Abstract

Gravitational lensing time delays offer an avenue to measure the Hubble parameter H0, with some analyses suggesting a tension with early-type probes of H0. The lensing measurements must mitigate systematic uncertainties due to the mass modelling of lens galaxies. In particular, a core component in the lens density profile would form an approximate local mass sheet degeneracy (MSD) and could bias H0 in the right direction to solve the lensing tension. We consider ultralight dark matter (ULDM) as a possible mechanism to generate such galactic cores. We show that cores of roughly the required properties could arise naturally if an ultralight axion of mass m~10^-25 eV makes up a fraction of order ten percent of the total cosmological dark matter density. A relic abundance of this order of magnitude could come from vacuum misalignment. Stellar kinematics measurements of well-resolved massive galaxies (including the Milky Way) may offer a way to test the scenario. Kinematics analyses aiming to test the core hypothesis in massive elliptical galaxies should not, in general, adopt the perfect MSD limit, as ignoring the finite extent of an actual physical core could lead to significant systematic errors.