Astronomy

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# Extragalactic Background Light Measurements and Applications

Asantha Cooray  Department of Physics & Astronomy, University of California,
Irvine, CA 92697<EMAIL_ADDRESS>
###### Abstract

This review covers the measurements related to the extragalactic background
light (EBL) intensity from gamma-rays to radio in the electromagnetic spectrum
over 20 decades in the wavelength. The cosmic microwave background (CMB)
remains the best measured spectrum with an accuracy better than 1%. The
measurements related to the cosmic optical background (COB), centered at 1
$\mu$m, are impacted by the large zodiacal light associated with
interplanetary dust in the inner Solar system. The best measurements of COB
come from an indirect technique involving Gamma-ray spectra of bright blazars
with an absorption feature resulting from pair-production off of COB photons.
The cosmic infrared background (CIB) peaking at around 100 $\mu$m established
an energetically important background with an intensity comparable to the
optical background. This discovery paved the path for large aperture far-
infrared and sub-millimeter observations resulting in the discovery of dusty,
starbursting galaxies. Their role in galaxy formation and evolution remains an
active area of research in modern-day astrophysics. The extreme UV background
remains mostly unexplored and will be a challenge to measure due to the high
Galactic background and absorption of extragalactic photons by the
intergalactic medium at these EUV/soft X-ray energies. We also summarize our
understanding of the spatial anisotropies and angular power spectra of
intensity fluctuations. We motivate a precise direct measurement of the COB
between 0.1 to 5 $\mu$m using a small aperture telescope observing either from
the outer Solar system, at distances of 5 AU or more, or out of the ecliptic
plane. Other future applications include improving our understanding of the
background at TeV energies and spectral distortions of CMB and CIB.

###### keywords:

Cosmology, galaxies: galaxy evolution

##  1 Introduction

The extragalactic background light (EBL) is the integrated intensity of all of
the light emitted throughout the history of the universe across the whole of
the electromagnetic spectrum. While EBL is sometimes defined as the
extragalactic intensity spectrum from UV to infrared (e.g., see review in Dwek
& Krennrich 2012), the total energy content of the universe in the
electromagnetic spectrum spans close to 20 decades in the wavelength from
gamma-rays to radio. Across this whole range, the EBL spectrum captures
cosmological backgrounds associated with either primordial phenomena, such as
the cosmic microwave background (CMB), or photons emitted by stars, galaxies,
and active galactic nuclei (AGN) due to nucleosynthesis or other radiative
processes, including dust scattering, absorption and reradiation. The EBL may
also contain signals that are diffuse and extended, including high energy
photons associated with dark matter particle decays or annihilation.

In the UV to infrared portion of the electromagnetic spectrum, the EBL
spectrum captures the redshifted energy released from all stars and galaxies
throughout the cosmic history, including first stellar objects, primordial
black holes, and proto-galaxies. If precisely measured, the EBL spectrum can
be used as a constraint on models of galaxy formation and evolution, while
providing an anchor that connects global radiation energy density to star
formation, metal production, and gas consumption. The microwave background
spectrum at microns to mm-wave radio wavelengths, associated with CMB photons,
has been measured to a precision better than a percent and is described by a
black-body spectrum with a temperature of $2.7260\pm 0.0013$ (Fixsen 2009).
Such a measurement is facilitated by the fact that the CMB is the brightest of
the EBL components with a factor of 30 to 40 higher energy density than the
next brightest background at optical to infrared wavelengths. The CMB is also
a well-known probe of cosmology. The anisotropies come from both primordial
physics, at the epoch of last scattering when electrons and protons first
combined to form hydrogen, or secondary effects during the propagation of
photons. The latter includes effects associated with both gravitational and
scattering effects. The angular power spectrum of CMB spatial anisotropies has
now been measured down to a few arcminute scales with Planck and has been used
to determine cosmological parameters such as the energy density contents, the
spatial curvature, spectral index of primordial density perturbations laid out
after an inflation epoch, among others.

Despite the limitations on the accuracy of existing EBL intensity measurements
there have been some key breakthroughs due to intensity measurements of the
sky. A classic example is the cosmic IR background (CIB) peaking at 100
$\mu$m. Its intensity was measured with instruments such as DIRBE (Hauser et
al. 1998) and at wavelengths beginning 250 $\mu$m with FIRAS (Fixsen et al.
1998), both on COBE. The EBL intensity peaking at 100 microns was found to be
roughly comparable to that of the optical background, suggesting that CIB at
long wavelengths is energetically important as the optical/near-IR background
dominated by galaxies. This then motivated high resolution far-infrared and
sub-millimeter imaging from both space and ground and with increasing aperture
sizes and sensitivity more of the CIB background has been resolved to point
sources. These point sources are mainly dusty, star forming and starbursting
galaxies at high redshifts (see review in Casey et al. 2014). Their role in
galaxy formation and galaxy evolution remains one of the key topics in sub-mm
astronomical observations today.

With a precise measurement of the EBL intensity spectrum, a cosmic consistency
test can be performed as a function of the wavelength by comparing the
integrated light from all galaxies, stars, AGN and other point sources, to the
EBL intensity. Any discrepancies suggest the presence of new, diffuse emission
that is unresolved by telescopes. The possibilities for new discoveries with
profound implications for astronomy range from recombination signatures during
reionization and diffuse photons associated with dark matter annihilation and
their products. Related to the last scientific possibility important studies
have been carried out, with more expected over the coming years, whether there
is a diffuse signature at GeV energy scales in the cosmic gamma-ray background
(CGB) as measured by the Fermi-LAT that can be ascribed to dark matter (e.g.,
Ando & Ishiwata 2015).

In addition to the total EBL intensity significant information on the sources
of emission and their nature comes from measurements that focus on the
anisotropies of the intensity across the sky. These are in general quantified
and measured in terms of the angular power spectrum or the correlation
function. The well-studied example in the literature is the anisotropy power
spectrum of the CMB, resulting in high-precision cosmological parameter
measurements (e.g., Ade et al. 2015a). Anisotropy power spectra have also been
measured for CIB, COB, and CGB leading to inferences on the properties of the
source populations present at these wavelengths, especially on certain
physical details related to the faint sources that are below the individual
point source detection level.

