The Hubble tension: why do the measurements disagree?
Two careful ways of measuring how fast the universe expands give two different answers. Both measurements are solid; why they disagree is one of the open problems in cosmology.
3 established, 1 disputed, 1 debunked, 1 unresolved. We do not pick an answer; the evidence places each claim. How we decide
14 citations from 4 independent sources - 10 echo the same voices and count once. Graded A1-C1.
Added 3 Jul 2026 - updated 29 Sept 2026
What the four states mean
- Establishedsupported, uncontested
- Disputedcredible sources disagree
- Debunkedrefuted by the evidence
- Unresolvedno settled answer
Established 3supported, uncontested
Planck's measurements of the cosmic microwave background, assuming the base LambdaCDM model, give an inferred H0 of about 67 km/s/Mpc (67.4 plus or minus 0.5).▰▰▱▱▱ likely2 points
Supports- We present cosmological parameter results from the final full-mission Planck measurements of the CMB anisotropies.Planck 2018 results. VI. Cosmological parameters, A&A 641, A6, arXiv:1807.06209 authoritative
- Assuming the base-ΛCDM cosmology, the inferred late-Universe parameters are: Hubble constant H0 = (67.4 ± 0.5) km/s/MpcPlanck 2018 results. VI. Cosmological parameters, A&A 641, A6, arXiv:1807.06209 authoritative
Hubble Space Telescope observations of Cepheids in the host galaxies of Type Ia supernovae, used to calibrate the Hubble constant, give H0 of about 73 km/s/Mpc (73.04 plus or minus 1.04).▰▰▱▱▱ likely2 points
Supports- We report observations from HST of Cepheids in the hosts of 42 SNe Ia used to calibrate the Hubble constant (H0).Riess et al. 2022, ApJL 934, L7, arXiv:2112.04510 authoritative
- Our baseline result from the Cepheid-SN sample is H0=73.04+-1.04 km/s/Mpc, which includes systematics and lies near the median of all analysis variants.Riess et al. 2022, ApJL 934, L7, arXiv:2112.04510 authoritative
The early-universe and late-universe determinations disagree at high statistical significance, around 5 sigma.▰▰▱▱▱ likely2 points
Supports- We find a 5-sigma difference with H0 predicted by Planck+LCDM, with no indication this arises from measurement errors or analysis variations considered to date.Riess et al. 2022, ApJL 934, L7, arXiv:2112.04510 authoritative
- combining any three independent approaches to measure H0 in the late universe yields tension with the early Universe values between 4.0σ and 5.8σ.Tensions between the Early and the Late Universe (Verde, Treu & Riess 2019), Nature Astronomy 3, 891, arXiv:1907.10625 analysis
Disputed 1credible sources disagree
The discrepancy may require new physics beyond the standard LambdaCDM cosmological model.▰▰▰▱▱ roughly even chance1 against2 points
Supports- Discrepancies developing between observations at early and late cosmological time may require an expansion of the standard model, and may lead to the discovery of new physics.Tensions between the Early and the Late Universe (Verde, Treu & Riess 2019), Nature Astronomy 3, 891, arXiv:1907.10625 analysis
Against- The TRGB results alone do not demand additional new physics beyond the standard Lambda-CDM cosmological model.Measurements of the Hubble Constant: Tensions in Perspective (Freedman 2021), ApJ 919, 16, arXiv:2106.15656 analysis
Debunked 1refuted by the evidence
A photometric bias from crowding in Hubble Space Telescope observations of Cepheids could account for the Hubble tension.Refuted by
These observations provide the strongest evidence yet that systematic errors in HST Cepheid photometry do not play a significant role in the present Hubble Tension.
refuted2 against4 pointsThe claim rested on- High-resolution JWST observations can test confusion-limited HST observations for a photometric bias that could affect extragalactic Cepheids and the determination of the Hubble constant.Riess et al. 2023, Crowded No More (JWST), ApJ, arXiv:2307.15806 authoritative
- There are several advantages of halo TRGB distance measurements relative to Cepheid variables: these include low halo reddening, minimal effects of crowding or blending of the photometryThe Carnegie-Chicago Hubble Program VIII: An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch (Freedman et al. 2019), ApJ 882, 34, arXiv:1907.05922 primary
Refuted by- These observations provide the strongest evidence yet that systematic errors in HST Cepheid photometry do not play a significant role in the present Hubble Tension.Riess et al. 2023, Crowded No More (JWST), ApJ, arXiv:2307.15806 authoritative
- the discrepancy does not appear to be dependent on the use of any one method, team, or source.Verde, Treu & Riess 2019, Nature Astronomy 3, 891, arXiv:1907.10625 analysis
Unresolved 1no settled answer
How the Hubble tension will be resolved (new physics, a systematic error, or a combination) is not yet settled; the discrepancy persists across methods and improved data.▰▰▰▱▱ likely2 points
Supports- While no specific proposal makes a strong case for being highly likely or far better than all othersDi Valentino et al. 2021 review, Class. Quantum Grav. 38, 153001, arXiv:2103.01183 analysis
- We find a 5-sigma difference with H0 predicted by Planck+LCDM, with no indication this arises from measurement errors or analysis variations considered to date.Riess et al. 2022, ApJL 934, L7, arXiv:2112.04510 authoritative
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