Not by itself. The Hubble tension is a persistent mismatch between measurements of the universe’s present expansion rate and the value inferred from early-universe observations using the standard cosmological model, ΛCDM. It is a serious problem that calls for an explanation, but it does not yet prove that ΛCDM is wrong: measurement effects, assumptions in the inference, and new physics remain possible explanations.
What is the Hubble tension?
The Hubble constant, written H0, describes how quickly the universe is expanding today. On sufficiently large scales, more distant galaxies recede faster in proportion to their distance. The tension arises because methods that measure the nearby universe generally give a higher H0 than the value inferred by observing the early universe and calculating how it would evolve under ΛCDM.
NASA’s overview gives rounded ranges of about 70–76 km/s/Mpc for space-telescope measurements and 67–68 km/s/Mpc for cosmic microwave background (CMB)-derived results. These are broad illustrative ranges, not universal confidence intervals; individual probes and analyses need not agree with every value in their respective range.
Why do the two approaches give different answers?
Nearby-universe measurements build a distance ladder
The local distance ladder calibrates distances in stages. Geometric measurements of nearby objects anchor distances to Cepheid variable stars. Cepheids in more distant galaxies then help calibrate Type Ia supernovae, whose observed brightnesses provide distances to still more distant galaxies. Combining those distances with redshifts allows researchers to estimate the present expansion rate.
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Each calibration step can carry uncertainty into the next. The Particle Data Group’s 2024 review discusses possible supernova-related systematics, including host-galaxy extinction and redshift evolution. Other local methods use different indicators or anchors, so a result’s meaning depends in part on what was measured and how shared uncertainties were treated.
Early-universe measurements infer H0 through a model
The CMB is radiation from the early universe. Researchers fit a cosmological model to its observed patterns, then use that model to infer the universe’s later expansion, including H0. This makes the result model-dependent in a way a local distance measurement is not: the inferred present-day value follows from both the early-universe data and the assumptions used to connect those data to today.
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What do the newer results show?
Recent results add cross-checks, but they do not all answer the same question or use one common measurement pipeline.
| Result | Reported H0 | What it measures or tests |
|---|---|---|
| H0 Distance Network Collaboration, 2026; reported by the Center for Astrophysics | Harvard & Smithsonian | 73.50 ± 0.81 km/s/Mpc; the report describes this direct measurement as just over 1% precision | Combines overlapping local indicators and accounts for shared uncertainties using covariance weighting. The report says it differs by approximately 5–7 standard deviations from recent CMB and BAO determinations. |
| ACT DR6 analysis, 2025; NASA NTRS record | For selected extensions, 69.9 (+0.8/−1.5), 69.1 ± 0.5, or 69.6 ± 1.0 km/s/Mpc | Tests baseline ΛCDM and extensions against CMB measurements and combinations of other data. These figures are extension-fit results, not a single baseline ΛCDM value; the analysis found no statistically significant preference for departure from baseline ΛCDM among the models tested. |
| SN H0pe team, 2024; reported by NASA | 75.4 +8.1/−5.5 km/s/Mpc | A gravitational-lensing time-delay measurement of a multiply imaged Type Ia supernova. Its broad uncertainty makes it a cross-check rather than a precision tie-breaker. |
| NASA overview of space-telescope and CMB-derived measurements | About 70–76 km/s/Mpc for space-telescope measurements; 67–68 km/s/Mpc for CMB-derived results | Rounded overview ranges, not confidence intervals for any one probe or a uniform comparison of analyses. |
A network of local measurements strengthens the local result
The 2026 H0 Distance Network result combines Cepheids, the tip of the red giant branch, Mira variables, megamasers, Type Ia and Type II supernovae, surface-brightness fluctuations, Tully–Fisher measurements, and the Fundamental Plane. Because indicators can share calibration uncertainties, the collaboration used covariance weighting rather than treating every input as independent. Its reported 5–7-standard-deviation comparison is specifically against recent CMB and baryon acoustic oscillation (BAO) determinations, not a universal significance figure for every possible combination of probes.
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Webb’s Cepheid observations check one proposed local error
NASA reported in 2024 that James Webb Space Telescope infrared observations provided a sharper cross-check of Cepheid measurements made with Hubble. The comparison reduced concern that crowding of stars or visible-light dust effects alone could account for the discrepancy. In that report, SH0ES lead Adam Riess said the observations let the team rule out a measurement error as the cause “with very high confidence.” That is his characterization of this Cepheid cross-check; it does not establish that every possible local systematic has been eliminated.
A lensing result offers a separate route, but with wide uncertainty
For SN H0pe, light from one supernova was split into multiple images by gravitational lensing. Measuring time delays between the images provides another way to estimate H0. NASA’s 2024 report described it as only the second measurement by this method and the first using a standard candle. Its uncertainty is large, so it does not settle the disagreement.
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Does the evidence point to new physics?
New physics is one possibility, not an established conclusion. The H0 Distance Network authors say that if the tension is real—as the growing body of evidence suggests—it may point beyond the standard cosmological model. Their phrasing is conditional. Meanwhile, the ACT DR6 analysis found no statistically significant preference for departure from baseline ΛCDM among the extensions it tested; models introduced to raise H0 were generally not favored by those data.
Those findings are not contradictory. The local network strengthens evidence for a higher local H0, while ACT DR6 tests whether particular changes to the cosmological model improve the fit to its selected data. A result that disfavors some tested extensions does not rule out every possible form of new physics, and a local mismatch does not identify which assumption or measurement is responsible.
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What could still explain the mismatch?
The cited evidence does not establish a single resolution. The main possibilities are:
- Remaining measurement systematics: Calibration errors or correlated uncertainties could affect multiple measurements. Agreement across different indicators makes a single overlooked local error less persuasive, but does not prove all shared or complex systematics are absent.
- Assumptions in early-universe inference: The CMB-derived value depends on the model used to evolve early-universe observations to the present. Different assumptions could change that inference.
- Physics beyond baseline ΛCDM: A change to the standard cosmological model could potentially alter the inferred expansion history, but the ACT DR6 tests do not establish that such a change is needed.
The Particle Data Group’s 2024 review described the SH0ES–Planck difference as about 5σ and noted the investigation of systematics and new physics. That dated comparison is distinct from the H0 Distance Network’s newer, approximately 5–7σ comparison with recent CMB and BAO determinations.
How to judge a new H0 claim
A headline number is most informative when read alongside its method. Check:
Quick Recap
- Whether it is direct or inferred: A local distance measurement and an early-universe value calculated under ΛCDM are not interchangeable kinds of result.
- Which indicators and anchors were used: A distance ladder, a network of multiple local indicators, a CMB fit, and lensing time delays have different dependencies.
- The uncertainty and its scope: A central value alone does not show how precisely it was measured or which uncertainties were included.
- How shared errors were handled: Measurements that share calibrations may not be independent; covariance treatment can matter.
- Which model and external data were tested: An extension’s result depends on the added physics and the data combination. Do not mistake an extension-fit value for a baseline ΛCDM estimate.
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