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The cosmic background radiation, more commonly called the cosmic microwave background radiation(CMBR) is electromagnetic radiation that fills the Universe. The radiation can only be detected with a radio telescope which makes it show as a faint glow. This glow is strongest in the microwave area of the radio spectrum.
The cosmic background radiation is radiation left over from early development of the universe, and is a landmark proof of the Big Bang theory. Before the formation of stars and planets, the Universe was smaller, much hotter, and filled with a uniform glow from its white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled and stable atoms could form, they eventually could no longer absorb the thermal radiation and the universe became transparent instead of being an opaque fog. The photons that from that time have been propagating ever since, growing fainter and less energetic. The CMBR has a thermal black body spectrum at a temperature of 2.725 K, so it peaks in the microwave range frequency of 160.2 Ghz(1.9 mm wavelength).
The CMBR is isotropic to roughly one part in 100,000. The Far-Infrared Absolute Spectrophotometer (FIRAS) instrument has measured the spectrum of the cosmic background radiation, making it the most precisely measured black body spectrum in nature. It is the main prediction of the Big Bang and Inflationary Cosmology predicts that after about 10-37 seconds the new universe underwent explosive growth that smoothed out nearly all inhomogeneities. This was followed by a type of phase transition that set the fundamental forces and elementary particles in their present form. The early universe was made up of a plasma of photons, electrons, and baryons. As the universe expanded adiabatic cooling caused the plasma to cool(at about 3,000 K) until electrons and protons combined to form hydrogen.
The Big Bang theory suggests that the cosmic microwave background fills all of observable space, and that most of the radiation energy in the universe is in the cosmic microwave background. Once you consider it, it only makes sense that there would be a background radiation that is directly related to the expansion of the Universe.
We’ve also recorded an entire episode of Astronomy Cast all about the Cosmic Microwave Background. Listen here, Episode 5: The Big Bang and Cosmic Microwave Background.