The Hubble Tension: Why Measurements of Cosmic Expansion Disagree

Why the Hubble Tension Is Back in the News in 2026

In April 2026, the H₀DN collaboration published an Astronomy & Astrophysics paper: a joint analysis of local distance indicators gave H₀ = 73.50 ± 0.81 km/s/Mpc. The advance goes beyond another decimal place. The authors connected several calibration routes in a statistical network and explicitly accounted for shared uncertainties. This tests the robustness of the local distance scale; it is not a separate telescope observing a new substance.

October 10 brought another result: a TDCOSMO preprint on light-travel delays in 11 gravitational-lens systems. Lenses bypass the stellar distance ladder and can test some of its conclusions another way. The paper was submitted to Physical Review Letters, rather than announced as a definitive solution. As of October 29, the question remained open: new data enrich comparisons between methods without yet identifying one cause for the discrepancy.

What the Hubble Constant and Its Unusual Units Mean

For sufficiently nearby galaxies, the overall expansion flow approximately follows v ≈ H₀d: recession speed is proportional to distance. Taking H₀ = 70 km/s/Mpc in a teaching example gives 1,400 km/s at 20 megaparsecs and 2,800 km/s at 40 megaparsecs. This is a velocity increment per unit distance, not a galaxy’s yearly acceleration. A megaparsec is about 3.26 million light-years.

More generally, H(t) is the rate of change of the cosmic scale divided by that scale, and the zero subscript denotes the present epoch. “Constant” therefore does not require H to be unchanged at all times. An individual galaxy also moves under its neighbours’ gravity: the Milky Way is not a perfect marker of the overall flow. At large redshifts, the simple linear formula likewise requires an appropriate cosmological description.

Why Measuring a Distance Is Harder Than Seeing a Galaxy

Redshift is measured using known spectral lines, comparing their laboratory positions with those in a galaxy’s light. Redshift alone cannot independently determine H₀, however. Calculating a distance from redshift while assuming H₀, then using that distance to test H₀, would be circular. Another way to establish the absolute scale is needed.

Apparent brightness does not directly give distance: a faint nearby source and a powerful distant one can look alike. Geometry comes first, using stellar parallaxes or maser measurements in NGC 4258, for example. Such anchors establish the scale for bright indicators visible farther away. Every step must propagate both a distance and its uncertainty; otherwise the final digits would imply more confidence than the underlying data support.

How Cepheids Calibrate Supernovae

Cepheids are pulsating stars whose brightness-variation period is related to luminosity. Measuring the period and calibrating that relation with objects at known distances allows distances to other galaxies to be estimated. A Type Ia supernova observed in one of those galaxies supplies the next step: its intrinsic brightness is linked to the galaxy’s distance. More distant supernovae then extend the scale into regions where cosmic expansion is easier to separate from local motions.

Type Ia supernovae require standardisation, accounting for light-curve properties, colour and other effects. Every explosion cannot simply be assigned exactly the same power. In the well-known 2022 SH0ES analysis, Cepheids in galaxies hosting 42 supernovae contributed to a result of 73.04 ± 1.04 km/s/Mpc. This is an important reference with extensively tested analysis variants, rather than a new detection in October 2026.

What Webb Checked, and Why Other Stars Matter

Neighbouring stars can blend with a Cepheid’s image and bias its measured brightness. Webb separates such sources better in the near infrared. Tests reported by NASA in 2023–2024 confirmed agreement with Hubble’s Cepheid measurements and narrowed the room for explanations based on crowding. Checking this particular effect does not eliminate every conceivable systematic uncertainty across cosmology.

Other stellar indicators include the tip of the red giant branch, or TRGB, and JAGB carbon stars. The updated CCHP report of March 2025 gave a principal TRGB result around 70.4 km/s/Mpc, with separate statistical, systematic and supernova uncertainty components. Local estimates therefore do not form one perfectly uniform camp. Indicators, anchors and samples differ, so the dispute cannot be reduced to a duel between two telescopes.

Continuing such checks also has an engineering aspect: servicing Hubble is again being explored as a way to extend access to the operating observatory. NASA’s October 29, 2026 request concerns a reboost, servicing or a controlled end of mission. The decision to investigate options does not resolve the H₀ discrepancy or guarantee another flight.

Why the Cosmic Microwave Background Gives a Value Near 67

The cosmic microwave background is observed today, but its temperature and polarisation patterns preserve information about the early Universe. Researchers measure angular scales and other statistical properties in that pattern. A physical model then reconstructs cosmic contents and expansion history, yielding H₀ for the present epoch. This is not a direct instrument reading of a “past speed” to compare with a “present speed”.

