Simulating Momentum Exchange in the Dark Sector

TL;DR

This paper explores how elastic scattering between dark matter and dark energy affects cosmic structure formation, revealing nonlinear effects that could tighten constraints on dark sector interactions beyond current linear observational bounds.

Contribution

First N-body simulations of dark matter-dark energy scattering showing significant nonlinear effects on structure formation, highlighting potential for improved constraints.

Findings

Nonlinear power spectrum deviations are about ten times larger than linear ones.

Strong enhancement of halo concentration and mass functions due to scattering.

Nonlinear probes can provide tighter constraints on dark sector interactions.

Abstract

Low energy interactions between particles are often characterised by elastic scattering. Just as electrons undergo Thomson scattering with photons, dark matter particles may experience an analogous form of momentum exchange with dark energy. We investigate the influence such an interaction has on the formation of linear and nonlinear cosmic structure, by running for the first time a suite of N-body simulations with different dark energy equations of state and scattering cross sections. In models where the linear matter power spectrum is suppressed by the scattering, we find that on nonlinear scales the power spectrum is strongly enhanced. This is due to the friction term increasing the efficiency of gravitational collapse, which also leads to a scale-independent amplification of the concentration and mass functions of halos. The opposite trend is found for models characterised by an increase of the linear matter power spectrum normalisation. More quantitatively, we find that power spectrum deviations at nonlinear scales ($k \approx 10\, h/$Mpc) are roughly ten times larger than their linear counterparts, exceeding $100%$ for the largest value of the scattering cross section considered in the present work. Similarly, the concentration-mass relation and the halo mass function show deviations up to $100%$ and $20%$, respectively, over a wide range of masses. Therefore, we conclude that nonlinear probes of structure formation might provide much tighter constraints on the scattering cross section between dark energy and dark matter as compared to the present bounds based on linear observables.