Hubble constant hunter’s guide

Measurements of the Hubble constant and, more generally, measurements of the expansion rate and distances over the interval $0<z<1$ appear to be inconsistent with the predictions of the standard cosmological model ($\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$) given observations of cosmic microwave background temperature and polarization anisotropies. Here we consider a variety of types of departures from $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ that could, in principle, restore concordance among these datasets, and we explain why we find almost all of them unlikely to be successful. We single out the set of solutions that increases the expansion rate in the decade of scale factor expansion just prior…