The familiar Planck 2018 result is 67.4 ± 0.5 km/s/Mpc under base flat ΛCDM. Λ denotes a cosmological constant, while cold dark matter is part of the matter description. Changing the model can alter inferred parameters and uncertainties. The difficulty is that the standard description successfully fits many observations, so it cannot be adjusted arbitrarily just to obtain a preferred H₀. “Planck 2018” identifies a set of results, not a new measurement in autumn 2026.

What the H₀DN Distance Network Changed

Several connected routes can be described jointly instead of relying on one chain: Cepheids, red giants, masers, supernovae and galaxy-property indicators. Some share a geometric anchor or photometric calibration, however. Treating them as wholly independent votes and averaging them would artificially shrink the uncertainty. H₀DN includes these relationships through covariance, a mathematical description of uncertainties that vary together.

The baseline network gave 73.50 ± 0.81 km/s/Mpc, or about 1.1% relative uncertainty. Removing Cepheids or TRGB changed the central estimate little; replacing Type Ia supernovae with galaxy indicators also kept it close while increasing the uncertainty. These checks reduce dependence on a single branch. They do not make the baseline dataset unquestionable, or independent of the earlier measurements used to build the network.

How Lenses Measure H₀ Without the Stellar Ladder

A lensing galaxy’s gravity can produce multiple images of a distant variable quasar. A change in source brightness appears in those images at different times because light follows different paths through the gravitational field. Measuring the delays, redshifts and lens mass distribution yields distances sensitive to H₀. The method therefore combines light curves, images and stellar motions in the lensing galaxy.

The October 10, 2026 TDCOSMO preprint obtained 71.1 with an upper uncertainty of 2.4 and lower uncertainty of 2.3 km/s/Mpc from 11 systems. Its primary analysis assumes flat ΛCDM and uses matter-density constraints from DES-Dovekie. It accounts for the mass-distribution ambiguity known as the mass-sheet degeneracy. Independence from the local ladder does not mean an absence of assumptions: lens models and the chosen cosmology remain part of the inference.

What Gravitational-Wave Standard Sirens Add

The waveform and amplitude from merging compact objects allow distance to be estimated within a theory of gravity. If an electromagnetic counterpart identifies a host galaxy, its redshift can be added to the distance estimate. GW170817 was used this way. Gravitational waves provide a different distance scale, rather than a sound recording of space: “siren” is an analogy with a standard candle.

For most mergers, the exact host galaxy is unknown. Researchers then use catalogues of possible hosts or statistical features of source masses, accounting for detector selection. In the August 4, 2026 version of the GWTC-5 analysis, 236 sources yielded 71.7 with a +9.4 and −7.5 km/s/Mpc interval at the 68% level. That uncertainty still spans both familiar ranges. A central value near 71 is not a vote for a compromise: precision is insufficient, and inclination, population and redshift uncertainties must remain in the analysis.

Why a Sigma Count Does Not Identify the Cause

Significance refers to particular results, uncertainty distributions and a model. For example, H₀DN compared its network with combined Planck, SPT and ACT data under flat ΛCDM and reported 7.1 sigma. Its comparison with BBN and DESI DR2 BAO gave 5.0 sigma. These are different tests; there is no single immutable “Hubble tension sigma”, and a new paper cannot inherit another comparison’s significance.

High statistical significance means the selected estimates are difficult to reconcile under the stated conditions. It does not establish the probability that a new particle or theory is true. A systematic effect can bias many objects similarly and does not vanish merely by enlarging a sample. Researchers therefore test photometry, dust, stellar properties, galaxy motions and lens mass distributions. “Systematics” refers to methodological biases, rather than random scatter about a mean.

Could New Physics Be the Answer?

Proposed explanations include changes to early-Universe physics: an additional energy component or different particle properties or interactions. These can alter the scale connecting the microwave background with late-time distances. Improving the fit to H₀ alone is insufficient, however. A new model must also survive other cosmological tests and make testable predictions distinct from the standard description.

Dark energy and its possible evolution are related but separate topics. “What is H₀ today?” and “How has accelerated expansion evolved?” are connected through cosmological equations, yet are not identical questions. An unusual H₀ estimate alone neither establishes evolving dark energy nor overturns a hot early Universe. Keeping these claims separate matters especially when headlines combine several cosmological puzzles under the word “crisis”.

Which Observations Can Separate a Model Problem from Bias?

Tests are especially valuable when they change a method’s vulnerable component: new geometric anchors, different stellar populations, independent lenses and sirens with securely identified hosts. Agreement between routes with different error sources would be stronger evidence than an even more precise repetition of one chain. If distinct answers persist, researchers must locate the physical or calibration step where they diverge.

A new measurement is best read with its full specification: the dataset and version, distance indicator, assumed model and uncertainty interval. For the October 2026 lenses, preprint status matters; for sirens, the current version and broad uncertainty; for the network, shared calibrations. The Hubble tension remains a testable problem of reconciling observations. It makes cosmology more interesting without establishing whichever scenario a reader might prefer.