Horizon problem

Cosmological fine-tuning problem

The horizon problem, also known as the homogeneity problem, is a cosmological fine-tuning problem within the Big Bang model of the universe. Observations of widely separated regions of space appear homogeneous, yet normal physical processes that create homogeneity require a causal connection and these regions are too far apart. Given the Einsteinian speed limit on communication, they have not had sufficient time to interact with each other since the Big Bang.

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