# The Horizon Problem

1. Mar 18, 2014

### alvallis

I have read that one of the problems with the big bang theory, which inflation solved/explained, is that although the universe seems to be isotonic, the different regions are so far apart from each other that they would have been unable to "talk" to each other, and so how is it that they "know" to behave the same way. I'm guessing that this means too far apart to have experienced the same thing at the same time. But I'm not sure. Can anyone shed any light (lol) on this problem?

2. Mar 18, 2014

### bapowell

It means that they are too far apart to have interacted via the exchange of light signals since the big bang. They are, in essence, not causally connected. How did such causally disconnected regions of the universe all end up having almost the exact same temperature? The idea is that such regions of the universe, while separated by such acausal distances today, were once in intimate contact just prior to inflation.

3. Mar 19, 2014

### Chalnoth

Well, during inflation. Inflation lasted long enough that the distance scales we observe were once Planck-length scales during inflation. What happened before inflation was therefore irrelevant, as inflation blew up the quantum foam of the vacuum to cosmological scales.

4. Mar 19, 2014

### George Jones

Staff Emeritus
I think that it takes more than inflation to do this. I think that the scale factor has increased by a factor of $10^{30}$ or so since inflation ended. If this is the case, then distances that are cosmological now were subatomic at the end of inflation.

5. Mar 19, 2014

### Chalnoth

Well, that's true. The entire current observable universe would have been about a millimeter across at the time inflation ended, if my calculations are correct.

But inflation is still the reason why quantum scales were blown up to cosmological scales, because it causally links scales which are currently cosmological with quantum fluctuations. If it weren't for this change in how the early universe expanded, then different parts of the universe would never have been in causal contact.