Astronomers have known since the 1920s, when Edwin Hubble noted that all galaxies are moving away from ours, that the universe is expanding. Theory holds that if you could rewind time toward the beginning, the universe would get denser and therefore hotter.

When the visible universe was only one-hundred-millionth its present size, the temperature was 273 million degrees above absolute zero, theorists say. Hydrogen, the chief component of the universe, would have been completely ionized at that extreme temperature, meaning instead of atoms there were just free electrons and nuclei, made of neutrons and protons.

Photons (the basic units of radiation) from early microwave radiation scatter easily off electrons, so they would have wandered erratically through the nascent universe. Radiation would have been scattered randomly like light in a heavy fog.

Eventually, the universe cooled enough so that protons and electrons could combine to form what scientists call neutral hydrogen. This occurred sometime around 380,000 years after the Big Bang. The fog began to lift and radiation was allowed to travel more or less directly.

This unleashed the universe's original radiation, now called cosmic microwave background (CMB) radiation. It, too, has spread out over time as the universe continues to expand. In fact, the CMB started out as something else, and because the universe has been expanding all along, the radiation's wavelengths were stretched over time to the microwave range.

The CMB fills the universe and is now very cold. Scientists have long figured that studying it would provide a picture of the universe at a time prior to anything that can be seen with conventional telescopes, which monitor visible light, infrared radiation or X-rays.

The CMB carries an imprint of the last scattering that occurred as it emerged from the fog. Taking its temperature allows a map to be made of this "surface of last scattering," scientists say. It's a bit like mapping a cloud: You can't see inside the cloud, but the light that emerges allows a detailed view of the cloud's surface and provides hints of the insides.

The Microwave Anisotropy Probe (MAP) observatory, launched June 30, 2001, measures the microwave background's extremely tiny variations. The CMB can range from 2.7251 to 2.7249 degrees Kelvin in two parts of the sky. Kelvin is a measure of degrees above absolute zero. The mission was renamed WMAP when its first findings were released.

The minute variations, called anisotropy, were first detected in 1992 by NASA's Cosmic Background Explorer (COBE) satellite. WMAP has been examining them in finer detail and with greater sensitivity. Scientists study the variations for clues about the size and geometry of the early universe, as well as the matter that was present when the microwave background was released.

This basic information in turn reveals the primordial structure of the universe.

The CMB was predicted to exist in 1948 by George Gamow. It was first observed accidentally in 1965 because it created noise in a radio receiver being designed at the Bell Telephone Laboratories. Arno Penzias and Robert Wilson made the discovery, which led to their receiving the 1978 Nobel prize in physics.

At first glance the microwave background seemed uniform. But with sensitive instruments, its fine variations were detected in 1992 by COBE. Scientists liken the situation to observing Earth from some far corner of space. At first it would just appear as a pale blue dot. Only upon close inspection would mountains, valleys and oceans be revealed.

The WMAP spacecraft sits about 1 million miles (1.5 million kilometers) from Earth, or four times the distance to the Moon. It hovers around a point of gravitational stability between the Earth and Sun. From this stable perch, going around the Sun in synch with Earth's orbit, WMAP has an unobstructed view of the sky, with the Sun, Earth and Moon always to its back. It is also free of interfering magnetic fields and microwave emissions nearer to Earth.