| Brane cosmology is a protoscience motivated by, but not
rigorously derived from, superstring theory and M-theory. The idea is to solve problems in cosmology using speculative particle physics
theories and in turn use cosmological observations to motivate ideas in string theory.
The central idea is that our visible, four-dimensional universe is entirely
restricted to a brane in a higher-dimensional space, called the bulk. The additional dimensions may be compact. 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.
As one of its attractive features, the model can "explain" the weakness of gravity
relative to the other fundamental forces of nature. In the brane picture, the other three forces (electromagnetism and the weak and strong nuclear forces) are
localised on the brane, but gravity has no such constraint and so much of its attractive
power "leaks" into the bulk. As a consequence, the force of gravity should appear significantly stronger on small
(sub-millimetre) scales, where less gravitational force has "leaked". Various experiments are currently underway to test
this.
The Randall-Sundrum, pre-big bang, ekpyrotic and cyclic scenarios are particular models
of brane cosmology which have attracted a considerable amount of attention.
See also: compactification, D-brane, M-brane,
Kaluza-Klein, holographic principle.
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