# The effect of early radiation in N-body simulations of cosmic structure formation

## TL;DR

This paper examines how including early radiation and general relativity effects in N-body simulations of cosmic structure formation influences large-scale results, finding percent-level impacts and validating three accurate approaches.

## Contribution

It introduces and compares three leading-order GR methods to incorporate early radiation effects into N-body cosmological simulations.

## Key findings

- Early radiation causes percent-level effects on large-scale power spectra.
- All three methods produce consistent results, validating their use.
- Two methods modify evolution directly; one is a post-processing approach.

## Abstract

Newtonian N-body simulations have been employed successfully over the past decades for the simulation of the cosmological large-scale structure. Such simulations usually ignore radiation perturbations (photons and massless neutrinos) and the impact of general relativity (GR) beyond the background expansion. This approximation can be relaxed and we discuss three different approaches that are accurate to leading order in GR. For simulations that start at redshift less than about 100 we find that the presence of early radiation typically leads to percent-level effects on the numerical power spectra at large scales. Our numerical results agree across the three methods, and we conclude that all of the three methods are suitable for simulations in a standard cosmology. Two of the methods modify the N-body evolution directly, while the third method can be applied as a post-processing prescription.