Basic Concept
When one looks out into the night sky, distances also correspond to time into the past. A galaxy measured at ten billion light years in distance appears to us as it was ten billion years ago, because the light has taken that long to travel to the viewer. If one were to look at a galaxy ten billion light years away in one direction, say "west", and another in the opposite direction, "east", the total distance between them is twenty billion light years. This means that the light from the first has not yet reached the second, because the 13.7 billion years that the universe has existed simply isn't a long enough time to allow it to occur. In a more general sense, there are portions of the universe that are visible to us, but invisible to each other, outside each other's respective particle horizons.
In standard physical theories, no information can travel faster than the speed of light. In this context, "information" means "any sort of physical interaction". For instance, heat will naturally flow from a hotter area to a cooler one, and in physics terms this is one example of information exchange. Given the example above, the two galaxies in question cannot have shared any sort of information; they are not in "causal contact". One would expect, then, that their physical properties would be different, and more generally, that the universe as a whole would have varying properties in different areas.
Contrary to this expectation, the universe is in fact extremely homogeneous. For instance, the cosmic microwave background radiation (CMB), which fills the universe, is almost precisely the same temperature everywhere in the sky, about 2.725 K. The difference in temperature is so slight that it has only recently become possible to develop instruments capable of measuring it. This presents a serious problem; if the universe had started with even slightly different temperatures in different areas, then there would simply be no way it could have evened itself out to a common temperature by this point in time. Quantum physics demands that this initial temperature difference should have actually existed at the Big Bang because of the uncertainty principle, such that there is no way that the universe could have formed with precisely the same properties everywhere.
The magnitude of this problem is quite large. According to the Big Bang model, as the density of the universe dropped while it expanded, it eventually reached a point where photons in the "mix" of particles were no longer immediately impacting matter; they "decoupled" from the plasma and spread out into the universe as a burst of light. This is thought to have occurred about 300,000 years after the Big Bang. The volume of any possible information exchange at that time was 900,000 light years across, using the speed of light and the rate of expansion of space in the early universe. Instead, the entire sky has the same temperature, a volume 1088 times larger.
Read more about this topic: Horizon Problem
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