Colloquium – Kenneth Wong: An Independent Measurement of H0 from Lensed Quasars

Strong gravitational lens systems with time delays between the multiple images are a powerful probe of cosmology, particularly of the Hubble constant (H0).  This independent method is an important test of the recent tension between early-Universe and late-Universe measurements of H0.  The TDCOSMO project has measured H0 from lensed quasars using deep Hubble Space Telescope and AO imaging, precise time delay measurements from dedicated monitoring observations, a measurement of the velocity dispersion of the lens galaxies, and a characterization of the mass distribution along the line of sight.  Assuming well-motivated physical profiles for the lens galaxies, our latest results constrain H0 to be ~74 km/s/Mpc for a flat Lambda CDM cosmology, with a precision of ~2%.  These results are consistent with independent determinations of H0 using type Ia supernovae calibrated by the distance ladder method, and are in >4-sigma tension with the results of Planck CMB measurements, hinting at possible new physics beyond the standard LCDM model and highlighting the importance of this independent probe.  Relaxing our assumptions on the form of the lens galaxy mass profiles and only allowing kinematic constraints, we can constrain H0 to ~8% precision.  Including information from external data sets can improve the precision to ~5%.  Future observations to increase the sample size and control systematic uncertainties will be key to resolving the current tension in H0.