electromagnetic radiation, notably in the microwave range, that arrives at the earth's surface from all directions in outer space. This microwave radiation forms a background to all the discrete radio sources that have been detected by radio telescopes (see RADIO ASTRONOMY,). It was first detected and reported by U.S. radio astronomers Arno A. Penzias and Robert W. Wilson in 1965, when they were working at the Bell Telephone Laboratories in Holmdel, N.J. Penzias and Wilson received the 1978 Nobel Prize in physics for this discovery.

Cosmic microwave background radiation was originally predicted to exist in 1948, as part of the big bang theory of the origin of the universe (see COSMOLOGY,). According to that generally accepted theory, such radiation is the lingering remains of the hot conditions that prevailed in an early period after the big bang. Scientists today think that for a while after the big bang the universe was so hot that radiation (photons) and the components of matter (such as electrons and protons) were intermixed; the universe was, so to speak, opaque. After about 300,000 years or so it became cool enough—about 3000 K (5000˚ F)—for a “decoupling” to occur. Electrons and protons could bind together to form atoms of hydrogen, allowing radiation photons to travel considerable distances before being absorbed; in other words, the universe became rather transparent.

The remnants of the radiation from that period are believed to account for the cosmic microwave background radiation we see today. Because of the continuing expansion of the universe, this background radiation has been red-shifted into the microwave region of the spectrum, so that it now represents a temperature of about 2.725 K (–455˚ F). The radiation is not entirely uniform. Tiny variations in temperature are observed that presumably originated in differences in density in the early universe—differences that eventually led to the formation of the large structures that exist today, such as galaxies and groups of galaxies.The tiny fluctuations, or anisotropy, of the cosmic microwave background radiation were detected by the Cosmic Background Explorer (COBE), a U.S. artificial earth satellite that was launched in 1989 to study the cosmic background and performed observations for four years. COBE also studied cosmic infrared background radiation; its data yielded approximate limits on the amount of star formation occurring in the universe and showed that, as was widely believed, the birth of many stars was obscured by dust. John C. Mather and George F. Smoot shared the 2006 Nobel Prize in Physics for their pioneering work in setting up and analyzing the COBE measurements. A more precise study of temperature differences in the cosmic microwave background has been carried out with the Wilkinson Microwave Anisotropy Probe (WMAP), a U.S. satellite launched into a “halo” orbit around the sun in 2001. In addition to confirming and elaborating on the COBE findings regarding the microwave anisotropy, data from WMAP suggested that ordinary matter accounts for only a small proportion of the universe. The data suggested that perhaps almost three-quarters of the universe was an enigmatic dark energy, which speeded up the universe's expansion, while the rest was a mysterious dark matter, which did not absorb or emit electromagnetic radiation.

There also is an X-ray cosmic background radiation, which long remained unexplained. But such powerful orbiting X-ray observatories as the U.S. Chandra X-ray Observatory and the European Space Agency's XMM-Newton, both launched in 1999, have been able to link most of the X-ray background with specific sources, such as distant active galaxies and quasars, whose strong X-ray production is believed to be associated with enormous amounts of matter being sucked into supermassive black holes.