Colloquia

I will introduce, for those unfamiliar with general relativity, the notion of the expansion of space, before going on to discuss a 5 sigma discrepancy between two inferences of the rate of that expansion today. One of those inferences is highly indirect and model dependent, relying on measurements of maps of the cosmic microwave background (CMB), a thermal relic of the hot big bang. I will explain what the CMB is, and show how well our standard cosmological model describes its statistical properties, and how we can use that model to infer the expansion rate today. I will then describe a symmetry under a scaling transformation of all relevant time scales in the problem that can potentially be exploited to reconcile the two inferences. Significant constraints on such a solution come from measurements of the CMB energy density and the abundances of light elements produced in the big bang. The former constraint can be circumvented by use of a ‘mirror world’ dark sector — a copy of the standard model of particle physics with little to no interactions with standard model particles other than via gravity.