What is the isotropy (or horizon) problem?
A) The universe is not as uniform as the standard cosmological model predicts it should be.
B) If the universe expanded at the rate originally predicted, then the parts seen in opposite directions would never have been in equilibrium with each other and should not exhibit the degree of isotropy observed.
C) Without modification, the standard theory suggests that an observer on a distant galaxy would not be able to see the Milky Way Galaxy because of the intense cosmological redshift.
D) As astronomers observe galaxies on the "horizon," the farthest distance obtainable, the standard theory predicts that the recessional velocity should approach the speed of light.

Question

Quizplus · Accepted Answer

The isotropy problem refers to the observation that the universe appears uniform in all directions despite the fact that, according to the Big Bang theory without inflation, regions now in opposite directions in the sky would not have had enough time to come into thermal equilibrium with each other. This uniformity is unexpected because these regions could not have exchanged information or energy due to the finite speed of light, yet they have nearly the same temperature.

What Is the Isotropy (Or Horizon) Problem