The existing EBL intensity measurements are due to a combination of ground and
space-based observations of the sky. Direct absolute intensity measurements
must account for a variety of foregrounds both within the Solar system, such
as Zodiacal light at optical and infrared wavelengths, to Milky Way, such as
the Galactic emission at radio, infrared, X-ray or gamma rays. A good example
of an indirect technique to measure EBL is the use of absorbed TeV spectra of
individual blazars and other AGNs at cosmological distances to infer the
number density of intervening infrared photons that are responsible of
electron-positron pair-production by interactions with TeV photons. This has
led to the best determined COB measurements in the literature, especially
given the fact that modeling and removing Zodiacal light remains a challenge
for direct EBL intensity measurements around 1 $\mu$m.

Figure 1:  Intensity of the extragalactic background ($\nu I_{\nu}$ in units
of nW m-2 sr-1) as a function of the wavelength in meters. We combine the
existing measurements from the literature to highlight the best determined
estimates for the background from gamma-ray to radio. The cosmic microwave
background (CMB) has the least uncertainty as the spectrum is determined to
better than 1%. Cosmic optical background (COB) has large uncertainties
involving direct measurements due to uncertain removal of the Zodiacal light
foreground. Here we show the indirect estimate of EBL at optical wavelengths
based on the TeV/gamma-ray absorption spectra of distant blazars. The UV/soft
X-ray background at a wavelength of 10 to 100 nm remains unexplored. From left
to right in increasing wavelength, the plotted datasets are: Fermi-LAT (the
total extragalactic background composed of diffuse and resolved point sources;
Ackermann et al. 2015) and EGRET (Sreekumar et al. 1998; we have removed three
data points from Strong et al. 2004 at highest energies) in the Gamma-ray
spectrum, COMPTEL (filled circles between Gamma-ray and X-rays;
Weidenspointner et al. 2000) between Gamma-ray and X-rays, HEAO1 A2 and A4
(Marshall et al. 1980; Rothschild et al. 1983), INTEGRAL (Churazov et al.
2007), SWIFT/BAT (Ajello et al. 2008), Nagoya balloon experiment (Fukada et
al. 1975), SMM (Watanabe et al. 2000), ASCA (Gendreau et al. 1995) and RXTE
(Revnivtsev et al. 2003) in the hard to soft X-ray regime in green symbols,
DXS and CHIPS in soft X-rays/Extreme UV (as discussed in Smith et al. 2014 as
a line at 0.25 KeV), HESS in optical (Abramowski et al. 2012; see Figure 2 for
other measurements), DIRBE (Hauser et al. 1998) and FIRAS (Lagache et al.
2000) in the far-infrared, FIRAS at microwaves (Mather et al. 1994; Fixsen et
al. 1996) and ARCADE (Kogut et al. 2011) in the radio. The area under each of
these backgrounds capture the total energy density of the photons in each of
those wavelength regimes. From gamma-rays to radio the integrated intensity
values in units of nW m-2 sr-1 for key EBL components are $\sim 0.015$ (gamma-
ray), $\sim 0.3$ (X-ray), $0.01-0.02$ (lower and upper limits at 4.9 nm for
Extreme UV), $24\pm 4$ (with an additional $\pm 5$ systematic; optical), $\sim
30\pm 10$ (CIB), $960$ (CMB), and $<0.001$ (radio), respectively.

We summarize existing EBL intensity measurements in Figure 1 where we plot the
spectral intensity $\lambda I_{\lambda}$ as a function of the wavelength
$\lambda$. In this figure the area under each of the spectral components
represents the total energy density associated with each of those backgrounds.
Those values are listed in the caption of Figure 1 where the estimates were
made using a statistical average of existing results from the literature. In
most of these measurements large systematics, associated with foreground
models, are likely to be still present. Here we briefly outline the
techniques, foregrounds, and systematics associated with EBL measurements. We
also discuss their applications for astrophysical and cosmological studies and
briefly summarize studies related to spatial anisotropies. We cover from short
to long wavelengths starting from the gamma-ray background.

###  1.1 Gamma-Ray

The early measurements of the Cosmic Gamma-Ray Background (CGB) intensity came
from SAS-2 between 40 and 300 MeV in 1978 (Fichtel et al. 1978), followed by
EGRET between 40 Mev and 10 Gev in 1998 (Sreekumar et al. 1998; Strong et al.
2004). These measurements have been superseded in this decade by Fermi-LAT
covering 100 MeV to 800 GeV with roughly 25-30 times better sensitivity than
EGRET, as well as overall an improvement in the flux calibration. The CGB
spectrum measured by Fermi-LAT shows a cutoff at energy scales around 280 GeV
(Ackermann et al. 2015). Below this cut-off the spectrum can be described by a
single power-law with a spectral index about 2.3 ($\pm 0.05$). The cutoff is
explained as the disappearance of the high energy photons that are pair-
producing via interactions with the infrared background photons that we
discuss later (Aharonian et al. 2006; Ackermann et al. 2012a; Dominguez et al.
2011; Gilmore et al. 2012).

The CGB spectrum below the cutoff is mostly explained in terms of a
combination of AGNs in the form of blazars and gamma-ray emission from star-
forming galaxies (SFGs). Small, but non-negligible depending on the exact
energy, comes from millisecond pulsars (MSPs), Type Ia supernovae, and gamma-
rays from galaxy clusters. At energies above $\sim$ 50 Gev blazars are able to
fully account for the background, with an estimate of $86^{+16}_{-14}$% in
explained by extrapolated blazar counts in Fermi-LAT Collaboration (2015).
Between 0.1 and 50 Gev, blazars account for about 20% of the CGB. The rest are
due to other populations of AGNs and SFGs. We refer the reader to a
comprehensive review by Fornasa and Sánchez-Conde (2015) on the source
populations contributing to CGB and existing population models in the
literature.

