Friday 25th of October, 11.00h, at Nikhef in H331.
Speaker: Leon Koopmans (RUG Groningen)
Title: “Time-delay cosmography: tensions between measurements of the Hubble constant from the early and later Universe”
Abstract:
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.