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extragalactic background light
The diffuse extragalactic background light (EBL) is all the accumulated radiation in the universe due to star formation processes, plus a contribution from active galactic nuclei (AGNs). This radiation covers the wavelength range between ~0.1 and 1000 µm (these are the ultraviolet, optical, and infrared regions of the electromagnetic spectrum). The EBL is part of the diffuse extragalactic background radiation (DEBRA), which by definition covers the overall electromagnetic spectrum. After the cosmic microwave background, 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.〔Aharonian, F. A., Very high energy cosmic gamma radiation: a crucial window on the extreme universe, River Edge, NJ: World Scientific Publishing, 2004〕 VHE photons coming from cosmological distances are attenuated by pair production with EBL photons. This interaction is dependent on the spectral energy distribution (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 that is orders of magnitude higher than the EBL. Different groups have claimed the detection of the EBL in the optical〔Bernstein R. A., 2007, ApJ, 666, 663〕 and near-infrared.〔Cambrésy L., Reach W. T., Beichman C. A., Jarrett T. H., 2001, ApJ, 555, 563〕〔Matsumoto T., et al., 2005, ApJ, 626, 31〕 However, it has been proposed that these analyses have been contaminated by zodiacal light.〔Mattila K., 2006, MNRAS, 372, 1253〕 Recently, two independent groups using different technique have claimed the detection of the EBL in the optical with no contamination from zodiacal light.〔Matsuoka Y., Ienaka N., Kawara K., Oyabu S., 2011, ApJ, 736, 119〕〔Mattila K., Lehtinen K., Vaisanen P., von Appen-Schnur G., Leinert C., 2011, Proceedings of the IAU 284 Symposium SED, arXiv:1111.6747〕
There are also other techniques that set limits to the background. It is possible to set lower limits from deep galaxy surveys.〔Madau P., Pozzetti L., 2000, MNRAS, 312, L9〕〔Keenan R. C., Barger A. J., Cowie L. L., Wang W. H., 2010, ApJ, 723, 40〕 On the other hand, VHE observations of extragalactic sources set upper limits to the EBL.〔Aharonian F., et al., 2006, Nature, 440, 1018〕〔Mazin D., Raue M., 2007, A&A, 471, 439〕〔Albert J., et al., 2008, Science, 320, 1752〕
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