The CMB is uniform to 1 part in 100,000 — yet opposite sky regions were never in causal contact. Inflation solves this.
CMB regions
~10⁴
ΔT/T
10⁻⁵
Mode
Standard
Causal contact
None
The Horizon Problem: The particle horizon at last scattering (~380,000 yr) subtended only ~1° on the CMB sky. Yet the CMB temperature is uniform to 1:100,000 across the entire sky — ~10,000 causally disconnected patches all at exactly T=2.725 K. Without prior causal contact, this requires absurd fine-tuning. Inflation solves this: all observable regions originated from a single causally connected patch that expanded exponentially. Quantum vacuum fluctuations in this patch seeded all structure.