N-Body Simulations for Coupled Scalar Field Cosmology

TL;DR

This paper develops and applies N-body simulations to coupled scalar field cosmologies, analyzing the effects of scalar coupling on structure formation, halo properties, and deviations from linear theory.

Contribution

It introduces a detailed numerical methodology for simulating coupled scalar field models and adapts halo identification algorithms for these cosmologies.

Findings

Scalar coupling increases the number of heavy halos.

Coupling suppresses inner halo density profiles.

The scalar field's potential is negligible except in background expansion.

Abstract

We describe in detail the general methodology and numerical implementation of consistent N-body simulations for coupled scalar field cosmological models, including the background cosmology and the generation of initial conditions (with the different couplings to different matter species taken into account). We perform fully consistent simulations for a class of coupled scalar field models with an inverse power-law potential and negative coupling constant, for which the chameleon mechanism does not operate. We find that in such cosmological models the scalar-field potential plays a negligible role except in the background expansion, and the fifth force that is produced is proportional to gravity in magnitude, justifying the use of a rescaled gravitational constant G in some earlier N-body simulations of similar models. We study the effects of the scalar coupling on the nonlinear matter power spectra and compare with linear perturbation calculations to investigate where the nonlinear model deviates from the linear approximation. For the first time, the algorithm to identify gravitationally virialized matter halos is adapted to the scalar field cosmology, and then used to measure the mass function and study the properties of virialized halos. We find that the net effect of the scalar coupling helps produce more heavy halos in our simulation boxes and suppresses the inner (but not the outer) density profile of halos compared with those predicted by lambda-CDM, while this suppression weakens as the coupling between the scalar field and dark matter particles increases in strength.