Abstract
We explore observational constraints on a cosmological brane world scenario in which the bulk is not empty. Rather, exchange of mass energy between the bulk and the brane is allowed. The evolution of matter fields to an observer on the brane is then modified due to new terms in the energy-momentum tensor describing this exchange. We investigate the constraints from various cosmological observations on the flow of matter from the bulk into the brane. Interestingly, we show that it is possible to have a $\ensuremath{\Lambda}=0$ cosmology to an observer in the brane which satisfies standard cosmological constraints including the cosmic microwave background (CMB) temperature fluctuations, Type Ia supernovae at high redshift, and the matter power spectrum. This model even accounts for the observed suppression of the CMB power spectrum at low multipoles. In this cosmology, the observed cosmic acceleration is attributable to the flow of matter from the bulk to the brane. A peculiar aspect of this cosmology is that the present dark-matter content of the Universe may be significantly larger than that of a $\ensuremath{\Lambda}\mathrm{CDM}$ cosmology. Its influence, however, is offset by the dark-radiation term. Possible additional observational tests of this new cosmological paradigm are suggested.
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