The literature also considers the possibility of a dark matter-induced signal
in the gamma-ray background (see Bertone et al. 2005 for a review). This could
be from dark matter that decays into standard particles (e.g., Bertone et al.
2007; Ibarra et al. 2008) or due to annihilation products (Abdo et al. 2010;
see also Palomares-Ruiz & Siegal-Gaskins 2010). These gamma-rays will form an
anisotropic signal associated with dark matter in our Galaxy. However, the
signal is also expected to be present in all galaxies. The integrated signal
from dark matter halos of all other galaxies will lead to a clustering signal
in the gamma-ray background and must be separated from the clustering of all
gamma-ray emitting faint extraglactic sources that also contribute to the
background. A direct detection of a signal associated with dark matter decay
has been attempted towards Galactic center (e.g., Abazajian & Kaplinghat 2012)
and nearby dwarf galaxies that are considered to be dark matter rich (e.g.,
Drlica-Wagner et al. 2015, Geringer-Sameth et al. 2015). The claims of gamma-
ray excesses toward the Galactic center have been questioned as whether due to
pulsars or other astrophysical foregrounds (e.g., Bartels et al. 2015; O’Leary
et al. 2015), while the signal towards dwarf galaxies remain at the level of a
3$\sigma$ detection (e.g., Hooper & Linden 2015).

Due to spatial resolution and the all-sky nature of the CBG measurements,
Fermi-LAT also provides a treasure trove of data beyond the energy spectrum.
In particular anisotropies or spatial fluctuations of the CGB have been
pursued to study the nature of faint sources that can account for the small
diffuse signals in the CBG below the point source detection of current high
energy instruments. The angular power spectrum of the CGB is mostly Poisson or
shot-noise like between multipole ell ranges of 150 to 500, corresponding to
30 arcminutes to 3 degree angular scales on the sky (e.g., Ackermann et al.
2012b). Moving beyond auto spectra, cross-correlations of the anisotropies can
also be pursued (e.g., Xia et al. 2011). For an example the diffuse CGB signal
from all of decaying dark matter in the universe can be studied via cross-
correlations with large-scale structure tracers of the same dark matter, such
as weak lensing maps (e.g., Camera et al. 2013). Existing cross-correlation
attempts using Fermi-LAT data include both the CMB lensing map from Planck
(Fornengo et al. 2015) and galaxy weak lensing in the CFHT Lensing Survey
(Shirasaki et al. 2014). These studies are likely to be improved in the near
future with wider area galaxy lensing surveys such as those expected from the
Dark Energy Survey (DES) and Large Synoptic Survey Telescope (LSST).

Over the next decade, CGB studies will be extended to higher energies with the
Cherenkov Telescope Array (CTA) with detections likely in 10 Gev to 10 TeV
energy range (Acharya et al. 2013). CTA will allow studies related to CBG
fluctuations at higher energies, especially TeV scales where there are still
no reliable measurements on the spectrum or intensity fluctuations. We also
lack a a complete understanding of the sources that contribute to CGB at 1 to
10 MeV scales, below the 100 MeV sensitivity of Fermi-LAT. This background
spectrum based on EGRET and COMPTEL (Figure 1) suggests the possibility of a
smooth connection to the cosmic X-ray background (CXB) at 10 to 100 KeV energy
scales, though subject to large uncertainties in COMPTEL background light
measurements. If continue to be confirmed such a smooth transition to X-rays
suggest a different source population for the MeV background than the
background at GeV energies. The leading possibility is a combination of
Seyfert galaxies and flat-spectrum radio quasars that appear as bright MeV
sources. Due to large uncertainties with EGRET and COMPTEL background
measurements at these energy scales, however, we recommend a future experiment
to improve background intensity measurements at MeV energy scales.

###  1.2 X-Ray

Cosmic X-ray background (CXB) is generally divided into high and soft
energies, with transition around an energy scale of 2 keV. Early measurements
of the hard CXB intensity came from HEAO1 between 2-30 keV and 10-400 keV with
A2 and A4 instruments. These measurements showed that the spectrum follows:
$I(E)=7.9\times 10^{-0.29}\exp[-E/41.1{\rm keV}\;{\rm
keVcm^{-2}s^{-1}sr^{-1}keV^{-1}}$, consistent with thermal bremsstrahlung
radiation with a temperature $\sim$ 40 keV (e.g., Marshall et al. 1980;
Rothschild et al. 1983; Garmire et al. 1992). Bulk of the energy density of
CXB is thus at 30 keV, but understanding the sources at such a high X-ray
energy has been slow. Previous surveys with SWIFT/BAT and Integral (Ajello et
al. 2008; Churazov et al. 2007) only resolved 1% of the background to point
sources at 30 keV. Current measurements in this energy scale are mainly from
NuSTAR (Harrison et al. 2013).

Recent models show that in order to match both the redshift distribution of
the faint X-ray sources in deep images with the Chandra X-ray Observatory and
the overall CXB spectrum requires an evolving ratio of Type 1 (AGNs with
visible nuclei) to Type 2 (with obscured broad-line regions) sources, such
that there are more Type 2 Seyferts at higher redshifts (e.g., Menci et al.
2004; Ueda et al. 2003; Gilli et al. 2007). Deep surveys with NuSTAR resolve
35% of the 8-24 KeV background to AGNs with obscuring columns up to 1025 cm-2
(Harrison et al. 2015), consistent with expectations from AGN population
synthesis models (Worsley et al. 2005). Additional evidence for the presence
of such highly absorbed AGNs exist as IR-bright sources in AKARI with 50% of
the mid-IR AGN samples currently undetected in deep Chandra surveys (e.g.,
Krumpe et al. 2014).

