On the Effects of Coupled Scalar Fields on Structure Formation

TL;DR

This paper investigates how coupled scalar fields, representing dark energy, influence cosmic structure formation through N-body simulations, revealing that mass rescaling and expansion rate modifications dominate over the fifth force effects.

Contribution

It provides the first detailed analysis of the relative importance of different scalar field effects on structure formation in coupled dark energy models using simulations.

Findings

Mass rescaling reduces halo concentration compared to LCDM.

The modified expansion rate significantly impacts structure growth.

The fifth force is less influential than mass rescaling and expansion modifications.

Abstract

A coupling between a scalar field (representing the dark energy) and dark matter could produce rich phenomena in cosmology. It affects cosmic structure formation mainly through the fifth force, a velocity-dependent force that acts parallel to particle's direction of motion and proportional to its speed, an effective rescaling of the particle masses, and a modified background expansion rate. In many cases these effects entangle and it is difficult to see which is the dominant one. Here we perform N-body simulations to study their qualitative behaviour and relative importance in affecting the key structure formation observables, for a model with exponential scalar field coupling. We find that the fifth force, a prominent example of the scalar-coupling effects, is far less important than the rescaling of particle mass or the modified expansion rate. In particular, the rescaling of particle masses is shown to be the key factor leading to less concentration of particles in halos than in LCDM, a pattern which is also observed in previous independent coupled scalar field simulations.