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The Hubble tension is a persistent disagreement between two ways of estimating how quickly the universe is expanding today. Measurements built from nearby stars and supernovae tend to produce a higher value for the Hubble constant, H0, than estimates inferred from the early universe’s cosmic microwave background (CMB). A 2026 collaboration combining multiple local distance indicators reported 73.50 ± 0.81 kilometers per second per megaparsec, while NASA’s broad illustrative ranges put local telescope measurements around 70–76 and CMB-based estimates around 67–68. The cause of the mismatch remains unknown.
What is the Hubble tension?
The Hubble constant, written H0, describes the universe’s present-day expansion rate. It is not a claim that expansion has proceeded at the same rate throughout cosmic history. The tension is that methods based on different observations and inference steps do not agree on the value of H0.
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NASA’s explanatory page gives broad illustrative ranges of about 70–76 kilometers per second per megaparsec for space-telescope measurements and about 67–68 for values inferred from the CMB. These are ranges, not a single matched pair of the latest measurements. The difference matters because the inferred rate affects estimates of the universe’s history and age; NASA associates the telescope-based estimates it describes with an age of about 13.8 billion years. NASA Science explains the Hubble tension.
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Why do different methods get different expansion rates?
The two main approaches are not simply separate instruments measuring the same nearby object. One builds a calibrated distance scale outward from nearby stars. The other reads patterns in ancient radiation and uses a cosmological model to infer what the present-day expansion rate should be. Each route depends on different observations, assumptions, and potential sources of uncertainty.
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- HOBBY MODEL KIT – Unassembled model packed in an envelope with easy to follow instructions. Ideal for ages 14 and up.
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| Route | What is observed | How H0 is obtained | Key qualification |
|---|---|---|---|
| Local distance ladder | Nearby stars, galaxies, and supernovae | Calibrate distances in stages, then compare distance with redshift | Uncertainties can enter at each calibration and measurement step |
| Early-universe inference | The CMB, radiation released shortly after the Big Bang | Fit the observed pattern with a cosmological model and infer the expansion history, including H0 | The result depends on the model used to connect early-universe observations to today |
The local distance ladder
Parallax provides geometric distances to nearby Cepheid variable stars. Cepheids then help calibrate distances to galaxies that also contain Type Ia supernovae. Because those supernovae are bright, they can extend the distance scale farther into the universe. Astronomers compare the resulting distances with galaxies’ redshifts to estimate the current expansion rate.
The chain is powerful, but it means that uncertainty in a nearby calibration can affect later steps. Crowded star fields and dust, for example, were raised as possible concerns in measuring distant Cepheids.
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The CMB route
The CMB is a snapshot of radiation from the early universe. Its detailed pattern contains information about cosmic conditions and evolution. Researchers use a cosmological model to translate that early-universe information into an inferred value for today’s H0. This is therefore a model-based inference, rather than a direct measurement of the present expansion rate using nearby distances. The distinction between these routes is central to understanding why disagreement may reflect measurement issues, model assumptions, or physics not yet understood. NASA LAMBDA provides background on cosmological observations and inference.
What does the latest local result say?
In 2026, the H0 Distance Network Collaboration (H0DN) reported a direct local measurement of 73.50 ± 0.81 km/s/Mpc, with just over 1% precision. The Center for Astrophysics report says the result remains approximately 5–7 standard deviations from recent determinations based on the CMB and baryon acoustic oscillations (BAO). Those figures describe this collaboration’s result and the comparisons in its report, not a universal measurement uncertainty or a claim that every method has the same statistical significance.
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H0DN combines many 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 using full covariance weighting. Combining multiple routes broadens the local evidence beyond a Cepheid-only ladder, but the reported result does not identify the cause of the disagreement. The Center for Astrophysics describes the H0DN result.
Could the disagreement be a measurement error?
A specific concern was that Cepheids in distant, crowded fields might appear brighter because neighboring stars blend into their images, while dust can affect measurements made in visible light. The James Webb Space Telescope’s sharper infrared imaging helps separate Cepheids from nearby stars and reduces dust effects.
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NASA and ESA report that Webb’s observations across the Hubble sample confirmed Hubble’s Cepheid measurements. That cross-check strengthened confidence in those measurements and did not make the tension disappear. It does not demonstrate that every possible systematic uncertainty in every method has been ruled out. NASA’s Webb report and ESA’s account describe the cross-check.
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What other methods can test the expansion rate?
Gravitational-wave standard sirens
A gravitational-wave signal from a merger can provide a luminosity distance. Pairing that distance with the redshift of the host galaxy offers an independent way to estimate H0. It does not rely on the same Cepheid-to-supernova calibration chain.
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Baryon acoustic oscillations
BAO patterns in the distribution of matter provide a useful cosmological ruler, but NASA LAMBDA notes that BAO data alone cannot determine H0; they must be combined with other information. The H0DN report’s comparison with recent BAO-based determinations should therefore be read in that context. NASA LAMBDA outlines the role of BAO and other cosmological evidence.
Strongly lensed supernovae
A massive foreground galaxy or cluster can bend a distant supernova’s light into multiple images. Because those paths have different lengths and pass through different gravitational fields, the images arrive at different times. Measuring the time delays can constrain distances and H0 geometrically. NASA describes the Nancy Grace Roman Space Telescope’s surveys as a future way to find more of these rare events; they are a potential source of additional evidence, not a result already resolving the tension. NASA describes Roman’s planned search for lensed supernovae.
Does the Hubble tension mean physics is wrong?
Not necessarily. The discrepancy could ultimately involve an underestimated uncertainty, a modeling issue, or a gap in the current understanding of cosmic evolution. NASA has discussed exotic particles, alternative theories of gravity, and early forms of dark energy as possibilities. None is an established explanation for the tension.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteThe important distinction is between evidence for a persistent mismatch and evidence for its cause. The disagreement has held up across multiple lines of measurement, but a statistically significant mismatch does not by itself identify new physics.
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