Cosmic background radiation
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When any patch of the sky is observed where no individual sources can be discerned, and the effects of interplanetary dust, and interstellar matter are taken into account, there is still radiation.
This radiation is known as Cosmic Background Radiation. The origin of this radiation depends on the region of the spectrum that we are observing. The most famous component is the Cosmic microwave background. This component is redshifted photons that have freely streamed from an epoch when the universe became transparent for the first time to radiation. Its discovery and detail observations of its properties are considered one of the major confirmations of the Big Bang. The Sunyaev Zeldovic theory shows the phenomena of radiant cosmic background radiation interacting with "electron" clouds distorting the spectrum of the radiation.
There is also background radiation in the infrared, x-rays, etc., with different causes; most of these are ultimately attributable to unresolved individual sources.
See also
Main: Timeline of cosmic microwave background astronomy, Cosmic microwave background radiation, Galaxy formation and evolution, Magnitude, Universe
Physics: Dark matter, Dirac sea, Hot dark matter, Irradiation, Sunyaev Zeldovic Effect
Other: List of astronomical topics
ca:Fons cosmològic de:Hintergrundstrahlung it:Radiazione cosmica di fondo nl:Kosmische achtergrondstraling ja:宇宙背景放射 fi:Kosminen taustasäteily