Gravitational lensing $H_0$ tension from ultralight axion galactic cores

TL;DR

This paper explores how ultralight axion dark matter could naturally form galactic cores that impact gravitational lensing measurements of the Hubble constant, potentially resolving existing tensions.

Contribution

It proposes a specific ultralight axion mass and abundance as a mechanism for core formation affecting $H_0$ measurements from lensing.

Findings

Ultralight axions of mass ~10^{-25} eV can form galactic cores.

Cores of this type could bias lensing $H_0$ measurements.

Stellar kinematics can test the core formation scenario.

Abstract

Gravitational lensing time delays offer an avenue to measure the Hubble parameter $H_0$, with some analyses suggesting a tension with early-type probes of $H_0$. 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 and could bias $H_0$ in the right direction to solve the lensing tension. We consider ultralight dark matter 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 $m\sim10^{-25}$ 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 lens galaxies should not, in general, adopt the perfect mass sheet limit, as ignoring the finite extent of an actual physical core could lead to significant systematic errors.