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What the Hubble constant measures
The Hubble constant, written H0, describes the present-day expansion rate of the universe. It is conventionally expressed in kilometers per second per megaparsec (km/s/Mpc): in the nearby-universe formulation, recession speed is proportional to distance, with H0 as the proportionality factor.
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To compare estimates, it helps to ask what was observed, how the present-day value was obtained, and which calibrations or model assumptions connect the observations to H0.
How the two approaches differ
| Question | Nearby-galaxy distance ladder | Early-universe CMB inference |
|---|---|---|
| What is observed? | Geometrically calibrated distances, distance indicators in more distant galaxies, and recession or redshift information. | Temperature and polarization patterns in radiation released in the early universe. |
| How is H0 obtained? | Distances are calibrated in stages and compared with galaxy recession to estimate the present expansion rate. | The observed CMB is fitted with ΛCDM or related cosmological assumptions; the fit yields an inferred value at redshift zero. |
| Where do important assumptions or errors enter? | Geometric calibration, Cepheid measurements, supernova calibration, and the distance-redshift comparison. | The cosmological model and the data and assumptions used in the CMB fit. |
NASA’s LAMBDA explanation emphasizes that the CMB route is an inference: the radiation comes from the early universe, while the quoted value is extrapolated to the present through a model. A CMB-derived estimate is therefore not a direct observation of today’s expansion rate.
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How the nearby-galaxy distance ladder works
Start with geometric calibration
The ladder begins with geometric distance measurements, including parallax measurements of nearby Cepheids. These calibrate the relationship between a Cepheid’s observable properties and its luminosity.
Calibrate brighter markers farther away
Calibrated Cepheids can be measured in more distant galaxies, including galaxies that also host Type Ia supernovae. Those supernovae are brighter, so they extend distance measurements to galaxies far enough away to compare their distances with the stretching of light caused by cosmic expansion.
Build an expansion estimate
Combining those calibrated distances with recession information produces a nearby-universe estimate of H0. It is a chain of linked measurements, so errors or biases in calibration and in the individual indicators can affect the result. Cepheid crowding and dust, supernova calibration, and which redshift-distance observations are used are among the relevant considerations.
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What published estimates say—and what they do not
NASA Science’s undated explainer, accessed in 2026, gives broad ranges of around 70–76 km/s/Mpc for space-telescope local measurements and around 67–68 km/s/Mpc for CMB-derived estimates. NASA calls the difference the Hubble tension. These are source-reported ranges, not a single matched pair of results with identical inputs or uncertainty treatment.
The gap alone does not identify its cause. It could reflect remaining measurement systematics, a limitation in the assumptions used to infer the early-universe value, or a more fundamental feature of cosmology. A reported estimate and a proposed explanation for that estimate are different kinds of claims.
What Webb’s Cepheid check established
In a 2024 account, ESA reported that the SH0ES team used Webb infrared observations to check Cepheids in five Type Ia supernova host galaxies, covering about 1,000 Cepheids and reaching NGC 5468 at a reported distance of 130 million light-years. Webb’s Cepheid observations agreed with Hubble’s optical measurements across the Hubble-observed range.
Infrared observations help address concerns such as crowding and dust that can affect Cepheid measurements. The agreement strengthens confidence in this particular distance-ladder check; it does not establish that every possible source of ladder error has been eliminated, nor does it explain why CMB-based estimates differ.
Why “early versus nearby” is not the whole comparison
A 2024 analysis by Leandros Perivolaropoulos in Physical Review D argues that the disagreement may chiefly be between distance-ladder results and other determinations, rather than a simple division into two camps. The paper compiled 20 distance-ladder measurements and 33 “one-step” measurements described as independent of both the CMB sound-horizon scale and the distance ladder.
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| Sample in the 2024 paper | Reported fit | Qualification |
|---|---|---|
| Distance-ladder sample | 72.8 ± 0.5 km/s/Mpc | Fit to the paper’s compiled 20 distance-ladder measurements. |
| One-step sample | 69.0 ± 0.48 km/s/Mpc | Fit to the paper’s compiled 33 one-step measurements. |
| One-step sample with two outliers removed | 68.3 ± 0.5 km/s/Mpc | Fit after the paper’s removal of two outliers. |
| Restricted one-step sample | 68.5 ± 0.8 km/s/Mpc | Fit to the paper’s restricted sample. |
These are analyses of selected samples, not universal values for every method in either category. Perivolaropoulos’s interpretation is the paper’s argument, not a settled consensus; it illustrates why method, sample selection, and dependence among measurements matter when comparing results.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A newer multi-indicator nearby estimate
NASA reported on April 13, 2026, that findings published April 10 in Astronomy & Astrophysics from the Local Distance Network put the nearby-universe expansion rate at about 73.5 km/s/Mpc. NASA said other estimates had typically ranged from 73 to 76 km/s/Mpc. The network combines different distance markers and accounts for their relative robustness, rather than relying on a single indicator.
Adam Riess, a coauthor affiliated with Johns Hopkins University and the Space Telescope Science Institute, told NASA: “The power of this work is that it doesn’t depend on any single method.” This result adds a multi-indicator comparison to the evidence; it does not by itself settle the contrast with early-universe inference.
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No final resolution is established. Remaining calibration or other systematic effects are possible explanations, as is a change to cosmological physics. NASA lists exotic particles, modified gravity, and early dark energy among proposed possibilities; these remain hypotheses, not demonstrated causes. The Local Distance Network and Webb Cepheid check strengthen aspects of nearby-universe evidence, while the 2024 analysis underscores that results depend on how methods and samples are grouped. Neither point alone proves what accounts for the tension.
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