Isolating nonlinearities of light propagation in inhomogeneous cosmologies

A new formulation for light propagation in geometric optics by means of the bilocal geodesic operators is considered. We develop the bigonlight Mathematica package, uniquely designed to apply this framework to compute optical observables in numerical relativity. Our package can be used for light propagation on a wide range of scales and redshifts and accepts numerical as well as analytical input for the spacetime metric. In this paper we focus on two cosmological observables, the redshift and the angular diameter distance, specializing our analysis to a wall universe modeled within the post-Newtonian approximation. With this choice and the input metric in analytical form, we are able to estimate non-linearities of light propagation by comparing and isolating the contributions coming from Newtonian and post-Newtonian approximations as opposed to linear perturbation theory. We also clarify the role of the dominant post-Newtonian contribution represented by the linear initial seed which, strictly speaking, is absent in the Newtonian treatment. We found that post-Newtonian nonlinear corrections are below 1%, in agreement with previous results in the literature.