BPS Branes in Cosmology

TL;DR

This paper investigates the 5d bulk and brane actions in M/string theory cosmology, clarifies the role of the 4-form field in BPS branes, and argues that a full 5d approach is necessary for consistent ekpyrotic universe models.

Contribution

It demonstrates that adding a potential V(Y) to the bulk brane action is insufficient, requiring a more general 5d metric ansatz for consistent cosmological solutions.

Findings

The effective 4d description suggests contraction in the ekpyrotic scenario.

Adding V(Y) alone does not solve the 5d equations with the usual metric ansatz.

A complete 5d treatment is essential for accurate cosmological modeling.

Abstract

The possibility to study the M/string theory cosmology via 5d bulk & brane action is investigated. The role of the 4-form field in the theory of BPS branes in 5d is clarified. We describe arguments suggesting that the effective 4d description of the universe in the ekpyrotic scenario (hep-th/0103239) should lead to contraction rather than expansion of the universe. To verify these arguments, we study the full 5d action prior to its integration over the 5th dimension. We show that if one adds the potential V(Y) to the action of the bulk brane, then the metric ansatz used in the ekpyrotic scenario does not solve the dilaton and gravitational equations. To find a consistent cosmological solution one must use a more general metric ansatz and a complete 5d description of the brane interaction instead of simply adding an effective 4d bulk brane potential V(Y).