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This is a very fundamental doubt. I think i am missing something in inflation.

The inflation theory solves the horizon problem by stating all of the universe started from a very small region in causal contact and suddenly space expanded exponentially to a large factor. Than the universe expanded and caught up with the space.

My question is if inflation didn't happen, than also everything at the very beginning was in causal contact. And as universe expanded although the particle horizon was smaller than the total universe, but since everything started from a small dense region in causal contact, the causality should be maintained. How come we need inflation to explain this?

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  • $\begingroup$ Because there is no causal contact between different areas of the universe today $\endgroup$ – Jim May 14 '15 at 15:35
  • $\begingroup$ We don't need inflation, it's just the flavor of the day. There have been plenty of other proposals in the past and extensions of general relativity might provide different frameworks to fit the phenomenology. My take on why inflation is popular is because it allows for a connection between cosmology and field theory, whereas a more classical approach to the problem trough e.g. Einstein-Cartan does not. The latter will tell you absolutely nothing about why the universe is the way it is, even if it can fit the observations to the theory (and I don't know if that's even true), inflation might. $\endgroup$ – CuriousOne May 14 '15 at 20:17
  • $\begingroup$ "My question is if inflation didn't happen, than also everything at the very beginning was in causal contact." Everything at the very beginning wasn't in causal contact, if the expansion trends were simply extrapolated back from today. $\endgroup$ – user102008 Jun 22 '16 at 6:41
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Here's the horizon problem:

Look at the sky. Look at one side of the sky. Then look at the other side of the sky. The light from one side has just now reached you, as has the light from the other side. When we look back to the earliest observable moments of the universe, the Cosmic Microwave Background (CMB), we do the same thing. We look at light that was emitted at about the same time and has only now reached us but from opposite sides of the universe. However, we still see that everything looks the same, it has been able to homogenize. This means (barring some freak coincidence of nature) that those two regions must have been in contact at some point before the light was emitted. Clearly they aren't in contact now. After 13.8 billion years, the light is only just reaching us, halfway between the two sources. So there's no way that those points are yet in contact with each other. If we look even farther back, the same thing arises. If the universe had been expanding along the same trend that it currently is, we see that regions that could never have had any communication with each other are homogenized. This is a problem.

How could it be that two points on the opposite sides of the observable universe are homogenized even though the light from each side has only had time enough to travel half the distance between them? They couldn't have communicated. Inflation answers this question. It says that the universe was initially in a causally connected state. Everything was able to communicate and homogenize. Then the universe expanded so fast that the different regions became causally disconnected in a small amount of time.

This is how it helps. Without inflation, the universe would have started as a region where things that couldn't have ever been in contact were homogenized. But with inflation, we can now say these regions were in contact but became disconnected due to rapid expansion.

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  • $\begingroup$ Was inflation expansion of space(as in vacuum) or the universe as a whole with whatever state of matter within it? $\endgroup$ – ruskin23 May 14 '15 at 16:48
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    $\begingroup$ It was a growth of the scale factor, which is like a stretching of space around the matter in it $\endgroup$ – Jim May 14 '15 at 17:53

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