| Several contemporary problems of fundamental physics including the gauge-hierarchy and cosmological constant problems can be addressed within the so-called "brane world" theories. Our primary focus is the investigation of the low energy consequences of such theories.;We first consider the Kaluza-Klein scenario in which gravity propagates in the 4 + n dimensional bulk of spacetime and the Standard Model particles are confined to a 3-brane. We calculate the gauge boson self-energy corrections arising from the exchange of virtual gravitons and present our results in the STU-formalism. We find that the new physics contributions to S, T and U decouple in the limit that the string scale MS goes to infinity. The oblique parameters constrain the lower limit on MS. Taking the quantum gravity cutoff to be M S, S-parameter constraints impose MS > 1.55 TeV for n = 2 at the 1sigma level. T-parameter constraints impose MS > 1.25 (0.75) TeV for n = 3 (6).;Next, we examine the cosmology and hierarchy of scales in models with branes immersed in a five-dimensional curved spacetime subject to radion stabilization. When the radion field is time-independent and the inter-brane spacing is stabilized, the universe can naturally find itself in the radiation-dominated epoch. This feature is independent of the form of the stabilizing potential. We recover the standard Friedmann equations without assuming a specific form for the bulk energy-momentum tensor. In the models considered, if the observable brane has positive tension, a solution to the hierarchy problem requires the presence of a negative tension brane somewhere in the bulk. We find that the string scale can be as low as the electroweak scale.;Finally, we present static solutions to Einstein's equations corresponding to the most general network of N orthogonal families of (2 + N)-branes in (4 + N)-dimensional AdS spacetimes. The bulk cosmological constant can take a different value in each cell enclosed by intersecting branes and the extra dimensions can be compact or noncompact. In each family of branes the inter-brane spacing is arbitrary. The extra dimensions may be any or all of the manifolds, R1, R1/Z2, S1and S1/Z 2. Only when the extra dimensions are R1 or/and R1/Z2, can networks consisting solely of positive tension branes be constructed. |
