Guth, Alan

Alan is Professor of Physics, MIT. He is the originator of the now widely accepted cosmological model of the early universe, which he called Inflation and which seeks to explain how the universe came to be so uniform and why it began so close to the critical density. Inflation modifies the Big Bang theory by proposing that the expansion of the universe was propelled by a repulsive gravitational force. Here is how Alan describes it: “The inflationary universe theory is an add-on to the standard Big Bang theory, and basically what it adds on is a description of what drove the universe into expansion in the first place. In the classic version of the Big Bang theory, that expansion was put in as part of the initial assumptions, so there's no explanation for it whatever. The classical Big Bang theory was never really a theory of a bang; it was really a theory about the aftermath of a bang. Inflation provides a possible answer to the question of what made the universe bang, and now it looks like it's almost certainly the right answer. Inflationary theory takes advantage of results from modern particle physics, which predicts that at very high energies there should exist peculiar kinds of substances which actually turn gravity on its head and produce repulsive gravitational forces. The inflationary explanation is the idea that the early universe contains at least a patch of this peculiar substance. It turns out that all you need is a patch; it can actually be more than a billion times smaller than a proton. But once such a patch exists, its own gravitational repulsion causes it to grow, rapidly becoming large enough to encompass the entire observed universe. The inflationary theory gives a simple explanation for the uniformity of the observed universe, because in the inflationary model the universe starts out incredibly tiny. There was plenty of time for such a tiny region to reach a uniform temperature and uniform density, by the same mechanisms through which the air in a room reaches a uniform density throughout the room. And if you isolated a room and let it sit long enough, it will reach a uniform temperature as well. For the tiny universe with which the inflationary model begins, there is enough time in the early history of the universe for these mechanisms to work, causing the universe to become almost perfectly uniform. Then inflation takes over and magnifies this tiny region to become large enough to encompass the entire universe, maintaining this uniformity as the expansion takes place.” One of the intriguing consequences of inflation is that quantum fluctuations in the early universe can be stretched to astronomical proportions, providing the seeds for the large scale structure of the universe. The predicted spectrum of these fluctuations was calculated by Guth and others in 1982. These fluctuations can be seen today as ripples in the cosmic background radiation, but the amplitude of these faint ripples is only about one part in 100,000. Nonetheless, these ripples were detected by the COBE satellite in 1992, and they have now been measured to much higher precision by the WMAP satellite and other experiments. The properties of the radiation are found to be in excellent agreement with the predictions of the simplest models of inflation. Working with Professor Edward Farhi and others, Guth has explored the question of whether it is in principle possible to ignite inflation in a hypothetical laboratory, thereby creating a new universe. The answer is a definite maybe. They showed that it cannot be done classically, but with quantum tunneling it might be theoretically possible. The new universe, if it can be created, would not endanger our own universe. Instead it would slip through a wormhole and rapidly disconnect completely. Another intriguing feature of inflation is that almost all versions of inflation are eternal—once inflation starts, it never stops completely. Inflation has ended in our part of the universe, but very far away one expects that inflation is continuing, and will continue forever. Is it possible, then, that inflation is also eternal into the past? Recently Guth has worked with Alex Vilenkin and Arvind Borde to show that the inflating region of spacetime must have a past boundary, and that some new physics, perhaps a quantum theory of creation, would be needed to understand it.