# Brane-World Gravity

**Update available: **
http://www.livingreviews.org/lrr-2010-5

**Roy Maartens
**

Institute of Cosmology & Gravitation

Portsmouth University

Portsmouth PO1 2EG, U.K.

http://www.icg.port.ac.uk/people/staff/maartens.html

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Abstract

The observable universe could be a 1+3-surface (the “brane”) embedded in a
1+3+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 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 ( 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. General relativity cannot describe gravity
at high enough energies and must be replaced by a quantum gravity theory, picking up
significant corrections as the fundamental energy scale is approached. 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 discusses 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.