Brane-World Gravity

TL;DR

This paper reviews brane-world gravity models where our universe is a 3+1-dimensional surface in higher-dimensional space, exploring their geometry, dynamics, and potential observable effects in cosmology and astrophysics.

Contribution

It provides a comprehensive analysis of simple brane-world models, focusing on warped 5-dimensional models and modifications of gravity at low energies, with implications for testing M theory predictions.

Findings

Analysis of warped 5D brane-world models based on Randall--Sundrum scenarios.

Discussion of low-energy modifications to 4D gravity in Dvali--Gabadadze--Porrati models.

Exploration of co-dimension two branes in 6D models.

Abstract

The observable universe could be a 1+3-surface (the "brane") embedded in a 1+3+\textit{d}-dimensional spacetime (the "bulk"), with Standard Model particles and fields trapped on the brane while gravity is free to access the bulk. At least one of the \textit{d} extra spatial dimensions could be very large relative to the Planck scale, which lowers the fundamental gravity scale, possibly even down to the electroweak ($\sim$ TeV) level. This revolutionary picture arises in the framework of recent developments in M theory. The 1+10-dimensional M theory encompasses the known 1+9-dimensional superstring theories, and is widely considered to be a promising potential route to quantum gravity. At low energies, gravity is localized at the brane and general relativity is recovered, but at high energies gravity "leaks" into the bulk, behaving in a truly higher-dimensional way. This introduces significant changes to gravitational dynamics and perturbations, with interesting and potentially testable implications for high-energy astrophysics, black holes, and cosmology. Brane-world models offer a phenomenological way to test some of the novel predictions and corrections to general relativity that are implied by M theory. This review analyzes the geometry, dynamics and perturbations of simple brane-world models for cosmology and astrophysics, mainly focusing on warped 5-dimensional brane-worlds based on the Randall--Sundrum models. We also cover the simplest brane-world models in which 4-dimensional gravity on the brane is modified at \emph{low} energies -- the 5-dimensional Dvali--Gabadadze--Porrati models. Then we discuss co-dimension two branes in 6-dimensional models.