Purely kinetic k essence as unified dark matter -: art. no. 011301

We examine k-essence models in which the Lagrangian p is a function only of the derivatives of a scalar field phi and does not depend explicitly on phi. The evolution of phi for an arbitrary functional form for p can be given in terms of an exact analytic solution. For quite general conditions on the functional form of p, such models can evolve to a state characterized by a density rho scaling with the scale factor a as rho=rho(0)+rho(1)(a/a(0))(-3), but with a sound speed c(s)(2)<<1 at all times. Such models can serve as a unified model for dark matter and dark energy, while avoiding the problems of the generalized Chaplygin gas models, which are due to a non-negligible sound speed in these models. A dark-energy component with c(s)<<1 serves to suppress cosmic microwave background fluctuations on large-angular scales.