New measurements using gravitational lensing suggest the universe’s current expansion rate does not agree with signals from the early cosmos.
Cosmologists are facing a major unresolved issue: the measured speed at which the universe expands does not agree across different methods, and solving this conflict may reveal new physics. To guard against hidden errors in traditional measurements that rely on supernovae and other distance markers, astronomers continue to search for alternative techniques.
In recent work, researchers including a team from the University of Tokyo examined the universe’s expansion by applying new observational strategies and data from cutting-edge telescopes. Their approach relies on the fact that light from extremely distant objects can reach Earth along several routes. Studying how these paths differ helps scientists refine large-scale models of how the universe behaves and grows.
Measuring the Expansion of Space
We know the universe is immense and still enlarging, even if its full size remains uncertain. Its rate of expansion can be estimated, although the process is not straightforward. The farther astronomers look, the faster distant objects appear to be moving away.
For every 3.3 million light-years (or one megaparsec) from Earth, objects at that distance seem to recede at roughly 73 kilometers per second. This rate of 73 kilometers per second per megaparsec (km/s/Mpc) is known as the Hubble constant.
Traditional Distance Ladders and a New Technique
Astronomers have relied on several approaches to calculate the Hubble constant, but all of them until now have depended on what are known as distance ladders. These ladders are built from familiar objects such as supernovae and Cepheid variable stars, which are considered consistent enough across galaxies to serve as reliable markers. By studying large numbers of these objects over many years, researchers have narrowed the possible values for the Hubble constant. Even so, some uncertainty has persisted, which is why scientists are eager for more independent techniques.
A recent study led by Project Assistant Professor Kenneth Wong and postdoctoral researcher Eric Paic of the University of Tokyo’s Research Center for the Early Universe presents such a technique, called time-delay cosmography. The team showed that this method can reduce the field’s dependence on distance ladders and may benefit other areas of cosmology as well.
Using Gravitational Lensing to Track Cosmic Expansion
“To measure the Hubble constant using time-delay cosmography, you need a really massive galaxy that can act as a lens,” said Wong. “The gravity of this ‘lens’ deflects light from objects hiding behind it around itself, so we see a distorted version of them. This is called gravitational lensing. If the circumstances are right, we’ll actually see multiple distorted images, and each will have taken a slightly different pathway to get to us, taking different amounts of time.
“By looking for identical changes in these images that are slightly out of step, we can measure the difference in time they took to reach us. Coupling this data with estimates on the distribution of the mass of the galactic lens that’s distorting them is what allows us to calculate the acceleration of distant objects more accurately. The Hubble constant we measure is well within the ranges supported by other modes of estimation.”
Why the Hubble Constant Remains a Problem
It may seem surprising that so much effort goes into refining a value that has been measured many times. However, this number plays a central role in how scientists interpret the evolution of the universe, and there is a longstanding conflict between measurement methods. The widely accepted value of 73 km/s/Mpc comes from observations of nearby galaxies. Another approach looks back to the early universe by analyzing the cosmic microwave background (CMB), the radiation left over from the big bang. When researchers calculate the Hubble constant using the CMB, they arrive at 67 km/s/Mpc.
This difference, known as the Hubble tension, has sparked debate about whether the discrepancy is caused by experimental limitations or signals something deeper. The work by Wong, Paic and their collaborators helps explore the roots of this tension at a time when a clear explanation is still lacking.
Evidence for a Real Physical Discrepancy
“Our measurement of the Hubble constant is more consistent with other current-day observations and less consistent with early-universe measurements. This is evidence that the Hubble tension may indeed arise from real physics and not just some unknown source of error in the various methods,” said Wong. “Our measurement is completely independent of other methods, both early- and late-universe, so if there are any systematic uncertainties in those methods, we should not be affected by them.”
“The main focus of this work was to improve our methodology, and now we need to increase the sample size to improve the precision and decisively settle the Hubble tension,” said Paic. “Right now, our precision is about 4.5%, and in order to really nail down the Hubble constant to a level that would definitively confirm the Hubble tension, we need to get to a precision of around 1-2%.”
Expanding the Sample and Improving Accuracy
The team is confident it can achieve that level of accuracy. In this study, they analyzed eight time-delay lens systems, each involving a foreground galaxy that hides a distant quasar (a supermassive black hole that is accreting gas and dust, causing it to shine brightly).
They combined this information with new observations from modern space-based and ground-based telescopes, including the James Webb Space Telescope. Future work will expand the number of lens systems, refine the measurements, and continue evaluating potential sources of error.
Understanding Mass Distribution and the Role of Collaboration
“One of the largest sources of uncertainty is the fact that we don’t know exactly how the mass in the lens galaxies is distributed. It is usually assumed that the mass follows some simple profile that is consistent with observations, but it is hard to be sure, and this uncertainty can directly influence the values we calculate,” said Wong.
“The Hubble tension matters, as it may point to a new era in cosmology revealing new physics. Our project is the result of a decades-long collaboration between multiple independent observatories and researchers, highlighting the importance of international collaboration in science.”
