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What Is Inflation Theory?

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What Is Inflation Theory?

A timeline of the history of the universe. (June 2009)

NASA / WMAP Science Team
Question: What Is Inflation Theory?
I've heard of the big bang theory, but now someone's telling me that there's also some cosmic inflation theory, which is a different way of describing the inflation of the universe. Do scientists still believe in the big bang theory? What is inflation theory? What about chaotic inflation and eternal inflation?
Answer:

Inflation theory brings together ideas from quantum physics and particle physics to explore the early moments of the universe, following the big bang. According to inflation theory, the universe was created in an unstable energy state, which forced a rapid expansion of the universe in its early moments. One consequence is that the universe is vastly bigger than anticipated, far larger than the size that we can observe with our telescopes. Another consequence is that this theory predicts some traits - such as the uniform distribution of energy and the flat geometry of spacetime - which was not previously explained within the framework of the big bang theory.

Developed in 1980 by particle physicist Alan Guth, inflation theory is today generally considered a widely-accepted component of the big bang theory, even though the central ideas of the big bang theory were well established for years prior to the development of inflation theory.

The Origins of Inflation Theory

The big bang theory had proven quite successful over the years, especially having been confirmed through discovery of the cosmic microwave background (CMB) radiation. Despite the great success of the theory to explain most aspects of the universe which we saw, there were three major problems remaining:

The big bang model seemed to predict a curved universe in which energy wasn't distributed at all evenly, and in which there were a lot of magnetic monopoles, none of which matched the evidence.

Particle physicist Alan Guth first learned of the flatness problem in a 1978 lecture at Cornell University by Robert Dicke. Over the next couple of years, Guth applied concepts from particle physics to the situation and developed an inflation model of the early universe.

Guth presented his findings at a January 23, 1980, lecture at the Stanford Linear Accelerator Center. His revolutionary idea was that the principles of quantum physics at the heart of particle physics could be applied to the early moments of the big bang creation. The universe would have been created with a high energy density. Thermodynamics dictate that the density of the universe would have forced it to expand extremely rapidly.

<>I'm skipping a lot of the technical language here, but for those who are interested in more detail, essentially the universe would have been created in a "false vacuum" with the Higgs mechanism turned off (or, put another way, the Higgs boson didn't exist). It would have gone through a process of supercooling, seeking out a stable lower-energy state (a "true vacuum" in which the Higgs mechanism switched on), and it was this supercooling process which drove the inflationary period of rapid expansion.

How rapidly? The universe would have doubled in size every 10-35 seconds. Within 10-30 seconds, the universe would have doubled in size 100,000 times, which is more than enough expansion to explain the flatness problem. Even if the universe had curvature when it started, that much expansion would cause it to appear flat today. (Consider that the size of the Earth is large enough that it appears to us to be flat, even though we know that the surface we stand on is the curved outside of a sphere.)

Similarly, energy is distributed so evenly because when it started out, we were a very small part of the universe, and that part of the universe expanded so quickly that if there were any major uneven distributions of energy, they'd be too far away for us to perceive. This is a solution to the homogeneity problem.

Refining the Theory

The problem with the theory, as far as Guth could tell, was that once the inflation began, it would continue forever. There seemed to be no clear shut-off mechanism in place.

Also, if space was continually expanding at this rate, then a previous idea about the early universe, presented by Sidney Coleman, wouldn't work. Coleman had predicted that phase transitions in the early universe took place by the creation of tiny bubbles that coalesced together. With inflation in place, the tiny bubbles were moving away from each other too fast to ever coalesce.

Fascinated by the prospect, the Russian physicist Andre Linde attacked this problem and realized there was another interpretation which took care of this problem, while on this side of the iron curtain (this was the 1980's, remember) Andreas Albrecht and Paul J. Steinhardt came up with a similar solution.

This newer variant of the theory is the one that really gained traction throughout the 1980s and eventually became part of the established big bang theory.

Other Names for Inflation Theory

Inflation Theory goes by several other names, including:

  • cosmological inflation
  • cosmic inflation
  • inflation
  • old inflation (Guth's original, 1980 version of the theory)
  • new inflation theory (name for the version with the bubble problem fixed)
  • slow-roll inflation (name for the version with the bubble problem fixed)

There are also two closely related variants of the theory, chaotic inflation and eternal inflation, which have some minor distinctions. In these theories, the inflation mechanism didn't just happen once immediately following the big bang, but rather happens over and over in different regions of space all of the time. They posit a rapidly-multiplying number of "bubble universes" as part of the multiverse. Some physicists point out that these predictions are present in all versions of inflation theory, so don't really consider them distinct theories.

Being a quantum theory, there is a field interpretation of inflation theory. In this approach, the driving mechanism is the inflaton field or inflaton particle.

Note: While the concept of dark energy in modern cosmological theory also accelerates the expansion of the universe, the mechanisms involved appear to be very different from those involved in inflation theory. One area of interest to cosmologists is the ways in which inflation theory might lead to insights into dark energy, or vice versa.
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