Astronomers have used time-delay cosmography on gravitationally lensed quasars to measure the universe's expansion rate, finding a value that aligns with local observations but conflicts with early-universe estimates. This result, from a team including University of Tokyo researchers, bolsters the Hubble tension and suggests possible new physics at play. The study analyzed eight lens systems with data from advanced telescopes like the James Webb Space Telescope.
Cosmologists continue to puzzle over the Hubble tension, a discrepancy in measurements of the universe's expansion rate. Traditional methods, such as those using supernovae and Cepheid variable stars in distance ladders, yield a Hubble constant of about 73 kilometers per second per megaparsec (km/s/Mpc) based on nearby objects. In contrast, analyses of the cosmic microwave background radiation from the early universe give a lower value of 67 km/s/Mpc.
A recent study led by researchers from the University of Tokyo's Research Center for the Early Universe, including Project Assistant Professor Kenneth Wong and postdoctoral researcher Eric Paic, introduces an independent approach: time-delay cosmography. This technique exploits gravitational lensing, where massive galaxies bend light from distant quasars, creating multiple images that arrive at Earth with slight time delays.
"To measure the Hubble constant using time-delay cosmography, you need a really massive galaxy that can act as a lens," Wong explained. "The gravity of this 'lens' deflects light from objects hiding behind it around itself, so we see a distorted version of them. ... 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 team's analysis of eight such systems produced a Hubble constant consistent with the 73 km/s/Mpc local value, with a precision of about 4.5%. "Our measurement of the Hubble constant is more consistent with other current-day observations and less consistent with early-universe measurements," Wong noted. "This is evidence that the Hubble tension may indeed arise from real physics and not just some unknown source of error."
Paic emphasized the need for improvement: "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. ... 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%."
Challenges remain, particularly in modeling the mass distribution within lens galaxies, which introduces uncertainty. The researchers plan to expand their dataset and refine techniques using observations from space- and ground-based telescopes to resolve the tension, potentially unveiling new cosmological insights.