Orateur
Description
The value of the Hubble constant as measured using the classical distance ladder method is 4 to 6sigma higher than the value inferred from a ΛCDM fit to the cosmic microwave background (CMB). While the possibility that unaccounted for systematic effect are responsible for this discrepancy, none are currently universally accepted. Moreover, we now have several independent local probes of the Hubble constant (supernovae, strongly lensed quasars), such that none of the suggested systematics could simultaneously explain all measurements. Consequently, increasing attention is given to the possibility that this “Hubble tension” indicates new physics beyond ΛCDM. In this talk, I would like to review promising solutions to this tension. I will first argue that this discrepancy should be interpreted as a tension between our understanding of the early and late universe cosmology, rather than a tension between a few datasets. In particular, I will show that the Hubble tension is better understood when recast in terms of a tension between measurement of the ``sound horizon’’ at recombination. I will then explain why data currently disfavor modifications of the universe dynamics at low-redshift over new physics in the pre-recombination era. 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 the presence of the EDE.
