Unifying Dark Matter and Dark Energy with non-Canonical Scalars

Abstract

Non-canonical scalar fields with the Lagrangian $\mathcal{L}=X^{\alpha }-V(\varphi )$, possess the attractive property that the speed of sound, $c_s^2=(2\alpha -1{)}^{-1}$, can be exceedingly small for large values of $\alpha$. This allows a non-canonical field to cluster and behave like warm/cold dark matter on small scales. We demonstrate that simple potentials including $V=V_0{\coth }^2\varphi$ 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. In all of these models the kinetic term $X^{\alpha }$ plays the role of dark matter, while the potential term $V(\varphi )$ plays the role of dark energy.

Comments: 19 pages, 9 figures. Comprehensively revised and expanded. New section on Cascading dark energy