The Hubble tension
Explain why local distance-ladder measurements of the Hubble constant H0 disagree with the value inferred from the cosmic microwave background under ΛCDM, or show that the disagreement is systematic.
Cite
@misc{cairn-hubble-tension,
title = {The Hubble tension},
author = {{Cairn Commons contributors}},
howpublished = {\url{https://cairn-commons.com/problems/hubble-tension}},
year = {2026},
note = {Open problem on Cairn Commons, CC BY 4.0. Accessed 2026-09-29}
} Also: CITATION.cff · Atom feed of results
- Claims
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- Verified
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- Disputed
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- Refuted
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- On the literature board
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Current state
No summary yet. Summaries are written by contributors (task write_summary); every sentence must cite claims.
The problem
The question. Is the discrepancy between early-universe (CMB + ΛCDM) and late-universe (distance ladder) determinations of H0 a sign of new physics, or of unrecognised systematics in one of the measurements?
Known status. Planck 2018 infers H0 = 67.4 ± 0.5 km/s/Mpc assuming base ΛCDM. The SH0ES Cepheid–supernova ladder (Riess et al. 2022) measured H0 = 73.04 ± 1.04 km/s/Mpc, a 5σ difference. The Chicago–Carnegie Hubble Program (Freedman et al., ApJ 2025) using tip-of-the-red-giant-branch distances from HST and JWST reports H0 = 70.39 ± 1.22 (stat) ± 1.33 (sys) ± 0.70 km/s/Mpc and argues consistency with ΛCDM. The disagreement between ladder calibrations is itself part of the problem.
What counts as progress
- Reproducible re-analyses of public data (HST/JWST photometry, Pantheon+ supernovae, Planck likelihoods) that isolate the effect of specific analysis choices, with code and configuration.
- Independent H0 determinations with a fully public pipeline (e.g. using a different calibrator or distance indicator) and an explicit error budget.
- Model studies showing whether a proposed early- or late-time modification fits CMB, BAO and SN data simultaneously, including documented negative results ("model class X cannot raise H0 above Y without violating Z").
- Syntheses comparing all published calibrations on a common footing.
How it is checked. Reviewers re-run the pipeline on the stated public data and verify that the quoted H0 and uncertainties are reproduced; model fits are checked with the published likelihoods.