The Hubble Tension is one of the great mysteries of cosmology. Solving it might require a fundamental change in how we understand the universe - but scientists have to prove it actually exists first. A new paper from a collective of cosmologist researchers known as the TDCOSMO Collaboration adds further fuel to that first with updated measurements of the “Late Universe” measurement of the Hubble Constant using gravitational lenses of quasars, which shows that the Tension might exist after all.
So what is the Hubble Constant, and why is there a “tension”? Cosmologists agree that the universe is expanding, and appears to be doing so at an increasing rate. They know this because, the farther away (and therefore farther back) you look in the universe, objects seem to be moving away from us at an increasing rate. However, depending on what method you use to find that rate, there are two different answers, which differ by about 10%.
One technique is to look using “Late Universe” objects, such as quasars that we can actively see with current telescopes. This gives a measurement of around 73 kilometers per second per megaparsec (about 3.3 million light years). In other words, for every megaparsec between us and an object, the speed at which it's moving away from us increases by 73 kilometers per second.
Fraser discusses potential solutions to the Hubble Tension.The other technique is to look at the “Early Universe” - primarily the Cosmic Microwave Background, which is some of the earliest light from the Universe. When measured using this technique, the value appears to be about 67 km per second per megaparsec.
That 6 km/s difference may not seem like much on a cosmological scale, but it has persisted through all attempts over past decades to reconcile it. And that likely means there’s something wrong with how we understand at least one of those measurements. Since they are based on our current standard model of the universe, that means that our standard model is wrong, and we need a new one. Hence the name the “Hubble tension”.
Several research groups have suggested that there is no actual Hubble tension, and the discrepancy can be explained by errors in the data collection methods in one or the other of the methods. But this paper provides two nails in the coffin of that argument. First, it uses a different technique than the standard “distance ladder” traditionally used to measure Late Universe objects. Second, it uses some of the most modern data available for its calculations, and strongly aligns with the Late Universe values previously found, softening some of the argument that the tension only exists because of signal inaccuracies.
Fraser discusses gravitational lensing, the technique used in the paper to calculate the Hubble Constant in a novel way.Traditional methods of establishing the Late Universe method have relied on what cosmologists call the “local distance ladder”, where they measure distances to increasingly distant objects whose properties are thought to be well known, such as supernovae and Cepheid variables, which have a well known luminosity based on their spin rate. Using those values and searching throughout the observable universe for supernovae will baseline how bright a supernova should be, and what its spectral profile should look like. Calculating the redshift from those supernovae gives a value for the Hubble Constant that typically aligns with the Late Universe side.
The paper describes a technique completely separate from that method, though. They use a set of eight gravitational lenses that have a large galaxy passing in front of a quasar farther away from us in time. This creates a gravitational lens of the quasar - essentially recreating it in multiple separate “images”, each of which takes a slightly different path to get to Earth. The cosmologists could then measure differences in those images when a specific event happened to the quasar. By using the properties of the lensing galaxy, they could then estimate the different paths the light took to get to us - and therefore have an estimate of how the expansion of the universe is affecting its path.
A critical feature of the paper is the use of highly accurate measurements of the movement of stars in the foreground galaxy to eliminate what cosmologists call the Mass-Sheet Degeneracy (MSD). MSD is used to describe how hard it is to differentiate the physical properties of a lensing galaxy from a “mass sheet” of just generic mass without any intricacies of a galaxy, which can dramatically mess up the calculation of the Hubble constant. Accounting for accurate “stellar kinematics” in the lensing galaxies allows the researchers to accurately interpret their weight, as the faster the stars within them are moving, the larger their mass must be. Advanced telescopes like the James Webb Space Telescope (JWST) are critical enabling factors for those stellar kinematic equations, as previous data sets had been unable to resolve individual star movements in these massive, far-away galaxies.
NASA video that goes into detail about the Hubble tension. Credit - NASA Goddard YouTube ChannelAfter all was said and done, and the researchers calculated their new number for the Hubble constant, it came out as 74.3 km/s/Mpc - almost exactly in line with Late Universe calculations. So even with a completely new method, and updated, more accurate data, they proved that the Late Universe measurements are indeed accurate, for their way of measuring. And, importantly, that they are statistically different from Early Universe measurements - therefore the Hubble Tension is real.
While this work doesn’t offer any solution to it, it does offer further validation that it exists, and disproves some of the arguments that it is just an data artifact that would be resolved with better methods of data collection. Even using data from our most powerful telescopes, and a completely different calculation method, still shows there’s something fishy in cosmology. We just need some testable hypotheses, and likely a whole bunch more data collection time, to figure it out.
Learn More:
University of Tokyo / EurekaAlert - A speed camera for the universe
TDCOSMO Collaboration et al. - TDCOSMO 2025: Cosmological constraints from strong lensing time delays
UT - If The Supernova Standard Candle Is Wrong, It Could Solve The Hubble Tension
