Inflationary Universe

An extraordinary burst of expansion in the very early stages of the universe inflated the size of the cosmos by a factor of 1050. This contrasts with the standard big-bang model, which has the universe expanding at an ever-decreasing rate as gravity tries to pull all the matter back together.

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Big Bang Universe

Cosmic Background Radiation

Phase Transitions

Alan Guth: An Eternity of Bubbles?

The big-bang theory does a remarkable job of describing the universe we see today: It explains the expansion of the universe, predicts the correct abundances of hydrogen and helium (the most common elements in the universe), and accounts for the cosmic background radiation. Few scientists today doubt its validity.

       Despite its successes, the standard big-bang theory was too simple to be complete. For example, it offered no reason why the temperature of the background radiation remains remarkably constant over the entire sky, varying by no more than one part in 100,000. In the standard big-bang model, the constituents of the early universe could not all interact with one another, so there was no way for them all to reach the same temperature. Another problem is that the universe appears very nearly flat, existing right on the knife edge between being open and closed. In the standard big-bang model, the only way to explain these observations is to have the universe start out with a uniform temperature and at the critical density.

       In 1980, the American physicist Alan Guth devised a way around these problems. He theorized that shortly after the Big Bang (10-35 seconds, or 100 billion trillion trillionths of a second, to be exact), the universe underwent a period of extraordinarily rapid expansion, inflating its size by a factor of 1050.

   Before this inflationary period, the universe’s constituents would have been in contact with one another, so they would have reached the same temperature. And the rapid inflation would make the universe’s expansion appear very flat, in the same way that the surface of a balloon blown up by such a huge factor would resemble the Great Plains. Inflation ended by 10-30 seconds after the Big Bang, and since then the universe has expanded just as it would have in the standard big-bang model.

   Guth based his argument on the Grand Unified Theories, or GUTs, that unite gravity, electromagnetism, and the weak and strong forces into one. These theories predict that as the universe cooled after the Big Bang, the forces separated into their individual identities at what are called phase transitions. Water undergoes a similar phase transition when it freezes into ice as the temperature drops. If conditions are right, you can supercool water below the freezing point without ice forming. If the universe behaved similarly, then space would have had an excess energy that counteracted gravity, driving the inflation.

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