Brane cosmology refers to several theories in

particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

and

cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe, the cosmos. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', with the meaning of "a speaking of the wo ...

related to

string theory In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and intera ...

superstring theory Superstring theory is an attempt to explain all of the particles and fundamental forces of nature in one theory by modeling them as vibrations of tiny supersymmetric strings. 'Superstring theory' is a shorthand for supersymmetric string t ...

and

M-theory In physics, M-theory is a theory that unifies all Consistency, consistent versions of superstring theory. Edward Witten first conjectured the existence of such a theory at a string theory conference at the University of Southern California in 1 ...

Brane and bulk

The central idea is that the visible, four-dimensional

spacetime In physics, spacetime, also called the space-time continuum, is a mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum. Spacetime diagrams are useful in visualiz ...

is restricted to a

brane In string theory and related theories (such as supergravity), a brane is a physical object that generalizes the notion of a zero-dimensional point particle, a one-dimensional string, or a two-dimensional membrane to higher-dimensional objec ...

inside a higher-dimensional space, called the "bulk" (also known as "hyperspace"). If the additional

dimension In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus, a line has a dimension of one (1D) because only one coo ...

s are

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, 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 In particle physics and string theory (M-theory), the Arkani-Hamed, Dimopoulos, Dvali model (ADD), also known as the model with large extra dimensions (LED), is a model framework that attempts to solve the hierarchy problem (''Why is the force of ...

idea, can explain the weakness of

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

relative to the other

fundamental forces In physics, the fundamental interactions or fundamental forces are interactions in nature that appear not to be reducible to more basic interactions. There are four fundamental interactions known to exist: * gravity * electromagnetism * weak int ...

of nature, thus solving the hierarchy problem. In the brane picture, the

electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

weak Weak may refer to: Songs * Weak (AJR song), "Weak" (AJR song), 2016 * Weak (Melanie C song), "Weak" (Melanie C song), 2011 * Weak (SWV song), "Weak" (SWV song), 1993 * Weak (Skunk Anansie song), "Weak" (Skunk Anansie song), 1995 * "Weak", a son ...

and strong nuclear force are localized on the brane, but gravity has no such constraint and propagates on the full spacetime, called the 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". In

2004 2004 was designated as an International Year of Rice by the United Nations, and the International Year to Commemorate the Struggle Against Slavery and Its Abolition (by UNESCO). Events January * January 3 – Flash Airlines Flight 60 ...

, various experiments were under way to test this. Extensions of the large extra dimension idea with

supersymmetry Supersymmetry is a Theory, theoretical framework in physics that suggests the existence of a symmetry between Particle physics, particles with integer Spin (physics), spin (''bosons'') and particles with half-integer spin (''fermions''). It propo ...

in the bulk appear 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

dimensions, but ordinary particles are confined in a potential well which is narrow along

N

spatial directions and flat along three others, and proposed a particular five-dimensional model. In 1998/99, Merab Gogberashvili published a number of articles on

arXiv arXiv (pronounced as "archive"—the X represents the Chi (letter), Greek letter chi ⟨χ⟩) is an open-access repository of electronic preprints and postprints (known as e-prints) approved for posting after moderation, but not Scholarly pee ...

where he showed that if we consider the universe as a thin shell (mathematical

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for "brane") expanding in 5-dimensional space then there is a possibility of obtaining a single scale for particle theory corresponding to the 5-dimensional

cosmological constant In cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: ), alternatively called Einstein's cosmological constant, is a coefficient that Albert Einstein initially added to his field equations of general rel ...

and the thickness of the universe, thus solving the hierarchy problem. Gogberashvili also showed that the four-dimensionality of the universe is the result of the

stability Stability may refer to: Mathematics *Stability theory, the study of the stability of solutions to differential equations and dynamical systems ** Asymptotic stability ** Exponential stability ** Linear stability **Lyapunov stability ** Marginal s ...

requirement found in mathematics since the extra component of the

Einstein field equations In the General relativity, general theory of relativity, the Einstein field equations (EFE; also known as Einstein's equations) relate the geometry of spacetime to the distribution of Matter#In general relativity and cosmology, matter within it. ...

giving the confined solution for

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic pa ...

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. For instance, the related Chung-Freese model, which has applications for spacetime metric engineering, followed in 2000. Later, the ekpyrotic and cyclic proposals appeared. The ekpyrotic theory hypothesizes that the origin of the

observable universe The observable universe is a Ball (mathematics), spherical region of the universe consisting of all matter that can be observation, observed from Earth; the electromagnetic radiation from these astronomical object, objects has had time to reach t ...

occurred when two parallel branes collided.

Empirical tests

As of now, no experimental or observational evidence of

s, as required by the Randall–Sundrum models, has been reported. An analysis of results from the

Large Hadron Collider The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It was built by the CERN, European Organization for Nuclear Research (CERN) between 1998 and 2008, in collaboration with over 10,000 scientists, ...

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.

References

External links

Brane cosmology on arxiv.org
* – Cosmological consequences of the brane world scenario are reviewed in a pedagogical manner.
Dimensional Shortcuts – evidence for sterile neutrino; (August 2007; Scientific American)
* – These notes (32 pages) give an introductory review on brane cosmology. * – Lectures (24 pages) presented at the First Aegean Summer School on Cosmology,

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