The Hubble Constant is a measure of the expansion rate of the Universe, but in recent years a dichotomy, or tension at the 5-sigma level, has appeared between its measurement in the early Universe for example via the Cosmic Microwave Background, and measurements in the late Universe via a wide range of methods. This presentation introduces recent results from time-delay cosmography, where the Hubble constant is inferred from time-delay measurements between the light arriving from multiple gravitationally-lensed quasars. This method can estimate the Hubble constant to several per cent precision in one step. Careful analyses of the systematics, in a fully blind study, have shown the tension to persist under the assumption of the standard cosmological model. To alleviate this tension, either the early or late universe measurements still suffer from hidden systematics, common to all of them, or the standard cosmological model needs modification. Some possibilities of the latter scenario are shortly discussed.

BIO
Leon Koopmans is full professor and Scientific Director at the Kapteyn Astronomical Institute, University of Groningen, the Netherlands. He holds a MSc (1995) and PhD (2000; Cum Laude) in astrophysics from the University of Groningen. He spent time as postdoctoral fellow at the University of Manchester (UK), California Institute of Technology (US) and Space Telescope Science Institute (US), before returning to the Netherlands in 2004. His expertise: 21-cm cosmology, gravitational lensing, dark matter, galaxy structure, evolution & formation, stellar dynamics, interferometry. He has two major research lines, studying galaxies using strong gravitational lensing and the physics of the infant Universe during the Cosmic Dawn and Epoch of Reionization (CD/EoR). He is the Chair of Science Advisory Committee (SAC) ASTRON and Int'l SKA CD/EoR Science Team. He is founding chair and core member of SKA Science Working Group. He received both the Dutch VIDI and VICI career awards for his work on strong lensing, as well as ERC Advanced grant and ERC-starter/consolidator grant for 21-cm Cosmology.