Price of shifting the Hubble constant

Abstract

An anisotropic measurement of the baryon acoustic oscillation (BAO) feature fixes the product of the Hubble constant and the acoustic scale H₀r_d. Therefore, regardless of the dark energy dynamics, to accommodate a higher value of H₀ one needs a lower r_d and so necessarily a modification of early time cosmology. One must either reduce the age of the Universe at the drag epoch or else the speed of sound in the primordial plasma. The first can be achieved, for example, with dark radiation or very early dark energy, automatically preserving the angular size of the acoustic scale in the cosmic microwave background (CMB) with no modifications to post-recombination dark energy. However, it is known that the simplest such modifications fall afoul of CMB constraints at higher multipoles. As an example, we combine anisotropic BAO with geometric measurements from strong lensing time delays from H0LiCOW and megamasers from the Megamaser Cosmology Project to measure r_d, with and without the local distance ladder measurement of H₀. We find that the best fit value of r_d is indeed quite insensitive to the dark energy model and is also hardly affected by the inclusion of the local distance ladder data.