Time-Delay Cosmography with Strong Lensing: TDCOSMO2025 Results and Future Prospects for LSST and Euclid

Time-delay cosmography with lensed quasars is a one-step method for estimating the Hubble constant in the local Universe independently of the cosmic distance ladder. It does not require any intermediate calibration and relies on measuring the time delays between multiple images of strongly lensed quasars, which are inversely proportional to the Hubble constant. In this talk, I will present cosmological constraints from eight strongly lensed quasars (hereafter, the TDCOSMO-2025 sample), based on a new blind analysis by the TDCOSMO collaboration designed to prevent experimenter bias. Building on previous work, we have improved our modeling of line-of-sight eLects, the surface brightness profiles of lens galaxies, and stellar orbital anisotropy, and we have corrected for projection eLects in the lens dynamics. Our uncertainties are maximally conservative, accounting for the mass-sheet degeneracy in the deflectors' mass density profiles, constrained by new measurements of stellar velocity dispersions from spectra obtained with the James Webb Space Telescope (JWST), the Keck Telescopes, and the Very Large Telescope (VLT). Our primary result, H_0 = 71.6+3.9−3.3 km/s/Mpc, is derived from the TDCOSMO-2025 sample combined with Ω_m constraints from the Pantheon+ Type Ia supernova (SN) dataset. We also present measurements of the Hubble constant combining TDCOSMO- 2025 with external datasets from the Sloan Lens ACS (SLACS) and Strong Lenses in the Legacy Survey (SL2S) lens sample, further improving the precision. The Hubble constant measurement is robust against the addition of external lens samples, the choice of different cosmological models beyond the ΛCDM model, and the use of the Ω_m prior from other datasets, such as the DESI DR2 BAO or the DES Year-5 SN sample. Finally, I will discuss the upcoming challenges and opportunities for this method in the era of large sky surveys. With LSST and Euclid expected to deliver time delays and high-resolution imaging for thousands of lensed quasars, this technique, which has so far been applied to only a few systems, will soon become a powerful statistical probe.