Horizon problem

When we look at the CMB, it comes from 46 billion comoving light-years away. When the light was emitted, the universe was much younger – only 300,000 years old. Physical causes, starting at the earliest time and traveling at the speed of light, would have only reached as far as the smaller circles. The two points indicated on the diagram would not have been able to contact each other because their spheres of causality do not overlap.

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. This creates a difficulty in explaining the observed homogeneity without a mechanism that sets the same initial conditions everywhere. It was first pointed out by Wolfgang Rindler in 1956.

The most commonly accepted hypothesis to explain the horizon problem is cosmic inflation. Alternative solutions propose a cyclic universe or a variable speed of light.

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