Extragalactic background light measurements and applications

Abstract

This review covers the measurements related to the extragalactic background light intensity from γ-rays to radio in the electromagnetic spectrum over 20 decades in 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), centred at 1 μm, 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 γ-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 μm established an energetically important background with an intensity comparable to the optical background. This discovery paved the way for large aperture far-infrared and sub-millimetre 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 (EUV) 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 and 5 μm 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.

Non-thermal emission in radio galaxy lobes – II. Centaurus A, Centaurus B, and NGC 6251

“…Uncertainties in the EBL density arise also from the fact that it is based on large datasets with inhomogeneous error estimates (in addition to other systematic uncertainties, such as redshift-dependent galaxy luminosity functions; Franceschini & Rodighiero 2017). The main EBL components, the CIB and the COB, have ∼30% uncertainties (Cooray 2016). A higher/lower COB power implies a lower/higher Ne,0 if the latter is determined via Compton/EBL modelling of Fermi-LAT data (e.g., the S1 region of Cen A); this in turn implies a higher/lower B in the synchrotron fit to the radio spectrum 4 .…”

Implications of gamma-ray observations on proton models of ultrahigh energy cosmic rays

“…The EBL is still quite uncertain, principally because a direct measurement is subject to several foregrounds [42]. However, there are different models, available in the literature, for the spectrum as a function of the photon energy and redshift.…”

Where does Earth’s atmosphere get its energy?

“…The CMB is of extragalactic origin (Cooray 2016). Recent precision measurements (de Bernardis 2015 and references therein) support the theory that the CMB is the remnant of the big bang (Noterdaeme et al 2011), ''of a time when the universe was very hot, which has now cooled down by its expansion'' (Stanev 2004).…”