Since the cosmic microwave background was first discovered in 1964, the science of cosmology — the study of the origin and structure of the universe — has significantly advanced. As the oldest light that we can see, the cosmic microwave background is a subject of great interest to scientists and continues to be studied, most notably with information provided by two satellite observatories: the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP).

The cosmic microwave background is a snapshot of how the universe looked at a very young age. Just after the Big Bang, the 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 all 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, which allowed atoms to form. Due to the presence of atoms, photons scattered less and the universe became transparent to radiation. Since then, the universe has continued to expand and cool, and the photons that existed when the young universe first became transparent at 3,000 K have been stretched along with the expansion of space. These photons are now seen as the cold 3 K radiation of the cosmic microwave background.

COBE, launched in 1989, was the first mission to detect slight fluctuations in the almost completely uniform distribution of the cosmic microwave background. Called anisotropies, these variations in temperature are just slightly different — only one part in 100,000 — from the general background temperature. WMAP, launched in 2001, followed up on the results from COBE by mapping the variations in the cosmic microwave background with incredible detail. Where COBE took a blurry baby picture of the universe, WMAP took one with superior focus and clarity.

The results of WMAP have offered support for the Big Bang theory and given further insight into the conditions of the early universe. Scientists have found that the universe is about 13.7 billion years old and that there was likely a period of rapid expansion just after the Big Bang. They have also learned that the early universe had variations in the density of matter — seen in the fluctuations of the cosmic microwave background — that eventually led to the formation of galaxies and to the large-scale structure of the universe that we see today. In addition, findings indicate that the geometry of the universe is flat and that it will continue to expand forever.

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