The universe is expanding about 10% faster than it should be, and a fresh set of measurements just made the problem harder to explain away.
The TDCOSMO collaboration, an international team of astrophysicists, has analyzed light from eight distant quasars warped by the gravity of galaxies sitting between them and Earth. Their result, posted as a preprint in June 2025, puts the Hubble constant at roughly 74 kilometers per second per megaparsec. That figure lands squarely in the range favored by other measurements of the nearby universe and well above the 67.4 km/s/Mpc predicted by observations of the cosmic microwave background, the faint afterglow of the Big Bang mapped by the European Space Agency’s Planck satellite.
The gap between those two numbers is known as the Hubble tension, and it has become one of the most consequential puzzles in modern physics. If neither camp’s measurements are wrong, something fundamental may be missing from the standard model of cosmology.
Why this measurement matters
Most previous estimates of the expansion rate relied on the cosmic distance ladder, a chain of calibrations that starts with nearby Cepheid variable stars and extends outward through Type Ia supernovae. Each link in that chain carries its own uncertainties, and skeptics have long argued that a subtle calibration error could be inflating the result.
Gravitational lensing time delays offer a completely different route to the same answer. When a quasar’s light passes close to a massive foreground galaxy, general relativity bends it along multiple paths. Each path has a slightly different length, so the quasar’s flickering arrives at Earth at slightly different times. Those tiny delays, sometimes just days apart, encode the geometry of the cosmos and, with it, the expansion rate. Because the method depends on gravity and geometry rather than stellar physics, it sidesteps the calibration issues that dog the distance ladder.
The fact that two independent approaches now converge on a higher Hubble constant makes it significantly harder to blame the discrepancy on a single measurement flaw.
Converging lines of evidence
TDCOSMO’s result does not stand alone. The SN H0pe team used the James Webb Space Telescope to observe a multiply imaged Type Ia supernova, deriving the expansion rate through detailed cluster modeling of the lensing galaxy cluster. That study marked the first time a lensed Type Ia supernova discovered by JWST was used for this purpose, and its result also pointed toward the higher end of the scale.
Earlier work strengthened the distance ladder itself. A joint analysis using both the Hubble Space Telescope and JWST, led by Nobel laureate Adam Riess and colleagues and published in the Astrophysical Journal Letters, cross-checked Cepheid distances measured by the two observatories. The telescopes agreed, reducing the chance that the tension stems from systematic errors in Cepheid photometry. That team’s headline number, 73.0 km/s/Mpc with an uncertainty of about 1.0, has anchored the late-universe side of the debate for several years.
On the other side, Planck’s measurement of 67.4 ± 0.5 km/s/Mpc, derived by fitting the cosmic microwave background to the Lambda-CDM cosmological model, remains the early-universe benchmark. No updated Planck analysis incorporating the 2025 lensing data has been released. The statistical tension between the two camps now exceeds five sigma by some estimates, a threshold physicists typically treat as strong evidence that the disagreement is not a fluke.
Three explanations, no consensus
Cosmologists broadly group possible explanations into three categories, and none has won the argument.
Hidden systematic errors. Even with multiple methods converging, each carries assumptions that could skew results. For lensing time delays, the mass distribution of the foreground galaxy must be modeled precisely. Different assumptions about a galaxy’s density profile can shift the inferred Hubble constant by several percent. The TDCOSMO team tested multiple lens models to address this, but their 2025 analysis has not yet completed formal peer review.
New physics beyond the standard model. If Lambda-CDM is incomplete, the early-universe prediction could be systematically off. Proposals include “early dark energy,” a brief burst of accelerated expansion shortly after the Big Bang that would raise the predicted Hubble constant, and time-varying dark energy that alters the expansion history after the cosmic microwave background formed. Each proposal, however, introduces its own conflicts with other cosmological datasets, and none has gained broad support.
Statistical convergence over time. It remains possible, though increasingly unlikely, that the tension will narrow as datasets grow. Future observatories are poised to test this. The Vera C. Rubin Observatory, expected to begin its main survey in 2025, will catalog billions of galaxies and discover thousands of supernovae, providing independent distance-ladder checks. The European Space Agency’s Euclid mission, already collecting data, will map the geometry of the universe over the past 10 billion years. Both could either confirm or erode the tension within the next few years.
What comes next for the expansion-rate puzzle
As of May 2026, the Hubble tension is not resolved, but the landscape of evidence has shifted. A measurement technique that bypasses the distance ladder entirely now agrees with the techniques that use it, and the combined weight of late-universe observations sits stubbornly above the early-universe prediction.
The TDCOSMO preprint still awaits peer review, and the collaboration has not released on-the-record statements endorsing a specific theoretical resolution. That caution is appropriate. Extraordinary claims about rewriting physics demand extraordinary scrutiny, and the full lens-model details from the eight-quasar sample will face intense examination once published in a journal.
Still, the direction of travel is clear. Each new independent measurement that lands near 73 or 74 km/s/Mpc tightens the case that the mismatch is real. If Rubin and Euclid confirm the pattern, physicists will face a stark choice: find the hidden error that has eluded a decade of searching, or accept that the universe operates by rules the standard model does not fully capture. Either outcome would reshape our understanding of the cosmos.
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*This article was researched with the help of AI, with human editors creating the final content.
