# Evidence for the Inflationary Period?

## Main Question or Discussion Point

What is the evidence for the Inflationary Period? Is there CMB/ WMAP data to support the theory of inflation? As far as I know, which is very little, the Inflationary Period was inserted into the standard model of cosmology to account for the Horizon Problem. What evidence for the period is there?

Kurdt
Staff Emeritus
Gold Member
The inflationary period was proposed to clear up three main points. The horizon problem as you have mentioned, the flatness problem and the abundances of relic particles.

With the flatness problem the observed universe posesses a density very close to the critical density. From the Friedman equation we can show how the density parameter evolves over time.

$$|\Omega_{tot}(t)-1|=\frac{|k|}{a^2H^2}$$

From that we see that if the total density parameter is 1 then it will remain so forever. If its is not 1 then it turns out that the difference between the total density parameter and 1 is an increasing function of time, so any slight curvature there is will just get worse and worse. As it gets worse with time, and its very close to 1 now it would be reasonable to assume that at the very beginning the universe had a total critical density equal to 1. There is no reason to prefer that choice over others.

With inflation the difference has a different behaviour over time.

$$|\Omega_{tot}-1| \propto e^{-\Lambda t}$$

This acts to bring $\Omega$ closer to 1.

I'm sure you're familiar with the horizon problem.

For relic particle abundances there was a question as to why the universe was radiation dominated for so long. There was a magnetic monopole particle that was predicted in modern particle physics that would have led to matter domination much earlier than normal particles. Since there are none of these particles (plus other speculated particles) floating about out there.

With inflation these particles density is reduced more quickly and thus they are diluted by inflation.

Inflation is one of the more speculative areas of cosmology but it does have some basis for being considered.

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The inflationary period was proposed to clear up three main points. The horizon problem as you have mentioned, the flatness problem and the abundances of relic particles.

With the flatness problem the observed universe posesses a density very close to the critical density. From the Friedman equation we can show how the density parameter evolves over time.

$$|\Omega_{tot}(t)-1|=\frac{|k|}{a^2H^2}$$

From that we see that if the total density parameter is 1 then it will remain so forever. If its is not 1 then it turns out that the difference between the total density parameter and 1 is an increasing function of time, so any slight curvature there is will just get worse and worse. As it gets worse with time, and its very close to 1 now it would be reasonable to assume that at the very beginning the universe had a total critical density equal to 1. There is no reason to prefer that choice over others.

With inflation the difference has a different behaviour over time.

$$|\Omega_{tot}-1| \propto e^{-\Lambda t}$$

This acts to bring $\Omega$ closer to 1.

I'm sure you're familiar with the horizon problem.

For relic particle abundances there was a question as to why the universe was radiation dominated for so long. There was a magnetic monopole particle that was predicted in modern particle physics that would have led to matter domination much earlier than normal particles. Since there are none of these particles (plus other speculated particles) floating about out there.

With inflation these particles density is reduced more quickly and thus they are diluted by inflation.

Inflation is one of the more speculative areas of cosmology but it does have some basis for being considered.
Excellent explanation, thank your for writing out.