Cascading Cosmology

TL;DR

This paper develops a covariant 5D effective action for the 6D cascading gravity model, deriving cosmological solutions and analyzing the role of the brane-bending mode i in cosmic acceleration, highlighting potential for acceleration without a cosmological constant.

Contribution

It introduces a fully covariant 5D effective action for cascading gravity, enabling detailed cosmological analysis and revealing the impact of the i mode on universe acceleration.

Findings

The i mode influences late-time cosmic acceleration.

A critical singularity prevents i from dominating completely.

Potential for acceleration without a cosmological constant exists.

Abstract

We develop a fully covariant, well-posed 5D effective action for the 6D cascading gravity brane-world model, and use this to study cosmological solutions. We obtain this effective action through the 6D decoupling limit, in which an additional scalar degree mode, \pi, called the brane-bending mode, determines the bulk-brane gravitational interaction. The 5D action obtained this way inherits from the sixth dimension an extra \pi self-interaction kinetic term. We compute appropriate boundary terms, to supplement the 5D action, and hence derive fully covariant junction conditions and the 5D Einstein field equations. Using these, we derive the cosmological evolution induced on a 3-brane moving in a static bulk. We study the strong- and weak-coupling regimes analytically in this static ansatz, and perform a complete numerical analysis of our solution. Although the cascading model can generate an accelerating solution in which the \pi field comes to dominate at late times, the presence of a critical singularity prevents the \pi field from dominating entirely. Our results open up the interesting possibility that a more general treatment of degravitation in a time-dependent bulk, or taking into account finite brane-thickness effects, may lead to an accelerating universe without a cosmological constant.