Presentation #555.01 in the session “Lancelot M. Berkeley Prize Plenary”.
The Hubble constant (H0) is a key parameter in cosmology that sets the expansion rate of the Universe, as well as its age and size. There is an intriguing tension in the H0 measurements from different methods. If the tension is not caused by unknown measurement errors, then reconciling the discrepant H0 measurements would require new physics beyond our current standard cosmological model. Independent methods to measure H0 are therefore necessary for assessing this tension. Strong gravitational lenses with measured time delays between the multiple images provide a competitive approach to determine H0. I will describe techniques for measuring H0 from lensed quasars, and present the results from the H0LiCOW program. Analysis of new lensed quasar systems is underway in the newly formed Time-Delay Cosmography organization. An exciting discovery of the first strongly lensed supernova offered a rare opportunity to perform a true blind test of our lens modeling techniques. I will show the bright prospects of gravitational lens time delays as an independent and competitive cosmological probe.