Hydrodynamical N-body simulations of coupled dark energy cosmologies

TL;DR

This work introduces fully self-consistent hydrodynamic N-body simulations of coupled dark energy cosmologies, implementing a scalar field that interacts with cold dark matter using a constant coupling. By modifying GADGET-2 and a Boltzmann code (CMBEASY), the authors capture both the altered background expansion and the nonlinear growth of structure, including a fifth force between CDM particles and a time-varying CDM mass. Key findings show a persistent CDM–baryon bias, lower halo inner densities, larger halo scale radii, and reduced baryon fractions in halos as the coupling strengthens, potentially alleviating small-scale tensions with LCDM. The methodology and results offer a robust framework to constrain coupled dark energy models using nonlinear structure formation data.

Abstract

If the accelerated expansion of the Universe at the present epoch is driven by a dark energy scalar field, there may well be a non-trivial coupling between the dark energy and the cold dark matter (CDM) fluid. Such interactions give rise to new features in cosmological structure growth, like an additional long-range attractive force between CDM particles, or variations of the dark matter particle mass with time. We have implemented these effects in the N-body code GADGET-2 and present results of a series of high-resolution N-body simulations where the dark energy component is directly interacting with the cold dark matter. As a consequence of the new physics, CDM and baryon distributions evolve differently both in the linear and in the nonlinear regime of structure formation. Already on large scales a linear bias develops between these two components, which is further enhanced by the nonlinear evolution. We also find, in contrast with previous work, that the density profiles of CDM halos are less concentrated in coupled dark energy cosmologies compared with LCDM, and that this feature does not depend on the initial conditions setup, but is a specific consequence of the extra physics induced by the coupling. Also, the baryon fraction in halos in the coupled models is significantly reduced below the universal baryon fraction. These features alleviate tensions between observations and the LCDM model on small scales. Our methodology is ideally suited to explore the predictions of coupled dark energy models in the fully non-linear regime, which can provide powerful constraints for the viable parameter space of such scenarios.

Figures (18)

• Figure 1: Left panel: Hubble functions as a function of redshift for the different coupled DE models with constant coupling investigated in this work and described in Table \ref{['Simulations_Table']} as compared to $\Lambda$CDM (black curve). Right panel: Ratio of the Hubble functions of each coupled DE model to the Hubble function for the reference $\Lambda$CDM cosmology as a function of redshift.
• Figure 2: Mass correction as a function of redshift for the coupled DE models with constant coupling.
• Figure 3: Matter power spectra at $z=60$ for interacting DE models with constant coupling as computed by CMBEASY.
• Figure 4: Comparison of the function $f(a)$ with its usual approximation $f=\Omega _{m}^{0.55}$ and with the new fit of Eqn. (\ref{['gf_fit']}) for a $\Lambda$CDM model and for a series of coupled DE models.
• Figure 5: Evolution of the growth function with redshift for the seven models of coupled DE investigated with the low-resolution simulations ran with our modified version of GADGET-2. The solid lines are the total growth functions as evaluated numerically with CMBEASY, while the triangles are the growth function evaluated from the simulations. The relative accuracy in reproducing the theoretical prediction is of the order of a few percent irrespective of the coupling value $\beta _{c}$.