The Online Encyclopedia and Dictionary






Cosmic microwave background radiation

WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003)
WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003)

The cosmic microwave background radiation (CMB) is a form of electromagnetic radiation that fills the whole of the universe. It has the characteristics of black body radiation at a temperature of 2.725 kelvins. It has a frequency in the microwave range.


CMB and the Big Bang

This radiation is regarded as the best available evidence of the Big Bang (BB) theory and its discovery in the mid-1960s curtailed interest for alternatives such as the steady state theory. The CMB gives a snapshot of the Universe when, according to standard cosmology, the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thus making the universe transparent to radiation. When it originated some 400,000 years after the Big Bang -- this time period is generally known as the "time of last scattering" or the period of recombination or decoupling -- the temperature of the Universe was about 3000 K. Since then the temperature of the radiation has dropped by a factor of roughly 1100 due to the expansion of the Universe. As the universe expands, the CMB photons are redshifted, cooling the radiation inversely proportional to the Universe's scale length. For details on reasoning that the radiation is used as evidence of the Big Bang, see Cosmic background radiation of the Big Bang.

After the creation of the CMB, there are a number of important events. The CMB created hydrogen atoms, but from observations of galaxies, it seems that most of the intergalactic medium consists of ionized material (since there are few absorption lines due to hydrogen atoms). This implies a period of reionization in which the material of the universe breaks down into hydrogen ions. The favored explanation for this is that starlight causes reionization although there is evidence that reionization began before there were large numbers of stars.

The period after the emission of the CMB and the observation of the first stars is semi-humorously referred to by cosmologists as the dark age, and is a period which is under intense study by astronomers.


One feature of the CMB is how closely it matches a black body. Although the temperature of the CMB varies from point to point (i.e. it contains small anisotropies), the spectrum in a particular direction very closely resembles a black body.

Another of the microwave background's salient features is a high degree of isotropy. There are however some anisotropies as well, the most pronounced of which is the dipole anisotropy (180 degree scales) which is at a level of about 10 −3 of the monopole. It is mostly due to the motion of the observer against the CMB, which is some 700 km/s for the Earth.

Variations due to external physics also exist; the Sunyaev-Zel'dovich effect is one of the major factors here, in which a cloud of high energy electrons scatters the radiation, transferring some energy to the CMB photons.

Even more interesting are anisotropies at a level of roughly 10 −5 on scales of roughly tens of arcminutes to several degrees. These very small variations are the result of the Sachs-Wolfe effect which causes photons from the cosmic microwave background to be gravitationally redshifted. According to inflationary theory, the origin of the variations is quantum fluctuations which expand during inflation and result in primordial fluctuations. The angular power spectrum of these variations can be calculated and produces a number of peaks and valleys. The location of these peaks and valleys can be correlated with cosmological parameters such as the Hubble constant, and the geometry of the universe.

Detection, prediction and discovery

Main article: Discovery of cosmic microwave background radiation

The CMB was predicted by George Gamow, Ralph Alpher, and Robert Hermann in the 1940s and was accidentally discovered in 1964 by Penzias and Wilson, who received a Nobel Prize in Physics in 1978 for this discovery. The interpretation of the CMB was a very controversial issue in the 1960s with some proponents of the steady state theory arguing that the CMB was the result of scattered starlight from distant galaxies. Using this model , and based on the study of narrow absorption line features in the spectra of stars, the astronomer Andrew McKellar wrote in 1941: "It can be calculated that the 'rotational' temperature of interstellar space is 2 K." However, during the 1970s the consensus view moved to the point of view that the CBR was the remant of the big bang. Among the observations that swung the astronomical community toward this point of view were the fact that the CBR was much smoother than would be expected from scattered star light.

Because water absorbs microwave radiation, a fact that is used to build microwave ovens, it is rather difficult to observe the CMB with ground-based instruments. CMB research therefore makes increasing use of air and space-borne experiments. Ground-based observations of the CMB are usually made from high altitude locations such as the Chilean Andes and the South Pole.


Of these experiments, the Cosmic Background Explorer (COBE) satellite that was flown in 1989-1996 is probably the most famous and which made the first detection of the large scale anisotropies (other than the dipole). Inspired by the COBE results, a series of ground and balloon-based experiments measured CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the scale of the first acoustic peak, which COBE did not have sufficient resolution to resolve. The first peak was measured with increasing sensitivity and by 2000 the Boomerang experiment reported that the highest power fluctions occur at one degree scales. Together with other cosmological data, these results implied that the geometry of the Universe is flat.

In June 2001, NASA launched a second CBR space mission, WMAP, to make detailed measurements of the anisotropies over the full sky. Results from this mission disclosed in 2003 provided a detailed measurement of the angular power spectrum down to degree scales, tightly constraining various cosmological parameters. The results are broadly consistent with those expected from cosmic inflation as well as various other competing theories, and are available in detail at NASA's data center for Cosmic Microwave Background (CMB) [ed. see links below],

A third space mission, Planck, is to be launched in 2007. Planck employs bolometer technology and will measure the CMB on smaller scales than WMAP. Unlike the previous two space missions, Planck is a collaboration between NASA and ESA (the European Space Agency).

CMB and non-standard cosmologies

During the mid-1990s, the lack of detection of anisotropies in the CMB led to some interest in nonstandard cosmologies (such as plasma cosmology) mostly as a backup in case detectors failed to find anisotropy in the CMB. The discovery of these anisotropies combined with a large amount of new data coming in has greatly reduced interest in these alternative theories.

Some supporters of non-standard cosmology argue that the primordial background radiation is uniform (which is inconsistent with the big bang) and that the variations in the CMB are due to the Sunyaev-Zel'dovich effect mentioned above (among other effects). Conversely, supporters of quasi-steady state model s continue to argue that the background radiation is scattered star light. However, even though the temperature of the radiation is roughly correct, the blackbody isotropy of the CMB cannot be reproduced by any "integrated starlight" method ever proposed.

See also

Physics and Astronomy
Timelines and lists


  • Seife, Charles (2003). Breakthrough of the Year: Illuminating the Dark Universe. Science 302 2038–2039.
  • Partridge, R. B. (1995). 3K: The Cosmic Microwave Background Radiation. New York: Cambridge University Press.

References and external links

  • NASA's data center for Cosmic Microwave Background (CMB)
  • Weisstein, E. W., "Cosmic Background Radiation ".
  • GSU hyperphysics's "3K Cosmic Background Radiation ".
  • Wilson, Robert Woodrow, "The Cosmic Microwave Background Radiation ". Nobel Lecture.
  • Wilkinson Microwave Anisotropy Probe (WMAP) Project . [ed. full-sky map of the oldest detected electromagnetic energy in the universe] -- Tests of the Big Bang: The CMB
  • COsmic Background Explorer  : NASA's COBE (Cosmic Background Explorer) satellite.
  • Hu, Wayne, "Introduction to the Cosmic Microwave Background ." Public talk presented at the IAS.
  • Cosmic Background Imager (CBI) Project
  • Boomerang (Stratospheric Balloon Borne Telescope) Boomerang
  • CMB Astrophysics Research Program -- Cosmic Microwave Background Radiation
  • Dept. Physics & Astronomy, "Cosmic Background Radiation ". Astronomy 162. University of Tennessee.
  • MAP Project . "Fluctuations in the Cosmic Microwave Background ". Department of Physics, Hallym University.
  • CMB and Large Scale Structure -- "One Day Cosmology Meeting ". MIT, April 4, 1997.

Last updated: 02-19-2005 17:38:33
Last updated: 05-06-2005 01:27:49