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| The stars (or rather galaxies) of the show. 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. Image Credit: ©2025 TDCOSMO Collaboration et al. (CC BY-ND 4.0) |
There is an important and unresolved tension in cosmology regarding the rate at which the universe is expanding, and resolving this could reveal new physics. Astronomers constantly seek new ways to measure this expansion in case there may be unknown errors in data from conventional markers such as supernovae. Recently, researchers including those from the University of Tokyo measured the expansion of the universe using novel techniques and new data from the latest telescopes. Their method exploits the way light from extremely distant objects takes multiple pathways to get to us. Differences in these pathways help improve models on what happens at the largest cosmological scales, including expansion.
The universe is big, and it’s getting bigger. How big is it? We don’t really know. But we do know how fast it’s expanding. It’s not a simple matter, however, as the expansion appears faster the farther away we observe. For every 3.3 million light years (or one megaparsec) of distance from us, we see things at that distance running away from us at increasing multiples of about 73 kilometers per second. In other words, the rate of expansion of the universe is 73 kilometers per second per megaparsec (km/s/Mpc), also known as the Hubble constant.
There are different ways to ascertain the Hubble constant, but until now, all have relied on so-called distance ladders. These are things like supernovae or special stars called Cepheid variable stars, both of which are thought to be well understood enough such that their presence even in other galaxies ought to afford us accurate measurements about them, including their distances. By observing enough of these over the decades, the Hubble constant has been increasingly constrained. But there has always been a degree of doubt about this method, so cosmologists welcome improvements. In their latest paper, a team of astronomers including Project Assistant Professor Kenneth Wong and postdoctoral researcher Eric Paic from the University of Tokyo’s Research Center for the Early Universe successfully demonstrated a method known as time-delay cosmography that they believe can mitigate the reliance on distance ladders and ought to have offshoots in other areas of cosmology as well.
“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.”
You may wonder why researchers go through such effort just to find a number they already know. That has to do with something critical to understanding the history of the universe, which at present remains unresolved. That value of 73 km/s/Mpc for the Hubble constant is correct based on observations of nearby objects, but there are other ways of measuring the rate of cosmic expansion which can also look at data from the distant past, in particular the radiation which permeates the universe resulting from the big bang, otherwise known as the cosmic microwave background (CMB). When researchers use the CMB to calculate the Hubble constant, they get a lower value of 67 km/s/Mpc. This discrepancy is called the Hubble tension, and the work of Wong, Paic and their collaborators helps to clarify the nature of it, as there is still some doubt as to whether it might be anything more than the result of experimental error.
“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%.”
The team is confident such gains in precision are possible. The current study used eight time-delay lens systems, each occluding a distant quasar (a supermassive black hole that is accreting gas and dust, causing it to shine brightly), and new data from the latest space-based and ground-based telescopes, including the James Webb Space Telescope. The team intends to increase the sample size, as well as improve various other measurements and rule out any systematic errors as yet unaccounted for.
“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.”
Published in journal: Astronomy and Astrophysics
Title: TDCOSMO 2025: Cosmological constraints from strong lensing time delays
Authors: 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
Source/Credit: University of Tokyo
Reference Number: astr120525_01