change in time? Can this gas be used to date the epoch when galaxies first formed? Analysis of the intergalactic medium at large distances requires both a high sensitivity to dim light and the ability to disperse the incoming light into its component wavelengths. The needed abilities are beyond current ground-based 4-m telescopes but within reach of the Hubble Space Telescope and the 8- and 10-m telescopes of the coming decade.

Still further back in time the universe consisted of smoothly distributed, hot gas. It emitted radiation that we should be able to see today. In 1965, astronomers did discover a bath of radio waves filling all space. It is believed that this radiation has been traveling freely through space, cooling as it goes, since the universe was only about 300,000 years old. At that time, the enormously hot energy of the cosmic fireball dominated the mass of the universe. Imprinted on this cosmic background radiation should be a record of the distribution of cosmic matter at that time, well before the epoch of galaxy formation. Irregularities in the distribution of matter at that time should be detectable now. They would show up today as variations in the intensity of the radiation detected by our radio telescopes pointed in different directions—and indeed all theories of the formation of galaxies demand the existence of such variations.

So far, to our puzzlement, no variations have been observed. From the measurements of the COBE, an orbiting satellite launched in 1989, and from other experiments, astronomers have recently determined that any variations in the intensity of the cosmic background radiation must be less than several parts in 100,000. Some theories of galaxy formation have been demolished by this fact. Revised theories that require the existence of large amounts of so-far undetected matter predict variations 10 times smaller. In the next decade, detectors now being developed should have the sensitivity required to challenge the new theories by looking for temperature variations of 1 part in 1 million. If no variations are found at these increased sensitivities, then theoretical extragalactic astronomy will be thrown into crisis. Something will be seriously wrong—either with our theories of galaxy formation or with our understanding of the cosmic background radiation. From such confrontations of theory with observations, deeper understanding emerges.

THE LIFE HISTORY OF THE UNIVERSE

The Big Bang Model

As we look further and further into space, will we come to an edge of space or a beginning of time? If the universe had a beginning, how did it begin? Will it have an end? These are questions in cosmology, the branch of astronomy concerned with the structure and evolution of the universe as a whole.

Every culture has had a cosmology. Aristotle's universe had an edge of



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