The soft CXB has been measured with ROSAT down to 0.1 keV energies. At such
low energies intensity measurements start to become challenging due to the
Galactic signal and there is a clear indication for thermal emission from hot
gas with a temperature of 106K associated with a hot component of the
interstellar medium (ISM) or the Galactic halo. Deep ROSAT imaging data
resolved 80% of the CXB at soft X-ray energies of 1 keV to to discrete
sources, mainly bright AGNs. Chandra X-ray Observatory, with spatial
resolution at the level of 0.5 arcsec, and XMM-Newton has resolved $>$ 90% of
the X-ray background at energies between 0.5 to 2 keV and $>$ 80% at hard 2 to
9 keV energies (Worsley et al. 2005; Xue et al. 2012). The dominant source is
AGNs with a non-negligible contribution from galaxy clusters (e.g., Wu & Xue
2001) and starbursting galaxies (e.g., Persic & Rephaeli 2003). The
uncertainty in the unresolved intensity is not in the source population but in
the overall normalization of the total XRB intensity.

With the background resolved to individual sources, anisotropy measurements of
the CXB intensity and their applications have been limited when compared to
similar studies at other wavelengths. In principle anisotropy studies can
uncover diffuse X-ray sources or faint sources below the detection level. A
recent example is the use of Chandra deep images to study the X-ray background
fluctuations in combination with fluctuations measured with Spitzer at 3.6
$\mu$m (Cappelluti et al. 2013). This signal has been explained as due to the
infrared and X-ray emission from direct collapse black holes (DCBHs) during
reionization (Yue et al. 2013). An X-ray surveyor, such as the planned Athena
mission, should facilitate more detailed studies on the diffuse background,
faint unresolved sources in current deep surveys such as DCBHs, and multi-
wavelength cross-correlation studies.

###  1.3 UV

EBL measurements at UV wavelengths exist with GALEX at 150 nm (Murthy et al.
2010), with Voyager 1 and 2 at 110 nm (Murthy et al. 1999), and with Voyager
UVS at 100 nm (Edelstein et al. 2000), though subject to both large
statistical and systematic uncertainties. In the extreme UV (EUV) wavelengths
below 100 nm, and down to 10 nm at energy scales of 0.1 KeV in soft X-rays,
there are no useful measurements of the Cosmic UV background (CUVB) in the
literature (Fig. 1). While technological developments can be expected, a
measurement of the extragalactic EUV background will likely remain challenging
due to absorption of the extragalactic photons by neutral hydrogen in our
Galaxy and the intergalactic medium at wavelengths below 91.2 nm. Furthermore
the Galactic soft X-ray/EUV background presents a considerable foreground that
limits a reliable measurement of the UV background. And the next best
measurements are in X-rays from a wavelength of 5 nm corresponding to energies
of 0.25 KeV.

While a reliable background intensity might be challenging, further EUV
observations are warranted since it is understood that bulk of the baryons
exists in the warm intergalactic medium (Cen & Ostriker 1999) with signatures
that involve emission and absorption lines around 50 nm (Tepper-Garcia et al.
2013). Based on Extreme UltraViolet Explorer (EUVE) data, there are some
indications that certain galaxy clusters like Coma show excess EUV emission,
especially at energies between 65 and 200 eV (Lieu et al. 1996). While the
wavelength regime between 10 to 100 nm remains a crucial wavelength regime for
further exploration, we recommend further attempts between 100 and 1000 nm in
the UV as there are possibilities for some significant discoveries on the
nature of warm intergalactic medium. Instruments on the planetary spacecraft
to outer Solar system may continue to provide opportunities for UV background
measurements, similar to past attempts with Voyager (Murthy et al. 1999;
Edelstein et al. 2000). In this respect one near-term possibility would be the
use of New Horizons’ Alice UV spectrometer (Stern et al. 2007) for a new
background measurement at wavelengths around 140-180 nm.

Figure 2:  The cosmic optical and infrared background light from 0.1 to 100
$\mu$m. The data points with error bars are direct estimates using DIRBE (red
circles: Wright 2004, Wright 2001; stars: Cambresy et al. 2001 at 1.25 and 2.2
$\mu$m; Gorjian et al. 2000 at 2.2 and 3.5 $\mu$m; Levenson et al. 2007 at 2.2
and 3.5 $\mu$m; open squares at 140 and 240 $\mu$m; Hauser et al. 1998), IRTS
(purple crosses; Matsumoto et al. 2005), Spitzer at 3.6 $\mu$m (open triangle;
Levenson & Wright 2008), Hubble (green circles; Bernstein 2007), UVS/STIS
(blue upper limits; Edelstein et al. 2000; Brown et al. 2000), CIBER (blue
circles; model-dependent based on fluctuation measurements; Zemcov et al.
2014), FIRAS (black line; with an overall uncertainty of 30% between 200
$\mu$m and 1.2mm; Lagache et al. 2000 also Fixsen et al. 1998), and IRAS (blue
square; 60 $\mu$m fluctuation-based estimate of EBL with IRAS; Miville-
Deschn̂es et al. 2002). The lower limits to the EBL are from integrated or
source counts using Hubble (Gardner et aL. 2000; Madau & Pozzetti 2000),
Spitzer/IRAC (Fazio et al. 2004), ISO (Elbaz et al. 1999), Spitzer/MIPS
(Papovich et al. 2004; Dole et al. 2004), Herschel/PACS (Berta et al. 2010),
Herschel/SPIRE (Béthermin et al. 2012), and SCUBA (Smail et al. 2002). The
blue shaded region is the estimate of EBL using the HESS TeV blazar absorption
spectra (Abramowski et al. 2012). Apart from recent Herschel measurements,
CIBER, and the estimate of EBL from HESS, all other measurements plotted here
are tabulated in Hauser & Dwek (2011). This figure is based on a previous
figure by Dole et al. (2006) that summarized these EBL and integrated galaxy
count measurements.

###  1.4 Optical/Near-Infrared

At optical and near-infrared (IR) wavelengths between 0.1 to 5 $\mu$m the EBL
intensity is predominantly due to stellar emission from nucleosynthesis
throughout the cosmic history (see Hauser & Dwek 2001 for a review). The
cosmic optical background (COB) spectrum also includes radiative information
from the reionization epoch. Due to redshifting of the UV photons to near-
infrared emission from primordial sources is primarily at wavelengths longward
of 1 $\mu$m (Santos et al. 2002; Salvaterra et al. 2003). This includes
diffuse Ly-$\alpha$ and free-free radiation in addition to direct emission by
stars and mini-quasars (e.g., Cooray & Yoshida 2004; Fernandez & Komatsu
2006).

