Lensed Quasars Refine Hubble Constant Measurement to 4.4% Uncertainty.

Analyses of eight strongly lensed quasars, utilising data from the James Webb Space Telescope and others, constrain the Hubble constant to 73.2 ± 1.9 km s Mpc⁻¹ within a flat Cold Dark Matter model, achieving 4.4% precision. Results remain consistent across alternative cosmological models and independent datasets.

Precise measurements of the Universe’s expansion rate remain a central challenge in modern cosmology. A new analysis leverages the phenomenon of gravitational lensing – where light from distant quasars is bent and magnified by intervening galaxies – to refine these measurements. By carefully analysing the time delays between multiple images of these lensed quasars, researchers can infer distances and, consequently, the Hubble Constant – a key parameter describing the Universe’s expansion. This work, detailed in a paper titled ‘TDCOSMO 2025: Cosmological constraints from strong lensing time delays’, presents constraints derived from eight strongly lensed quasars, incorporating new data from the James Webb Space Telescope, Keck Telescopes, and the Very Large Telescope.

The research was conducted by Simon Birrer, Elizabeth J. Buckley-Geer, Michele Cappellari, Frédéric Courbin, Frederic 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.

Refining the Universe’s Expansion Rate: A Precise Hubble Constant Measurement from Strong Gravitational Lensing

Ongoing cosmological research centres on precisely determining the Hubble constant, a fundamental parameter quantifying the universe’s expansion rate, and resolving discrepancies between locally measured values and those inferred from observations of the early universe. Recent studies utilise strong gravitational lensing – the bending and magnification of light from distant quasars by massive foreground galaxies – to constrain cosmological parameters. This offers an independent probe of the universe’s expansion history, complementing traditional distance ladder techniques and cosmic microwave background (CMB) observations, and potentially illuminating the origins of the Hubble tension.

A team of astronomers has presented a refined measurement of the Hubble constant derived from an analysis of eight strongly lensed quasars comprising the TDCOSMO-2025 sample. The study builds upon previous investigations by incorporating newly acquired stellar velocity dispersion measurements of the lensing galaxies, obtained through spectroscopic observations with the James Webb Space Telescope, the Keck Telescopes, and the Very Large Telescope. These observations provide crucial insights into the mass distribution within the lensing galaxies, enabling a more accurate reconstruction of the gravitational potential and a precise determination of the time delays between the multiple images of the quasar. Improved analytical methods further refine the assessment of these data, enhancing the precision of the derived cosmological parameters.

The research extends beyond the TDCOSMO-2025 sample, incorporating data from eleven lenses within the Sloan Lens ACS (SLACS) survey and four from the Strong Lenses in the Legacy Survey (SL2S). These additional samples benefit from Keck-KCWI resolved kinematics – measurements of stellar motions within the lensing galaxy – providing a more robust overall constraint on the Hubble constant. Researchers employed both integrated and resolved stellar kinematics, with resolved kinematics specifically applied to the well-studied RX J1131-1231, allowing for a detailed mapping of the velocity distribution within the lensing galaxy. This comprehensive approach strengthens the statistical significance of the results and minimises systematic uncertainties.

Significant attention focused on mitigating systematic uncertainties, including a detailed consideration of line-of-sight effects, lens galaxy surface brightness profiles, and orbital anisotropy. Researchers carefully modelled these effects to ensure reliable cosmological constraints.

The study highlights the power of strong gravitational lensing as a complementary probe of the universe’s expansion rate. By combining strong lensing measurements with traditional distance ladder techniques and CMB observations, researchers can obtain a more complete and accurate picture of the universe’s evolution. This multi-pronged approach is essential for resolving the Hubble tension and understanding the underlying physics driving the universe’s expansion. Future observations with next-generation telescopes will further refine these measurements and provide even more stringent constraints on cosmological parameters.

The study extends beyond the standard flat ΛCDM model – the prevailing cosmological model – presenting constraints on cosmological parameters within open ΛCDM, wCDM, and CDM cosmologies. The inference from the TDCOSMO sample remains consistent across all tested cosmological models, aligning with constraints derived from independent Baryonic Acoustic Oscillation and Type Ia Supernovae analyses. This consistency provides further support for the standard cosmological model and suggests that the universe is well-described by a flat, cold dark matter dominated geometry.

The team meticulously modelled the effects of dark matter halos surrounding the lensing galaxies, accounting for their contribution to the overall gravitational potential. These halos play a crucial role in shaping the observed image positions and time delays, and accurately modelling their properties is essential for obtaining reliable cosmological constraints. Researchers employed sophisticated numerical simulations to model the distribution of dark matter and account for its effects on the observed lensing signals.

The study obtained a precise measurement of the Hubble constant, providing an independent check on other methods and helping to narrow down the possible values of this fundamental parameter. By continuing to refine these measurements and explore new observational techniques, researchers hope to gain a deeper understanding of the universe’s expansion history and the underlying physics driving its evolution. The ongoing quest to understand the Hubble constant remains one of the most exciting and challenging endeavours in modern cosmology.

Researchers employed integrated stellar kinematics for five lenses, VLT-MUSE for two, and resolved kinematics from Keck for the well-studied RX J1131-1231. This multi-faceted approach leverages the strengths of different observational techniques and provides a comprehensive picture of the dynamics within the lensing galaxies. The combination of integrated and resolved kinematics allows for a detailed mapping of the velocity distribution and a precise determination of the mass profile.