# Rotation of the CMB polarisation by foreground lensing

## TL;DR

This paper studies how weak gravitational lensing causes a true physical rotation of the Cosmic Microwave Background (CMB) polarization, emphasizing the importance of second-order effects and clarifying previous inconsistencies.

## Contribution

It demonstrates that polarization rotation is a genuine second-order effect in lensing and clarifies its treatment, correcting prior misconceptions in the literature.

## Key findings

- Rotation of polarization is a second-order physical effect.
- No first-order polarization rotation for vector and tensor modes.
- Clarifies inconsistencies in previous theoretical treatments.

## Abstract

We investigate the weak lensing corrections to the CMB polarization anisotropies. We concentrate on the effect of rotation and show that the rotation of polarisation is a true physical effect which has to be taken into account at second order in perturbation theory. We clarify inconsistencies on the treatment of this rotation in the recent literature. We also show that at first order in perturbation theory there is no rotation of polarisation also for vector and tensor modes.