Dark energy and dark matter from an inhomogeneous dilaton

A cosmological scenario is proposed in which the dark matter (DM) and dark energy (DE) of the Universe are two simultaneous manifestations of an inhomogeneous dilaton. The equation of state of the field is scale dependent and pressureless at galactic and larger scales and it has a negative pressure as DE at very large scales. The dilaton drives an initial inflationary phase followed by a kinetic-energy-dominated one, as in the quintessential inflation model introduced by Peebles and Vilenkin, and soon after the end of inflation particle production seeds the first inhomogeneities that lead to galaxy formation. The dilaton is trapped near the minimum of the potential where it oscillates like a massive field, and the excess of kinetic energy is dissipated via the mechanism of gravitational cooling first introduced by Seidel and Suen. The inhomogeneities therefore behave like solitonic oscillations around the minimum of the potential, known as oscillatons, which we propose account for most of the DM in galaxies. Those regions where the dilaton does not transform enough kinetic energy into reheating, or carry an excess of it from regions that have cooled, evolve to the tail of the potential as DE, driving the acceleration of the Universe.