Brane cosmology, varying speed of light and inflation in models with one or more extra dimensions

Abstract

We summarise the approach to brane cosmology known as ``mirage cosmology'' and use it to determine the Friedmann equation on a 3-brane embedded in different bulk spacetimes all with one or more extra dimensions. Usually, when there is more than one extra dimension the junction conditions, central to the usual brane world scenarios, are difficult to apply. This problem does not arise in mirage cosmology because the brane is treated as a ``test particle'' in the background spacetime. We discuss in detail the dynamics of a brane embedded in two specific 10D bulk spacetimes, namely Sch-AdS$_5 \times$S$_5$ and a rotating black hole, and from the dynamics--which are now rather more complicated since the brane can move in all the extra dimensions--determine the new ``dark fluid'' terms in the brane Friedmann equation. Some of these, such as the cosmological constant term, are seen to be bulk dependent. However, for both bulks we show that there exists a critical brane angular momentum, $\ell_c$, and discuss its significance. We then show explicitly how this mirage cosmology approach matches with the familiar junction condition approach when there is just one extra dimension. The issue of a varying speed of light in mirage cosmology is reviewed and we find a scenario in which $c_{\bf eff}$ always increases, tending asymptotically to a constant $c_0$ as the universe expands. Finally some comments are made regarding brane inflation and limitations of the mirage cosmology approach are also discussed.