employing
some of the concepts explored in https://arxiv.org/abs/1712.05417 An
alternative ‘branch’ is to consider mild cryogenics, and a shift to longer
wavelengths to accommodate silicon main optics.606060The Voyager concept is a
well-developed package of upgrades; https://arxiv.org/abs/2001.11173 In
addition to technical feasibility, the downtime, cost, risk, and astrophysical
reach are all important considerations. The utility as pathfinders for CE is,
as well.

The tension between observing and upgrading the instruments was discussed at
several points in the meeting. In terms of numbers of events, a pause of a
year or two can be more than recovered due to the growth in event rate as the
cube of the detector sensitivity. There is also a gain in SNR for a given
event, and some improvement in localization. However, the gaps in GW
observations run a risk of missing a rare event and can deprive a short-lived
EM/particle mission of significant common time with GW observation. The bigger
picture includes the KAGRA and LIGO-India detectors. The dates by which those
detectors can achieve LIGO- or Virgo-like sensitivities are still in flux.
However, once there is a network of 5 detectors, one (or two) detectors can be
taken offline for improvements while retaining a network with the capability
of localization.

The greatest challenge in reaching the target sensitivity is in the
frequencies below roughly 50 Hz. Here, many different noise sources, some
related and some independent, contribute to an excess above the ‘fundamental’
noise sources of thermal noise (in the coatings and ultimately suspensions)
and quantum radiation pressure noise. This domain has always proven to be
difficult, and advances in the sensitivity often require in-vacuum
modifications or significant changes to sensors and servo-control systems.
This domain is also central to the observational science goals, and not only
for the current detectors – also for CE and ET. This is one of the motivations
in the instrument science community for continuing to pursue upgrades; they
provide an opportunity to learn about the limitations of the instruments, and
thus to improve the designs for the following phases.

An important additional concern is maintaining a sizable team of instrument
scientists who have experience with the full range of activities: identifying
weaknesses in the current designs, undertaking R&D to develop approaches to
move forward, being part of the engineering, fabrication, assembly, test, and
installation; and then the commissioning of the completed instrument. This
gives a strong incentive to maintain a rhythm of upgrades that is roughly one
‘PhD cycle’ in length.

The best path for upgrades of the detectors in the current 3- and 4 km
observatories will evolve most significantly with the time scales for
realizing CE and ET. If those next-generation-observatories follow the fastest
path to realization that seems feasible, more modest upgrades of the current
detectors will be best. If there are delays in CE and ET, more ambitious
programs will be motivated.

Pedro Marronetti of the NSF reviewed the range of options for NSF support of
upgrades. There are proposal opportunities ranging from one to hundreds of
millions of dollars. Evidently one of the considerations in the US is the
overall availability of large-scale funding; a very significant upgrade to the
current LIGO observatories may well delay the start of funding of CE.

###  5.3 Einstein Telescope Project

Einstein Telescope616161http://www.et-gw.eu is a pan-European concept for a
next-generation gravitational-wave observatory.

####  5.3.1 ET Concept

The ET
concept626262https://gwic.ligo.org/3Gsubcomm/docs/ET-0007B-20_ETDesignReportUpdate2020.pdf
is based on a triangular array with 10 km long arms and three co-located
detectors. Each detector consists of two interferometers in a xylophone
configuration, where one interferometer maximises sensitivity at low
frequencies while the other interferometer maximises high frequency
performance. The output signals from the two interferometers are then combined
to provide broadband sensitivity for each detector. The triangular system has
the ability to resolve the polarisation of gravitational waves without
additional GW observatories, and the 6-interferometer implementation makes a
very good use of the underground facility. In addition, a combination of the
output signals of all three co-located detectors (null stream) allows a
validity check of detections due to the redundancy of the signals and thus a
slight reduction of the detection threshold, which is equivalent to an
increase in sensitivity. The main challenge is the complexity of installing,
commissioning and operating multiple instruments in the same facility.

The xylophone configuration allows the decoupling of technologies that are
difficult to coexist in a single detector, such as cryogenics ($\sim$20K) and
high power operation. The ET-Xylophone concept envisages a cryogenic low-
frequency detector with low circulating power (18 kW) and a high-frequency
detector at room temperature with high circulating power (3 MW). This split
into two interferometers, with partly very different requirements, increases
the complexity and the commissioning effort. Experience gained on one
interferometer cannot necessarily be transferred to the other type.

