A rolling tachyon field for both dark energy and dark halos of galaxies

Abstract

Distance measurements to type Ia supernovae (SNe Ia) indicate that the universe is accelerating and it is spatially flat. Otherwise, the rotation curves for galaxies or galaxy clusters reveal the existence of a dark matter component. Then, approximately 0.35 of the total energy density of the universe consists of dark matter, and 0.65 of the energy density corresponds to dark energy. These two dark components of the universe have different properties such that: dark matter clusters gravitationally at galactic scales while dark energy, dominating at large scale, does not cluster gravitationally. It seems difficult to find a candidate which explains the observations at both large and small scales. In this paper we take into account a scalar field, more precisely a rolling tachyon arising from string theory, which seems be capable to explain the observations at different scales. We examine this scalar field behavior at galactic scales and we fit the rotational curve data of spiral galaxies. Our results are comparable with the curves obtained on one hand from a complex scalar field, which only plays the role of the dark matter at galactic scales, and on the other hand from the MOND model.