A de Sitter tachyon thick braneworld

Abstract

Among the multiple 5D thick braneworld models that have been proposed in the last years,in order to address several open problems in modern physics, there is a specific one involving atachyonic bulk scalar field. Delving into this framework, a thick braneworld with a cosmologicalbackground induced on the brane is here investigated. The respective field equations — derivedfrom the model with a warped 5D geometry — are highly non-linear equations, admitting anon-trivial solution for the warp factor and the tachyon scalar field as well, in a de Sitter 4Dcosmological background. Moreover, the non-linear tachyonic scalar field, that generates the branein complicity with warped gravity, has the form of a kink-like configuration. Notwithstanding, thenon-linear field equations restricting character does not allow one to easily find thick branesolutions with a decaying warp factor which leads to the localization of 4D gravity and othermatter fields. We derive such a thick brane configuration altogether in this tachyon-gravitysetup. When analyzing the spectrum of gravity fluctuations in the transverse traceless sector, the4D gravity is shown to be localized due to the presence of a single zero mode bound state,separated by a continuum of massive Kaluza-Klein (KK) modes by a mass gap. It contrasts withprevious results, where there is a KK massive bound excitation providing no clear physicalinterpretation. The mass gap is determined by the scale of the metric parameter H. Finally, thecorrections to Newton's law in this model are computed and shown to decay exponentially. It is infull compliance to corrections reported in previous results (up to a constant factor) withinsimilar braneworlds with induced 4D de Sitter metric, despite the fact that the warp factor andthe massive modes have a different form.