Why do early- and late-universe measurements of H₀ disagree?
The expansion rate today inferred from the CMB assuming ΛCDM (~67.4 km/s/Mpc) sits ~5σ away from the distance-ladder value measured with Cepheids and supernovae (~73 km/s/Mpc). Either a systematic error persists on both sides, or the standard cosmological model is incomplete.
Why it is hard
Every cross-check — tip of the red giant branch, surface brightness fluctuations, gravitational wave standard sirens — shrinks error bars rather than resolving the discrepancy. Proposed new physics (early dark energy, extra relativistic species, modified gravity) tends to fix H₀ while breaking other observables.
Current evidence
Planck CMB fits give H₀ = 67.4 ± 0.5; SH0ES/HST + JWST distance ladders give ~73. JWST observations have largely ruled out crowding-induced bias in Cepheid photometry, strengthening the case that the tension is real rather than instrumental.
Possible approaches
Independent calibrators (TRGB, Miras, megamasers, standard sirens), time-delay lensing cosmography, higher-resolution CMB (Simons Observatory, CMB-S4), and theories that change pre-recombination physics without spoiling the CMB acoustic fit.
Related topics
Sources
- Planck and the cosmic microwave background — European Space Agency
- NASA Celebrates Edwin Hubble’s Discovery of a New Universe — NASA Science
- DESI DR2 Lyman-alpha Results and Cosmological Constraints — Dark Energy Spectroscopic Instrument
- Gravitational-Wave Science — LIGO Scientific Collaboration