Purely KinetickEssence as Unified Dark Matter

We examine k-essence models in which the Lagrangian p is a function only of the derivatives of a scalar field φ and does not depend explicitly on φ. The evolution of φ 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 ρ scaling with the scale factor a as ρ=ρ₀+ρ₁(a/a₀)^-3, but with a sound speed cₛ²1 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 cₛ1 serves to suppress cosmic microwave background fluctuations on large-angular scales.