Chapter 16
It may happen that small differences in the initial conditions produce very great ones in the final phenomena.
Henri Poincare

Despite its successes, the standard big bang cosmology has some problems that are difficult to resolve. Among these are:
  • The horizon problem
  • The flatness problem
  • The structure problem
  • The relic problem
These can be summarized thusly: (1) The universe is observed to be highly homogeneous and isotropic, but how did it become so when all regions observable universe were not in mutual causal contact at early times? (2) The universe is nearly flat today, but this implies that it must have had an Omega very nearly equal to one at early times. Unless the universe is exactly flat, this seems to require fine tuning. Why is the universe so flat? (3) What formed the perturbations that lead to the structure we see around us? Why is structure the same everywhere, even though different parts of the universe were not causally connected early in the big bang model? (4) GUTs predict massive particles that are not observed. What happened to these "relics" of the GUT epoch?

The inflationary model addresses all these issues by presuming that what we call the observable universe is actually a very small portion of the initial universe that underwent a de Sitter phase of exponential expansion around the time of the GUT epoch. This model posits that what became our observable universe was small enough to be in causal contact at the big bang; it then grew at an exponential rate during the inflationary epoch. The exponential growth had the effect of flattening out any curvature, stretching the geometry of the universe so much that it became flat. Any massive GUT particles were diluted, spread out over this now fantastically huge domain to an extremely tiny density, so that they no longer are observable. Quantum fluctuations in the vacuum are preserved and "blown up" to large scales by the expansion, providing the seeds for structure formation.

The source of this exponential growth was a negative pressure produced by a nonzero vacuum energy. A nonzero vacuum energy could result from quantum processes in the early universe. In quantum field theory, a field is associated with each particle, and the field in turn is related to a potential, the latter being a function which describes the energy density of the field. The right potential would result in a false vacuum, a situation in which the field was zero but the corresponding potential was not zero. The false vacuum state could have provided a vacuum energy that would behave exactly like a positive (repulsive) cosmological constant, resulting in a temporary de Sitter phase during which the patch of universe grew by a factor of perhaps 10100 or more. Eventually, however, this vacuum energy was converted into real particles and the field found its way to the true vacuum, bringing the inflation to a halt. The universe then continued to evolve from this point as in our standard model.

The inflationary model is an area of active research. It makes some predictions about the structures in the universe which are consistent with the WMAP data, and it predicts that the present universe should be flat, in agreement with observations. However, the particle that might have provided the vacuum energy density is still unidentified, even theoretically; it is sometimes called the inflaton because its sole purpose seems to be to have produced inflation. Despite these outstanding questions, it seems difficult to understand how the horizon problem could be explained unless something like inflation occurred. Research continues along all these lines of investigation.

For more information see Questions and Answers related to Chapter 16.

Cambridge University has tutorial pages on the subject of cosmological inflation.

Original content © 2005 John F. Hawley