Echoes from the Big Bang; The Once and Future Cosmos; Special Editions; by Robert R. Caldwell and Marc Kamionkowski; 6 Page(s)

Cosmologists are still asking the same questions that the first stargazers posed as they surveyed the heavens. Where did the universe come from? What, if anything, preceded it? How did the universe arrive at its present state, and what will be its future? Although theorists have long speculated on the origin of the cosmos, until recently they had no way to probe the universe's earliest moments to test their hypotheses. In recent years, however, researchers have identified a method for observing the universe as it was in the very first fraction of a second after the big bang. This method involves looking for traces of gravitational waves in the cosmic microwave background (CMB), the cooled radiation that has permeated the universe for nearly 15 billion years.

The CMB was emitted about 400,000 years after the big bang, when electrons and protons in the primordial plasma-the hot, dense soup of subatomic particles that filled the early universe-first combined to form hydrogen atoms. Because this radiation provides a snapshot of the universe at that time, it has become the Rosetta stone of cosmology. After the CMB was discovered in 1965, researchers found that its temperature-a measure of the intensity of the black body radiation-was very close to 2.7 kelvins, no matter which direction they looked in the sky. In other words, the CMB appeared to be isotropic, which indicated that the early universe was remarkably uniform. In the early 1990s, though, a satellite called the Cosmic Background Explorer (COBE) detected minuscule variations-only one part in 100,000 in the radiation's temperature. These variations provide evidence of small lumps and bumps in the primordial plasma. The inhomogeneities in the distribution of mass later evolved into the large-scale structures of the cosmos: the galaxies and galaxy clusters that exist today.