TDCOSMO III: Dark matter substructure meets dark energy -- the effects of (sub)halos on strong-lensing measurements of $H_0$

TL;DR

This study investigates the impact of dark matter subhalos and line-of-sight halos on strong-lensing measurements of the Hubble constant, finding that while they do not bias results, they add a quantifiable uncertainty.

Contribution

The paper explicitly quantifies the effect of dark matter substructure on $H_0$ measurements using mock lens systems, providing a fitting function for the additional uncertainty.

Findings

Omitting dark substructure does not bias $H_0$ estimates.

Substructure introduces an uncertainty of 0.7-2.4%.

Uncertainty scales with lensing volume and time delay.

Abstract

Time delay cosmography uses the arrival time delays between images in strong gravitational lenses to measure cosmological parameters, in particular the Hubble constant $H_0$. The lens models used in time delay cosmography omit dark matter subhalos and line-of-sight halos because their effects are assumed to be negligible. We explicitly quantify this assumption by analyzing mock lens systems that include full populations of dark matter subhalos and line-of-sight halos, applying the same modeling assumptions used in the literature to infer $H_0$. We base the mock lenses on six quadruply-imaged quasars that have delivered measurements of the Hubble constant, and quantify the additional uncertainties and/or bias on a lens-by-lens basis. We show that omitting dark substructure does not bias inferences of $H_0$. However, perturbations from substructure contribute an additional source of random uncertainty in the inferred value of $H_0$ that scales as the square root of the lensing volume divided by the longest time delay. This additional source of uncertainty, for which we provide a fitting function, ranges from $0.7 - 2.4\%$. It may need to be incorporated in the error budget as the precision of cosmographic inferences from single lenses improves, and sets a precision limit on inferences from single lenses.