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The Horizon Problem

The uniformity of cosmic background radiation--varying by no more than one part in 10,000, where ever you look--posed a problem to Standard Big Bang cosmology. Suppose the universe began 14 billion years ago. We look to the west, we detect cosmic background radiation. We turn our radio antennas to the east, we detect cosmic background radiation--at exactly the same temperature. The radiation from the east and the radiation from the west are separated by 28 billion light years. Common sense tells us that the radiation from the east could not possibly be causally connected to that from the west, because information cannot travel faster than the speed of light. Nor could the regions they traveled from ever have been in communication.

It's as though 200 students one of those huge introductory chemistry classes were taking a test in a huge auditorium. And yet each student scores exactly 93% on the test. There had to be some cheating going on. But how?

The horizon problem gets worse if we travel back to the time when radiation was released from matter. The universe was 100,000 years old--meaning that the horizon was 100,000 light years across. But the east and west photons reaching our radio antennae today were then separated by 10 million light years. That's 100 times the horizon!

Again, how can such two causally disconnected regions have one and same temperature? Isotropy was simply an initial condition specified by the Standard Big Bang model. But such ad hoc assumptions don't make for a very satisfying theory.

The horizon problem was finally explained by inflation.

Inflation and the Observable Universe

The instant before inflation began, 10^-35 seconds after the Big Bang, the "stuff" that expanded to become our universe was only about 10^-24 centimeter in diameter. All matter and energy were in close and uniform contact.
JPEG Image (22.6 KB); Caption

Within the briefest instant, the universe expanded exponentially by a factor of about 10^50, stretching once intimately connected matter and energy to the farthest reaches of the universe. The information contained in the pre-inflationary universe didn't have to travel the speed of light--indeed it couldn't have--it traveled at the speed of inflation.

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Copyright, (c) 1995: Board of Trustees, University of Illinois

NCSA. Last modified 11/2/95.