Galaxy-lens determination of $H_0$: constraining density slope in the context of the mass sheet degeneracy

Understand

Gravitational lensing offers a competitive method to measure $H_0$ with the goal of 1% precision.

• A major obstacle comes in the form of lensing degeneracies, such as the mass sheet degeneracy (MSD), which make it possible for a family of density profiles to reproduce the same lensing observables but return different values of $H_0$.
• The modeling process artificially selects one choice from this family, potentially biasing $H_0$.
• The effect is more pronounced when the profile of a given lens is not perfectly described by the lens model, which will always be the case to some extent.