Structure formation in multiple dark matter cosmologies with long-range scalar interactions

Long-range scalar forces with gravitational strength can be ruled out by accurate tests of scalar-tensor theories in the Solar system. However, such tests are based on the motion of celestial bodies made of standard baryonic particles, which leaves room for possible scalar interactions in the dark sector of the universe if a new scalar degree of freedom couples only to dark matter particles. In particular, an interaction between cold dark matter (CDM) and a classical scalar field playing the role of the cosmic dark energy (DE) might provide such long-range dark interactions without conflicting with Solar system bounds. Although presently available observations allow us to constrain such interactions to a few per cent of the gravitational strength, some recent studies have shown that if CDM is composed by two different particle species having opposite couplings to the DE field, such tight constraints can be considerably relaxed, allowing for long-range scalar forces of order gravity without significantly affecting observations both at the background and at the linear perturbations level. In the present work, we extend the investigation of such multiple dark matter scenarios to the non-linear regime of structure formation, by presenting the first N-body simulations ever performed for these cosmologies. Our results highlight some characteristic footprints of long-range scalar forces that arise only in the non-linear regime for specific models that would be otherwise practically indistinguishable from the standard ΛCDM scenario both in the background and in the growth of linear density perturbations. Among these effects, the formation of `mirror' cosmic structures in the two CDM species, the suppression of the non-linear matter power spectrum at k ≳ 0.1 h Mpc<SUP>-1</SUP> and the fragmentation of collapsed haloes, represent peculiar features that might provide a direct way to constrain this class of cosmological models.