The importance of local measurements for cosmology

We explore how local, cosmology-independent measurements of the Hubble constant and the age of the Universe help to provide a powerful consistency check of the currently favored cosmological model (flat ΛCDM) and model-independent constraints on cosmology. We use cosmic microwave background (CMB) data to define the model-dependent cosmological parameters, and add local measurements to assess consistency and determine whether extensions to the model are justified. At current precision, there is no significant tension between  the locally measured Hubble constant and age of the Universe (with errors of 3% and 5%, respectively) and the corresponding parameters derived from the CMB. However, if errors on the local measurements could be decreased by a factor of two, one could decisively conclude if there is tension or not. We also compare the local and CMB data assuming simple extensions of the flat, ΛCDM model (including curvature, dark energy with a constant equation of state parameter not equal to −1, non-zero neutrino masses and a non-standard number of neutrino families) and find no need for these extra parameters; in particular, we constrain the effective number of neutrino species to be less than 4 at 95% confidence. We show that local measurements provide constraints on the curvature and the equation of state of dark energy nearly orthogonal to those of the CMB. We argue that cosmology-independent measurements of local quantities at the percent level would be very useful to explore cosmology in a model-independent way.