N-body simulations for coupled dark energy: halo mass function and density profiles

TL;DR

The paper conducts N-body simulations of cosmologies with DM–DE coupling using a RP potential, showing DM particle masses vary as $m_c = m_{0} e^{-\\sqrt{16\\pi G/3} \\beta \\phi}$ and DM–DM gravity strengthens to $G^{*}=G(1+4\\beta^{2}/3)$. It analyzes the halo mass function, density profiles, and non-linear bias, finding that nonlinear evolution constrains the coupling to $\\beta \ ightarrow 0.1$ and produces higher halo concentrations for larger $\\beta$. The $z=0$ mass function remains consistent with ΛCDM fits (e.g., Jenkins et al.), while outer halo profiles differ by ~10% and inner profiles can be significantly steeper (with $r_{s}$ as small as $\\sim 0.022\ h^{-1}$ Mpc for $\\beta=0.25$). The results imply RP-coupled DE models are largely compatible with nonlinear observables but do not automatically resolve the halo-concentration problem, motivating further exploration of DM mass evolution scenarios.

Abstract

We present the results of a series of N-body simulations in cosmologies where dark matter (DM) is coupled to dark energy (DE), so easing the cosmic coincidence problem. The dark-dark coupling introduces two novel effects in N-body dynamics: (i) DM particle masses vary with time; (ii) gravity between DM particles is ruled by a constant $G^{*}$, greater than Newton's constant $G$, holding in other 2-body interactions. As a consequence, baryons and DM particle distributions develop a large scale bias. Here we investigate DE models with Ratra-Peebles (RP) potentials; the dark-dark coupling is set in a parametric range compatible with observations, for as concern background and linear perturbation properties. We study the halo mass function, the halo density profile and the behavior of the non-linear bias. We find that non-linear dynamics puts additional constraints to the coupling parameter. They mostly arise from density profiles, that we find to yield higher concentrations, in coupled RP models, with respect to (uncoupled) dynamical DE cosmologies. Such enhancement, although being a strong effect in some coupling parameter range, is just a minor change for smaller but significant values of the coupling parameter. With these further restrictions, coupled DE models with RP potential are consistent with non-linear observables.

Figures (10)

• Figure 1: Left panel: density parameters for radiation, baryons, DM and DE. Just after radiation equivalence, a DE plateau occurs, due to the dark--dark coupling. In the text this plateau is denominated $\phi$--MDE stage. Right panel: evolution of the state parameter $w$ for different value of $\beta$
• Figure 2: Mass function at $z=0$ for $\alpha =2$ and $\alpha =0.143$. For $\alpha =0.143$ we report three curves, for different values of $\beta$. They are all practically indistinguishable and are well fitted the approximation of Jenkins et al (2001).
• Figure 3: DM and baryons linear perturbation growth for two different values of $\beta$. The dependence on $\alpha$ is weak and could not be appreciated in this plot.
• Figure 4: Linear bias as a function of $\beta$ for three values of $\alpha$. Notice the very weak dependence on $\alpha$.
• Figure 5: Behaviour of the integrated bias $B$ for $\beta =0.15$ and for $\beta =0.25$. Notice that $B$ tends to the predicted linear bias (dashed horizontal lines) at large scales.