The horizon problem relates to the isotropy of the microwave background radiation.

Question

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The horizon problem is due to the fact that regions of the universe that are now separated by vast distances were in thermal equilibrium shortly after the big bang, which is inconsistent with the observed isotropy of the cosmic microwave background radiation. The horizon problem refers to the puzzle of how regions of the universe, which are now separated by vast distances, could have had enough time to come into thermal equilibrium and have the same temperature observed in the cosmic microwave background radiation. This is related to the isotropy (sameness in all directions) of the radiation.

The Horizon Problem Relates to the Isotropy of the Microwave