the particles, and it is estimated that detectors of the next decade need to be approximately 100 times more sensitive before the particles can be found—if they exist.
The goal of physics is to explain nature with as simple a theory as possible, and the goal of cosmology is to explain the large-scale structure and evolution of the universe in terms of that theory. Astronomers and physicists today believe that many properties of the present universe probably depend on what happened during the first instants after the Big Bang. One such property, ironically, is the apparent uniformity of the universe on the large scale, as evidenced by the cosmic background radiation. Although such uniformity and homogeneity have been assumed in the Big Bang model, they still must be explained, or at least be made plausible. It seems unlikely to many scientists that the universe would have been created so homogeneous by accident.
In the 1970s an important change occurred in theoretical cosmology. Physicists with expertise in the theory of subatomic particles joined astronomers to work on cosmology. The physicists brought a fresh stock of ideas and a new set of intellectual tools to bear on the question of why the universe has the properties it does, not just what those properties are. Particle physicists in the United States and in the Soviet Union proposed a modification to the Big Bang model called “the inflationary universe” that has caused a major change in cosmological thinking. The essential feature of the inflationary universe model is that, shortly after the Big Bang, the infant universe went through a brief and extremely rapid expansion, after which it returned to the more leisurely rate of expansion of the standard Big Bang model. By the time the universe was a tiny fraction, perhaps 10−32, of a second old, the period of rapid expansion, or inflation, was over. The epoch of rapid expansion could have taken a patch of space so tiny that it had already homogenized and quickly stretched it to a size larger than today's entire observable universe. Thus the inflationary expansion would make the universe appear homogeneous over an extremely vast region, far larger than any region from which we have data.
The inflationary model makes specific predictions about the formation of structures in the universe. In particular, processes in the early universe would have determined the nature of the initial inhomogeneities that later condensed into galaxies and clusters of galaxies. Some of the predictions of this model will be tested by the COBE satellite and by galaxy surveys to be made in the 1990s with new optical telescopes. Whether these and other observations will confirm the inflation model or lead theorists to a different approach altogether is, of course, unknown. But astronomers and physicists will be working together to learn the past history of the observable universe.