Non-singular Brane/Bubble Cosmologies in the Presence of String Cloud

We investigate the cosmological evolution of a four-dimensional brane/bubble universe embedded in a five-dimensional Anti-de Sitter bulk containing a uniformly distributed cloud of strings. The endpoints of the strings attached to the brane induce an effective matter component in the four-dimensional universe. We derive the Friedmann equations for both the Randall-Sundrum braneworld and the shellworld (dark bubble) scenarios and show that one can obtain a nonsingular bounce in both the brane and bubble universes under certain conditions of the black hole mass and the four-dimensional cosmological constant. Though the bounce due to the spatial curvature occurs outside the black hole horizon, however, the bounce obtained due to the presence of string cloud in the braneworld scenario suffers from the Cauchy horizon instability as the bounce occurs inside the Cauchy horizon of the black hole. In contrast, for the shellworld scenario, the bounce can be made to occur outside the black hole horizon, thereby avoiding the horizon instability. We also analyze the field fluctuations on the brane/bubble and show that the fluctuations remain finite and well-behaved at the bounce point.