Background image: The temperature distribution of the cosmic microwave background as seen by the Planck satellite in 2013. Red spots indicate slightly above-average temperature, blue spots slightly below-average temperature. The variations are on the order of 1 in
The Cosmic Microwave Background is the hot plasma of photons and baryons at the time of formation of stable hydrogen, some 380,000 years after the big bang, redshifted by the subsequent expansion of space. However the plasma was not perfectly uniform. Baryons oscillated around lumps of dark matter, subject to the competition between the force of gravity and the radiation pressure from the photons.
The oscillations (and other effects) give rise to a complex spatial fluctuation pattern in the cosmic microwave background. The statistical properties of this fluctuation pattern carry information about the constituents of the Universe at the time of last scattering as well as the subsequent evolution of the Universe. This imposes constraints on the amount of dark energy, dark matter, baryonic matter and neutrinos. The black-body spectrum of the radiation was confirmed with high precision by the Cosmic Background Explorer (COBE) satellite in 1990, as well as the basic details of the fluctuation pattern measured in 1992. These measurements led to the 2006 Nobel Prize for the discoveries. More recently, there is evidence of polarization of the MWB.
The cosmic microwave background has been studied in great detail across the whole sky by the Wilkinson Microwave Anisotropy Probe (WMAP) 2001-2010 and currently by the Planck satellite, providing an important foundation for our current understanding of the Universe.