Unifying dark matter and dark energy with non-canonical scalars

Non-canonical scalar fields with the Lagrangian L = X-alpha -V (phi), 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 derive a general condition on the potential in order to facilitate the kinetic term X-alpha to play the role of dark matter, while the potential term V (phi) playing the role of dark energy at late times. We demonstrate that simple potentials including V = V-0 coth(2) phi 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.