ESTIMATING SMALL ANGULAR SCALE COSMIC MICROWAVE BACKGROUND ANISOTROPY WITH HIGH-RESOLUTION N-BODY SIMULATIONS: WEAK LENSING

We estimate the impact of weak lensing by strongly nonlinear cosmological structures on the cosmic microwave background. Accurate calculation of large ℓ multipoles requires N-body simulations and ray-tracing schemes with both high spatial and temporal resolution. To this end, we have developed a new code that combines a gravitational Adaptive Particle–Particle, Particle–Mesh solver with a weak-lensing evaluation routine. The lensing deviations are evaluated while structure evolves during the simulation so that all evolution steps—rather than just a few outputs—are used in the lensing computations. The new code also includes a ray-tracing procedure that avoids periodicity effects in a universe that is modeled as a three-dimensional torus in the standard way. Results from our new simulations are compared with previous ones based on Particle-Mesh simulations. We also systematically investigate the impact of box volume, resolution, and ray-tracing directions on the variance of the computed power spectra. We find that a box size of 512 h−1 Mpc is sufficient to provide a robust estimate of the weak-lensing angular power spectrum in the ℓ-interval (2000–7000). For a reaslistic cosmological model, the power [ℓ(ℓ + 1)Cℓ/2π]1/2 takes on values of a few μK in this interval, which suggests that a future detection is feasible and may explain the excess power at high ℓ in the Berkeley–Illinois–Maryland Association and Cosmic Background Imager observations.