We consider the low-energy description of 5D models of supergravity with boundaries comprizing a vector multiplet and the universal hypermultiplet in the bulk. We analyze the spontaneous breaking of supersymmetry induced by the vacuum expectation value of superpotentials on the boundary branes. When supersymmetry is broken, the moduli corresponding to the radion, the zero mode of the vector multiplet scalar field and the dilaton develop a potential in the effective action. We compute the resulting soft breaking terms and give some indications on the features of the corresponding particle spectrum. We consider some of the possible phenomenological implications when supersymmetry is broken on the hidden brane.

We study the low-energy effects of space-time non-commutativity on light propagation in a background electromagnetic field. Contrary to some of the previous claims, we find no polarization rotation for vanishing time-space commutator [x i ,x 0 ] = 0, although dispersion relation is modified, allowing for propagation faster than the vacuum speed of light. For non-zero [x i ,x 0 ], as allowed with a proper quantization, a naive rotation effect is found to be actually absent when physical fields are defined through SeibergWitten map. We also consider non-commutative QED weakly coupled to small mass particles such as axions. Non-commutativity is found to dominate the inverse oscillation length, compared to axion mass and QED effects, for mixing particle masses smaller than 10 −12 eV . Conventional constraints on axion coupling based on photon-axion transition rates are unmodified, however induced ellipticity is proportional to the non-commutativity squared length scale. This last effect is found to be too small to account for the ellipticity reported by the PVLAS experiment, yet unexplained by conventional QED or axion physics.

We derive the effective cosmological equations for a non-Z 2 symmetric codimension one brane embedded in an arbitrary D-dimensional bulk spacetime, generalizing the D = 5, 6 cases much studied previously. As a particular case, this may be considered as a regularized codimension (D-4) brane avoiding the problem of curvature divergence on the brane. We apply our results to the case of spherical symmetry around the brane and to partly compactified AdS-Schwarzschild bulks.

We consider the spontaneous breaking of supersymmetry in five dimensional supergravity with boundaries where the supersymmetry breaking mechanism is provided by the even part of the bulk prepotential. The supersymmetric action comprises boundary brane terms with detuned tensions. We analyse the possible vacua with spontaneously broken supersymmetry. A class of solutions corresponds to rotated branes in an AdS 5 bulk. In particular parallel branes which are rotated with respect to the bulk preserve 1/4 of supersymmetry. We analyse more general vacua using the low energy effective action for gravity coupled to the radion field. Supersymmetry breaking implies that the radion field acquires a potential which is negative and unbounded from below. This potential is modified when coupling the boundary branes to a bulk four-form field. For a brane charge larger than the deficit of brane tension, the radion potential is bounded from below while remaining flat when the charge equals the deficit of brane tension.