At optical and near-IR the few attempts at absolute measurements involve DIRBE
on COBE in several band-passes between 1.25 $\mu$m and 240 $\mu$m (Hauser et
al. 1998; Cambresy et al. 2001; Gorjian et al. 2000; Levenson et al. 2007;
Wright 2001; Wright 2004), IRTS, a small JAXA mission, between 1 and 4 $\mu$m
(Matsumoto et al. 2005), Voyager (Edelstein et al. 2000) and Hubble (Brown et
al. 2000; Bernstein et al. 2002; Bernstein 2007). Because DIRBEs confusion
limit was 5th magnitude at 2.2 $\mu$m, all recent EBL measurements using DIRBE
require subtraction of stellar light using ancillary measurements, such as
2MASS (Wright 2001; Levenson et al. 2007). While Hubble has been used for
optical (Bernstein et al. 2002; Bernstein 2007) and far-UV (Brown et al. 2000)
EBL measurements, the instrument was not designed for absolute photometry and
required a careful subtraction of instrumental emission and baselines (e.g.
dark current). Those measurements are subject to large uncertainties (e.g.,
Mattila 2003).

The dominant limitation for direct EBL intensity spectrum at these wavelengths
is the Zodiacal light associated with scattered Solar light from micron-size
dust interplanetary dust (IPD) particles near the Earth’s orbit. Existing
measurements with DIRBE on COBE make use of model to remove Zodiacal light
(Kelsall et al. 1998) or slight variations (Wright 2001). The Zodiacal light
foreground limits the accuracy of EBL intensity to roughly an order of
magnitude at wavelengths about 1 micron (Hauser et al. 1998; Gorjian et al.
2000; Wright 2001; Cambresy et al. 2001; Matsumoto et al. 2005; Levenson &
Wright 2007). Techniques to remove Zodiacal light includes monitoring of
Fraunhofer lines in the dust scattered spectrum relative to the Solar spectrum
and use of the equivalent width of the lines to estimate the column density of
dust. In the case of HST results on the optical background the zodiacal
emission based on the observed strength of the reflected Fraunhofer lines from
a ground-based measurement (Bernstein et al. 2002). The sounding rocket
experiment CIBER (Zemcov et al. 2013) is capable of absolute photometry
(Tsumura et al. 2013) and results related to the optical/near-IR EBL are soon
expected (Matsuura et al. in preparation). If Spitzer/IRAC shutter operations
are allowed and successful, a carefully planned program should also be able to
improve the absolute EBL measurements at 3.6 and 4.5 $\mu$m over the coming
years.

In Figure 2 we summarize EBL intensity measurements between optical and IR
wavelengths using absolute photometry, model-dependent estimates based on the
EBL fluctuations, and the integrated galaxy light (IGL) from galaxy counts
(lower limits). In general, the summed contribution of galaxies to the EBL
does not reproduce the EBL measured by absolute photometric instruments. For
example, at $\lambda=3.5\,\mu$m the EBL measured by DIRBE from absolute
photometry is $13.0\pm 4.8\,$nW m-2 sr-1 (Levenson et al. 2007), while the
deepest pencil beam surveys with Spitzer at 3.6 $\mu$m give $6{-}9\,$nW m-2
sr-1 (Fazio et al. 2004; Sullivan et al. 2007), with the best determination of
$9.0^{+1.7}_{-0.9}\,$nW m-2 sr-1 (Levenson & Wright 2008). At shorter
wavelengths centered around 1 $\mu$m, and especially considering EBL
measurements from IRTS (Matsumoto et al. 2005) this divergence is even more
pronounced.

Note that model-dependent fluctuations-based estimates of EBL are in between
IGL and absolute photometry measurements (at 1 $\mu$m, such estimates of EBL
are from CIBER; Zemcov et al. 2014; Figure 2). Fluctuation measurements and
angular power spectra of source-subtracted optical and near-IR background
intensity have been presented with Spitzer at 3.6 $\mu$m and above (Kashlinsky
et al. 2005; Kashlinsky et al. 2012; Cooray et al. 2012), AKARI at 2.4, 3.2
and 4.1 $\mu$m (Matsumoto et al. 2011), Hubble/NICMOS at 1.1 and 1.6 $\mu$m
(Thompson et al. 2007), CIBER at 1.1 and 1.6 $\mu$m (Zemcov et al. 2014), and
Hubble/ACS and WFC3 in 5 bands from 0.6 and 1.6 $\mu$m (Mitchell-Wynne et al.
2015). These measurements generally reveal a picture in which source counts,
mainly galaxies, dominate the fluctuations with some evidence for additional
components such as intra-halo light (IHL; Cooray et al. 2012; Zemcov et al.
2014), associated with diffuse stars in extended dark matter halos due to
galaxy mergers and tidal stripping, and a signal from galaxies present during
reionization at $z>8$ (Mitchell-Wynne et al. 2015). Spitzer fluctuations have
been also cross-correlated with far-infrared maps from Herschel/SPIRE (Thacker
et al. 2015) and X-ray from Chandra (Cappelluti et al. 2013). The former
provides allows a quantification of the total dust content as a function of
redshift while the latter has been used to argue for the presence of
primordial direct collapse black holes (DCHBs; Yue et al. 2013). Over the
coming decade significant improvements in the study of near-IR and optical
fluctuations will come from planned cosmological missions such as Euclid and
WFIRST. The small explorer SPHEREx (Doré et al. 2014), recently selected by
NASA for a Phase A study, has the ability to conduct 3D intensity mapping of
spectral lines such as H$\alpha$ at $z\sim 2$ and Ly-$\alpha$ at $z>6$ during
reionization over large areas on the sky.

