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Satellite Measurements of Cosmic Radiation Support Inflation Theory

By Vinu G. Ipe
Staff reporter

The inflationary theory of the beginning of the universe received a boost last week when scientists reported that data from the Cosmic Background Explorer (COBE) satellite were consistent with the theory's predictions. The theory was first promulgated by Professor of Physics Alan H. Guth '68 about a decade ago.

The COBE results are significant because they contain observations of anisotropies, or non-uniformities, in the microwave radiation that pervades the universe, usually referred to as the cosmic background radiation. These non-uniformities are left over from concentrations of matter that formed 300,000 years after the Big Bang. As time passed and the universe expanded, these clumps attracted more and more matter, eventually growing into the large-scale structures of the modern universe.

COBE, launched by NASA in 1989, contains several instruments which measure the background radiation. The instruments, called differential microwave radiometers, record temperatures at three wavelengths in the microwave spectrum. The temperature variations recorded were on the order of 1 part in 100,000; the ambient temperature of the cosmic background is about 3 K.

Guth said the non-uniformity in the mass density imprints itself on the cosmic background radiation by creating a gravitational potential well which causes photons to lose energy. Since the energy of a photon is directly related to its frequency, these photons have a lower frequency -- a shift which COBE can detect. Each photon's frequency shift tells the depth of the gravitational potential well from which it originated.

Professor of Physics Rainer Weiss '55 cautioned, "The COBE results are only measurements, not a confirmation of the inflationary theory. COBE adds another benchmark in our efforts to understand the world."

Guth said, "In the inflationary model, the universe goes through a short period -- very early in its history and lasting for a fraction of a second -- of very rapid expansion during which matter is dominated by a very strange state called the false vacuum. This `vacuum' causes a gravitational repulsion which drives the enormous expansion of the early universe.

"Then at some point the false vacuum becomes unstable and decays, and the energy that was contained in the false vacuum is released and becomes the energy that makes up everything in the universe. Because of quantum uncertainties in the processes which end inflation, it ends at slightly different times in different places, and that leads to anisotropies, or slightly different mass densities in different places." It is these anisotropies that have been detected by COBE in the form of non-uniformities in the cosmic background radiation.

"The COBE results are the most important since the discovery of the microwave background radiation itself. I regard it as an observation which is really going to open a new era in astronomy. I feel very gratified that COBE is consistent with the inflationary theory. Certainly when I first calculated what kind of a distribution of non-uniformities inflation would give rise to, it sounded very abstract to me. It never seemed real that somebody would actually someday measure those non-uniformities," Guth said.