Snyder Geometry and Quantum Field Theory

We find that, in presence of the Snyder geometry, the notion of translational invariance needs to be modified, allowing a momentum dependence of this symmetry. This step is necessary to build the maximally localized states and the Feynman rules of the corresponding quantum field theory.Comment: 10 pages, LaTeX, no figure

Gravity on a fuzzy sphere

We propose an action for gravity on a fuzzy sphere, based on a matrix model. We find striking similarities with an analogous model of two dimensional gravity on a noncommutative plane, i.e. the solution space of both models is spanned by pure U(2) gauge transformations acting on the background solution of the matrix model, and there exist deformations of the classical diffeomorphisms which preserve the two-dimensional noncommutative gravity actions.Comment: 14 pages, no figures, LaTe

Hawking radiation for a scalar field conformally coupled to an AdS black hole

The decomposition in normal modes of a scalar field conformally coupled to an AdS black hole leads to a Heun equation with simple coefficients thanks to conformal invariance. By applying the Damour-Ruffini method we can relate the critical exponent of the radial part at the horizon surface to the Hawking radiation of scalar particles.Comment: 9 pages, no figure

Matrix model for noncommutative gravity and gravitational instantons

We introduce a matrix model for noncommutative gravity, based on the gauge group $U(2) \otimes U(2)$. The vierbein is encoded in a matrix $Y_{\mu}$, having values in the coset space $U(4)/ (U(2) \otimes U(2))$, while the spin connection is encoded in a matrix $X_\mu$, having values in $U(2) \otimes U(2)$. We show how to recover the Einstein equations from the $\theta \to 0$ limit of the matrix model equations of motion. We stress the necessity of a metric tensor, which is a covariant representation of the gauge group in order to set up a consistent second order formalism. We finally define noncommutative gravitational instantons as generated by $U(2) \otimes U(2)$ valued quasi-unitary operators acting on the background of the Matrix model. Some of these solutions have naturally self-dual or anti-self-dual spin connections.Comment: 28 pages, LaTeX, no figure

Extended BRS symmetry in topological field theories

A class of topological field theories like the $BF$ model and Chern-Simons theory, when quantized in the Landau gauge, enjoys the property of invariance under a vector supersymmetry, which is responsible for their finiteness. We introduce a new type of gauge fixing which makes these theories invariant under an extended $BRS$ symmetry, containing a new type of field, the ghost of diffeomorphisms. The presence of such an extension is naturally related to the vector supersymmetry discussed before.Comment: 6 pages, LaTeX, no figure

Non-abelian instantons on a fuzzy four-sphere

We study the compatibility between the $BPST SU(2)$ instanton and the fuzzy four-sphere algebra. By using the projective module point of view as an intermediate step, we are able to identify a non-commutative solution of the matrix model equations of motion which minimally extends the SU(2) instanton solution on the classical sphere $S^4$. We also propose to extend the non-trivial second Chern class with the five-dimensional noncommutative Chern-Simons term