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Brane cosmology refers to several theories inparticle physics andcosmology related tostring theory,superstring theory andM-theory.
The central idea is that the visible, four-dimensionalspacetime is restricted to abrane inside ahigher-dimensional space, called the "bulk" (also known as "hyperspace"). If the additionaldimensions arecompact, 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.
Some versions of brane cosmology, based on thelarge extra dimension idea, can explain the weakness ofgravity relative to the otherfundamental forces of nature, thus solving thehierarchy problem. In the brane picture, theelectromagnetic,weak andstrong 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". Various experiments are currently under way to test this.[1] Extensions of the large extra dimension idea withsupersymmetry in the bulk appear to be promising in addressing the so-calledcosmological constant problem.[2][3][4]
One of the earliest documented attempts to apply brane cosmology as part of a conceptual theory is dated to 1983.[5]
The authors discussed the possibility that the Universe has dimensions, but ordinary particles are confined in a potential well which is narrow along spatial directions and flat along three others, and proposed a particular five-dimensional model.
In 1998/99,Merab Gogberashvili published onarXiv a number of articles where he showed that if the Universe is considered as a thin shell (a mathematicalsynonym for "brane") expanding in 5-dimensional space then there is a possibility to obtain one scale for particle theory corresponding to the 5-dimensionalcosmological constant and Universe thickness, and thus to solve thehierarchy problem.[6][7] Gogberashvili also showed that the four-dimensionality of the Universe is the result of thestability requirement found in mathematics since the extra component of theEinstein field equations giving the confined solution formatter fields coincides with one of the conditions of stability.[8]
In 1999, there were proposed the closely relatedRandall–Sundrum scenarios, RS1 and RS2. (SeeRandall–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 forspacetime metric engineering, followed in 2000.[9]
Later, theekpyrotic andcyclic proposals appeared. The ekpyrotic theory hypothesizes that the origin of theobservable universe occurred when two parallel branes collided.[10]
As of now, no experimental or observational evidence oflarge extra dimensions, as required by the Randall–Sundrum models, has been reported. An analysis of results from theLarge Hadron Collider in December 2010 severely constrains the black holes produced in theories with large extra dimensions.[11] Therecent multi-messenger gravitational wave event GW170817 has also been used to put weak limits on large extra dimensions.[12][13]
