Simulating momentum exchange in the dark sector

Low-energy interactions between particles are often characterized 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 non-linear 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 non-linear 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 haloes. The opposite trend is found for models characterized by an increase of the linear matter power spectrum normalization. More quantitatively, we find that power spectrum deviations at non-linear scales (k ≈ 10 h Mpc−1) are roughly 10 times larger than their linear counterparts, exceeding 100 per cent 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 per cent, respectively, over a wide range of masses. Therefore, we conclude that non-linear 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.