A novel approach to relativistic ray-tracing technique in N-body simulations

Abstract

N-body simulations are fundamental to cosmology and essential for high-precision investigations. Theoretical predictions are made in the simulations and compared to observations, owing to their known cosmological parameters, and hence makes them crucial for future tests of general relativity (GR) on cosmological scales - specifically, a test of GR via cosmic magnification in the weak lensing regime. Whilst N-body simulations are the theoretical tool that is requisite for generating mock weak lensing galaxy catalogues, it is ray-tracing technique that permits the study of light propagation within the catalogues; to trace simulated light paths through spacetime. However, there must be assurance that ray-tracing technique does not ignore the effects of gravitational lensing. Most studies simulate light paths along straight trajectories in three-dimensional space. Yet on larger scales, this assumption will lead to inaccuracies; photons follow a curved trajectory as they pass a gravitational field. A small number of relativistic ray-tracing codes have been developed to address this issue. Therefore, I investigate an existing relativistic ray-tracing algorithm and evaluate, experiment and modify the code for the purpose of testing GR physics via cosmic magnification in future research. A novel experimentation design has been applied to the code - a proof-of-concept methodological approach - that ultimately seeks to input an extended list of particle values and output gravitational lensing results. More specifically, the modified code assigns the non-gridded gravitational potentials from a Gadget-2 dark matter halo simulation file onto a three-dimensional grid, in which the values are interpolated and integrated through the remainder of the modified code. Gadget-2 simulation files are a popular choice for cosmologists and astrophysicists to conduct their studies of light propagation via ray-tracing, hence this modified design is a contribution to the field. Additionally, the results are further verified by establishing contour plots of the displacement angles, which will be compared to the projected mass surface density, S, in the next stage of research; by inputting the solutions to the geodesic equations and outputting a projection of the three-dimensional mass distribution onto a two-dimensional surface. This modified relativistic ray-tracing algorithm will be employed for an upcoming test of GR on cosmological scales, where the simulation will be populated with real data from the EMU radio galaxy survey. Theoretical predictions of cosmic magnification will be made, via the cross-correlation of distant radio galaxies and nearby optical galaxies, in which the lensing measurements will indicate if there is conformity to GR.

Date of Award	2022
Original language	English