Gauge-Higgs unification on the brane

From the quantum field theory point of view, matter and gauge fields are generally expected to be localised around branes or topological defects occurring in extra dimensions. Here I discuss a simple scenario where, by starting with a five dimensional SU(3) gauge theory, we end up with several 4-D parallel branes with localised "chiral" fermions and gauge fields to them. I will show that it is possible to reproduce the electroweak model confined to a single brane, allowing a simple and geometrical approach to the fermion hierarchy problem. Some nice results of this construction are: Gauge and Higgs fields are unified at the 5-D level; and new particles are predicted: a left-handed neutrino of zero hypercharge, and a massive vector field coupling together the new neutrino to other left-handed leptons.Comment: Contribution to the proceedings of the RTN workshop "The Quest for Unification: Theory Confronts Experiment", Corfu, Greece, Sept 11-18, 200

Phases of massive gravity

We systematically study the most general Lorentz-violating graviton mass invariant under three-dimensional Eucledian group using the explicitly covariant language. We find that at general values of mass parameters the massive graviton has six propagating degrees of freedom, and some of them are ghosts or lead to rapid classical instabilities. However, there is a number of different regions in the mass parameter space where massive gravity can be described by a consistent low-energy effective theory with cutoff $\sim\sqrt{mM_{Pl}}$ free of rapid instabilities and vDVZ discontinuity. Each of these regions is characterized by certain fine-tuning relations between mass parameters, generalizing the Fierz--Pauli condition. In some cases the required fine-tunings are consequences of the existence of the subgroups of the diffeomorphism group that are left unbroken by the graviton mass. We found two new cases, when the resulting theories have a property of UV insensitivity, i.e. remain well behaved after inclusion of arbitrary higher dimension operators without assuming any fine-tunings among the coefficients of these operators, besides those enforced by the symmetries. These theories can be thought of as generalizations of the ghost condensate model with a smaller residual symmetry group. We briefly discuss what kind of cosmology can one expect in massive gravity and argue that the allowed values of the graviton mass may be quite large, affecting growth of primordial perturbations, structure formation and, perhaps, enhancing the backreaction of inhomogeneities on the expansion rate of the Universe.Comment: 36 pages, 1 figur

Gravitational Lorentz Violation and Superluminality via AdS/CFT Duality

A weak quantum mechanical coupling is constructed permitting superluminal communication within a preferred region of a gravitating AdS_5 spacetime. This is achieved by adding a spatially non-local perturbation of a special kind to the Hamiltonian of a four-dimensional conformal field theory with a weakly-coupled AdS dual, such as maximally supersymmetric Yang-Mills theory. In particular, two issues are given careful treatment: (1) the UV-completeness of our deformed CFT, guaranteeing the existence of a ``deformed string theory'' AdS dual, and (2) the demonstration that superluminal effects can take place in AdS, both on its boundary as well as in the bulk. Exotic Lorentz-violating properties such as these may have implications for tests of General Relativity, addressing the cosmological constant problem, or probing "behind'' horizons. Our construction may give insight into the interpretation of wormhole solutions in Euclidean AdS gravity.Comment: 23 pages LaTex. Typo in Eq. (37) corrected. References adde

Quasilocalized gravity without asymptotic flatness

We present a toy model of a generic five-dimensional warped geometry in which the 4D graviton is not fully localized on the brane. Studying the tensor sector of metric perturbation around this background, we find that its contribution to the effective gravitational potential is of 4D type (1/r) at the intermediate scales and that at the large scales it becomes 1/r^{1+alpha}, 0<alpha=< 1 being a function of the parameters of the model (alpha=1 corresponds to the asymptotically flat geometry). Large-distance behavior of the potential is therefore not necessarily five-dimensional. Our analysis applies also to the case of quasilocalized massless particles other than graviton.Comment: 9 pages, 1 figure; to be published in Phys. Rev.

Cosmological attractors in massive gravity

We study Lorentz-violating models of massive gravity which preserve rotations and are invariant under time-dependent shifts of the spatial coordinates. In the linear approximation the Newtonian potential in these models has an extra ``confining'' term proportional to the distance from the source. We argue that during cosmological expansion the Universe may be driven to an attractor point with larger symmetry which includes particular simultaneous dilatations of time and space coordinates. The confining term in the potential vanishes as one approaches the attractor. In the vicinity of the attractor the extra contribution is present in the Friedmann equation which, in a certain range of parameters, gives rise to the cosmic acceleration.Comment: 26 pages, 1 figur