ET is built underground at a depth of about 200-300 m to minimize seismic
disturbances and the gravitational interaction of the mirrors with their
environment (Newtonian Noise). This requires an underground infrastructure
with a total of about 30 km of tunnels and various underground chambers with
heights up to 30 m.

####  5.3.2 Project Status

As a pan-European project, one of the main organizational challenges is the
coordination of activities at the political level. ET was placed on various
national roadmaps. Most significantly, with the political support of Italy,
the Netherlands, Belgium, Spain and Poland, ET was included on the ESFRI
roadmap in June 2021, which is equivalent to a quality label at the European
level and an important milestone for ET. The internal organization of ET is
being advanced at both the project and the collaboration level, on the one
hand by establishing the ET project with the support of the involved funding
agencies and by the formalization of the collaboration on the other hand. The
collaboration already includes several well-functioning specific boards that
take care of the instrumental aspects (Instrument Science Board, ISB), the
scientific output of the project (observational Science Board, OSB), the
survey of the site candidates (Site preparation board, SPB), the computing
needed for the data analysis (Computational Infrastructure board, CIB), and
various others. There are currently two candidate sites for the observatory:
the Italian island of Sardinia and an area called Euregio Meuse-Rhine, the
border region between the Netherlands, Belgium and Germany. Studies are
underway at both sites to explore their scientific suitability in terms of
seismic disturbances and subsurface geology. The site decision is envisaged in
2024 or 2025. The next few years will be characterized by the so-called
Preparatory Phase of the ET project, during which the organizational structure
of ET will be established, the technical studies will be refined and deepened,
the site decision will be prepared both on the scientific and on the policy
level, and funding will be pursued. The projected timeline foresees a start of
construction in 2026 with a construction phase lasting up to 8 years. After
the installation of the vacuum system and the first detector, a first data
taking can be expected in the second half of the thirties.

At present, formal Coordinators of the ET Project are INFN in Italy and Nikhef
in the Netherlands. A Board of Governmental Representatives (BGR) has been
established with ministerial delegates from various interested countries. The
Coordinators report to the BGR. In addition a Board of Scientific
Representatives (BSR) has been established that includes scientists from
various countries. Moreover the Coordinators have set up an ET Project
Directorate. The ET Project is now in the Preparatory phase and important
deliverables of this phase include the agreement on the type of legal entity,
governance, project organization, technical design, financial planning, and
site selection.

###  5.4 NEMO

Presenter: Bram Slagmolen

The Neutron star Extreme Matter Observatory (NEMO636363Publications of the
Astronomical Society of Australia, 37, E047. doi:10.1017/pasa.2020.39) is a
concept for an interferometric gravitational wave detector focused on
observing the late in-spiral and post-merger signatures of Binary Neutron
Stars to infer their equation of state, in an infrastructure of $\sim$4 km. To
have a realistic chance of observing these signatures a sensitivity window
reaching at least $10^{-24}\rm Hz^{-1/2}$ in a kHz bandwidth centred near 2
kHz is needed. This would be a comparable sensitivity in this frequency range
to other next-generation detectors such as Cosmic Explorer and the Einstein
Telescope but would be achieved with a specialized detector.

The sensitivity requirements at the low-frequencies ($<$ 1kHz) are relaxed to
enable high bandwidth control loops to mitigate opto-mechanical instabilities
and reduce the cost with less complex test mass suspension systems. The
planned optical configuration is similar to the current observatories using a
dual-recycled Fabry-Perot Michelson interferometer. Compared to other
detectors however, it will employ a long signal recycling cavity to tune and
maximise its sensitivity in the 1–4 kHz band. In addition, alternative signal
enhancement techniques can also be utilised to further improve the target
sensitivity.646464Classical and Quantum Gravity, 37(7):07LT02, Mar 2020;
Communications Physics, 4(1):27, 2021

NEMO is envisioned to employ evolution of present technologies, such as the
use of longer laser wavelengths656565Opt. Express, 28(3):3280–3288, Feb 2020
and cryogenically cooled silicon test masses666666Phys. Rev. D 102, 122003.
doi: 10.1103/PhysRevD.102.122003. This will make a NEMO detector a pathfinder
for technology that will enhance the next generation detectors. A NEMO
pathfinder is supported by the Australian Astronomy Decadal
Plan676767https://arxiv.org/abs/1912.06305, and a conceptual design proposal
is currently in preparation.

