Brane cosmology refers to several theories
in particle physics and cosmology related
to string theory, superstring theory and M-theory.
== Brane and bulk ==
The central idea is that the visible, three-dimensional
universe is restricted to a brane inside a
higher-dimensional space, called the "bulk"
(also known as "hyperspace").
If the additional dimensions are compact,
then the observed universe contains the extra
dimension, and then no reference to the bulk
is appropriate.
In the bulk model, at least some of the extra
dimensions are extensive (possibly infinite),
and other branes may be moving through this
bulk.
Interactions with the bulk, and possibly with
other branes, can influence our brane and
thus introduce effects not seen in more standard
cosmological models.
== Why gravity is weak and the cosmological
constant is small ==
Some versions of brane cosmology, based on
the large extra dimension idea, can explain
the weakness of gravity relative to the other
fundamental forces of nature, thus solving
the hierarchy problem.
In the brane picture, the electromagnetic,
weak and strong nuclear force are localized
on the brane, but gravity has no such constraint
and propagates on the full spacetime, called
bulk.
Much of the gravitational attractive power
"leaks" into the bulk.
As a consequence, the force of gravity should
appear significantly stronger on small (subatomic
or at least sub-millimetre) scales, where
less gravitational force has "leaked".
Various experiments are currently under way
to test this.
Extensions of the large extra dimension idea
with supersymmetry in the bulk appears to
be promising in addressing the so-called cosmological
constant problem.
== Models of brane cosmology ==
One of the earliest documented attempts to
apply brane cosmology as part of a conceptual
theory is dated to 1983.The authors discussed
the possibility that the Universe has
(
3
+
N
)
+
1
{\displaystyle (3+N)+1}
dimensions, but ordinary particles are confined
in a potential well which is narrow along
N
{\displaystyle N}
spatial directions and flat along three others,
and proposed a particular five-dimensional
model.
In 1998/99 Merab Gogberashvili published on
arXiv a number of articles where he showed
that if the Universe is considered as a thin
shell (a mathematical synonym for "brane")
expanding in 5-dimensional space then there
is a possibility to obtain one scale for particle
theory corresponding to the 5-dimensional
cosmological constant and Universe thickness,
and thus to solve the hierarchy problem.
It was also shown that the four-dimensionality
of the Universe is the result of the stability
requirement found in mathematics since the
extra component of the Einstein field equations
giving the confined solution for matter fields
coincides with one of the conditions of stability.
In 1999 there were proposed the closely related
Randall–Sundrum scenarios, RS1 and RS2.
(See Randall–Sundrum model for a nontechnical
explanation of RS1).
These particular models of brane cosmology
have attracted a considerable amount of attention.
Later, the pre-big bang, ekpyrotic and cyclic
proposals appeared.
The ekpyrotic theory hypothesizes that the
origin of the observable universe occurred
when two parallel branes collided.
== Empirical tests ==
As of now, no experimental or observational
evidence of large extra dimensions, as required
by the Randall–Sundrum models, has been
reported.
An analysis of results from the Large Hadron
Collider in December 2010 severely constrains
the black holes produced in theories with
large extra dimensions..
The recent multi-messenger gravitational wave
event GW170817 has also been used to put weak
limits on large extra dimensions.
== See also ==
Kaluza–Klein theory
Loop quantum cosmology
