Randall–Sundrum models (also called 5-dimensional warped geometry theory) imagines that the real world is a higher-dimensional universe described by warped geometry. 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. Such models require two fine tunings; one for the value of the bulk cosmological constant and the other for the brane tensions.
What is your point? I see no unanswered question.
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Usually, one still call a RS model to a huge collection of scenarios which generalize the idea of "our Universe" being a hypersurphase of a higher dimensional spacetime.
Among the generalizations one finds:
- More than 2 branes: Randall and Sundrum considered models with two and one brane.
- Thick branes: instead of "thin" hypersurphases
- Domain Wall scenarios: which were already thought 20 years earlier by Rubakov and Shaposhnivok (abbreviated RS as well.... coincidence???? hehehe)
- Models with matter... etc.
Randall Sundarum model was proposed in late 1990s. The objective was to solve what is known as the hierarchy problem. Hierarchy problem is a very intriguing one which addresses the question that why in nature two fundamental scales, namely the weak scale and Planck scale are so many orders of magnitude away from each other. In the proposal of RS model, the weak scale is generated from the Planck scale by what they termed as the exponential hierarchy. The model is based on a five dimensional universe. Randal Sundram metric is a new solution of Einstein's equations. This metric is a non-factorizable metric. See: hep-th/9906064
In addition to the normal fermions, scalars and gauge bosons, there are additional spin-2 excitations whose masses are predicted. Interestingly the masses are within the scope of discovery of LHC. In addition to this there are no light Kaluza-Klein modes; even though they are not invisible. They should be observable as spin-2 resonances.
As far as experimental tests are concerned, no experimental observation is reported as an observational evidence as required by Randall Sundrum models. LHC results severely constrain Randall Sundrum models.
http://en.wikipedia.org/wiki/Brane_cosmology#Empirical_tests
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