In a decelerating hot Big Bang model, widely separated regions of the observable universe have nearly equal conditions despite having no shared past light cone. Inflation supplies an earlier era with a shrinking comoving Hubble radius, allowing a formerly causal patch to grow beyond the later horizon.
The "horizon problem" is a concept from cosmology that pertains to the uniformity of the Cosmic Microwave Background Radiation (CMB) and the large-scale structure of the universe. The problem arises in the context of the Big Bang cosmology and is associated with the observation that regions of the universe that are now separated by vast distances appear to have very similar temperatures and physical properties, despite being too far apart to have ever interacted with each other.
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