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 (3 + 1)-dimensional brane or branes.

The models were proposed in 1999 by Lisa Randall and Raman Sundrum because they were dissatisfied with the universal extra dimensional models then in vogue. Suchmodels require two fine tunings; one for the value of the bulk cosmological constant and theother for the brane tensions. Later, while studying RS models in the context of the AdS/CFT correspondence,they showed how it can be dual to technicolor models.

There are two popular models. The first, called RS1, has a finite size for the extra dimension with two branes, one at each end. The second, RS2, is similar to the first, but one brane has been placed infinitely far away, so that there is only one brane left in the model.

The RS1 model

The RS1 model attempts to address the hierarchy problem. The warping of the extra dimension is analogous to the warping of spacetime in the vicinity of a massive object, such as a black hole. This warping, or red-shifting, generates a large ratio of energy scales so that the natural energy scale at one end of the extra dimension is much larger than at the other end.

:$mathrm\{d\}s^2=\{1over\; k^2\; y^2\}(mathrm\{d\}y^2+eta\_\{mu\; u\},mathrm\{d\}x^mu,\; mathrm\{d\}x^\; u)$

where "k" is some constant and η has "+++" metric signature. This space has boundaries at "y" = 1/"k" and "y" = 1/"Wk", with $0le\; \{1\; over\; k\}\; le\; \{1\; over\; Wk\}$ where "k" is around the Planck scale and "W" is the warp factor and "Wk" is around a TeV. The boundary at "y" = 1/"k" is called the Planck brane and the boundary at "y" = 1/"Wk" is called the TeV brane. The particles of the standard model reside on the TeV brane. The distance between both branes is only −ln("W")/"k", though.

In another coordinate system,

:$varphi\; stackrel\{mathrm\{def\{=\}\; -\{pi\; ln(ky)over\; ln(W)\},$

so that

:$0le\; varphi\; le\; pi$

and

:$mathrm\{d\}s^2=left\; (\{ln(W)over\; pi\; k\}\; ight\; )^2,\; mathrm\{d\}varphi^2+e^\{2ln(W)varphiover\; pi\}eta\_\{mu\; u\},mathrm\{d\}x^mu,\; mathrm\{d\}x^\; u.$

Check out the 5 dimensional warped geometry theory article for a nontechnical explanation.

The RS2 model

The RS2 model uses the same geometry as RS1, but there is no TeV brane. The particles of the standard model are presumed to be on the Planck brane. This model was originally of interest because it represented an infinite 5-dimensional model which, in many respects, behaved as a 4-dimensional model. This setup may also be of interest for studies of the AdS/CFT conjecture. Or possibly in the field of theoretical desserts.

ee also

*Kaluza-Klein theory
*Large extra dimension
*DGP model

Further reading

*cite journal |last=Randall |first=Lisa |authorlink= |coauthors=Sundrum, Raman |year=1999 |month= |title=Large Mass Hierarchy from a Small Extra Dimension |journal=Physical Review Letters |volume=83 |issue=17 |pages=3370–3373 |doi=10.1103/PhysRevLett.83.3370 |url= |accessdate= |quote=
*cite book |title=Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions |last=Randall |first=Lisa |authorlink= |coauthors= |year=2005 |publisher=HarperCollins |location=New York |isbn=0060531088 |pages=

External links

* [http://www.physics.harvard.edu/people/facpages/randall.html Lisa Randall's web page at Harvard University]
* [http://physics-astronomy.jhu.edu/people/faculty/sundrum.html Raman Sundrum's web page at Johns Hopkins University]