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Asked by strangequark to Arttu, Ceri, James_M, Monica, Philip on 19 Jun 2011.
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James M Monk answered on 19 Jun 2011:
This is explained by the process of inflation, which is a very rapid expansion of the Universe that happened early on. Although objects within the Universe cannot move faster than light, the expansion of the Universe is a change of space-time itself, which is more like changing the scale on a map. The rapid expansion caused regions that were connected to suddenly become disconnected, therefore these different regions now have the same temperature because they were in thermal equilibrium before the inflation.
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Arttu Rajantie answered on 19 Jun 2011:
This is known as the horizon problem. The cosmic microwave background consists of radiation emitted when the universe was around 400 000 years old. If the universe contained only radiation and matter, gravity would slow down its expansion. In that case light would have only been able to travel a relatively short distance by that time. On our sky it would correspond to an angle of roughly two degrees. Any two points separated by more than this on the sky would not have been able to interact in any way before the radiation was emitted. Yet, the temperature of the cosmic microwave background is almost exactly the same 2.7K in every direction.
The solution is, as James mentions, that the universe was not always slowing down. Instead, the expansion was accelerating at very early times, which we call inflation. This allowed light to travel a much longer distance before the cosmic microwave background radiation was emitted, and if the acceleration lasted long enough, it would have been able to cross the whole sky we see today. Then we can understand why the whole sky has the same temperature.
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