The approximate gravitational lensing multiple plane mass sheet degeneracy

TL;DR

This paper reviews the mass sheet degeneracy in multi-plane gravitational lensing, analyzing how internal and external mass sheets affect lens modeling and the constraints on cosmological parameters.

Contribution

It extends the understanding of the mass sheet degeneracy to multiple lens planes, including non-linear effects and time delay considerations.

Findings

Mass sheet degeneracy persists with multiple lens planes.

Internal and external mass sheets can be treated similarly in models.

Residual degeneracy involving time delays and H0 remains even with multiple planes.

Abstract

Strong gravitational lensing has to deal with many modeling degeneracies, the most notable being the Mass Sheet Degeneracy (MSD). We review the MSD when one needs to model more lens planes, each one with an internal mass sheet. We take into account the non-linear lens-lens coupling and line of sight effects, the latter treated as external mass sheets with associated shear. If second order shear terms on external and internal mass sheets can be neglected, we show that the MSD is always retained, and the mass sheets influence can be reabsorbed in the redefinition of angular diameter distances. In particular, internal and external mass sheets can be placed on the same footing. The version of the MSD discussed here does not require any particular relation between the internal mass sheets in the different planes. Even when including time delays from all sources, a residual degeneracy involving time delays, mass sheets and $ H_0 $ remains. We develop a framework which shows what can actually be constrained in multiple plane lens systems.