The universe is expanding, and after decades of work, astronomers still cannot agree on how fast. That disagreement, known as the Hubble tension, has only grown sharper in recent months as new data from the James Webb Space Telescope and the largest-ever 3D map of the cosmos have landed on opposite sides of the divide. As of spring 2025, neither camp has found a flaw in the other’s methods, raising the real possibility that something fundamental is missing from our picture of the cosmos.
At the heart of the dispute is a single number: the Hubble constant, or H0, which describes the present-day expansion rate of the universe. Teams that measure it by studying stars and exploding supernovae in nearby galaxies consistently get values near 70 to 73 kilometers per second per megaparsec (km/s/Mpc). Teams that infer it from the faint afterglow of the Big Bang and the large-scale clustering of galaxies favor something closer to 67 km/s/Mpc. That gap of roughly 5 to 9 percent may sound small, but it has persisted through every cross-check and calibration improvement thrown at it, and it now exceeds the threshold where statisticians start taking notice.
JWST closes the door on a leading explanation
One of the most persistent criticisms of local Hubble constant measurements has targeted Cepheid variable stars, the pulsating beacons astronomers use as cosmic yardsticks. In crowded stellar fields, light from neighboring stars can blend with a Cepheid’s signal, potentially inflating its apparent brightness and throwing off distance calculations. If that bias were large enough, it could account for the higher local H0 values.
A preprint submitted to The Astrophysical Journal in May 2025 directly tests that concern. Using JWST’s sharper infrared vision to image the galaxy pair NGC 3447/3447A, which hosted a Type Ia supernova and sits in an unusually clean stellar environment, the study performs a three-way comparison of Cepheid brightness measurements from the Hubble Space Telescope, JWST, and a background-free reference field. The offsets came in smaller than 0.03 magnitudes, with scatter of roughly 0.12 magnitudes. Those numbers are too small to rescue the early-universe value. For systems with similar stellar densities, crowding is effectively ruled out as the culprit behind the tension.
“This has been the go-to escape hatch for people who thought the local measurements were wrong,” said Adam Riess, the Nobel laureate at Johns Hopkins University whose SH0ES team has led Cepheid-based H0 measurements for over a decade. With JWST now confirming Hubble Space Telescope photometry at this level of precision, that escape hatch is closing.
Independent distance markers tell a consistent story
Cepheids are not the only tool in the local toolkit. The Chicago-Carnegie Hubble Program, an independent effort that relies on JWST observations of older, redder stars rather than Cepheids, has been building its own distance ladder using the tip of the red giant branch (TRGB) and the J-region asymptotic giant branch (JAGB) as distance indicators. Anchored to a geometric distance measurement in the galaxy NGC 4258 and calibrated through Type Ia supernova host galaxies, the program reports a TRGB-based best estimate of H0 at approximately 70.4 km/s/Mpc, with combined uncertainties under 2 km/s/Mpc, as detailed in an independent analysis.
A separate calibration update from the same program, covering 11 supernova host galaxies, found that JWST-based TRGB distances agree with prior Hubble Space Telescope TRGB distances to within about 1 percent on average. Depending on which supernova light-curve fitting method is used, that update places H0 between roughly 68.4 and 69.6 km/s/Mpc, as reported in a March 2025 preprint. Both estimates sit above the early-universe value, but the spread between them highlights a subtlety that matters: the way astronomers standardize supernova brightness can shift the inferred expansion rate by more than a kilometer per second per megaparsec, comparable to the quoted statistical errors.
The early-universe side holds firm
If local measurements are converging on a higher number, the early-universe side is not budging either. The Dark Energy Spectroscopic Instrument (DESI) collaboration, operating from a mountaintop in Arizona, released its first-year baryon acoustic oscillation (BAO) results using millions of galaxies, quasars, and hydrogen gas clouds to map the universe’s expansion history at percent-level precision. BAO measurements work by detecting a characteristic spacing in the distribution of galaxies, a relic of sound waves that rippled through the infant universe. That spacing acts as a standard ruler, and when combined with data from the European Space Agency’s Planck satellite, which mapped the cosmic microwave background (the oldest light in the universe), it consistently points toward H0 near 67.4 km/s/Mpc.
