MR. SATADRU BAG

We obtain a closed system of equations for scalar perturbations in a multi-component braneworld. Our braneworld possesses a phantom-like equation of state at late times, weff < ¿1, but no big-rip future singularity. In addition to matter and radiation, the braneworld possesses a new effective degree of freedom ¿ the `Weyl fluid' or `dark radiation'. Setting initial conditions on super-Hubble spatial scales at the epoch of radiation domination, we evolve perturbations of radiation, pressureless matter and the Weyl fluid until the present epoch. We observe a gradual decrease in the amplitude of the Weyl-fluid perturbations after Hubble-radius crossing, which results in a negligible effect of the Weyl fluid on the evolution of matter perturbations on spatial scales relevant for structure formation. Consequently, the quasi-static approximation of Koyama and Maartens provides a good fit to the exact results during the matter-dominated epoch. We find that the late-time growth of density perturbations on the brane proceeds at a faster rate than in ¿CDM. Additionally, the gravitational potentials ¿ and ¿ evolve differently on the brane than in ¿CDM, for which ¿ = ¿. On the brane, by contrast, the ratio ¿ / ¿ exceeds unity during the late matter-dominated epoch (z $\lesssim$ 50). These features emerge as smoking gun tests of phantom brane cosmology and allow predictions of this scenario to be tested against observations of galaxy clustering and large-scale structure.