We identify the effective theory appropriate to the propagation of massless bulk fields in braneworld scenarios, to show that the dominant low-energy effect of asymmetric warping in the bulk is to modify the dispersion relation of the effective 4-dimensional modes. We show how such changes to the graviton dispersion relation may be bounded through the effects they imply, through loops, for the propagation of standard model particles. We compute these bounds and show that they provide, in some cases, the strongest constraints on nonstandard gravitational dispersions. The bounds obtained in this way are the strongest for the fewest extra dimensions and when the extradimensional Planck mass is the smallest. Although the best bounds come for warped 5-D scenarios, for which M5 is O(TeV), even in 4 dimensions the graviton loop can lead to a bound on the graviton speed which is comparable with other constraints.

Recently, a scenario has been proposed in which the gravitational scale could be as low as the TeV scale, and extra dimensions could be large and detectable at the electroweak scale. Although supersymmetry is not a requirement of this scenario, it is nevertheless true that its best-motivated realizations arise in supersymmetric theories ͑such as M theory͒. We argue here that supersymmetry can have robust, and in some instances fatal, implications for the expected experimental signature for TeV-scale gravity. The signature of the supersymmetric version of the scenario differs most dramatically from what has been considered in the literature because mass splittings within the gravity supermultiplet in these models are extremely small, implying in particular the existence of a very light spin-one superpartner for the graviton. We compute the implications of this graviphoton, and show that it can acquire dimension-four couplings to ordinary matter which can strongly conflict with supernova bounds.

As part of a programme for the general study of the low-energy implications of supersymmetry breaking in brane-world scenarios, we study the nonlinear realization of supersymmetry which occurs when breaking N = 2 to N = 1 supergravity. We consider three explicit realizations of this supersymmetry breaking pattern, which correspond to breaking by one brane, by one antibrane or by two (or more) parallel branes. We derive the minimal field content, the effective action and supersymmetry transformation rules for the resulting N = 1 theory perturbatively in powers of κ = 1/M P lanck . We show that the way the massive gravitino and spin-1 fields assemble into N = 1 multiplets implies the existence of direct brane-brane contact interactions at order O(κ). This result is contrary to the O(κ 2 ) predicted by the sequestering scenario but in agreement with recent work of Anisimov et al. Our low-energy approach is model independent and is a first step towards determining the low-energy implications of more realistic brane models which completely break all supersymmetries.