New Insights on CMB Lensing Bias from Anisotropic Rotation

The paper titled Efficient estimation of rotation-induced bias to reconstructed CMB lensing power spectrum explores the impact of anisotropic cosmic birefringence on the cosmic microwave background (CMB) lensing power spectrum. This power spectrum is crucial for understanding cosmological parameters, including the sum of neutrino masses and the equation of state for dark energy. The authors, Hongbo Cai, Yilun Guan, Toshiya Namikawa, and Arthur Kosowsky, highlight that upcoming CMB experiments, particularly those focusing on polarization lensing signals, are vulnerable to biases introduced by this anisotropic rotation.

The study identifies a specific type of noise, referred to as $N_L^{(1)}$-like noise, which arises from anisotropic rotation rather than lensing effects. The authors demonstrate that for experiments similar to CMB-S4, a scale-invariant anisotropic rotation field with a standard deviation of 0.05 degrees can significantly suppress the small-scale lensing power spectrum, comparable to the effects of massive neutrinos with a total mass of 50 meV. This finding suggests that the rotation field could complicate neutrino mass measurements in future CMB experiments.

Additionally, the paper provides an analytic expression and a simulation-based estimator for this noise, facilitating better forecasting and mitigation strategies for future experiments. The authors also investigate how a non-scale-invariant rotation power spectrum affects the reconstructed lensing power spectrum, concluding that an excess of power in the small-scale rotation spectrum leads to increased bias. This work offers a numeric framework that can enhance the accuracy of future CMB lensing measurements by accounting for the biases caused by anisotropic rotation.