The Hubble Tension: measuring the expansion history of the universe with gravitational time delays.

The standard LCDM model gives a successful description of many astrophysical observations. However, in the past few years a tension has developed between local determinations of the current expansion rate of the Universe (the Hubble constant) and the value predicted from early universe probes. If confirmed, this so-called Hubble Tension would require additional physical ingredients beyond LCDM, e.g. early dark energy, or new particles. Gravitational time delay between multiple images of strongly lensed variable sources provide a way to measure the Hubble constant independent of all other methods. I will briefly describe the method and show that it can reach 1% precision using a combination of strong lensing and stellar kinematics. I will then present the results of a new milestone paper from the TDCOSMO collaboration (the Union of H0LICOW, STRIDES, SHARP and COSMOGRAIL).