What is the Hubble tension and why does it worry cosmologists?
Two ways of measuring how fast the universe is expanding give stubbornly different answers, and nobody knows why.
The answer
The Hubble tension refers to a persistent discrepancy between two independent measurements of the universe's expansion rate (the Hubble constant, H₀). Observations of the cosmic microwave background by the Planck satellite yield H₀ = 67.4 km/s/Mpc, while local measurements using Cepheid variable stars and Type Ia supernovae by Adam Riess's SH0ES team give H₀ = 73.0 km/s/Mpc - a roughly 8% disagreement at over 5-sigma statistical significance. JWST observations in 2023-2024 confirmed the Cepheid measurements, ruling out systematic errors in local distance measurements. Proposed explanations include early dark energy, modified gravity, new relativistic particles in the early universe, or decaying dark matter. The DESI experiment's 2024 results suggesting evolving dark energy added another dimension. If no measurement error is found, the Hubble tension may require fundamentally new physics beyond the standard cosmological model.
Source: The Astrophysical Journal
- Hubble tension
- expansion rate
- cosmological crisis
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