cosmic microwave background (CMB), electromagnetic radiation filling the universe that is a residual effect of the big bang 13.8 billion years ago. Because the expanding universe has cooled since this primordial explosion, the background radiation is in the microwave region of the electromagnetic spectrum.

Discovery of the cosmic background

Beginning in 1948, the American cosmologist George Gamow and his coworkers, Ralph Alpher and Robert Herman, investigated the idea that the chemical elements might have been synthesized by thermonuclear reactions that took place in a primeval fireball. According to their calculations, the high temperature associated with the early universe would have given rise to a thermal radiation field, which has a unique distribution of intensity with wavelength (known as Planck’s radiation law), that is a function only of the temperature. As the universe expanded, the temperature would have dropped, each photon being redshifted by the cosmological expansion to longer wavelength, as the American physicist Richard C. Tolman had already shown in 1934. By the present epoch the radiation temperature would have dropped to very low values, about 5 kelvins above absolute zero (0 kelvin [K], or −273 °C [−460 °F]) according to the estimates of Alpher and Herman.

Interest in these calculations waned among most astronomers when it became apparent that the lion’s share of the synthesis of elements heavier than helium must have occurred inside stars rather than in a hot big bang. In the early 1960s physicists at Princeton University, New Jersey, as well as in the Soviet Union, took up the problem again and began to build a microwave receiver that might detect, in the words of the Belgian cleric and cosmologist Georges Lemaître, “the vanished brilliance of the origin of the worlds.”

The actual discovery of the relict radiation from the primeval fireball, however, occurred by accident. In experiments conducted in connection with the first Telstar communication satellite, two scientists, Arno Penzias and Robert Wilson, of the Bell Telephone Laboratories, Holmdel, New Jersey, measured excess radio noise that seemed to come from the sky in a completely isotropic fashion (that is, the radio noise was the same in every direction). When they consulted Bernard Burke of the Massachusetts Institute of Technology, Cambridge, about the problem, Burke realized that Penzias and Wilson had most likely found the cosmic background radiation that Robert H. Dicke, P.J.E. Peebles, and their colleagues at Princeton were planning to search for. Put in touch with one another, the two groups published simultaneously in 1965 papers detailing the prediction and discovery of a universal thermal radiation field with a temperature of about 3 K.

Precise measurements made by the Cosmic Background Explorer (COBE) satellite launched in 1989 determined the spectrum to be exactly characteristic of a blackbody at 2.735 K. The velocity of the satellite about Earth, Earth about the Sun, the Sun about the Galaxy, and the Galaxy through the universe actually makes the temperature seem slightly hotter (by about one part in 1,000) in the direction of motion rather than away from it. The magnitude of this effect—the so-called dipole anisotropy—allows astronomers to determine that the Local Group (the group of galaxies containing the Milky Way Galaxy) is moving at a speed of about 600 km per second (km/s; 400 miles per second [miles/s]) in a direction that is 45° from the direction of the Virgo cluster of galaxies. Such motion is not measured relative to the galaxies themselves (the Virgo galaxies have an average velocity of recession of about 1,000 km/s [600 miles/s] with respect to the Milky Way system) but relative to a local frame of reference in which the cosmic microwave background radiation would appear as a perfect Planck spectrum with a single radiation temperature.

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The COBE satellite carried instrumentation aboard that allowed it to measure small fluctuations in intensity of the background radiation that would be the beginning of structure (i.e., galaxies and clusters of galaxies) in the universe. The satellite transmitted an intensity pattern in angular projection at a wavelength of 0.57 cm after the subtraction of a uniform background at a temperature of 2.735 K. Bright regions at the upper right and dark regions at the lower left showed the dipole asymmetry. A bright strip across the middle represented excess thermal emission from the Milky Way. To obtain the fluctuations on smaller angular scales, it was necessary to subtract both the dipole and the galactic contributions. An image was obtained showing the final product after the subtraction. Patches of light and dark represented temperature fluctuations that amount to about one part in 100,000—not much higher than the accuracy of the measurements. Nevertheless, the statistics of the distribution of angular fluctuations appeared different from random noise, and so the members of the COBE investigative team found the first evidence for the departure from exact isotropy that theoretical cosmologists long predicted must be there in order for galaxies and clusters of galaxies to condense from an otherwise structureless universe. These fluctuations correspond to distance scales on the order of 10⁹ light-years across (still larger than the largest material structures seen in the universe, such as the enormous grouping of galaxies dubbed the “Great Wall”).

The Wilkinson Microwave Anisotropy Probe (WMAP) was launched in 2001 to observe the fluctuations seen by COBE in greater detail and with more sensitivity. The conditions at the beginning of the universe left their imprint on the size of the fluctuations. WMAP’s accurate measurements showed that the early universe was 63 percent dark matter, 15 percent photons, 12 percent atoms, and 10 percent neutrinos. Today the universe is 72.6 percent dark energy, 22.8 percent dark matter, and 4.6 percent atoms. Although neutrinos are now a negligible component of the universe, they form their own cosmic background, which was discovered by WMAP. WMAP also showed that the first stars in the universe formed half a billion years after the big bang.

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