In the past few years, a tension has emerged between the current expansion rate of the universe (H0) and the value predicted by early universe probes under the assumption of a standard LCDM cosmology. The tension is statistically significant: combinations of local probes are 4-6 sigma away from H0 as inferred by Planck, for example. Efforts to uncover systematic uncertainties are under way, but they have been unsuccessful so far. If the tension is real, most proposals to resolve it require changing the expansion history before recombination in a non-trivial way, possibly as a result of early dark energy, or sterile neutrinos. I will review measurements of H0, including the results of the well known methods based on the local distance ladder and on the cosmic microwave background. I will explain in detail the approach based on gravitational time delays, that have recently reached comparable precision, providing an independent verification of the tension. I will conclude by discussing the prospects of reaching sub-percent precision on multiple independent methods as a way to control systematics, and verify whether indeed this is the first hint of new physics beyond LCDM.
Aut. Qtr. Colloq. committee: R. Blandford (Chair), B. Feldman, A. Kapitulnik, B. Lev and V. Khemani
Location: Hewlett Teaching Center, Rm. 200