The importance of local measurements for cosmology

TL;DR

This paper demonstrates how local, model-independent measurements of the Hubble constant and universe age serve as crucial consistency checks for the flat LambdaCDM model and help constrain cosmological parameters independently of the CMB.

Contribution

It shows that current local measurements are consistent with CMB-derived parameters and highlights the potential of improved local data to detect tensions or new physics.

Findings

No significant tension between local and CMB measurements of H0 and age.

Constraints on neutrino species: Neff < 4 at 95% confidence.

Local measurements provide orthogonal constraints on curvature and dark energy.

Abstract

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 LambdaCDM) 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, $\Lambda$CDM 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 Neff < 4 at 95% confidence. We show that local measurements provide constraints on the curvature and 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.