Motivating Inflation

Two observations, in particular, are extremely puzzling under the Big Bang model.

The flatness problem

Why is OmegaTotal ~ 1? Why not 106? Why not 42? Why not 0.0021034011031?

The density of the Universe changes with time, as the Universe expands. So OmegaM, the ratio of the actual density to the critical density also changes:

(Strictly speaking, this holds only for a matter dominated universe. But it's only recently that dark energy has started affecting the expansion of the universe, so early on the universe behaved like a matter dominated universe....)

Let's look at two examples.

In the universe that is slightly overdense at z=104, the density parameter today (at z=0) would be 100. In the universe that is slightly underdense at early times, we ought to measure a density parameter today of 0.001. Omega very quickly diverges from 1, unless it is equal to 1.

It gets worse. Look at this figure (from Ned Wright's Cosmology Tutorial): if the density of the universe had been ever so slightly non-critical 1 nanosecond after the big bang, we would have a drastically different universe:

The Smoothness Problem

Looking at the microwave background, it is very smooth to 1 part in 105. Everywhere. But at the time of recombination, regions of the universe which are now separated by more than 2 degrees on the sky were never in causal contact. How did all these regions of space "know" that they should all be at exactly the same temperature?

(figure courtesy Wang 2014)

Why is the universe flat? Why is the universe smooth?

1980: Alan Guth dreams up inflation.