What is the horizon problem and how does it impact our understanding of the early universe?

The horizon problem is a challenge in cosmology where different regions of the universe appear to have the same temperature and properties, even though they are too far apart to have interacted. This poses a problem for our understanding of the early universe because it suggests that there was not enough time for these regions to reach thermal equilibrium through standard processes. This raises questions about the initial conditions and evolution of the universe that are still being studied by scientists.