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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Nearby-galaxy methods build distances outward from geometric measurements and compare them with galaxies’ recession; early-universe methods fit the cosmic microwave background (CMB) with a cosmological model to infer the expansion rate today. These different evidence chains produce estimates that do not currently agree, a discrepancy known as the Hubble tension.
What the Hubble constant measures
The Hubble constant, written H0, describes the universe’s present-day expansion rate. It is conventionally expressed in kilometers per second per megaparsec (km/s/Mpc): in the nearby-universe formulation, recession velocity is proportional to distance, with H0 as the proportionality factor. A value of 73 km/s/Mpc, for example, corresponds to about 73 km/s more recession speed for each additional megaparsec of distance in that relationship.
The two broad approaches do not observe the same thing and then calculate the answer in the same way. Nearby-galaxy distance ladders calibrate distance indicators and pair distances with recession information. CMB analyses observe radiation from the early universe and infer the value of H0 at the present epoch through a cosmological model.
How nearby-galaxy distance-ladder measurements work
Start with geometric distances
A distance ladder links methods that work at different ranges. At its nearby end, geometric measurements—including parallax—help calibrate Cepheid variable stars. Cepheids’ calibrated brightness then provides a way to estimate distances in more remote galaxies.
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Use Cepheids to calibrate supernovae
Some galaxies contain both Cepheids and Type Ia supernovae. The Cepheids help establish the host galaxy’s distance, which in turn calibrates the supernova’s intrinsic brightness. Because Type Ia supernovae are bright, astronomers can use them in more distant galaxies than the Cepheid measurements typically reach.
Compare distance with cosmic recession
With supernovae extending the calibrated distance scale, astronomers can compare galaxy distances with information about the stretching of light as the universe expands. The relationship between distance and recession in the nearby universe yields a local estimate of H0.
Potential error sources include the geometric calibration, Cepheid crowding and dust, supernova calibration, and how recession information is sampled against distance. A distance ladder is a chain: an error or bias in a calibration step can affect later steps built on it.
How early-universe CMB estimates work
The CMB is radiation released when the universe was much younger. Its temperature and polarization patterns preserve information about early-universe conditions. In NASA LAMBDA’s explanation, researchers fit those observations with the standard ΛCDM cosmological model; the fit yields an inferred H0 at redshift zero, meaning the present epoch.
That value is not a direct measurement of today’s expansion rate. It depends on the model used to connect the early-universe evidence to the universe now. The fit, its assumptions, and the data combined with the CMB therefore matter when comparing its result with a local distance-ladder estimate.
What the reported values show
NASA’s undated explainer gives broad ranges of about 70–76 km/s/Mpc for space-telescope local measurements and about 67–68 km/s/Mpc for CMB-derived estimates. These are source-reported ranges, not a matched pair from one analysis: they summarize different measurements and methods. NASA calls the difference between such local and CMB-based estimates the Hubble tension.
| Result | Reported value | What it represents |
|---|---|---|
| Space-telescope local measurements | About 70–76 km/s/Mpc | Broad range in NASA’s explainer; not one single measurement. |
| CMB-derived estimates | About 67–68 km/s/Mpc | Broad range in NASA’s explainer for values inferred by fitting early-universe observations. |
| Local Distance Network | About 73.5 km/s/Mpc | Nearby-universe estimate reported by NASA in 2026; NASA says other estimates have typically ranged from 73–76 km/s/Mpc. |
| Distance-ladder sample in a 2024 analysis | 72.8 ± 0.5 km/s/Mpc | Fit to the paper’s compiled distance-ladder sample. |
| One-step sample in the same 2024 analysis | 69.0 ± 0.48 km/s/Mpc | Fit to measurements the paper classifies as independent of both the CMB sound-horizon scale and the distance ladder. |
| One-step sample after removing two outliers | 68.3 ± 0.5 km/s/Mpc | Fit reported in the same paper after two outliers were removed. |
| Restricted one-step sample in the same 2024 analysis | 68.5 ± 0.8 km/s/Mpc | Fit to the paper’s restricted sample. |
The 2024 figures come from Leandros Perivolaropoulos’s analysis in Physical Review D, which compiled 20 distance-ladder measurements and 33 one-step measurements. They describe that paper’s chosen samples and fits, not universal values for every measurement in either category. The paper argues that the discrepancy may chiefly be between distance-ladder measurements and other determinations, rather than a simple split between all early- and late-universe results. That is the author’s interpretation, not a settled consensus.
What Webb’s Cepheid observations checked
One concern about Cepheid measurements is that crowded fields and dust can affect how the stars appear in optical observations. Webb observes in infrared wavelengths, providing a cross-check on Cepheid measurements made with Hubble.
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In 2024, ESA reported that the SH0ES team observed five Type Ia supernova host galaxies containing about 1,000 Cepheids. The observations reached NGC 5468, 130 million light-years away. Across the range observed by Hubble, Webb’s infrared Cepheid measurements agreed with Hubble’s optical measurements.
This agreement strengthens confidence in that part of the distance ladder and addresses specific concerns about crowding and dust. It does not show that every possible systematic error in the ladder is absent, nor does it explain why CMB-based inference gives a different value.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the estimates are not a simple two-team contest
- The evidence differs: local methods use calibrated distances and recession information, while CMB estimates derive today’s value by fitting early-universe observations.
- Assumptions differ: ladder results depend on calibrations and distance-indicator measurements; CMB results depend on the cosmological model and the data used in the fit.
- Published estimates may not be independent: combined-data results and one-step techniques can use different inputs and assumptions. A reported value should be interpreted in light of how it was obtained, rather than assigned to one of two supposedly uniform camps.
- A discrepancy is not an explanation: disagreement alone does not establish whether the cause is an unaccounted-for systematic, a limitation in how measurements are grouped and compared, or new cosmological physics.
The 2024 compilation illustrates the importance of that distinction: its one-step sample, described as independent of both the CMB sound-horizon scale and the distance ladder, does not simply reproduce the ladder sample’s fitted value. Its analysis is one reason to compare method-specific samples and correlations rather than treating every published estimate as a direct vote for one of two methods.
What is—and is not—known about the Hubble tension
The estimates remain in disagreement; the evidence described here does not establish a final resolution. Possible explanations include remaining calibration or other systematic effects, changes to cosmological physics, or issues in how different determinations are grouped and compared. NASA has listed exotic particles, modified gravity, and early dark energy among proposed possibilities. They remain hypotheses, not demonstrated causes.
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In its April 13, 2026 report on the Local Distance Network, NASA described a framework that combines multiple distance markers while accounting for their relative robustness. The reported result, published April 10 in Astronomy & Astrophysics, was about 73.5 km/s/Mpc. Coauthor Adam Riess, 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.” The network adds evidence from a multi-indicator nearby-universe approach; it does not by itself settle why the CMB-based inference differs.
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