John Mather, at NASA's Goddard Space Flight Center, leads the design of the COBE satellite (Cosmic Background Explorer), launched in 1989 to measure with unprecedented precision the cosmic background radiation accidentally detected by Penzias and Wilson in 1965. Mather confirms that the radiation's spectrum matches, with extraordinary accuracy, that of a perfect black body at a temperature of 2.725 kelvin, validating with complete mathematical precision the predictions of the Big Bang model. George Smoot, at the University of California, Berkeley, leads the satellite instrument designed to detect extraordinarily small variations — on the order of one part in a hundred thousand — in the temperature of that radiation depending on the direction of the sky observed. In 1992, Smoot announces the detection of these tiny anisotropies, which represent the primordial density seeds from which, through progressive gravitational attraction over billions of years, all galaxies and large-scale structures of the observable universe would later form. The finding, which Stephen Hawking called the scientific discovery of the century, if not of all history, provides the oldest possible observational map of the universe, frozen at the moment light was first able to travel freely, 380,000 years after the Big Bang, and constitutes the observational foundation on which all subsequent cosmological models rest.