What causes the Hubble tension?
In plain words
Two careful ways of measuring how fast the universe expands today give different answers. The cause could be a measurement error or new physics.
Precise statement
Write $H0 = h x 3.24 \times 10^{-18}\ \mathrm{s}^{-1}$. The SH0ES Cepheid plus type Ia supernova ladder gives $h = 0.730 \pm 0.010$, while Planck CMB in LCDM gives $h = 0.674 \pm 0.005$. Identify whether the discrepancy comes from distance-ladder systematics, from physics before recombination that shrinks the sound horizon, or from late-time physics, with a model that fits all data at least as well as LCDM.
What would settle it
Independent one-percent $H0$ measurements that agree with one side, plus a model or systematic that removes the discrepancy without spoiling other fits.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
2025 to 2026 JWST cross-checks by SH0ES find Cepheid, TRGB and JAGB distances agree within about 0.03 mag, while CCHP finds $h$ near 0.70.
Problems built on this one
Special cases
- Can pre-recombination physics raise the Hubble constant to 0.73? · open
- Do TRGB, JAGB and Cepheid ladders give the same Hubble constant? · open
- Does the tension persist in measurements independent of ladders and rulers? · open
- Does a local underdensity bias the locally measured Hubble constant? · open
- Are the oldest stars too old for a Hubble constant of 0.73? · open
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