In 1967 Arno Penzias and Robert Wilson made the first definitive detection of 3 K microwave radiation from the sky. The radiation was soon confirmed as the redshifted afterglow from the creation of the universe. For this discovery Penzias and Wilson won the 1978 Nobel Prize in Physics. This Cosmic Microwave Background (CMB) radiation allows astronomers to probe the distribution of matter and energy at a time slightly less than 400,000 years after the Big Bang. Soon after its discovery, astronomers recognized that accurate measurements of the CMB could provide vital information regarding the structure and evolution of the universe. Many experiments have been done since then, confirming this vision. The 2006 Nobel Prize in Physics was awarded to John Mather and George Smoot for their measurements of the spectrum and anisotropy of the CMB with the Cosmic Background Explorer (COBE) experiment.
Launched in July 2001, the Wilkinson Microwave Anisotropy Probe (WMAP) has made an extraordinarily detailed, all-sky high-resolution map of the brightness of the cosmic microwave background. It is sensitive to temperature variations at the level of 1 part per million. By analyzing these tiny variations, the WMAP team, led by Bennett, Page and Spergel, has measured the fundamental cosmological parameters, including the geometry, age and composition of the universe to unprecedented precision. We now know that the universe is geometrically flat, its age is 13.75 ± 0.13 billion years, and that it is composed of dark energy (72.1 ± 1.5 %), dark matter (23.3 ± 1.3 %) and baryons (4.6 ± 0.1 %). The WMAP measurements have opened the era of precision cosmology and have had an enormous impact across astronomy, cosmology and physics.