| | Research: Cosmic X-Ray Background Radiation |
 | | The cosmic X-ray background (XRB), like the cosmic microwave background (CMB), is radiation which originates at high redshift and has an extremely uniform distribution of intensity across the sky, which is to say it is extremely isotropic. |
 | | CMB radiation is a `relic' from a time when the Universe was hot and dense enough for baryonic matter (made of protons, neutrons, and electrons) and radiation to exist in a state of thermal equilibrium with each other. |
 | | AGN are the best candidates for the source of the X-ray background because at low redshift (nearby), where there are fewer of them and they are bright enough for us to examine their individual high-energy spectra in detail, we see that they produce radiation which is almost as hard as the X-ray background itself. |
| www.astro.uiuc.edu /~pmricker/research/xrb (858 words) |