Resource: Evidence for the Big Bang Theory

Print Background Essay

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.

To learn more about the history and fate of the universe, check out History of the Universe.

To learn more about the electromagnetic spectrum, check out The Electromagnetic Spectrum: FRONTLINE.

To learn more about the cosmic microwave background, check out WMAP: "Baby Picture" of the Universe.

To learn more about the size of the universe, check out How Big Is the Universe?.

Print Questions for Discussion

• Why didn't Wilson and Penzias expect to find microwaves from space in 1964?
• For what did Wilson and Penzias win the Nobel Prize?
• Scientists who predicted that the Big Bang would have left a signature of microwave energy throughout the universe were working on a way to detect this when they heard about Wilson and Penzias's inadvertent discovery. What does this say about science, scientific discovery, and communication?
• If microwave radiation in the universe were visible, it would be bright and astonishingly uniform. How does this support the Big Bang theory?
• Do you think microwaves are the only form of radiation left from the Big Bang? Why or why not?