Asymmetric Swiss-cheese brane-worlds

TL;DR

This paper explores an asymmetric brane-world cosmology with black holes, revealing how bulk asymmetry influences universe expansion, pressure singularities, and constraints from nucleosynthesis and observations.

Contribution

It introduces the pressure singularity censorship to bound bulk asymmetry and analyzes its impact on early universe behavior and observational constraints.

Findings

Asymmetry in bulk cosmological constants affects pressure singularities.

Pressure singularity censorship bounds bulk asymmetry.

Constraints from nucleosynthesis limit brane tension and asymmetry.

Abstract

We study a brane-world cosmological scenario with local inhomogeneities represented by black holes. The brane is asymmetrically embedded into the bulk. The black strings/cigars penetrating the Friedmann brane generate a Swiss-cheese type structure. This universe forever expands and decelerates, as its general relativistic analogue. The evolution of the cosmological fluid however can proceed along four branches, two allowed to have positive energy density, one of them having the symmetric embedding limit. On this branch a future pressure singularity can arise for either (a) a difference in the cosmological constants of the cosmological and black hole brane regions (b) a difference in the left and right bulk cosmological constants. While the behaviour (a) can be avoided by a redefinition of the fluid variables, (b) establishes a critical value of the asymmetry over which the pressure singularity occurs. We introduce the pressure singularity censorship which bounds the degree of asymmetry in the bulk cosmological constant. We also show as a model independent generic feature that the asymmetry source term due to the bulk cosmological constant increases in the early universe. In order to obey the nucleosynthesis constraints, the brane tension should be constrained therefore both from below and from above. With the maximal degree of asymmetry obeying the pressure singularity censorship, the higher limit is 10 times the lower limit. The degree of asymmetry allowed by present cosmological observations is however much less, pushing the upper limit to infinity.