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Cosmic Background Radiation
When any patch of the sky is observed where no individual sources can be
discerned, and the effects of the interplanetary dust, and interstellar
matter are taken into account, there is still is 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. Certainly the
most famous component is the Cosmic microwave background radiation, a
remainder of the epoch when the universe, still hot, became transparent for
the first time to 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.