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Brane cosmology
 
 
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Brane cosmology refers to several theories in particle physics
Particle physics

Particle physics is a branch of physics that studies the elementary particle constituents of matter and radiation, and the interactions between them....
 and cosmology
Physical cosmology

Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of our universe and is concerned with fundamental questions about its formation and evolution....
 motivated by, but not exclusively derived from, superstring theory
Superstring theory

Superstring theory is an attempt to explain all of the Elementary particle and fundamental forces of nature in one theory by modelling them as vibrations of tiny supersymmetry strings....
 and M-theory
M-theory

In theoretical physics, M-theory is a new limit of string theory in which 11 dimensions of spacetime may be identified. Because the dimensionality exceeds the dimensionality of five superstring theories in 10 dimensions, it was originally believed that the 11-dimensional theory is more fundamental and unifies all string theories ....
.

Brane and bulk
The central idea is that the visible, four-dimensional universe
Universe

The universe is defined as everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and physical constants that govern them....
 is restricted to a brane
Brane

In theoretical physics, a membrane, brane, or p-brane is a spatially extended mathematical concept that appears in string theory and its relatives that exists in a static number of dimensions....
 inside a higher-dimensional space, called the "bulk". The additional dimension
Dimension

In mathematics, the dimension of a space is roughly defined as the minimum number of coordinates needed to specify every point within it. For example: a point on the unit circle in the plane can be specified by two Cartesian coordinates but one can make do with a single coordinate , so the circle is 1-dimensional even though it exists in...
s are compact
Compact dimension

In string theory, a model used in theoretical physics, a compact dimension is curled up in itself and very small . Anything moving along this dimension's direction would return to its starting point almost instantaneously, and the fact that the dimension is smaller than the smallest particle means that it cannot be observed by conventional me...
, in which case the observed universe contains the extra dimensions, and then no reference to the bulk is appropriate in this context.






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Encyclopedia


Brane cosmology refers to several theories in particle physics
Particle physics

Particle physics is a branch of physics that studies the elementary particle constituents of matter and radiation, and the interactions between them....
 and cosmology
Physical cosmology

Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of our universe and is concerned with fundamental questions about its formation and evolution....
 motivated by, but not exclusively derived from, superstring theory
Superstring theory

Superstring theory is an attempt to explain all of the Elementary particle and fundamental forces of nature in one theory by modelling them as vibrations of tiny supersymmetry strings....
 and M-theory
M-theory

In theoretical physics, M-theory is a new limit of string theory in which 11 dimensions of spacetime may be identified. Because the dimensionality exceeds the dimensionality of five superstring theories in 10 dimensions, it was originally believed that the 11-dimensional theory is more fundamental and unifies all string theories ....
.

Brane and bulk


The central idea is that the visible, four-dimensional universe
Universe

The universe is defined as everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and physical constants that govern them....
 is restricted to a brane
Brane

In theoretical physics, a membrane, brane, or p-brane is a spatially extended mathematical concept that appears in string theory and its relatives that exists in a static number of dimensions....
 inside a higher-dimensional space, called the "bulk". The additional dimension
Dimension

In mathematics, the dimension of a space is roughly defined as the minimum number of coordinates needed to specify every point within it. For example: a point on the unit circle in the plane can be specified by two Cartesian coordinates but one can make do with a single coordinate , so the circle is 1-dimensional even though it exists in...
s are compact
Compact dimension

In string theory, a model used in theoretical physics, a compact dimension is curled up in itself and very small . Anything moving along this dimension's direction would return to its starting point almost instantaneously, and the fact that the dimension is smaller than the smallest particle means that it cannot be observed by conventional me...
, in which case the observed universe contains the extra dimensions, and then no reference to the bulk is appropriate in this context. In the bulk model, 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


As one of its attractive features, the model can explain the weakness of gravity relative to the other fundamental forces of nature, thus solving the so-called hierarchy problem
Hierarchy problem

In theoretical physics, a hierarchy problem occurs when the fundamental parameters of some Lagrangian mechanics are vastly different from the parameters measured by experiment....
. In the brane picture, the other three forces (electromagnetism
Electromagnetism

Electromagnetism is the physics of the electromagnetic field, a field which exerts a force on Elementary particles with the property of electric charge and which is reciprocally affected by the presence and motion of such particles....
 and the weak
Weak interaction

The weak interaction is one of the four fundamental interactions of nature. In the Standard Model of particle physics, it is due to the exchange of the heavy W and Z bosons....
 and strong
Strong interaction

In particle physics, the strong interaction, or strong force, or color force, holds quarks and gluons together to form protons, neutrons and other particles....
 nuclear forces) are localized on the brane
Brane

In theoretical physics, a membrane, brane, or p-brane is a spatially extended mathematical concept that appears in string theory and its relatives that exists in a static number of dimensions....
, 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 (subatomic or at least sub-millimetre) scales, where less gravitational force has "leaked". Various experiments are currently underway to test this.

Models of brane cosmology


The Randall-Sundrum
Randall-Sundrum model

In physics, Randall-Sundrum models imagine that the real world is a higher-dimensional Universe described by warped geometry. More concretely, our Universe is a five-dimensional anti de Sitter space and the elementary particles except for the graviton are localized on a -dimensional brane or branes....
 (RS1 and RS2; see 5 dimensional warped geometry theory
5 dimensional warped geometery theory

Introduction The 5-dimensional warped geometry theory is a braneworld theory developed by physicist Lisa Randall and Raman Sundrum while trying to solve the hierarchy problem of the Standard Model....
 for a nontechnical explanation of RS1), pre-big bang, ekpyrotic
Ekpyrotic

The ekpyrotic universe, or ekpyrotic scenario, is a physical cosmology about the Origin of the Universe and Shape of the Universe of the universe....
 and cyclic
Cyclic model

Cyclic model refers to several cosmological models in which the universe follows infinite, self-sustaining cycles ....
 scenarios are particular models of brane cosmology which have attracted a considerable amount of attention. The ekpyrotic theory hypothesizes that the origin of the observable universe
Observable universe

In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion....
 occurred when two parallel branes collided.

See also

  • Kaluza-Klein theory
  • M-theory
    M-theory

    In theoretical physics, M-theory is a new limit of string theory in which 11 dimensions of spacetime may be identified. Because the dimensionality exceeds the dimensionality of five superstring theories in 10 dimensions, it was originally believed that the 11-dimensional theory is more fundamental and unifies all string theories ....
  • String theory
    String theory

    String theory is a developing branch of theoretical physics that combines quantum mechanics and general relativity into a quantum gravity. The String s of string theory are one-dimensional oscillating lines, but they are no longer considered fundamental to the theory, which can be formulated in terms of points or surfaces too....
  • Loop quantum cosmology
    Loop quantum cosmology

    Loop quantum cosmology is a finite, symmetry reduced model of loop quantum gravity, theorizing that our universe expands and then eventually contracts over and over, rebirthing for infinity....


External links

  • – Cosmological consequences of the brane world scenario are reviewed in a pedagogical manner.
  • – These notes (32 pages) give an introductory review on brane cosmology.
  • – Lectures (24 pages) presented at the First Aegean Summer School on Cosmology, Samos
    Samos Island

    Samos is a Greece island in the North Aegean sea, south of Chios, north of Patmos and the Dodecanese, and off the Ionian coast of Turkey....
    , September 2001.