Horizon Problem (No Inflation)

The Cosmic Microwave Background is strikingly uniform in temperature across the entire sky — regions separated by more than ~2 degrees on the sky were causally disconnected at the time of last scattering and could never have exchanged information or reached thermal equilibrium by any standard mechanism. Yet they share the same temperature to one part in 100,000. How did regions separated by distances greater than the causal horizon at recombination come to be at the same temperature? This is the horizon problem, and it is a direct crisis for ΛCDM without an inflationary epoch.

Inflation solves the horizon problem by proposing a period of exponential expansion that stretched a tiny, causally connected region to encompass our entire observable universe, erasing any initial inhomogeneity. However, inflation is not part of core ΛCDM — it is an auxiliary hypothesis with its own serious theoretical baggage, including the trans-Planckian problem, the question of how inflation began, and the measure problem in eternal inflation. Without inflation, ΛCDM simply has no explanation for why the CMB is uniform. With inflation, the explanation works statistically but imports dozens of new free parameters and raises questions about uniqueness and falsifiability that remain unresolved.