8 Minutes
New measurements using gravitational lensing reinforce a growing discrepancy in how fast the universe is expanding. By timing light that takes multiple routes around massive galaxies, astronomers are testing the Hubble constant with a method that avoids traditional distance ladders — and the results deepen the gap between early- and late-universe estimates.
Timing delays in quasar light bent by massive galaxies offer a fresh way to measure the universe’s expansion, and the results deepen the rift between early- and late-universe estimates. This growing tension may be a clue that something fundamental about cosmic physics is still undiscovered.
A montage of eight time-delay gravitational lens systems. There’s an entire galaxy at the center of each image, and the bright points in rings around them are gravitationally lensed images of quasars behind the galaxy. These images are false-color and are composites of data from different telescopes and instruments.
A new lens on cosmic expansion
For decades cosmologists have measured the universe's expansion rate — the Hubble constant (H0) — using multiple independent techniques. Nearby measurements based on Cepheid stars and Type Ia supernovae favor a value near 73 kilometers per second per megaparsec (km/s/Mpc). Observations of the cosmic microwave background (CMB), relic radiation from the early universe, prefer a lower value close to 67 km/s/Mpc. This mismatch, the so-called Hubble tension, is now one of the most pressing puzzles in cosmology.
To avoid possible shared systematics in distance-ladder methods, researchers at the University of Tokyo and international collaborators have applied time-delay cosmography — a gravitational-lensing technique that measures arrival-time differences of light from background quasars — to re-evaluate H0. Their latest analysis of eight time-delay lens systems finds a present-day expansion rate more consistent with the higher, local measurements and less consistent with early-universe estimates inferred from the CMB.
How time-delay cosmography measures H0
Gravitational lensing occurs when the gravity of a foreground galaxy bends light from a bright, distant source such as a quasar. If the geometry and mass distribution are favorable, observers see multiple images of the same background object. Because each image corresponds to a different light path — and therefore a different travel time — intrinsic brightness variations in the background source appear in each image with measurable delays.
Time-delay cosmography links these arrival-time differences to the absolute scale of the universe. A longer or shorter measured delay implies a different overall expansion rate when the lens geometry and mass profile are correctly modeled. Importantly, this method does not rely on intermediate distance indicators like Cepheids or supernovae, offering an independent probe of H0.
The University of Tokyo team, led by Project Assistant Professor Kenneth Wong and postdoctoral researcher Eric Paic, combined precise time-delay measurements with high-resolution imaging and spectroscopy from modern observatories, including data from the James Webb Space Telescope and complementary ground-based telescopes. Modeling the lens mass distribution and accounting for mass along the line of sight allowed them to infer a value of H0 that aligns with other late-universe probes.
Why the result matters: evidence for a real discrepancy
Finding consistency across multiple late-universe measurements strengthens the case that the Hubble tension reflects more than measurement error. "Our measurement of the Hubble constant is more consistent with other current-day observations and less consistent with early-universe measurements," the research team reports. Because time-delay cosmography is independent of both traditional distance ladders and CMB-derived analysis, it helps isolate whether the tension arises from unknown systematics or points to new physics.
If the tension persists as measurements grow more precise, it could indicate modifications to the standard cosmological model (Lambda Cold Dark Matter, or LCDM). Possible explanations discussed in the community include early dark energy, nonstandard neutrino properties, or subtle departures from general relativity on cosmological scales. Each proposed solution has different observational signatures, so independent probes such as lensing time delays are essential to discriminate among them.
Limitations and the path to precision
Despite its promise, time-delay cosmography has challenges. One of the largest sources of uncertainty is modeling the mass distribution of the lensing galaxy. Astronomers typically adopt parameterized mass profiles that fit available data, but if the true mass distribution departs from assumed forms — or if undetected structures along the line of sight contribute extra lensing — the inferred H0 can shift.
In the current study the team analyzed eight lens systems and reached about 4.5% precision on H0. To decisively adjudicate the Hubble tension, they estimate a target precision near 1–2% is necessary. Achieving that goal requires a larger sample of well-characterized lenses, deeper and higher-resolution imaging, improved spectroscopy to map stellar dynamics, and rigorous treatment of line-of-sight mass structures.
Ongoing and upcoming surveys — including wide-field optical surveys and targeted follow-up with space telescopes like JWST and the Roman Space Telescope — will expand the sample of time-delay lenses by discovering new systems and refining measurements for known ones. The international collaboration model exemplified by TDCOSMO (the Time Delay COSMOgraphy collaboration) will remain critical, combining resources, telescopes, and modeling expertise across institutions.
Related technologies and observational advances
Advances in adaptive optics, integral-field spectroscopy, and high-contrast imaging have tightened constraints on lens mass profiles and host galaxy structure. Computational improvements — such as flexible Bayesian modeling frameworks and machine-learning tools for lens detection and mass reconstruction — accelerate analyses and quantify uncertainties more robustly.
Meanwhile, multiwavelength observations help separate contributions from stars, dark matter, and baryonic structures in lens galaxies. By cross-checking lens models against stellar kinematics and population synthesis, astronomers can reduce degeneracies that currently limit precision.
Expert Insight
"Time-delay cosmography offers one of the cleanest independent checks on the Hubble constant," says Dr. Maya Patel, an astrophysicist at the Institute for Cosmology who is not part of the University of Tokyo team. "If future samples and improved models keep pointing to a higher H0, we will have to take seriously the possibility that our cosmological model is missing an ingredient. That would be an exciting opportunity — it would mean new physics is within reach of observation."
Expanding the sample and international collaboration
The study underlines a simple truth in modern cosmology: resolving foundational disagreements requires both precision and redundancy. Increasing the number of suitable lens systems from eight to dozens — and ultimately to hundreds — will lower statistical errors. Equally important is the independent replication of results by multiple teams using distinct modeling choices and data sets to expose hidden systematic effects.
Wong and Paic emphasize that their work focused on refining methodology; improving hardware, observation strategies, and community-wide modeling standards will be needed to drive uncertainties down to the 1–2% range. As lens discoveries ramp up in the coming years, time-delay cosmography will be a cornerstone technique for testing whether the Hubble tension is a doorway to new physics or a challenge solvable within existing cosmological theory.
Conclusion
Gravitational lensing time delays are maturing into a powerful, independent probe of the universe's expansion. The latest results strengthen the case that the Hubble tension is not merely a fluke of measurement, but a real discrepancy that could reveal physics beyond the standard model. With improved instruments, larger lens samples, and deeper international cooperation, cosmologists are preparing for a decisive era that may reshape our understanding of cosmic history.
Source: scitechdaily
Comments
Reza
Pretty cool progress, 4.5% is impressive. Need hundreds of lenses though, and standardization, fast...
datapulse
Is the mass-modeling uncertain enough to fake this? sounds fishy, but JWST data helps... curious how robust
astroset
Wait, so lensing keeps siding with the local H0? Mind blown. If true, new physics incoming... wow
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