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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →The Hubble tension is a persistent mismatch between measurements of the universe’s present-day expansion rate and the lower rate inferred from early-universe observations using the standard ΛCDM cosmological model. Possible explanations range from overlooked measurement errors to new physics that changes how the universe expanded in its early or later history. None has been confirmed.
What is the Hubble tension?
The Hubble constant, written as H0, describes how quickly the universe is expanding today. It is expressed in kilometres per second per megaparsec (km/s/Mpc): a larger value means that, on average, galaxies at a given distance recede faster.
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The tension is a difference between two ways of estimating H0. One measures distances and galaxy redshifts in the relatively nearby universe. The other observes the cosmic microwave background (CMB), the afterglow of the Big Bang, and uses a cosmological model to infer what today’s expansion rate should be. NASA gives broad explanatory ranges of about 70–76 km/s/Mpc for local telescope measurements and 67–68 km/s/Mpc for CMB-based estimates. These ranges summarize results; they are not a single, matched-data calculation of statistical significance.
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There is strong evidence for a mismatch between the two approaches, but its cause is unknown. The latest direct local result identified here is from the H0 Distance Network Collaboration: H0 = 73.50 ± 0.81 km/s/Mpc, reported on April 2, 2026. The Center for Astrophysics | Harvard & Smithsonian said this result differs by approximately 5–7 standard deviations from recent CMB and baryon acoustic oscillation (BAO) determinations. That significance is the report’s approximate comparison, not a universal figure independent of the datasets and analyses being compared.
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The network combines several local distance indicators and accounts for correlated uncertainties, so the result does not depend on a single kind of local measurement. It strengthens the case that the mismatch is not simply an isolated result from one distance method. It still does not establish that every possible systematic error has been eliminated, or prove that new physics is responsible.
Why do the two methods give different answers?
Local measurements build a distance ladder
A local distance-ladder analysis starts with nearby objects whose distances can be calibrated geometrically, including through parallax. Cepheids—stars whose regular brightness changes help estimate distance—then calibrate Type Ia supernovae in their host galaxies. More distant Type Ia supernovae extend the scale. Comparing the distances of galaxies with their redshifts gives an estimate of the current expansion rate.
The H0 Distance Network broadens this local approach by combining 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. The collaboration says it models shared uncertainties using full covariance weighting, which matters because different indicators may rely on overlapping data or calibration steps.
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Early-universe measurements infer today’s rate through a model
The CMB records conditions in the early universe. Analyses use it, along with measurements of large-scale structure such as BAO, to constrain cosmological parameters. To derive today’s H0, they extrapolate forward under a model—usually ΛCDM, which describes the universe with cold dark matter and a cosmological constant as dark energy. The local ladder measures distances in the comparatively recent universe; the early-universe method infers today’s rate from an earlier snapshot and the assumed history connecting the two.
What could explain the Hubble tension?
The leading proposals differ in which part of cosmic history they change. Some alter early-universe physics and the calibration of the distance scale; others change expansion at later times. There are also proposals involving measurement systematics or more fundamental changes to gravity and dark energy. A candidate must do more than raise the inferred H0: it must also fit the other observations that constrain cosmic history.
| Proposal | What it changes | Main challenge |
|---|---|---|
| Early dark energy, extra relativistic particles, primordial magnetic fields, or modified recombination | Early expansion or the physics of recombination, potentially reducing the sound horizon used to calibrate distances. | Must remain consistent with CMB structure, BAO, supernovae, primordial element abundances, and other cosmological observations. Tested examples have not been shown to resolve the discrepancy. |
| Evolving dark energy or another late-time expansion change | The universe’s expansion history after the early era, changing the distance–redshift relation. | Must fit supernova and BAO distance measurements together; changing one part of the late-time model does not ensure agreement with independent probes. |
| Measurement systematics | Calibration or distance estimates, for example through Cepheid crowding or dust. | Independent checks make a single hidden error less plausible, but do not demonstrate that every possible systematic in every method is absent. |
| Local cosmic underdensity, or “Hubble bubble” | The expansion inferred nearby, if our region had unusually low matter density. | A 2026 review concludes that a local bubble or void has long been ruled out as a significant contribution; it is a historical proposal rather than a leading viable explanation. |
| Modified gravity, exotic particles, or interacting dark energy | Fundamental physics or interactions that could affect expansion in the early universe, late universe, or both. | These remain proposed research directions, not established causes, and must satisfy the same constraints from independent cosmological data. |
Early-universe physics and the sound horizon
The sound horizon is a characteristic distance set by how sound waves travelled through the early universe before atoms formed and the CMB was released. It provides a calibrated scale for interpreting later observations. If new physics made the early universe expand faster for a time, or changed recombination, the sound horizon could be smaller. A smaller calibration scale can allow analyses of early-universe data to infer a higher value of today’s H0.
