Reheating and the horizon problem - chicken and egg?

In summary, Alan Guth discusses the chicken and egg problem of inflation and reheating. He says that because reheating didn't cause the CMB, it is a waste of hydrogen gas.
  • #1
GeorgeDishman
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One of the benefits of inflation often mentioned in beginner treatments is that it solves the horizon problem by taking a volume which was in thermal equilibrium and expanding it so much that it is now larger than the observable universe thus explaining how the CMB temperature is so uniform.

Reheating says the majority of the particle contents of the universe were created as the potential oscillated around the minimum (the "true vacuum" value) at the end of inflation.

This combination of descriptions appears to create a "chicken and egg" paradox, the matter was produced at the end of inflation by reheating but needed to have reached equilibrium before inflation in order to be solve the horizon problem.

What is the correct timeline and dependency for these processes?
 
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  • #2
There is no need to reach global equilibrium before inflation. Inflation blows up the tiniest region to immense size and washes out any initial state. All that is left after is the inflaton field that will be the same everywhere in the observable universe.

The point is that inflation creates a homogeneous initial condition for the remaining history of the universe.
 
  • #3
So are you saying that because the inflaton field when it started to decay had a homogeneous density when its energy was converted to matter through reheating, that resulted in the homogeneous observed horizon temperature? That makes sense to me, thanks.
 
  • #4
As long as you keep in mind that reheating didn't cause the CMB ;)
 
  • #5
Well there might be a few interesting steps in between but I relate that to Edward Harrison's quote "Hydrogen is a light, odorless gas, which, given enough time, turns into people." :wink:
 
  • #6
A waste of perfectly good hydrogen gas, in some cases.
 
  • #7
This is a few years old now but I just came across lectures by Alan Guth in which he discusses this and answers my questions. He says there was sufficient time for the normal processes of thermal to act to produce equilibrium within a small patch before inflation then expanded to greater than our current horizon. The relevant section is here and lasts about a minute:

 

1. What is the horizon problem?

The horizon problem is a fundamental issue in cosmology that arises when trying to explain the uniformity of the cosmic microwave background radiation (CMB) in different regions of the universe. According to the Big Bang theory, the universe was once in a state of rapid expansion known as inflation. However, this inflation should have caused regions of the universe that are now separated by vast distances to have never been in causal contact, making it difficult to explain how the CMB is so uniform.

2. How does reheating relate to the horizon problem?

Reheating is a period of rapid energy transfer that occurs after the universe undergoes inflation. During this time, the universe rapidly heats up and becomes filled with particles, leading to the formation of matter and radiation. Reheating is important for solving the horizon problem because it allows for the different regions of the universe to become causally connected, explaining how the CMB is so uniform.

3. What is the chicken and egg analogy in relation to reheating and the horizon problem?

The chicken and egg analogy is often used to explain how reheating and the horizon problem are interconnected. Just as it is impossible to determine which came first - the chicken or the egg - it is impossible to determine which came first - the uniformity of the CMB or the causal connection between different regions of the universe. Reheating is the key factor that allows for both the egg and the chicken (the horizon problem and the uniformity of the CMB) to exist simultaneously.

4. How does the theory of inflation help to solve the horizon problem?

The theory of inflation is proposed as a solution to the horizon problem. During the inflationary period, the universe underwent a rapid expansion, stretching out initially tiny regions into the vast observable universe we see today. This expansion allowed for the different regions of the universe to become causally connected, explaining the uniformity of the CMB. Reheating then solidifies this solution by filling the universe with matter and radiation.

5. Are there alternative theories to explain the horizon problem?

Yes, there are alternative theories that attempt to explain the uniformity of the CMB without relying on the concept of inflation and reheating. Some propose modifications to the theory of gravity, while others suggest alternative models of the universe with different properties. However, the theory of inflation remains the most widely accepted and successful explanation for the horizon problem.

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