Constraints on late time violations of the equivalence principle in the dark sector

If dark energy is dynamical due to the evolution of a scalar field, then in general it is expected that the scalar is coupled to matter. While couplings to the standard model particles are highly constrained by local experiments, bounds on couplings to dark matter (DM) are only obtained from cosmological observations and they are consequently weaker. It has recently been pointed out that the coupling itself can become non-zero only at the time of dark energy domination, due to the evolution of dark energy itself, leading to a violation of the equivalence principle (EP) in the dark sector at late times. In this paper we study a specific model and show that such late-time violations of the EP in the DM sector are not strongly constrained by the evolution of the cosmological background and by observables in the linear regime (e.g. from the cosmic microwave background radiation), although the model is not preferred over ΛCDM. A study of perturbations in non-linear regime is necessary to constrain late-time violations of the equivalence principle much more strongly.