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.