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The Hubble tension is a persistent disagreement about how fast the universe is expanding today. Measurements built from nearby stars and supernovae generally give a higher expansion rate than estimates inferred from the cosmic microwave background (CMB), the afterglow of the early universe. The difference is not simply a dispute between two instruments: the methods observe different things and use different steps to arrive at today’s rate.
What does the Hubble tension mean?
The Hubble constant, written H0, describes the universe’s expansion rate today. It is expressed in kilometres per second per megaparsec (km/s/Mpc): broadly, how quickly the distance between galaxies increases for each megaparsec of distance. It is a present-day rate, not a claim that expansion has proceeded at the same speed throughout cosmic history.
The tension is the mismatch between two main ways of estimating H0. A local distance ladder builds outward from nearby objects whose distances can be measured, while an early-universe approach uses the CMB and a cosmological model to infer the expansion history and its present-day value. NASA’s explanatory page gives broad illustrative ranges of about 70–76 km/s/Mpc for telescope-based measurements and about 67–68 km/s/Mpc for CMB-derived estimates. Those ranges summarize the discrepancy; they are not one matched pair of latest measurements. NASA Science explains the Hubble constant and the tension.
How do the two main approaches estimate expansion?
| Approach | What is observed | How it produces H0 | Where uncertainties can enter |
|---|---|---|---|
| Local distance ladder | Nearby stars and galaxies, including Cepheids and Type Ia supernovae | Geometric distances calibrate Cepheids; Cepheids calibrate supernova distances; astronomers compare distance with galaxy redshift to estimate today’s expansion rate. | Uncertainty can enter at each calibration step and in the observations used to measure distances and redshifts. |
| Early-universe inference | The CMB, radiation released shortly after the Big Bang | A cosmological model uses the CMB’s observed pattern to infer the universe’s expansion history and calculate a present-day value. | The result depends on the model assumptions used to connect early-universe observations to expansion today. |
The local distance ladder
Parallax provides geometric distance measurements for nearby Cepheid variable stars. Cepheids’ brightness patterns then help calibrate distances to galaxies that also contain Type Ia supernovae. Because those supernovae are bright, they extend the scale to more distant galaxies. Comparing distance with redshift—the shift in a galaxy’s light associated with cosmic expansion—yields an estimate of H0. This is a chain of calibrated distances, rather than a direct reading from one faraway galaxy.
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The CMB route
The CMB records conditions in the young universe. Its pattern is measured today, then interpreted using a cosmological model that describes how the universe evolved. The resulting H0 is therefore an inference from early-universe evidence, not a local distance measurement. NASA’s LAMBDA resource notes that baryon acoustic oscillation (BAO) data alone cannot determine H0; BAO must be combined with other information. NASA LAMBDA provides cosmology data and resources.
What do the latest measurements show?
A 2026 report from the Center for Astrophysics gives the H0 Distance Network Collaboration’s direct local measurement as 73.50 ± 0.81 km/s/Mpc, with just over 1% precision. The report says the result remains approximately 5–7 standard deviations from recent determinations based on the CMB and BAO. Those figures describe the report’s comparison and its methods; they are not a claim that every analysis or dataset produces precisely the same difference. Read the Center for Astrophysics report on the H0 Distance Network.
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The network combines multiple distance indicators: Cepheids, the tip of the red giant branch, Mira variables, megamasers, Type Ia and Type II supernovae, surface-brightness fluctuations, the Tully–Fisher relation, and the Fundamental Plane. Its analysis accounts for shared uncertainties and correlations through full covariance weighting. Broadening the local evidence is important because a result based on several indicators is not just a repeat of one Cepheid-and-supernova measurement. But the network result does not identify why local and early-universe estimates differ; the disagreement remains unresolved in the sources available here.
For context rather than as a measurement of the tension itself, NASA associates its described telescope-based expansion estimates with an age of about 13.8 billion years for the universe. The age and the present-day expansion-rate disagreement are related cosmological quantities, but they are not interchangeable.
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Did the James Webb Space Telescope resolve the Cepheid concern?
One proposed concern was that Cepheids in crowded, distant fields might appear brighter because of blended neighboring stars, or that dust could affect visible-light measurements. Webb’s sharper infrared imaging helps separate Cepheids from nearby stars and reduces dust effects. NASA and ESA report that Webb observations spanning the Hubble sample confirmed Hubble’s Cepheid measurements, so this cross-check did not make the tension disappear. It strengthens confidence in that part of the local distance ladder, but it does not rule out every possible systematic uncertainty in every measurement method. NASA’s Webb report on the expansion-rate cross-check and ESA’s account of Webb’s confirmation describe the observations.
What other ways can astronomers measure H0?
Gravitational-wave standard sirens
A merging pair of compact objects can produce gravitational waves whose signal helps estimate the event’s luminosity distance. If astronomers identify the host galaxy, its redshift supplies another part of the measurement. This offers an independent route, though it is distinct from both the traditional local ladder and the CMB inference.
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Strongly lensed supernovae
A massive foreground galaxy or cluster can bend a distant supernova’s light into multiple images. The images travel along different paths and arrive at different times. Measuring those delays helps constrain distances and H0 geometrically. NASA describes future Roman Space Telescope surveys as a way to find more of these rare events; that prospect is a source of future evidence, not a result that has already resolved the tension. NASA describes how Roman may use lensed supernovae.
What could be causing the disagreement?
No cause is established. The mismatch could reflect uncertainties or unrecognized effects in one or more measurement routes; it could also indicate that the cosmological model used to interpret early-universe evidence needs revision. NASA lists exotic particles, alternative theories of gravity, and early forms of dark energy among ideas proposed by researchers. These remain possibilities, not discoveries or settled explanations. NASA’s overview discusses proposed explanations.
Best Value
The key distinction is between evidence that the measurements disagree and evidence explaining why. A precise new measurement can sharpen the disagreement without revealing its cause.
Further reading for a technical treatment
Readers who want a deeper, specialist reference can consult The Hubble Constant Tension, edited by Eleonora Di Valentino and Dillon Brout. Springer lists hardcover and softcover editions and chapters covering measurement methods such as Cepheids, Type Ia supernovae, strong lensing, and the CMB. It is an edited technical volume, rather than a beginner’s plain-English guide. See the book at Springer.
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