The

** horizon problem ** is a

cosmological question that remained unanswered until the theory of

cosmological inflation was put forward.
The problem is as follows: the

cosmic background radiation is the same

temperature to one part in 10,000, over the whole of the observable

universe, which implies the widely separated regions must have been in contact at one time, to reach

thermal equilibrium. You might think "According to the

big bang theory, everything began at a single point, as it expanded, equilibrium must have been possible, as the universe was so small..."
The 'classic' big bang theory was modeled using

general relativity, the

speed of light sets a limit on how far away two points can be to be able to

communicate in

*any* way. If you want to bring two opposed points in the universe to 186,000 miles apart, you would have to roll back time to

*less* than one second after the big bang. But light, needs the

*whole* second to travel between the two points, and so the universe cannot equilibrate. You can carry on rolling back

time, but light never has enough time to span the whole of the universe....
If you've followed the

inflation node, the rest should now make sense.. Playing the

film of the universe backwards, when you get to the period of inflation, you get an

exponential contraction, so fast it get's around the barrier imposed by the speed of light...

Calculations have

proved that the universe now has plenty of time to reach equilibrium, even though this period was

*infinitessemal* compared to the rest of the

evolution of the universe.