Genesis: The Sequel

Natural History, Feb, 2000 by Alan H. Guth

In the beginning was the big bang. But what exactly banged? How did it bang? And what happened before it banged? Inflation theory offers some answers.

Although the study of the origin of the universe is in some sense one of the oldest sciences, cosmology as we know it today--a branch of astronomy dealing with the origin and structure of the universe--was in its infancy in 1935, when the Hayden Planetarium first opened. Just six years earlier, Edwin Hubble had discovered the expansion of the universe. Not only did this revolutionary insight overturn the age-old assumption of a static universe, but it initiated an effort among physicists and astronomers to trace this newfound expansion to its beginning--to what we would today call "the big bang."

Over the decades, the big bang theory has become the framework of contemporary cosmology. It elegantly describes how the early universe expanded and cooled and how matter clumped to form galaxies and stars. While there's never any guarantee that a scientific theory is correct, the big bang theory has passed a number of persuasive tests, including the 1965 discovery, by American physicists Arno A. Penzias and Robert W. Wilson, of a microwave hiss that matched predictions for the afterglow of the heat of the big bang. In 1993 the COBE (Cosmic Background Explorer) satellite made far more precise measurements of this radiation, verifying that its properties are just what scientists expected. Yet for all its striking successes, the big bang theory in its traditional form remained incomplete.

In fact, the big bang theory has never really been the theory of a bang at all. It describes the aftermath of a bang--the ongoing ballooning of space itself as the matter of the universe flies apart. But the theory says nothing about what caused this spectacular expansion. It gives not even a clue about what banged, what caused it to bang, or what happened before it banged.

In other words, the theory sheds no light on the underlying physics of the primordial fireball--and with good reason. Not until the 1970s did physicists develop an accurate enough theory of elementary particles to justify extrapolation to the extraordinary temperatures and pressures of the early universe. At about this same time, the evidence for the big bang theory became convincing enough for the scientific community to take it seriously. Thus, in the study of the first fraction of a second of the universe's existence, particle physicists and cosmologists reached common ground. Their combined efforts led to what we now know as inflation theory--a description of the driving force behind the expansion of the universe and a source of plausible answers to the questions of what banged, how it banged, and more.

According to inflation theory, cosmic expansion was propelled by a peculiar material that turned gravity on its head, generating a repulsive, rather than an attractive, gravitational force. The proposal that the laws of physics should allow such a material is not a blind hypothesis but rather a prediction that arises from the combining of modern particle theory with general relativity. A patch of the early universe may have become filled with this repulsive-gravity material in a number of ways, but they would all have led to the same result. Inflation is like a wildfire taking over a forest: whether the fire was started by a match, a candle, or lightning, the outcome is much the same. The inflating patch would grow exponentially, doubling and redoubling. During the first trillionth of a trillionth or a trillionth or a second of the universe's existence, the volume of the universe grew by at least a factor of [10.sup.75]--about as much as it did during the next million years.

After a hundred or more doublings, the repulsive-gravity material would decay, much like a radioactive substance. The energy released would produce a hot, uniform soup of particles--the assumed starting point of traditional big bang theory. Here inflation theory joins the big bang theory, leaving all the successful big bang predictions intact. If inflation theory is right, essentially all the matter in the universe was created during the inflationary expansion. The universe is the ultimate free lunch.

Because inflation theory describes the bang itself, it can explain a number of previously mysterious features of the cosmos. One is the extreme uniformity of the universe on very large scales. If we divided space into cubes of 300 million light-years or more on each side, we would find that each cube closely resembled the others in all its average properties--mass density, galaxy density, light output, and so on. This large-scale uniformity is seen in galaxy surveys, but the cosmic background radiation provides the most dramatic evidence, showing that the temperature of the early universe was uniform to better than one part in 100,000.

Aside from inflation, no known mechanism can explain this uniformity. Before inflation theory was proposed, cosmologists had no choice but to postulate that the universe somehow began with an almost perfectly uniform temperature. Inflation theory changes this picture, inserting an enormous growth spurt into cosmic history. Prior to this spurt, when the region we are currently observing was less than [1/10.sup.75] the size that traditional big bang theory had assigned it, there was plenty of time for it to come to a uniform temperature, just as a slice of pizza cools to room temperature after being taken from the oven. Once this tiny region became uniform, inflation stretched it so much that it now encompasses everything we can see.