Two Expansion Rates, One Universe: The Hubble Tension at 5σ
The universe has two measured expansion rates. One comes from the early universe, encoded in the cosmic microwave background. The other comes from measuring distances to nearby galaxies. They disagree by approximately 5σ — the threshold physicists call discovery. Every systematic error has been hunted down. JWST has confirmed the distance ladder. DESI has found hints that dark energy is not constant. Something is either wrong with our cosmological model or with one of two extremely well-tested measurement chains. We are not sure which.