The diffuse extragalactic background light (EBL) is all the accumulated radiation in the

universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. A ...

due to

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

processes, plus a contribution from

active galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...

(AGNs). This radiation covers almost all wavelengths of the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from b ...

, except the microwave, which is dominated by the primordial

cosmic microwave background In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all space ...

. The EBL is part of the diffuse extragalactic background radiation (DEBRA), which by definition covers the entire electromagnetic spectrum. After the

, the EBL produces the second-most energetic diffuse background, thus being essential for understanding the full energy balance of the universe. The understanding of the EBL is also fundamental for extragalactic very-high-energy (VHE, 30 GeV-30 TeV) astronomy. VHE

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always m ...

s coming from cosmological distances are attenuated by

pair production Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers specifi ...

with EBL photons. This interaction is dependent on the

spectral energy distribution A spectral energy distribution (SED) is a plot of energy versus frequency or wavelength of light (not to be confused with a 'spectrum' of flux density vs frequency or wavelength). It is used in many branches of astronomy to characterize astron ...

(SED) of the EBL. Therefore, it is necessary to know the SED of the EBL in order to study intrinsic properties of the emission in the VHE sources.

Observations

The direct measurement of the EBL is a difficult task mainly due to the contribution of

zodiacal light The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction in ...

that is orders of magnitude higher than the EBL. Different groups have claimed the detection of the EBL in the optical and near-infrared. However, it has been proposed that these analyses have been contaminated by zodiacal light. Recently, two independent groups using different technique have claimed the detection of the EBL in the optical with no contamination from zodiacal light. There are also other techniques that set limits to the background. It is possible to set lower limits from deep galaxy surveys. On the other hand, VHE observations of extragalactic sources set upper limits to the EBL. In November 2018, astronomers reported that the EBL amounted to

Empirical modelings

There are empirical approaches that predict the overall SED of the EBL in the local universe as well as its evolution over time. These types of modeling can be divided in four different categories according to:Domínguez et al. 2011, MNRAS, 410, 2556 (i) Forward evolution, which begins with cosmological initial conditions and follows a forward evolution with time by means of semi-analytical models of galaxy formation. (ii) Backward evolution, which begins with existing galaxy populations and extrapolates them backwards in time. (iii) Evolution of the galaxy populations that is inferred over a range of

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in fr ...

s. The galaxy evolution is inferred here using some quantity derived from observations such as the star formation rate density of the universe.Khaire V., Srianand R., 2014, ApJ, 805, 33 (arXiv:1405.7038) (iv) Evolution of the galaxy populations that is directly observed over the range of redshifts that contribute significantly to the EBL.

References

{{DEFAULTSORT:Extragalactic Background Light Physical cosmology

Observations

Empirical modelings

See also

References