Erik M. Leitch of the University of Chicago explains.
The Cosmic Microwave Background radiation, or CMB for short, is a faint glow of light that fills the universe, falling on Earth from every direction with nearly uniform intensity. It is the residual heat of creation--the afterglow of the big bang--streaming through space these last 14 billion years like the heat from a sun-warmed rock, reradiated at night.
Since the early twentieth century, two concepts have transformed the way astronomers think about observing the universe. The first is that it is fantastically large; the portion of the universe visible today is a sphere nearly 15 billion light-years in radius, and that, we believe, is just the tip of the iceberg. The second is that light travels at a fixed speed. A simple consequence of these ideas is that as you look at more and more distant objects, you're seeing farther and farther back in time--sometimes very far back indeed. When you see Jupiter shining in the night sky, for example, you're looking about an hour back in time, whereas the light from distant galaxies captured by telescopes today was emitted millions of years ago.
The CMB is the oldest light we can see--the farthest back both in time and space that we can look. This light set out on its journey more than 14 billion years ago, long before the Earth or even our galaxy existed. It is a relic of the universe's infancy, a time when it was not the cold dark place it is now, but was instead a firestorm of radiation and elementary particles. The familiar objects that surround us today--stars, planets, galaxies and the like--eventually coalesced from these particles as the universe expanded and cooled.
This residual radiation is critical to the study of cosmology because it bears on it the fossil imprint of those particles, a pattern of miniscule intensity variations from which we can decipher the vital statistics of the universe, like identifying a suspect from his fingerprint.
When this cosmic background light was released billions of years ago, it was as hot and bright as the surface of a star. The expansion of the universe, however, has stretched space by a factor of a thousand since then. The wavelength of the light has stretched with it into the microwave part of the electromagnetic spectrum, and the CMB has cooled to its present-day temperature, something the glorified thermometers known as radio telescopes register at about 2.73 degrees above absolute zero.
Answer originally posted October 13, 2003.