Massive Gravity on a Non-extremal Brane

We consider a brane world scenario which arises as the near-horizon region of a non-extremal D5-brane. There is a quasi-localized massive graviton mode, as well as harmonic modes of higher mass which are bound to the brane to a lesser degree. Lorentz invariance is slightly broken, which may have observable effects due to the leakage of the metastable graviton states into the bulk. Unlike a brane world arising from an extremal D5-brane, there is no mass gap. We also find that a brane world arising from a non-extremal M5/M5-brane intersection has the same graviton dynamics as that of a non-extremal D5-brane. This is evidence that a previously conjectured duality relation between the dual quantum field theories of each p-brane background may hold away from extremality.Comment: 10 pages, 4 figures, LaTe

Ultimate Fate of Constrained Voters

We determine the ultimate fate of individual opinions in a socially-interacting population of leftists, centrists, and rightists. In an elemental interaction between agents, a centrist and a leftist can become both centrists or both become leftists with equal rates (and similarly for a centrist and a rightist). However leftists and rightists do not interact. This interaction step between pairs of agents is applied repeatedly until the system can no longer evolve. In the mean-field limit, we determine the exact probability that the system reaches consensus (either leftist, rightist, or centrist) or a frozen mixture of leftists and rightists as a function of the initial composition of the population. We also determine the mean time until the final state is reached. Some implications of our results for the ultimate fate in a limit of the Axelrod model are discussed.Comment: 10 pages, 6 figures, 2-column revtex format; for submission to J. Phys. A. Final version for JPA; very minor change

S^1-wrapped D3-branes on Conifolds

We construct a D3-brane wrapped on S^1, which is fibred over the resolved conifold as its transverse space. Whereas a fractional D3-brane on the resolved conifold is not supersymmetric and has a naked singularity, our solution is supersymmetric and regular everywhere. We also consider an $S^1$-wrapped D3-brane on the resolved cone over T^{1,1}/Z_2, as well as on the deformed conifold. In the former case, we obtain a regular supergravity dual to a certain four-dimensional field theory whose Lorentz and conformal symmetries are broken in the IR region and restored in the UV limit.Comment: Latex, 14 pages, minor correction

Penrose Limits of Non-standard Brane Intersections

The non-standard intersection of two 5-branes and a string can give rise to AdS_3\times S^3\times S^3\times S^1. We consider the Penrose limit of this geometry and study the supersymmetry of the resulting pp-wave solution. There is a one-parameter family of Penrose limits associated with the orthogonal rotation of the two foliating circles within the two 3-spheres. Supernumerary Killing spinors arise only when the rotation angle is 45 degrees, for which case we obtain the corresponding light-cone string action that has linearly-realised supersymmetry. We also obtain Penrose limits of other non-standard intersections that give rise to the product of AdS_3 or AdS_2 and two spheres. The resulting pp-waves are supported by multiple constant field strengths.Comment: Latex, 15 pages, references adde

Smooth Cosmologies from M-theory

We review two ways in which smooth cosmological evolution between two de Sitter phases can be obtained from M/string-theory. Firstly, we perform a hyperbolic reduction of massive IIA* theory to D=6 N=(1,1) SU(2)xU(1) gauged de Sitter supergravity, which supports smooth cosmological evolution between dS_4 x S^2 and a dS_6-type geometry. Secondly, we obtain four-dimensional de Sitter gravity with SU(2) Yang-Mills gauge fields from a hyperbolic reduction of standard eleven-dimensional supergravity. The four-dimensional theory supports smooth cosmological evolution between dS_2 x S^2 and a dS_4-type geometry. Although time-dependent, these solutions arise from a first-order system via a superpotential construction. For appropriate choices of charges, these solutions describe an expanding universe whose expansion rate is significantly larger in the past than in the future, as required for an inflationary model.Comment: Latex, 7 pages, Contribution to Quantum Theory and Symmetries