CMBR( Cosmic Microwave Background Radiation)

In cosmology, cosmic microwave background (CMB) radiation (also CMBR, CBR, MBR, and relic radiation) is a form of electromagnetic radiation filling the universe. With a traditional optical telescope, the space between stars and galaxies (the background) is pitch black. But with a radio telescope, there is a faint background glow, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum, hence the name cosmic microwave background radiation. The CMB's discovery in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned them the 1978 Nobel Prize.

The CMBR is well explained as radiation left over from an early stage in the development of the universe, and its discovery is considered a landmark test of the Big Bang model of the universe. When the universe was young, before the formation of stars and planets, it 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 enough, stable atoms could form. These atoms could no longer absorb the thermal radiation, and the universe became transparent instead of being an opaque fog. The photons that existed at that time have been propagating ever since, though growing fainter and less energetic, since the exact same photons fill a larger and larger universe. This is the source for the term relic radiation, another name for the CMBR.

Precise measurements of cosmic background radiation are critical to cosmology, since any proposed model of the universe must explain this radiation. The CMBR has a thermal black body spectrum at a temperature of 2.725 K, thus the spectrum peaks in the microwave range frequency of 160.2 GHz, corresponding to a 1.9 mm wavelength.[nb 1] The glow is almost but not quite uniform in all directions, and shows a very specific pattern equal to that expected if a fairly uniformly distributed red-hot gas is expanded to the current size of the universe. In particular, the spatial power spectrum (how much difference is observed versus how far apart the regions are on the sky) contains small anisotropies, or irregularities, which vary with the size of the region examined. They have been measured in detail, and match what would be expected if small thermal variations, generated by quantum fluctuations of matter in a very tiny space, had expanded to the size of the observable universe we see today. This is still a very active field of study, with scientists seeking both better data (for example, the Planck spacecraft ) and better interpretations of the initial conditions of expansion.