Hubble Tension (SH0ES 73 vs Planck 67, 5-Sigma)

The Hubble constant, H₀, describes how fast the universe is expanding — specifically, how many kilometers per second a galaxy recedes for every megaparsec of distance. Two fundamentally different methods of measuring H₀ yield results that are irreconcilably different. The SH0ES collaboration, using a cosmic distance ladder built from Cepheid variable stars and Type Ia supernovae in the nearby universe, measures H₀ ≈ 73 km/s/Mpc. The Planck satellite, inferring H₀ by fitting the temperature fluctuations of the Cosmic Microwave Background to the standard ΛCDM model, obtains H₀ ≈ 67.4 km/s/Mpc. The gap between these values stands at roughly 5 standard deviations — a level of discrepancy that, in physics, normally signals a fundamental error or a missing piece of physics.

Within ΛCDM, this conflict is deeply troubling because both measurements are self-consistent and internally robust. The Planck result uses the universe's oldest observable light — a snapshot from 380,000 years after the Big Bang — while SH0ES measures the universe as it is today. If ΛCDM were correct and complete, both methods would converge on the same H₀. Instead, the local universe appears to expand roughly 8–9% faster than the early-universe data predicts. Proposed patches — early dark energy, additional neutrino species, modified recombination — have each failed to resolve the tension without introducing equally problematic inconsistencies elsewhere in the model. The Hubble Tension is widely regarded as the most statistically robust crisis in modern cosmology.