Determine whether the low-multipole power deficit is a genuine nonlinear effect

Determine whether the apparent reduction of nonlinear angular power spectra relative to linear-theory predictions at multipoles l less than or approximately equal to 10 in the gevolution ray-tracing simulations is a genuine physical effect rather than statistical scatter caused by cosmic variance.

Background

The paper compares angular power spectra for fluctuations in the optical expansion observable $\vartheta$ and the weak-lensing convergence $\kappa$, obtained from linear perturbation theory and from ray tracing through a relativistic N-body simulation using gevolution. The simulated spectra agree well with linear theory at intermediate multipoles, but the authors observe substantial scatter among the 20 simulated observers at multipoles $l\lesssim 10$.

Because the number of observers is small, the observed suppression relative to linear theory may be attributable to cosmic variance rather than to a physical nonlinear modification. Resolving this issue requires additional observers and dedicated simulations and analyses; the result matters for interpreting large-scale deviations between nonlinear ray-tracing measurements and perturbative predictions.