The DESI results, published in the Journal of Cosmology and Astroparticle Physics with a companion systematic-error analysis in Monthly Notices of the Royal Astronomical Society, show that modeling uncertainties and data-processing choices are well controlled. There is little room in the BAO pipeline to hide an 8 percent shift in H0.
A separate study brings the tension closer to home. By extending the distance ladder to the Coma Cluster using 12 Type Ia supernovae and comparing those distances against DESI-calibrated Fundamental Plane measurements, researchers report a Coma distance of approximately 98.5 plus or minus 2.2 megaparsecs. When paired with the cluster’s recession velocity, that distance implies an expansion rate closer to the locally measured value than to the one derived from Planck and DESI. The result is not yet definitive, since the gravitational pull of surrounding large-scale structure can shift a cluster’s apparent velocity by enough to blur the comparison, but it adds another data point on the local side of the ledger.
A third path through gravitational lensing
Both the local distance ladder and the early-universe approach carry their own webs of assumptions. That is why a third, fully independent method has drawn increasing attention. Strong gravitational lensing, where a massive foreground galaxy bends and splits light from a distant quasar into multiple images, can yield a Hubble constant measurement without relying on any star-based distance indicator or on the cosmic microwave background.
The TDCOSMO collaboration has presented a full analysis of the doubly lensed quasar HE1104-1805, combining time delays between the quasar images, high-resolution lens models, corrections for matter along the line of sight, and the motion of stars within the lensing galaxy itself. The result is still a preprint and has not yet passed peer review, and the collaboration’s treatment of the lens galaxy’s mass profile and external convergence remains under active scrutiny. No published analysis yet combines the TDCOSMO lensing result with the JWST-era TRGB calibrations, so whether these independent paths converge on the same number is an open question.
Why the gap matters beyond astronomy
The Hubble constant is not just an abstract number. It sets the age of the universe, the distances to everything we observe, and the amount of dark energy required to explain the acceleration of cosmic expansion. If the local value near 73 km/s/Mpc is correct and the early-universe physics is also correct, then something is missing from the standard cosmological model, the framework that has successfully described the universe for over two decades.
Proposed explanations range from the conservative to the exotic. On the cautious end, subtle population differences among Type Ia supernovae or unrecognized correlations between supernova brightness and host-galaxy properties could shift local H0 estimates downward by a few km/s/Mpc. On the more radical end, theorists have proposed extra species of neutrino-like particles in the early universe, time-varying dark energy, or modifications to gravity that would shrink the sound horizon used to calibrate BAO measurements, pushing the early-universe H0 upward. So far, none of these proposals has gained consensus support, and most introduce new tensions with other observations.
Where the tension goes from here
Progress in the near term will likely come from tightening internal consistency within each method and from cross-checks that bridge the redshift gap between local and early-universe probes. Expanded JWST campaigns targeting additional supernova host galaxies should clarify whether the remaining spread among local H0 estimates (roughly 68 to 73 km/s/Mpc, depending on method and calibration choices) reflects genuine systematics or statistical scatter. DESI’s upcoming data releases, drawing on several more years of observations, will shrink BAO error bars further and stress-test the current analysis pipeline.
More strong-lensing systems with high-quality time delays and kinematic data could eventually turn gravitational lensing into a decisive tiebreaker. And the Vera C. Rubin Observatory, expected to begin its main survey in 2025, will discover thousands of new Type Ia supernovae per year, offering a statistical lever that previous surveys could not match.
For now, the universe’s exact expansion rate remains uncertain at the few-percent level. That sliver of disagreement between two of the most precise measurements in all of physics is either the signature of an undetected systematic error or a crack in the standard model of cosmology. Either outcome would reshape how we understand the universe. The data keep arriving, and so far, neither side is blinking.
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*This article was researched with the help of AI, with human editors creating the final content.