More generally, the observational science value to having a network node in
the southern hemisphere is significant. The community should continue to
explore means to realize a next-generation observatory there.

###  5.5 The Cosmic Explorer Project

A series of three presentations provided a view of the current state of Cosmic
Explorer and the path forward.

####  5.5.1 CE Concept and Status

Presenter: Joshua Smith

The US Cosmic Explorer concept is an outgrowth of studies performed by members
of the GW instrument science community in parametric explorations of concepts
for future detectors. The community participated over many years, via the
GWADW, GWPAW, and PAX workshops, the Amaldi meeetings, and the LSC working
groups and meetings, to explore the paths for significant improvements to the
detectors. Interaction in a variety of environments and covering both
observational science goals and instrument science paths to realization has
led to the specific concept of CE. The name “Cosmic Explorer” with the nominal
length ten times that of LIGO was described in the 2014 LSC Instrument White
Paper,686868https://dcc.ligo.org/LIGO-T1400316 and a growing level and focus
of activity has continued since that time to find the best path forward.

Guided by the experience with the LIGO and Virgo detector commissioning, the
CE team came to the conclusion that while making the detector longer evidently
increased the cost, it appeared to be the lowest risk path to better
sensitivity. Initial comparisons with observational science targets suggested
that the compromise of building a surface detector – which would be limited at
low frequencies by Newtonian noise coupling to seismic activity – would
preserve, and deliver, a wide range of exciting science targets; the great
length also suppresses the size of this noise source. There is an explicit
understanding that multiple generations of detectors, which can push the
sensitivity further, will be accommodated over the lifetime of the
observatories. With that starting point,696969https://arxiv.org/abs/1607.08697
the Cosmic Explorer Project could be proposed to the NSF for a serious pre-
conceptual-design phase study,707070NSF Award #1836814, Collaborative
Research: The Next Generation of Gravitational Wave Detectors leading to the
recently completed Cosmic Explorer Horizon Study.

The CEHS was carried out by a group of five institutions.717171MIT, Caltech,
Cal State Fullerton, Penn State, Syracuse Faculty and research scientists,
postdocs, grad and undergrad students contributed directly to the focused
research and writing. Civil engineering support was contracted. The LIGO Lab
and OzGrav, along with members of the gravitational-wave community broadly,
helped to form, critique, and share the concept. A CE Consortium has been
formed727272http://cosmicexplorer.org to focus this support to future work.

Cosmic Explorer is envisioned to be the next-generation US-led observatory
project consisting of two widely separated and non-parallel sites; one site
supports a detector with 20 km long arms, and the other a detector with 40 km
long arms. The 40 km system is designed to deliver 10 times better sensitivity
than the 4 km LIGO A+ design, with a primary observational science goal of
access to most neutron star and (stellar to intermediate mass) black hole
mergers in the universe. The 20 km detector is optimized for neutron star post
merger physics; its shorter length allows an optimized sensitivity in the
range of 1-3 kHz. The two would observe together, ideally with ET and
conceivably with a southern hemisphere detector, forming the next-generation
gravitational-wave detector network.

The two sites would use common technologies and fabrication processes for all
of the civil and vacuum infrastructure, and the vast majority of the detector
optical, mechanical, and electronic components. The experience with LIGO’s two
sites demonstrated the utility of commonality for cost savings in
construction, and the advantages for commissioning of solving problems once
and training staff who can work interchangeably at the two sites.

An initial ‘chalkboard’ budget for the CE project has been developed. The
costs of the design and construction phases are based on scaling from Initial
LIGO, Advanced LIGO, and engineering estimates made specifically for CE. The
cost for construction of the two sites and the detectors for them is roughly
estimated at a cost of $1.6B 2021 USD. Operations then follows, with a yearly
cost estimated to be $60M 2021 USD. It must be stressed that they are a first
effort and require refinement in the next phase of the Project.