Reference: “TDCOSMO 2025: Cosmological constraints from strong lensing time delays” by Simon Birrer, Elizabeth J. Buckley-Geer, Michele Cappellari, Frédéric Courbin, Frédéric Dux, Christopher D. Fassnacht, Joshua A. Frieman, Aymeric Galan, Daniel Gilman, Xiang-Yu Huang, Shawn Knabel, Danial Langeroodi, Huan Lin, Martin Millon, Takahiro Morishita, Veronica Motta, Pritom Mozumdar, Eric Paic, Anowar J. Shajib, William Sheu, Dominique Sluse, Alessandro Sonnenfeld, Chiara Spiniello, Massimo Stiavelli, Sherry H. Suyu, Chin Yi Tan, Tommaso Treu, Lyne Van de Vyvere, Han Wang, Patrick Wells, Devon M. Williams and Kenneth C. Wong, 5 December 2025, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202555801
This work was supported by NASA (grants 80NSSC22K1294 and HST-AR-16149), the Max Planck Society (Max Planck Fellowship), the Deutsche Forschungsgemeinschaft under Germany’s Excellence Strategy (EXC-2094, 390783311), the U.S. National Science Foundation (grants NSF-AST-1906976, NSF-AST-1836016, NSF-AST-2407277), the Moore Foundation (grant 8548), and JSPS KAKENHI (grant numbers JP20K14511, JP24K07089, JP24H00221).
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11 Comments
As a senior lay American male discoverer/investigator since 2009, I have been demonstrating and explaining a discrepancy in the Hubble constant for more than a decade; the misinterpretation of the true nature of gravity. My finding in 2009 was that gravity is induced by some as yet unidentified higher force to radiate out across the universe from all mass in expanding spheres of cohering pulsing angular lines of gravity force, in accordance with the inverse-square law of attraction. Also, gravity lensing supports that photons are affected by individual lines of gravity force and accelerate (blueshift) on expanding lines of gravity force when emitted by their sources and decelerate (redshift) on contracting lines of gravity force when arriving to earth. So, it stands to reason for me that photons from cosmic objects farther from earth will have greater redshift than those from closer to earth and scientists have yet to determine the age and/or acceleration/size of the universe.” I believe my most convincing demonstration is the one I uploaded last June, showing an invisible force causing start-stop rotation in two at first stopped aluminum (to rule out magnetism) disks: https://odysee.com/@charlesgshaver:d/5Gravity:c
There is no peer reviewed “demonstrations”, “explanations”, “discoveries”, “investigations” or “findings” of a CG Shaver in the scientific record. Your personal opinion has nothing to do with the science that the article describes.
Without gravity and redshift there would be no “Hubble tension.” Obviously, the “scientific record” is long overdue for some updating.
Funnily enough as a “lay” (programmer) senior (50+) curious individual from outside the cosmology area, intrigued by the new videos from presenters like Curt Jaimungal and Sabine Hossenfelder, my thoughts so far are also that gravitational effects will produce increasing redshift with distance, and the expansion/distance interpretation of redshift to be unnecessary. Distant time dilation : we already know one photon affects a subsequent (double slit single photon interference) so one photon redshifting and affecting the “terrain” of a subsequent is understood.
The cosmological redshift due to space expansion is observed in individual photons.
Please understand that “presenters” may not present the science at all, poorly and/or biased. For example, a short search finds a film maker Curt Jaimungal that discuss pseudoscientific (philosophy) or magic (religious) notions. And theoretical physicist Sabine Hossenfielder is well known for her diatribes against established science. I wouldn’t listen to any of those.
The cosmological redshift due to space expansion is observed in individual photons.
Please understand that “presenters” may not present the science at all, poorly and/or biased. I detailed your examples (film maker, biased scientist) but it got stuck in the comment filter. I wouldn’t listen to any of those.
As a layman doing independent research in theoretical physics, my opinion is that the present LCDM model of the universe is incorrect. What I propose is a Newtonian model of expanding universe,where expansion is due to the thermodynamic process of internal – energy changing into speed of individual units (gravitationaly bound super galaxy-clusters) of the universe. Here the acceleration of expansion is maximum in the beginning and zero at halfway, when the temperature will be approximately 1K.
This is essentially no better than Charles G. Shaver’s public record, with open Orcid and Academia.edu registrations of independent ‘research’ but no peer reviewed publicized research. As you correctly state, your personal opinion, and it has nothing to do with the science that the article describes.
In my ignorance, i wonder if this could be simply the effect of gravity.
Is it possible that the collective gravity of all the mass after the big bang held back rapid expansion? Then, as time increased the distance, the gravitational effect was lessened effectively accelerating particles, ie galaxies as they got farther from the core
The Friedmann equations, which describe say LCDM cosmology or in general give a theoretical description of the Hubble rate, are based in gravity (general relativity). That is why the article can point out that “new physics”, making distinguishing effects on gravity, is an alternative to measurement problems.
Not nearly good enough to be interesting: “Right now, our precision is about 4.5%, and in order to really nail down the Hubble constant to a level that would definitively confirm the Hubble tension, we need to get to a precision of around 1-2%.”