Tiny temperature fluctuations in the otherwise smooth cosmic background radiation represent the gravitational seeds in the early universe around which galaxies and galaxy clusters ultimately formed.
Predicted by George Gamow and his collaborators in the 1940s and detected by Arno Penzias and Robert Wilson in the 1960s, the cosmic background radiation is the faint echo of the Big Bang. Following the explosive birth of our cosmos, the universe both expanded and cooled off rapidly. After roughly 300,000 years, its temperature had fallen to about 3000 kelvin (5000° Fahrenheit) and a big change was taking place. Before this time, conditions were too hot for atoms to form—protons and electrons each went their separate ways—and photons of light could travel only short distances before interacting with the free electrons. It was as if the universe existed in a thick fog that kept light from penetrating.
But when the temperature reached 3000 kelvin, atomic nuclei finally captured electrons and formed stable atoms. Photons were then able to travel unimpeded—the fog lifted—and the universe became transparent to light. It’s that light we see as the background radiation, coming at us from all directions. However, in the 10 billion or more years since the Big Bang, the universe has expanded by a factor of a thousand, causing the temperature of the radiation to fall by the same amount. It now glows at just 3 kelvin (3° Celsius above absolute zero) in the microwave part of the electromagnetic spectrum, a faint reminder of our universe’s hot start. The background appears very smooth, varying by only one part in 100,000 across the sky.
A pair of radio astronomers working at Bell Laboratories, Arno Penzias (1933-) and Robert Wilson (1936-) are credited with discovering the cosmic microwave background radiation. Using an antenna originally designed to detect signals from the Echo satellite, the two chanced upon an annoying radio hiss that seemed to be coming from everywhere. After accounting for all possible sources of error, including pigeon droppings inside the antenna, they concluded they were seeing signals coming from all directions of space. After discussing their findings with Princeton physicist Robert Dicke, they realized they were seeing the faint echo of the Big Bang predicted by George Gamow and his colleagues in the 1940s, now glowing softly at a temperature of just 3 degrees above absolute zero. For the discovery, Penzias and Wilson shared the 1978 Nobel Prize for physics.