A Speed Camera for the Universe
hubie writes:
Researchers exploit gravitational lensing to see how fast the universe is really expanding:
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.
Included are some cool pics of the lensed quasars used in the study.
Journal Reference:Simon Birrer, Elizabeth J. Buckley-Geer, Michele Cappellari, et al., "TDCOSMO 2025: Cosmological constraints from strong lensing time delays," Astronomy and Astrophysics: November 25, 2025, doi:10.1051/0004-6361/202555801.
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