A leading possibility for the large difference between absolute photometry EBL
and IGL is likely an unsubtracted foreground component, such as an
underestimate of the Zodiacal light signal (Mattila 2006). If this difference,
however, is real it would have significant implications given that the nature
of the emission source must be diffuse and not point-like similar to galaxies.
Fortunately, there is also a third technique to measure the EBL. Given the
limitation of direct measurements possibly due to foregrounds, currently the
best estimates of optical/near-IR EBL come from this indirect technique that
makes use of the absorbed TeV/GeV spectra to constrain the optical and
infrared background due to pair production (shaded region in Figure 2 from
Abramowski et al. 2013). The modeling requires intrinsic spectrum for each
blazar, but since this is not observed or available, EBL is inferred through
statistical techniques that make use of a large sample of blazars over a wide
range of cosmological distances. The existing measurements come from High
Energy Stereoscopic System (HESS) array in Namibia (Aharonian et al. 2006;
Abramowski et al. 2013) and Fermi/LAT (Ackermann 2012a; Gong & Cooray 2013).
The discrepancy between absolute photometry and IGL-implied intensity is less
severe when comparing galaxy counts to the EBL inferred from absorbed TeV
spectra. The measurements are such that deep galaxy counts have effectively
resolved all of the optical and near-IR photons to individual galaxies. The
overall uncertainties, however, are still that the measurements leave the
possibility for small signals such as IHL an reionization consistent with
fluctuation measurements.

Given the large uncertainties, including systematics, with TeV measurements,
and especially absolute photometry measurements, it is crucial that we improve
on the optical and near-IR EBL intensity level. It is also clear that simply
repeating an absolute photometry experiment like DIRBE or IRTS at 1 AU will
not improve the current EBL spectrum at wavelengths less than 5 $\mu$m due to
limitations coming from the foreground model. Improvements in EBL measurements
will only come with a parallel improvement in our understanding of the IPD
distribution in the Solar system, if measurements are limited to 1 AU, or from
observations that are conducted outside of the zodiacal light dust cloud. In
Figure 3 we show the expected zodiacal light intensity as a function of the
radial distance from the Sun, based on the in-situ dust measurements from
Pioneer 10; the dust density is dropping more rapidly than the $r^{-1}$
profile expected from the Poynting-Robertson effect (Hanner et al. 1974). At
distances of Jupiter these dust density measurements suggest a decrease in the
zodiacal light intensity of roughly two orders of magnitude from the intensity
level near Earth orbit. One possibility is the out-of-Zodi EBL measurements,
such as the proposed piggy-back ZEBRA instrument on a planetary spacecraft to
Jupiter or Saturn distances, or to travel outside of the ecliptic plane. Given
the relatively small cost of the instrument necessary for the required
observations we encourage attempts to measure EBL as a by-product of planetary
spacecraft that explore the outer Solar system (Cooray et al. 2010).

Figure 3:  Zodiacal light intensity as a function of the radial distance from
the Sun, based on the Pioneer 10 dust density estimates (Hanner et al. 2010)
normalized to a radial profile from the Sun of $r^{-1.5}$. For reference we
also show the optical to near-IR spectrum of the Galactic interstellar medium
and two predictions from semi-analytical models on the total IGL from $z<5$
galaxy populations (from Primack et al. 2008). The Zodical light intensity
estimates as a function of the radial distance is based out of the
calculations for ZEBRA concept instrument for the outer Solar system in Cooray
et al. (2009).

###  1.5 Far-Infrared

Absolute photometry measurements between 10 $\mu$m and 1000 $\mu$m,
corresponding to the far-infrared EBL peak at around 100 $\mu$m (Figure 2),
are mainly from DIRBE (Hauser et al. 1998) and FIRAS (Fixsen et al. 1998;
Lagache et al. 2000). In general these measurements have an overall
uncertainty at the level of 30%, all due to uncertain corrections associated
with the foreground sky model. This background is generally referred to as the
cosmic infrared background (CIB) in some literature, especially in the context
of CMB experiments that also overlap in frequency ranges as the sub-millimeter
wavelengths.

With the absolute photometry measurements establishing a cosmologically
important energy density in the universe at long infrared wavelengths,
subsequent observations have focused on resolving this background to point
sources with large aperture telescopes. While the fraction resolved was low
with Spitzer/MIPS at 70 and 160 $\mu$m, significant improvements in our
ability to detect and study distant galaxies came over the last five years
with Herschel/PACS and SPIRE between 70 to 500 $\mu$m. In particular,
Herschel/SPIRE resolved 15% (250 $\mu$m) to 5% (500 $\mu$m) of the background
directly to sources (Oliver et al. 2010). Using statistics such as $P(D)$,
probability of deflections, in deep SPIRE images, Glenn et al. (2010) resolved
60% (250 $\mu$m) to 43% (500 $\mu$m) of the background to source counts,
especially sources below the individual point source detection level in maps.
Methods involving stacking analysis resolve more of the background with recent
analysis suggesting a resolved fraction greater than 90% (Viero et al. 2015).
The sources that make up the far-infrared/submillimeter background are dusty,
star-forming galaxies predominantly at high redshifts ($z>1$). They are likely
the dominant contribution to the cosmic star-formation rate density during the
peak epoch of galaxy formation at $z\sim 2$-3. We refer the reader to the
comprehensive review by Casey et al. (2014) for properties of these galaxies.

