On the Choice of Lens Density Profile in Time Delay Cosmography

Understand

Time delay lensing is a mature and competitive cosmological probe.

• However, it is limited in accuracy by the well-known problem of the mass-sheet degeneracy: too rigid assumptions on the density profile of the lens can potentially bias the inference on cosmological parameters.
• I investigate the degeneracy between the choice of the lens density profile and the inference on the Hubble constant, focusing on double image systems.
• By expanding lensing observables in terms of the local derivatives of the lens potential around the Einstein radius, and assuming circular symmetry, I show that three degrees of freedom in the radial direction are necessary to achieve a few percent accuracy in the time-delay distance.