The Trouble with Hubble: Signs of New Physics?

  • Nov. 25, 2019, 2:00 pm US/Central
  • Curia II
  • Vivian Poulin, LUPM (CNRS & Université de Montpellier)

The value of the Hubble constant as measured using the classical distance ladder method is 4 to 6 sigma higher than the value inferred from a ΛCDM fit to the cosmic microwave background (CMB). Interestingly, we now have several independent local probes of the Hubble constant (supernovae, strongly lensed quasars), such that none of the potential systematic effects could simultaneously explain all measurements. Consequently, increasing attention is given to the possibility that this “Hubble tension” indicates new physics beyond ΛCDM. After reviewing the various measurements of the Hubble constant, I will explain why data currently disfavor any modification of the universe dynamics at low-redshift over new physics in the pre-recombination era. In particular I will argue that the tension is better understood when recast in terms of a tension between measurement of the “sound horizon’’ at recombination. Finally, I will entertain the idea that these observations might indicate that our Universe has undergone anomalous expansion due to the presence of an early dark energy (EDE) at redshift z ~ 3500. Such idea, if confirmed, could have far-reaching implications for our understanding of the current epoch of dark energy domination. While undetectable for Planck, I will show that future CMB experiment should be able to unambiguously tell us about (or exclude) the presence of the EDE.