Unifying dark matter and dark energy with non-canonical scalars

Abstract

Non-canonical scalar fields with the Lagrangian L= Xα- V(ϕ) , possess the attractive property that the speed of sound, cs2=(2α-1)-1, can be exceedingly small for large values of α. This allows a non-canonical field to cluster and behave like warm/cold dark matter on small scales. We derive a general condition on the potential in order to facilitate the kinetic term Xα to play the role of dark matter, while the potential term V(ϕ) playing the role of dark energy at late times. We demonstrate that simple potentials including V= Vcoth 2ϕ and a Starobinsky-type potential can unify dark matter and dark energy. Cascading dark energy, in which the potential cascades to lower values in a series of discrete steps, can also work as a unified model.