Structure Formation by Fifth Force: Power Spectrum from N-Body Simulations

TL;DR

This paper develops a numerical framework to study how a scalar field coupled to dark matter influences nonlinear structure formation, revealing small-scale differences from standard cosmology while maintaining similar large-scale predictions.

Contribution

It introduces a new simulation approach for scalar-field-coupled dark matter models, enabling detailed analysis of their impact on structure formation at small scales.

Findings

Scalar field tracks dark matter distributions in voids and clusters.

Small-scale structure predictions differ from LCDM, especially in halo mass functions.

Large-scale observables like CMB and Hubble expansion remain similar to LCDM.

Abstract

We lay out the framework to numerically study nonlinear structure formation in the context of scalar-field-coupled cold dark matter models (phiCDM models) where the scalar field phi serves as dynamical dark energy. Adopting parameters for the scalar field which yield a realistic CMB spectrum, we generate the initial conditions for our Nbody simulations, which follow the spatial distributions of dark matter and the scalar field, by solving their equations of motion using the multilevel adaptive grid technique. We show that the spatial configuration of the scalar field tracks both the voids and clusters of dark matter. The phiCDM model differs from standard LCDM at small scales with observable modifications of, e.g., the mass function of halos as well as the matter power spectrum. Nevertheless, the predictions of both models for the Hubble expansion and the CMB spectrum are virtually indistinguishable. Hence, galaxy cluster counts and weak lensing observations, which probe structure formation at small scales, are needed to falsify this class of models.