Since COBE/DIRBE and FIRAS CIB intensity measurements the experimental focus
has been on measurements related to the spatial anisotropies and the angular
power spectrum of CIB intensity (Haiman & Knox 2000; Knox et al. 2001; Amblard
& Cooray 2007). The power spectrum at 60 and 100 $\mu$m with IRAS allowed
studies related to the spatial distribution properties of Galactic dust and an
estimate of the total intensity of extragalactic sources through clustering
and Poisson noise at small angular scales (e.g., Miville-Deschn̂es et al.
2002). At 90 and 170 $\mu$m fluctuation measurements have also been attempted
with ISO (Lagache & Puget 2000; Matsuhara et al. 2000). Significant
improvements in our ability to remove Galactic emission and detect
extragalactic fluctuations have come from more recent experiments including
Spitzer/MIPS at 160 $\mu$m (Lagache et al. 2007), AKARI at 90 $\mu$ (Matsuura
et al. 2011), and BLAST at 250, 350 and 500 $\mu$m (Viero et al. 2009).
Currently the best measurements of CIB power spectrum at a few degree to ten
arcsecond angular scales are from Herschel/SPIRE (Amblard et al. 2010; Viero
et al. 2013; Thacker et al. 2013), while best measurements at larger angular
scales and spanning the whole sky are from Planck (Ade et al. 2011; Ade et al.
2013a). While there was a mismatch between Herschel/SPIRE and Planck CIB power
spectra at overlapping angular scales with first measurements this difference
has mostly gone away with the latest flux calibration of Planck/HFI data. In
combination, Planck and Herschel allow studies of the CIB angular power
spectrum from large linear scales with Planck to non-linear 1-halo term
(Cooray & Sheth 2002) with Herschel maps. The measurements are useful to
describe the spatial distribution of faint galaxies that make up the CIB and
the relation between far-IR luminosity to dark matter halo mass (Shang et al.
2011; De Bernardis & Cooray 2012; Xia et al. 2012; Viero et al. 2013; Ade et
al. 2013a).

Moving beyond the anisotropy power spectra, CIB fluctuations have also been
used for cross-correlation studies. For example, far-IR galaxies are
correlated with unresolved near-IR background detected with Spitzer and the
cross-correlation of near and far-IR backgrounds improve models related to the
dust distribution within dark matter halos (Thacker et al. 2015). Sources that
make up CIB are mostly at $z>1$. The foreground dark matter potentials that
are responsible for lensing of the CMB is mostly at $z\sim 1$-2 and CIB
provides one of the best tracers of the projected dark matter related to CMB
lensing (Song et al. 2002). This cross-correlation of CMB lensing with CIB
maps has been studied with Planck (Ade et al. 2013b) and for detections of CMB
lensing signal in the B-modes of CMB polarization (Hanson et al. 2013; Ade et
al. 2014).

Additional future applications involving the far-IR/submm background include
lensing of CIB fluctuations, that is CIB at $z\sim 3$ is expected to be
gravitationally lensed by structures at $z<1$, the search for a diffuse CIB
components including intra-halo dust in dark matter halos that extend beyond
the dusty disks of star-forming galaxies, and a detailed statistical
comparison of dust in emission seen with CIB vs. dust seen in absorption
through extinction studies (Ménard et al. 2007). While the recent focus has
been on CIB fluctuations and its applications, the absolute CIB intensity
still remains uncertain at the level of 30% and must be improved down to sub-
percent level if detailed comparisons are to be made on the sources
responsible for CIB vs. diffuse emission sources at far-IR wavelengths. The
proposed Primordial Inflation Explorer (PIXIE; Kogut et al. 2014) has the
capability to achieve such a measurement. Its’ sensitivity should also be
adequate for a detection of the CIB dipole allowing a comparison of the CIB
and CMB dipoles.

###  1.6 Microwaves

As shown in Figure 1, the cosmic microwave background (CMB), peaking at mm
wavelengths between sub-mm and radio, is the dominant background intensity
across all wavelengths in the electromagnetic spectrum. Its total integrated
intensity of 960 nW m-2 sr-1 is roughly a factor of 30 higher than the
integrated intensity of the infrared background, which remains the next
highest energetic component of the universe. Since its’ accidental discovery
roughly 50 years ago, the high background intensity or photon energy density
has facilitated its wide applications in cosmology, especially with spatial
anisotropies and polarization using a large number of ground and space-based
experiments, including COBE, WMAP and Planck. For recent reviews on CMB theory
and experimental data summaries we refer the reader to Durrer (2015) and
Komatsu et al. (2014).

While most of the experimental work in CMB concentrate on anisotropies and
polarization, the best measurement of CMB spectrum, and correspondingly the
best measurement of any EBL component from gamma-ray to radio, comes from
COBE/FIRAS. It is described by a Planck function with a blackbody temperature
of $2.7260\pm 0.0013$ (Fixsen 2009), with spectral departure from blackbody
currently limited by the data to be at the level in $\delta
I_{\nu}/I_{\nu}<10^{-5}$ (Mather et al. 1994; Fixsen et al. 1996). Distortions
to the spectrum are expected at the micro and nano-Kelvin level (for a general
review see Chluba 2014). Detection and detailed study of these distortions,
generated both during the early universe and at late times, remain a primary
scientific goal for a next generation CMB experiment, such as PIXIE (Kogut et
al. 2014), with sensitivity at least a factor of 30-100 better than FIRAS.

A well-known cosmological test related to the CMB temperature anisotropy power
spectrum involves the location of the first acoustic peak in the multi-polar
space (Kamionkowski et al. 1994). The CMB power spectrum from experiments like
Planck now reveal the multiple acoustic peak structure in the anisotropy power
spectrum from multipole moments 2 to $\sim$ 2500 and across at least eight
peaks. Along with constraints on cosmological parameters (Ade et al. 2015a),
these observations now provide evidence for an initial spectrum of scale-
invariant adiabatic density perturbations as expected under models involving
inflation. It has been argued for a while that the smoking-gun signature of
inflation would be the detection of stochastic background of gravitational
waves associated with it. These gravitational-waves produce a distinct
signature in the polarization of CMB in the form of a contribution to the
curl, or magnetic-like, component of the polarization (Kamionkowski et al.
1997). While polarization from density, or scalar, perturbations dominate, due
to the fact they have no handedness, there is no contribution to curl mode
polarization from density perturbations. Thus the current generation, and one
focus of next generation measurements, involves CMB polarization and
especially detailed characterization of the B-modes of polarization.

