[Show abstract][Hide abstract] ABSTRACT: We present a solution to the vacuum Einstein Equations which represents a collapse of a gravitational wave in 5 dimensions. Depending on the focal length of the wave, the collapse results, either in a black string covered by a horizon, or in a naked singularity which can be removed.
[Show abstract][Hide abstract] ABSTRACT: We study the possibility that a generalised real scalar field minimally coupled to gravity could explain both the galactic and the cosmological dark components of the universe. Within the framework of Einstein's Relativity we model static galactic halos by considering the most general action built from the scalar field and its first derivatives. Although the gravitational configuration is static, the scalar field may be either static, or homogeneous and linear in time. In the case of the static scalar field, the models we look at inevitably posses unphysical negative energies, and we are led to a sort of no-go result. In the case of the homogeneous scalar field, on the contrary, we find that compact objects with flat rotational curves and with the mass and the size of a typical galaxy can be successfully modeled and the Tully-Fisher relation recovered. We further show that the homogeneous scalar field deduced from the galactic halo spacetimes has an action compatible with the kinetic Unified Dark Matter models recently proposed by Scherrer. Therefore, such a homogeneous kinetic Unified Dark Matter not only may correctly mimic the galactic dynamics, but could also be used to model the present-day accelerated expansion in the universe. Comment: Minor cahanges, some misprints corrected. References added. Matches the print version. To appear in Phys. Rev. D
[Show abstract][Hide abstract] ABSTRACT: We show that short-range interactions between the fundamental particles in the universe can drive a period of accelerated expansion. This description fits the early universe. In the present day universe, if one postulates short-range interactions or a sort of "shielded gravity", the picture may repeat. Comment: 4 pages, 1 figure. Discussion expanded. Some references added. Matches version in print. To appear in Phys. Lett. A