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. 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 low energies – the 5-dimensional
Dvali–Gabadadze–Porrati models. Then we discuss co-dimension two branes in 6-dimensional
models.
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