In transit to us, CMB photons also encounter the large scale structure that
defines the local universe; thus, several aspects of photon properties, such
as the frequency or the direction of propagation, are affected. In the
reionization epoch, variations are also imprinted when photons are scattered
via electrons, moving with respect to the CMB. Though these secondary effects
are in some cases insignificant compared to primary fluctuations, they leave
certain imprints in the anisotropy structure and induce higher order
correlations. A well-studied example of such a secondary effect with current
generation CMB experiments is lensing of the CMB (Lewis & Challinor 2006),
with a significant detection of the lensing effect in Planck (Ade et al.
2015b). The lensing of CMB is useful for cosmological applications involving
structure formation and signatures leftover by a massive neutrinos. A Stage IV
CMB experiment will be able to reach the neutrino mass threshold expected
given the neutrino oscillation experiments (Abazajian et al. 2015). AS
discussed with respect to Gamma-ray background, CMB lensing traces the large-
scale structure that is also visible at other wavelengths. Therefore,
improvements in our understanding of the nature of dark matter and faint
sources at each of the backgrounds will likely come from cross-correlation
studies. These are new topics in cosmology that will likely be improved over
the coming years.

###  1.7 Radio

The cosmic radio background (CRB) has been measured at multiple frequencies
and in recent years with the balloon-borne ARCADE-2 experiment from 3 to 90
GHz (Kogut et al. 2011). When compared to CMB, Galactic synchrotron
background, and extragalactic point sources, ARCADE 2 measured an excess radio
background. For example, at 3 GHz ARCADE measured the equivalent antenna
temperature to be 65 mK, once corrected for CMB and Galactic emission. At the
same frequency the known radio galaxy counts contribute about 30 mK. If
undetected radio sources account for the excess seen in ARCADE 2 they will
need to form an extra peak in the Euclidean-normalized number counts of radio
sources at flux densities around 1-100 nJy, but an explanation involving point
sources is ruled by various deep radio observations and other arguments (e.g.,
Condon et al. 2012; Holder 2014; Vernstrom et al. 2014). The excess has also
motivated alternative suggestions, such as decaying WIMP dark matter (e.g.,
Fornengo et al. 2011). A reanalysis of the Galactic synchrotron emission using
multiple components instead of the single slab model for the Galactic plane
synchrotron emission used by the ARCADE team, however, suggests that there is
likely no excess over the background produced by known sources (Subrahmanyan &
Cowsik 2013). Future attempts to improve the radio background will thus likely
also involve improvements to understanding and modeling of the Galactic radio
foreground.

Current and next-generation experiments will likely focus more on the long
wavelength radio background at frequencies around 100 MHz. These experiments
are driven by the need to characterize the background intensity spectrum to
study the global signature associated with 21-cm spin-flip transition of HI
from the epoch of reionization. The global signal involves a strong absorption
feature around 60-100 mK, associated with adiabatic cooling of gas, followed
by a weak emission during the epoch of reionization (Furlanetto 2006).
Detection of the expected absorption feature in the background intensity
spectrum at frequencies around 60 MHz is challenging due to the large Galactic
foreground at these low radio frequencies. Technology development studies are
underway to pursue such a measurement from the Moon, including the lunar
orbiter DARE (Dark Ages Radio Explorer; Jones et al. 2014). There are also a
host of experiments underway at frequencies above 100 MHz focused on the
intensity fluctuations, especially the power spectrum of 21-cm background
during reionization and for absolute measurements of the sky intensity (see
review by Pritchard & Loeb 2012).

##  2 Summary

This review covers the measurements related to the extragalactic background
light (EBL) intensity from gamma-rays to radio in the electromagnetic spectrum
over 20 decades in the wavelength. The Cosmic Microwave Background (CMB)
remains the best measured spectrum with an accuracy better than 1%. The
measurements related to the Cosmic Optical Background (COB), centered at 1
$\mu$m, are impacted by the large Zodiacal light intensity associated with
interplanetary dust-scattered sunlight in the inner Solar system. The best
measurements of COB come from an indirect technique involving the absorption
of Gamma-ray photons emitted by bright blazars and other active sources in the
universe. The Cosmic Infrared Background (CIB) at wavelengths centered around
100 $\mu$m established an energetically important intensity level comparable
to the optical background. This eventually resulted in the discovery of dusty,
starbursting galaxies with large aperture telescopes and a deeper
understanding of their importance in galaxy formation and evolution. The soft
X-ray/extreme UV extragalactic background at wavelengths of 10 to 100 nm
remains mostly unexplored, but is unlikely to be achieved easily due to the
absorption of the extragalactic photons by the intervening neutral
intergalactic medium and the interstellar medium of our Galaxy. We also
summarize our understanding of spatial anisotropies of these backgrounds and
the cosmological/astrophysical applications with angular power spectra of
intensity fluctuations across the sky. We motivate a precise direct
measurement of the COB between 0.1 to 5 $\mu$m using a small aperture
telescope observing from either the outer Solar system or out of the ecliptic
plane. Other future applications include improving our understanding of the
background at TeV energies, improving the MeV background over the previous
measurements with COMPTEL, radio background, and the spectral distortions to
CMB and CIB.

## Funding Statement

My research is funded by NSF (CAREER AST-0645427, AST-1310310) and NASA.

## Competing Interests

I have no competing interests.

## Acknowledgments

I thank my collaborators in CIBER, SPHEREx, CANDLES, HerMES, H-ATLAS, ZEBRA,
especially Jamie Bock, Bill Reach, Michael Zemcov, Yan Gong, Ranga Chary, and
many others for conversations related to topics covered in this review. Nick
Timmons and Ketron Mitchell-Wynne are acknowledged for help with figures and
collection of data in figures 1 and 2 from the literature to electronic
format. The radial intensity of Zodiacal light in figure 3 comes from
calculations performed by Bill Reach for the ZEBRA concept instrument in
Cooray et al. (2009). Figure 2 is an update to a similar figure from Dole et
al. (2006) that discussed the measurements related to optical and infrared
extragalactic background light and the integrated light from galaxy/source
counts.

## Data Accessibility

Electronic files listing the wavelength and intensity (in units of nW m-2 sr-1
as plotted in Figure 1) and the electronic versions of the figures are
available from herschel.uci.edu.

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