Page  1  |  2  |  3

Alan Guth

If the inflationary theory of cosmology is right, it means that the universe is far larger than we had thought. Most likely the universe is also much older than we thought, and it includes not just one, but an infinity of big bangs.

       The inflationary theory was developed to explain observable features of the universe, and it has been remarkably successful. One example is the nature of the cosmic background radiation, which cosmologists interpret as the afterglow of the heat of the big bang itself.

Alan Guth

       Astronomers have made high precision measurements of this radiation, finding that it arrives at Earth with the same intensity from all directions, to the extraordinary accuracy of about 1/1000 of a percent.  Tracing the history of this radiation backwards in time, cosmologists conclude that the temperature and the density of matter in the universe must have been uniform to this accuracy when the cosmic background radiation was released, about 300,000 years after the big bang. Without inflation, this extreme uniformity of the early universe must be assumed, but cannot be explained. Calculations show that without inflation there would not have been nearly enough time for this uniformity to come about, so one is forced to assume, without explanation, that the universe was uniform from its very beginning.

       Despite its name, the classical form of the big bang theory is not really a theory of a bang at all. It really describes only the aftermath of the bang. It describes how the early, hot, dense universe expanded and cooled; it describes how the light chemical elements were synthesized during this expansion, and how the matter coagulated to form galaxies and stars. But it says nothing about what banged or what caused it to bang, and therefore it makes no predictions about the uniformity of the universe just after the bang.

Learn more about:

Inflationary Universe

The Big Bang

Cosmic Background Radiation

       Inflation, on the other hand, can explain the “bang” of the big bang. It relies on a proposal, originating in modern particle physics, that extraordinarily high densities can lead to a form of matter that would turn gravity on its head, causing it to become repulsive rather than attractive. 

       For reasons that are not important here, this form of matter is called a “false vacuum.”  Inflation is the proposal that the expansion of the universe that we see today is the result of the gravitational repulsion of a false vacuum that filled the universe during a small fraction of a second of its early history.

        In the inflationary theory the extreme uniformity of the universe was established early, before inflation began. At this time the region destined to become the presently observed universe was tiny—more than a billion times smaller than the size of a single proton. For such a small region, there was plenty enough time for uniformity to arise by the same kind of mundane processes by which the air in a room spreads out to uniformly fill the volume. After this uniformity was established, inflation took over to stretch the region to become large enough to include all the stars and galaxies that we see today.

Alan Guth is the Victor Weisskopf Professor of Physics at the Massachusetts Institute of Technology. He is the originator of the inflationary universe theory, and recently authored a popular-level book, THE INFLATIONARY UNIVERSE: THE QUEST FOR A NEW THEORY OF COSMOS ORIGINS.

Page  1  |  2  |  3

[Home]  [Unsolved Mysteries]

PBS Online]  [Thirteen Online]