####  5.5.2 Cosmic Explorer Reference Design

Presenter: Stefan Ballmer

The Cosmic Explorer reference design calls for a 40 km and a 20 km L-shaped
surface-based gravitational-wave detector. The design is guided by the science
goals outlined in the CE Horizon Study, capable of observing frequencies up to
about 5 kHz from neutron star mergers, and mergers of black holes out to high
redshifts ($\sim$10). Notably, the characteristic strain from binary black
hole mergers at a reference frequency of 10Hz exhibits a plateau behavior for
redshifts above a few, due to a combination the characteristics of the
luminosity-distance – redshift relation and the frequency shifting of
stronger-emission late-inspiral signals. The signal will drop away once the
merger frequency has been shifted below the reference frequency. The result is
a “threshold sensitivity” for reaching cosmological distances with
gravitational-wave interferometers.

The CE reference design is guided by three principles: i) “Build on what
works”, taking advantage of the success of Advanced LIGO’s design as much as
possible; ii) “Let observational science drive the design”, i.e., match the
antenna length to known sources and address the known issues with Advanced
LIGO’s design; and iii) “Keep it flexible”, designing an observatory that will
be able to accommodate any new technology developments.

Other than the increase in arm length (from 4 km to 40 km/20 km, including
larger vacuum system), the reference design asks for larger test masses (320
kg, 70 cm diameter, compared to Advanced LIGO’s 40 kg, 34 cm diameter), a
second input mode cleaner to address the frequency stabilization requirements,
beam reduction telescopes on the arm-side of the beam splitter, a low-loss
signal recycling cavity, a scaled up filter cavity to rotate the squeezed
vacuum for quantum noise suppression, and a homodyne readout scheme.

The required R&D is outlined in much more detail in the CE R&D white
paper,737373https://dcc.cosmicexplorer.org/CE-P2100005 as well as the
CEHS.747474https://dcc.cosmicexplorer.org/public/0163/P2100003/007/ce-horizon-
study.pdf

####  5.5.3  CE Path Forward

Presenter: Matt Evans

##### Cosmic Explorer Timeline mapped onto the NSF Major Facilities Lifecycle

The CE team is using the NSF Major Facilities Guide to structure the work
going forward. Evidently if other funding agencies can participate in the
project, additional direction will come from the practice in those agencies.
The Cosmic Explorer Horizon Study (CEHS) fits well as a conclusion to the NSF
‘Development Phase’, “in which initial ideas emerge and a broad consensus is
built for the potential long-term needs, priorities, and general
requirements”. With the support of GW future science from the Astro2020
Decadal and for Cosmic Explorer from the Dawn meeting, we anticipate NSF
recommendation to enter the ‘Design Stage’ “where detailed, construction-ready
budget estimates, schedules, technical specifications and drawings, and
management processes are developed”.

The presentation followed by a day the “Megaprojects” round table as well as
the other Dawn meeting sessions, and an effort was made to incorporate some of
the ideas from those presentations and discussion in what follows (and what
will be planned for the future).

A sketch of the plan forward is laid out in the CEHS which calls for an
overall Design phase of around 7-9 years at cost of order $100M 2021 USD:

  * •

Conceptual Design 3 years

  * •

Preliminary Design 2 years

  * •

Final Design 2-4 years

This will be followed by a Construction phase of some 5 years, and the
Operations. The infrastructure will be designed for a lifetime of order 50
years. This will allow funding agencies to capitalize on future research and
development breakthroughs, should the operational life-span of CE be extended
beyond the initial mandate (which is expected to be 20 years).

The baseline plan for obtaining funding for the near-term activities of
Conceptual Design is for a multi-institution proposal to the NSF be made which
would support a Project office, enable an initial Project Execution Plan to be
written, and to work intensively to identify a ‘home’ for Cosmic Explorer in
an organization capable of managing this scale of project. In parallel, R&D
activities in the broader community will be coordinated by the Project Office
but supported by NSF (and ideally other US and international funding
agencies), in a mode resembling Advanced LIGO in its coordination of the
nascent LIGO Scientific Collaboration R&D.

##### Core Activities along the path to realization

We outline the work to be done according to the NSF Major Facility Guide (MFG)
phases.

###### Conceptual Design Phase

The development of the Project governance, schedule, and budget will be the
lead activity in this phase. Identification of a structure, and likely a host
organization, for the Project is critical.

The observational science goals will continue to be refined, informed by the
greater scientific community’s research into multi-messenger applications of
GWs, and the growing body of GW observations. The goals will be flowed down to
the requirements for Cosmic Explorer.