Title: Variable sound speed in interacting dark energy models—

Abstract: The nature of both Dark Matter and Dark Energy remains one of the most open questions in cosmology. A priori there is no reason why these two sectors should not interact and current experimental bounds leave some parameter space to be explored. Additionally, given the fact that an interacting dark energy and dark matter sector seems to be able to resolve recent “tensions” between high and low redshift probes, these interacting models should be studied with renewed vigour. I will discuss a self-consistent and physical approach to interacting dark energy models described by a Lagrangian, focusing on a pure momentum case. I will focus on the phenomenology of these models, examining how different forms of the coupling can effect the scalar field Dark Energy, by introducing a non unitary sound speed. I will also examine the cosmological implications of these models