Opposite sides of the universe have the same temperature, yet according to the standard Big Bang theory, these points are too far apart for light to have traveled from one to the other in the age of the universe; that is, they cannot have exchanged heat to even out their temperature. Why, then, do they have the same temperature?
A) Light (and heat) could travel much faster in the early universe, allowing them to exchange heat while the universe was young.
B) The expansion of the universe has always been the same everywhere; therefore, all parts of the universe have the same temperature regardless of whether they have ever exchanged heat or not.
C) It is pure coincidence that opposite sides of the universe have the same temperature.
D) Opposite sides of the universe were originally close together and evened out their temperature, and then a rapid inflation of the universe carried them far apart.

Question

Quizplus · Accepted Answer

The concept of inflation in the early universe explains how regions now far apart were once close enough to reach thermal equilibrium before the rapid expansion carried them beyond each other's horizon.

Opposite Sides of the Universe Have the Same Temperature, Yet