Structure Formation by the Fifth Force III: Segregation of Baryons and Dark Matter

TL;DR

This paper uses N-body simulations to study how a chameleon scalar field with different couplings to baryons and dark matter influences large-scale structure formation and causes segregation of these matter types within halos.

Contribution

It introduces a novel simulation approach for coupled scalar fields and analyzes their impact on matter segregation and halo density profiles in cosmology.

Findings

Baryon-dark matter segregation increases with strong fifth force regions.

Halo density profiles depend on environment due to scalar field effects.

The scalar field exhibits chameleon behavior affecting structure formation.

Abstract

In this paper we present the results of $N$-body simulations with a scalar field coupled differently to cold dark matter (CDM) and baryons. The scalar field potential and coupling function are chosen such that the scalar field acquires a heavy mass in regions with high CDM density and thus behaves like a chameleon. We focus on how the existence of the scalar field affects the formation of nonlinear large-scale structure, and how the different couplings of the scalar field to baryons and CDM particles lead to different distributions and evolutions for these two matter species, both on large scales and inside virialized halos. As expected, the baryon-CDM segregation increases in regions where the fifth force is strong, and little segregation in dense regions. We also introduce an approximation method to identify the virialized halos in coupled scalar field models which takes into account the scalar field coupling and which is easy to implement numerically. It is find that the chameleon nature of the scalar field makes the internal density profiles of halos dependent on the environment in a very nontrivial way.