The Big Bang theory states that approximately 14 billion years ago the universe burst into being from an unknown cosmic trigger. The early universe, contained within just a few millimeters, was very hot and densely packed with matter. Ever since that initial explosion, the universe has continued to expand and cool.

Although generally accepted as the model for the origin and evolution of the universe, the Big Bang theory is not complete. For example, it does not explain what caused the initial expansion or why galaxies formed. However, there is significant evidence that the universe did originate from such an event. Three key pieces of observational evidence lend support to the Big Bang theory: the measured abundances of elements, the observed expansion of space, and the discovery of the cosmic microwave background (CMB).

The CMB refers to the uniform distribution of radiation that pervades the entire universe. Scientists have found that there is radiation at a temperature of just about 3 K in every direction of the sky. The CMB is a nearly perfect blackbody -- a theoretical object that absorbs and emits at all wavelengths and has a characteristic spectral curve dependent on its temperature. At a temperature of 3 K, the blackbody curve peaks in the microwave range of the electromagnetic spectrum. When the CMB was discovered, the temperature of the radiation strongly matched predictions from the Big Bang theory.

Scientists believe that, as a nearly perfect blackbody, the CMB must have been produced by the Big Bang. Just after the Big Bang, the very early universe would have been so hot and dense that matter could exist only as free electrons and nuclei. These free particles would continually scatter photons throughout the universe, making the universe opaque. About 380,000 years after the Big Bang, the universe had expanded and cooled to about 3,000 K. This allowed atoms to form, scattering fewer photons and making the universe transparent to radiation. Since then, the universe has continued to expand and cool, and the photons that existed when the universe first became transparent have also expanded and cooled. These photons are now detected as the 3 K radiation of the CMB -- cooled remnants of the Big Bang.

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