Local determination of the Hubble constant and the deceleration parameter

Abstract

The determination of the Hubble constant H0 from the cosmic microwave background by the Planck Collaboration (N. Aghanim, arXiv:1807.06209) is in tension at 4.2σ with respect to the local determination of H0 by the SH0ES collaboration [M. J. Reid, Astrophys. J. Lett. 886, L27 (2019)2041-821310.3847/2041-8213/ab552d]. Here we improve upon the local determination, which fixes the deceleration parameter to the standard ΛCDM model value of q0=-0.55, that is, uses information from observations beyond the local universe. First, we derive the effective calibration prior on the absolute magnitude MB of type Ia supernovae, which can be used in cosmological analyses in order to avoid the double counting of low-redshift supernovae. We find MB=-19.2334±0.0404 mag. Then we use the above MB prior in order to obtain a determination of the local H0 which uses only local observations and assumes only the cosmological principle, that is, large-scale homogeneity and isotropy. This is achieved by adopting an uninformative flat prior for q0 in the cosmographic expansion of the luminosity distance. We use the latest Pantheon sample and find H0=75.35±1.68kms-1Mpc-1, which features a 2.2% uncertainty, close to the 1.9% error obtained by the SH0ES Collaboration. Our determination is at the higher tension of 4.5σ with the latest results from the Planck Collaboration that assume the ΛCDM model. Furthermore, we also constrain the deceleration parameter to q0=-1.08±0.29, which disagrees with the Planck Collaboration at the 1.9σ level. These estimations only use supernovae in the redshift range 0.023≤z≤0.15.