Antigravitating braneworld solutions for a de Sitter brane in scalar-tensor gravity

Abstract

In the context of a five-dimensional theory with a scalar field nonminimally coupled to gravity, we look for solutions that describe novel black-string or maximally symmetric solutions in the bulk. The brane line element is found to describe a Schwarzschild--(anti)--de Sitter spacetime, and, here, we choose to study solutions with a positive four-dimensional cosmological constant. We consider two different forms of the coupling function of the scalar field to the bulk scalar curvature, a linear one and a quadratic one. In the linear case, we find solutions where the theory, close to our brane, mimics an ordinary gravitational theory with a minimally coupled scalar field giving rise to an exponentially decreasing warp factor in the absence of a negative bulk cosmological constant. The solution is characterized by the presence of a normal gravity regime around our brane and an antigravitating regime away from it. In the quadratic case, there is no normal-gravity regime at all; however, scalar field and energy-momentum tensor components are well defined and an exponentially decreasing warp factor emerges again. We demonstrate that, in the context of this theory, the emergence of a positive cosmological constant on our brane is always accompanied by an antigravitating regime in the five-dimensional bulk.