Multiple Lensing of the Cosmic Microwave Background anisotropies

TL;DR

This paper investigates gravitational lensing effects on CMB anisotropies using detailed ray-tracing through N-body simulated large-scale structures, comparing methods and analyzing impacts on temperature and polarization spectra.

Contribution

It introduces a comprehensive ray-tracing approach for CMB lensing, comparing multiple methods and assessing the influence of non-linear structure evolution on CMB power spectra.

Findings

Lensing power is accurately recovered across a wide multipole range.

Excess lensing potential power appears at small scales ($rac{3000}$).

Numerical resolution impacts B-mode polarization simulations.

Abstract

We study the gravitational lensing effect on the Cosmic Microwave Background (CMB) anisotropies performing a ray-tracing of the primordial CMB photons through intervening large-scale structures (LSS) distribution predicted by N-Body numerical simulations with a particular focus on the precise recovery of the lens-induced polarized counterpart of the source plane. We apply both a multiple plane ray-tracing and an effective deflection approach based on the Born approximation to deflect the CMB photons trajectories through the simulated lightcone. We discuss the results obtained with both these methods together with the impact of LSS non-linear evolution on the CMB temperature and polarization power spectra. We compare our results with semi-analytical approximations implemented in Boltzmann codes like, e.g., CAMB. We show that, with our current N-body setup, the predicted lensing power is recovered with good accuracy in a wide range of multipoles while excess power with respect to semi-analytic prescriptions is observed in the lensing potential on scales $\ell \gtrsim 3000$. We quantify the impact of the numerical effects connected to the resolution in the N-Body simulation together with the resolution and band-limit chosen to synthesise the CMB source plane. We found these quantities to be particularly important for the simulation of B-mode polarization power spectrum.