Hydrodynamical N-body simulations of coupled dark energy cosmologies

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.