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Early dark energy is one version of this idea: an additional component would briefly affect expansion before fading. Other explored options include extra relativistic particles, primordial magnetic fields, and altered recombination physics. These mechanisms are not interchangeable, and improving the fit to H0 alone does not show that any one of them is correct.
What recent ACT results say about selected models
An ACT collaboration analysis published in 2025 tested extensions to baseline ΛCDM using Atacama Cosmology Telescope Data Release 6 (ACT DR6) combinations. Its reported H0 constraints were:
| Tested model | Reported H0 (km/s/Mpc) |
|---|---|
| Early-dark-energy fit | 69.9 (+0.8/−1.5) |
| Primordial magnetic fields | 69.1 ± 0.5 |
| Modified recombination history | 69.6 ± 1.0 |
These are model- and dataset-dependent constraints, not preferred solutions. In the reported ACT analysis, none of the tested extensions had a statistically significant overall preference over baseline ΛCDM. The estimates also remained below the 2026 H0 Distance Network value. This constrains the specific models and data combinations tested; it does not rule out every possible form of new physics.
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A separate 2025 ACT collaboration analysis reported sound-horizon-independent estimates of 66.4 (+3.2/−3.7) km/s/Mpc from large-scale structure alone, and 64.3 (+2.1/−2.4) km/s/Mpc when combined with uncalibrated Pantheon+ supernovae. Their uncertainties are broad. They are different analyses from the local distance-network measurement and should not be treated as directly equivalent measurements of the same kind.
Changes to late-time expansion
Dark energy that evolves over time could change the relationship between distance and redshift, potentially affecting late-universe inferences of the expansion rate. A 2026 review also discusses proposals that change effective supernova luminosity; it notes that these are strongly constrained by inverse distance ladders combined with the cosmic distance-duality relation. Any late-time proposal must explain the pattern of supernova and BAO distances, not just adjust a parameter to move H0.
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More fundamental changes
Modified gravity, exotic particles, and interacting dark energy are broader possibilities. Some interacting dark-energy models combine early- and late-time changes; others focus on the transition between the inhomogeneous universe and the more homogeneous large-scale cosmos. Such ideas may be scientifically active without being established explanations. As with early-dark-energy proposals, their success depends on fitting multiple independent observations at once.
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Could measurement errors still explain it?
Potential sources of error in local distance measurements include calibration, dust, and the blending of Cepheids with nearby stars. NASA reports that observations by the James Webb Space Telescope, whose infrared resolution helps with crowding and dust complications, cross-checked Hubble’s Cepheid measurements. The discrepancy remained. Adam Riess of Johns Hopkins University characterized that Hubble/Webb check this way: “We’ve now spanned the whole range of what Hubble observed, and we can rule out a measurement error as the cause of the Hubble Tension with very high confidence.” This is Riess’s interpretation of that cross-check, not proof that all systematics across every method have been ruled out.
The 2026 distance-network result adds a different kind of check: it pools multiple distance indicators and accounts for their correlated uncertainties. Together, these checks make an explanation based only on one overlooked local measurement problem less plausible. They cannot logically exclude every unknown systematic or every issue shared across methods.
What would count as a convincing explanation?
A proposal becomes compelling if it accounts for the mismatch while also surviving independent tests. In practice, researchers need to assess whether a model:
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problems- Changes early sound-horizon physics, late-time expansion, or the distance measurements themselves—and whether that change is physically consistent.
- Fits CMB and BAO observations as well as supernova distances and independent local indicators.
- Improves the agreement across datasets that do not simply reuse the measurements on which the proposal was built.
- Introduces additional parameters or creates new conflicts with other constraints, including primordial element abundances.
A better fit to one selected dataset is not, by itself, independent confirmation. Current observations constrain representative proposals but do not identify a confirmed cause. The question “What could explain the Hubble tension?” therefore has several credible categories of answer, but no settled solution.
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