1992

Mapping the Cosmic Microwave Background

The discovery of the Cosmic Microwave Background in 1964 had exciting implications because it showed that theories like the Big Bang could be tested with actual observations. A new generation of astronomers became interested in observational cosmology—the precise measurement of certain characteristics of the universe that could enable a detailed understanding of its origin and evolution. To reach the needed precision, though, these measurements had to be conducted from space.

NASA’s continuing Explorer small satellite program, which had begun in 1958 with the Explorer 1 discovery of the Van Allen Radiation Belts, was a perfect platform. Astrophysicists worked out the mission concepts in the 1970s, and in the 1980s NASA approved the Cosmic Background Explorer (COBE) mission, which was launched into Earth orbit in 1989. COBE used very sensitive infrared and microwave radiation detectors to slowly and accurately build up a map of the variations in cosmic background radiation across the entire sky.

In 1992 cosmologists announced that the initial map was completed, and that the results were exciting. The main variation that COBE detected was a weak so-called dipole signature, only about 1/1000th of the brightness of the sky, due to the Doppler shift from our galaxy’s motion relative to the rest of the universe. Once that signal was removed, the next biggest COBE variation was from the faint microwave emission of our own Milky Way galaxy. Once that signal was removed, astronomers were delighted that there were still some small variations left, tiny fluctuations in background radiation at the scale of a few millionths of a degree.

Cosmologists believe that those tiny variations were formed during the inflation of the universe in the first 10-³² seconds after the Big Bang, concentrating normal and dark matter into the “seeds” that would eventually form galaxies and stars. COBE’s results were confirmed and extended by the Wilkinson Microwave Anisotropy Probe (WMAP) Explorer mission in 2003, helping to provide new, accurate estimates of the age of the universe.

SEE ALSO Big Bang (c. 13.7 Billion BCE), Recombination Era (c. 13.7 Million BCE), Einstein’s “Miracle Year” (1905), Dark Matter (1933), Earth’s Radiation Belts (1958), Cosmic Microwave Background (1964), Walls of Galaxies (1989), Age of the Universe (2001).

Maps of the microwave radiation permeating the universe, as measured by the COBE satellite. The plane of the Milky Way goes through the “equator” of these maps. At the top is the total signal; in the middle is the map with the relative motion of the solar system removed; at the bottom is the map with the signal from the galaxy removed.