The effects on structure of a momentum coupling between dark matter and quintessence

TL;DR

This work investigates a momentum coupling between dark matter and a quintessence scalar field using high-resolution N-body simulations (RAMSES) to study halos and substructure. It demonstrates that the sign of the coupling ($\gamma_0$) produces opposite effects on structure: positive coupling enhances large-scale power but suppresses small-scale power and steepness of inner density profiles, while negative coupling does the opposite and yields higher halo concentrations under certain gravity assumptions. The dynamical state of halos also differs markedly, with positive coupling associated with bimodal velocity distributions and overvirialised hosts, and negative coupling leading to more unimodal, sub-virial systems, especially when analyzed with Newtonian gravity. These distinct kinematic and structural signatures offer observational discriminants for coupled dark sector models and motivate future hydrodynamic studies and broader parameter exploration.

Abstract

Given the mysterious nature of dark matter and dark energy, and the persistent tensions in cosmological data, it is worthwhile exploring more exotic physics in the dark sector, such as a momentum coupling between dark matter and dark energy, specifically in the form of a quintessence field. In this study, using collisionless N-body numerical simulations with a modified version of the RAMSES code, we follow up previous work to investigate the consequences of this model on dark matter halos and their substructures. We consider both the sign of the coupling and the imprints on structure formation and halo properties at a statistical level. We find that there is a clear enhancement (reduction) of substructure if the sign of the coupling is negative (positive) and that the dynamical state of the dark matter halos, particularly host halos, is undervirialised (overvirialised) at redshift zero when compared to uncoupled models or a reference $Λ$CDM simulation. Furthermore, positive coupling leads to less concentrated, less cuspy halos, whereas negative coupling leads to the opposite.

Figures (24)

• Figure 1: Hubble parameter normalised by that of $\Lambda$CDM for the three values of the coupling constant.
• Figure 2: Modified Euler equation coefficients for the three values of the coupling constant.
• Figure 3: Density projection plots of all simulations considered in this work. The leftmost panel is our $\Lambda$CDM run, and the other three panels (from left to right) are coupled models with: $\gamma_0 = 0.3$, $\gamma_0 = -0.3$, $\gamma_0 = -20$.
• Figure 4: Power spectra normalised by the corresponding uncoupled model.
• Figure 5: Halo mass functions for all simulations considered in this study. The comparison line given is from Watson_2013, as defined in the Colossus Python package.