5.4. The General Idea of Inflation
The horizon problem especially is an extremely serious problem for the standard cosmology because at its heart is simply causality. Any solution to this problem is therefore almost certain to require an important modification to how information can propagate in the early universe. Cosmological inflation is such a mechanism.
Before getting into the details of inflation we will just sketch the
general idea here. The fundamental idea is that the universe undergoes
a period of accelerated expansion, defined as a period when
> 0, at early times.
The effect of this acceleration is
to quickly expand a small region of space to a huge size, diminishing
spatial curvature in the process, making the universe
extremely close to flat. In addition, the horizon size is greatly
increased, so that distant points on the CMB
actually are in causal contact and unwanted relics are tremendously
diluted, solving the monopole problem. As an unexpected bonus,
quantum fluctuations make it impossible for inflation to smooth out
the universe with perfect precision, so there is a spectrum of remnant
density perturbations; this spectrum turns out to be approximately
scale-free, in good agreement with observations of our current universe.