The Hubble tension persists, and gravitational lenses are adding fuel to the fire by showing that something still isn’t fitting together in our understanding of the cosmos.
Researchers have produced one of the most precise estimates yet for the current expansion rate of the universe, and the result reinforces the long-standing puzzle known as the Hubble tension. Different methods yield different values for this single quantity, and the latest measurement has just added another data point to the debate.
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Cosmologists agree that the universe is accelerating in its expansion. The Hubble constant quantifies this rate and can be determined in several ways. One route uses the cosmic microwave background (CMB), the afterglow of the Big Bang, while another relies on measuring distances to galaxies and how quickly they appear to be receding.
Both approaches have reached remarkable precision in the past decade, yet their results do not align within their uncertainties.
Among the alternatives explored to resolve this discrepancy is time-delay cosmography, which has just yielded a result that reinforces the impression that the numbers do not converge. This method rests on gravitational lensing: massive galaxies bend spacetime and warp the paths of light from more distant sources. The bending acts like a lens, magnifying and duplicating the distant light.
Hidden within these lensed images lies information about the true structure and scale of the universe. To extract it, researchers used some of the world’s most powerful telescopes, including the James Webb Space Telescope (JWST), the Keck Observatory, and the Very Large Telescope (VLT).
The Hubble tension is arguably the most consequential open question in cosmology today.
Professor Tommaso Treu, University of California, Los Angeles
“When a light source lies almost directly behind a massive foreground galaxy—the deflector—the observer can see multiple images of the source. The light from these images does not arrive simultaneously because the light paths differ. By measuring the delays between the images, we can infer the relative distances between us, the lens, and the source. From those distances, we reconstruct the expansion history of the universe and, ultimately, determine the Hubble constant,” explains co-author Professor Tommaso Treu.
Planck, the European Space Agency’s mission that mapped the CMB, previously pegged the Hubble constant at 67.4 kilometers per second per megaparsec, with one megaparsec equaling 3.26 million light-years. In plain terms, a pair of galaxies separated by 1 megaparsec would appear to recede from each other at about 67.4 km/s due to cosmic expansion.
A separate CMB analysis from another observatory, after about two decades of data collection, corroborated this tension and reinforced the mismatch with other measurements. When incorporating observations from Hubble and JWST, researchers arrived at a higher value around 72.8 km/s per megaparsec. The small uncertainties on each method do not overlap, underscoring a genuine discrepancy rather than statistical fluctuation.
Time-delay cosmography now yields a value of about 71.6 km/s per megaparsec, with an uncertainty of +3.9 and -3.3. This result aligns with the distance-based galaxy measurements but remains at odds with the CMB estimates. Although this method currently has larger uncertainties than the other two, the team plans to tighten its precision by analyzing more gravitational lenses with high-quality data for each system.
The team emphasizes that high-quality data are crucial for accurately tracing how stars move within the lensing galaxies, which in turn clarifies the light paths. A Keck observing campaign to study stellar motions in these lenses is already planned.
Resolving the Hubble tension is central to advancing our understanding of the universe. It remains possible that all measurements underestimate their uncertainties, and the methods could eventually converge. Alternatively, a fundamental flaw might lurk in our cosmological model.
“The Hubble tension is potentially the most important open issue in cosmology right now, and multiple independent measurements are needed to confirm whether it signals new physics or a systematic error,” Treu notes. Time-delay cosmography offers an independent check that can be especially valuable in this regard.
The study detailing these findings has been published in Astronomy & Astrophysics.
