Causality constraints on K-essence: Superfluid dark matter to modified gravity

ABSTRACT: Superfluid dark matter is a novel way of reconciling the apparent discrepancy between galactic phenomenology and LCDM cosmology. The advantage of these models is that on cosmological scales they can play the role of CDM, while on galactic scales there is a phase transition to a superfluid, and the nontrivial dynamics in this regime can reproduce the observed Tully-Fisher relation.

However, we find in general these models exhibit a breakdown of causality, which indicates they would resist having any standard UV completion. Constructing a class of causal k-essence theories, instead of dark matter we can consider these as scalar modifications of gravity.

In these theories the strength of the new force grows with increasing density, becoming most relevant in the densest astrophysical environments. We consider various observational constraints on this class of modified gravity theories, with the strongest constraint coming from equilibrium of neutron stars.