Brane-world cosmology with black strings
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Abstract
We consider the simplest scenario when black strings/cigars penetrate the cosmological brane. As a result, the brane has a Swiss-cheese structure, with Schwarzschild black holes immersed in a Friedmann-Lema\^{\i}tre-Robertson-Walker brane. There is no dark radiation in the model, the cosmological regions of the brane are characterized by a cosmological constant $\ensuremath{\Lambda}$ and flat spatial sections. Regardless of the value of $\ensuremath{\Lambda}$, these brane-world universes forever expand and forever decelerate. The totality of source terms in the modified Einstein equation sum up to a dust, establishing a formal equivalence with the general relativistic Einstein-Straus model. However in this brane-world scenario with black strings the evolution of the cosmological fluid strongly depends on $\ensuremath{\Lambda}$. For $\ensuremath{\Lambda}\ensuremath{\le}0$ it has positive energy density $\ensuremath{\rho}$ and negative pressure $p$ and at late times it behaves as in the Einstein-Straus model. For (not too high) positive values of $\ensuremath{\Lambda}$ the cosmological evolution begins with positive $\ensuremath{\rho}$ and negative $p$, but this is followed by an epoch with both $\ensuremath{\rho}$ and $p$ positive. Eventually, $\ensuremath{\rho}$ becomes negative, while $p$ stays positive. A similar evolution is present for high positive values of $\ensuremath{\Lambda}$, however in this case the evolution ends in a pressure singularity, accompanied by a regular behavior of the cosmic acceleration. This is a novel type of singularity appearing in brane-worlds.