Brane World Scenarios and the Cosmological Constant

TL;DR

This paper explores how brane world scenarios can naturally produce a flat four-dimensional universe without fine tuning, by adjusting an integration constant related to the cosmological constant, and discusses potential string theory realizations.

Contribution

It highlights the role of integration constants in brane world models as a mechanism for achieving a flat universe without fine tuning, revisiting an old idea in a new context.

Findings

The effective cosmological constant can be adjusted via a constant of integration.

Brane world scenarios can produce Poincare invariant four-dimensional worlds without fine tuning.

Potential string theory models may realize this mechanism.

Abstract

Brane world scenarios offer a way of ensuring that a Poincare invariant four dimensional world can emerge, without fine tuning, as a solution to the equations of motion of an effective action. We discuss the different ways in which this happens, and point out that the underlying reason is that there is a contribution to the effective cosmological constant which is a constant of integration, that maybe adjusted to ensure a flat space solution. Basically this is an old idea revived in a new context and we speculate that there may be string scenarios that provide a concrete realization of it. Finally we discuss to what extent this is a solution to the cosmological constant problem.