A spacecraft which is precisely measuring the cosmic microwave background (CMB) radiation. The Wilkinson Microwave Anisotropy Probe (WMAP) is a space mission of the National Aeronautics and Space Administration (NASA) that has put fundamental theories of the nature of the universe to a precise test. Since August 2001, WMAP has continually surveyed the full sky, mapping out tiny differences in the temperature of the cosmic microwave background radiation, which is the radiant heat from the big bang.

A fossil remnant of the hot big bang, the cosmic microwave background permeates the universe and is seen today with an average temperature of only 2.725 kelvins. Tiny variations about this average temperature were first discovered by NASA's Cosmic Background Explorer (COBE) mission. WMAP followed up on the COBE results by characterizing the detailed statistical nature of the temperature variations of the cosmic microwave background (called anisotropy), revealing a wealth of detail about the global properties of the universe.