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 µm, 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 µm 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 µm 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.