Why do we need inflation to explain the homogeneity of CMB?

In summary, the discussion revolves around the idea of whether or not inflation can explain the homogeneity of the cosmic microwave background (CMB) and the expansion of the universe. While some argue that inflation can account for the uniformity of the CMB, others question why the expansion of the universe should have been uniform in the first place. The concept of a uniform inflationary patch and a causal mechanism for creating homogeneity is contrasted with the idea of imposing uniformity on matter fields without inflation. Ultimately, the debate centers around whether or not inflation is a necessary or arbitrary explanation for the homogeneity of the universe.
  • #1
petergreat
267
4
Can't we simply assume that the initial condition for the universe is perfectly spherically symmetric, and the problem is solved? In other words, can't we make the CMB homogeneous just by imposing homogeneous initial conditions? The fluctuations can be explained by quantum effects. Of course there are other things such as the flatness problem which seem to be quite naturally explained by inflation, but I never feel sure about the homogeneity problem. After all, we know very little about how the big bang happened, and we can't rule out any particular kind of initial conditions, such as a homogeneous one.
 
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  • #2
Inflation nicely explains why the CMB almost perfectly homogenous in every direction. That is one of its more appealing features.
 
  • #3
Chronos said:
Inflation nicely explains why the CMB almost perfectly homogenous in every direction. That is one of its more appealing features.

Inflation in turn only begs the question as to why the expansion was uniform.
 
  • #4
friend said:
Inflation in turn only begs the question as to why the expansion was uniform.
Uniform expansion follows from the requirement that the field driving inflation be uniform in space across the initial inflationary patch. Why do you suggest that the expansion should not have been uniform?
 
  • #5
petergreat said:
After all, we know very little about how the big bang happened, and we can't rule out any particular kind of initial conditions, such as a homogeneous one.
I think this is precisely the point. Sure, the initial conditions could have been perfectly homogeneous. There is nothing that forbids this in principle. But, as you say, the IC's could have been inhomogeneous as well. In fact, there are lots more ways of not being homogeneous than there are of being homogeneous. So, given a generic initial state, one would not expect it to be perfectly homogeneous. By just putting such a condition in 'by hand' is considered by many to be a serious fine tuning of initial conditions. Inflation is preferred in this case because, while one still needs a sufficiently homogeneous region of spacetime in order to get inflation started, the region across which we demand such homogeneity is smaller than today's currently observable universe.
 
  • #6
bapowell said:
Uniform expansion follows from the requirement that the field driving inflation be uniform in space across the initial inflationary patch. Why do you suggest that the expansion should not have been uniform?

If you're going to suppose that fields can be uniform as the universe grows to astronomical dimensions, then we could just as easily suppose that the matter fields were uniform with a larger universe as well. I fail to see the difference.
 
  • #7
friend said:
If you're going to suppose that fields can be uniform as the universe grows to astronomical dimensions, then we could just as easily suppose that the matter fields were uniform with a larger universe as well. I fail to see the difference.
The difference is that the initially uniform inflationary patch can be made uniform through a causal mechanism (the scalar field is correlated across the inflationary spacetime). This is very different then imposing uniformity on the matter fields across cosmological distances without inflation. This is the horizon problem -- no causal mechanism could generate such homogeneity.

But that's not what I was asking. I was asking about your comment regarding why inflation begs the question that the expansion was uniform.
 

1. Why is inflation necessary to explain the homogeneity of the CMB?

Inflation is necessary to explain the homogeneity of the Cosmic Microwave Background (CMB) because it provides a solution to the horizon problem. This problem arises from the fact that the CMB is incredibly uniform in temperature across the entire observable universe, but regions that are currently separated by billions of light-years were once close enough to interact and reach thermal equilibrium. Inflation proposes that the early universe underwent a period of rapid expansion, causing these regions to be in contact and reach the same temperature before stretching out to their current distances. This explains the observed homogeneity of the CMB.

2. How does inflation explain the flatness of the universe?

Inflation also provides an explanation for the flatness of the universe. According to the Big Bang theory, the universe should be curved due to the presence of matter and energy. However, observations have shown that the universe is nearly flat. Inflation proposes that the universe underwent a period of exponential expansion, which stretched out any curvature and made the universe appear flat on a large scale. This is known as the flatness problem and inflation provides a solution to it.

3. Can the homogeneity of the CMB be explained without inflation?

No, the homogeneity of the CMB cannot be explained without inflation. The CMB is observed to have a nearly uniform temperature of 2.7 Kelvin across the entire observable universe, but without inflation, there is no known mechanism that could have caused this level of homogeneity. Inflation is currently the only proposed theory that can explain the observed uniformity of the CMB.

4. How does inflation help to unify different theories of the universe?

Inflation plays an important role in unifying different theories of the universe, such as the Big Bang theory and the theory of cosmic inflation. Before inflation was proposed, the Big Bang theory could not fully explain certain observations of the universe, such as the homogeneity of the CMB. Inflation provides a mechanism to solve these problems and brings together different theories to create a more complete understanding of the universe.

5. What evidence supports the theory of inflation?

There is a growing body of evidence that supports the theory of inflation. One of the strongest pieces of evidence is the observed uniformity of the CMB, which is consistent with predictions made by inflation. Other evidence includes the observed flatness of the universe, the absence of magnetic monopoles, and the large-scale structure of the universe. Additionally, the recent discovery of gravitational waves by the BICEP2 experiment provided further support for the theory of inflation.

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