2+1 Gravity and Closed Time-Like Curves

In this paper we report some results obtained by applying the radial gauge to 2+1 dimensional gravity. The general features of this gauge are reviewed and it is shown how they allow the general solution of the problem in terms of simple quadratures. Then we concentrate on the general stationary problem providing the explicit solving formulas for the metric and the explicit support conditions for the energy momentum tensor. The chosen gauge allows, due to its physical nature, to exploit the weak energy condition and in this connection it is proved that for an open universe conical at space infinity the weak energy condition and the absence of closed time like curves (CTC) at space infinity imply the total absence of CTC. It is pointed out how the approach can be used to examine cosmological solution in 2+1 dimensions.Comment: Talk given at the Workshop on ``Constraint Theory and Quantization Methods'',10 pages, Latex, IFUP-TH-43/9

Stationary Solutions in 2+1 Dimensional Gravity and Closed Time-Like Curves

We apply the reduced radial gauge to give the general solution of the metric in 2+1 dimensions in term of quadratures. It allows a complete controll on the support of the source. We use the result to prove that for a general stationary universe, conical at infinity, the weak energy condition and the absence of CTC at space infinity prevent the occurrrence of any CTC.Comment: Talk presented at Europhysics Conference on High Energy Physics, Marseille, France, July 22-28 1993, 5 pages, Latex, IFUP-49/9

Graviton-Form Invariants in D=11 Supergravity

We complete an earlier derivation of the 4-point bosonic scattering amplitudes, and of the corresponding linearized local supersymmetric invariants in D=11 supergravity, by displaying the form-curvature, F^2 R^2, terms.Comment: 2 pages. One typo corrected; note added on comparison with D=10 string expansion invariants. Accepted for publication in Phys. Rev. D1

Free Spin 2 Duality Invariance Cannot be Extended to GR

We show by explicit computation that the recently discovered duality invariance of D=4 linearized gravity fails, already at first self-interacting, cubic, approximation of GR. In contrast, the cubic Yang-Mills correction to Maxwell does admit a simple deformed duality.Comment: 11 Page

Counterterms/M-theory Corrections to D=11 Supergravity

We construct a local on-shell invariant in D=11 supergravity from the nonlocal four-point tree scattering amplitude.Its existence, together with earlier arguments, implies non-renormalizability of the theory at lowest possible, two loop, level. This invariant, whose leading bosonic terms are exhibited, may also express the leading, "zero-slope", M$-$theory corrections to its D=11 supergravity limit.Comment: 16 pages RevTe

The radial gauge propagators in quantum gravity

We give a general procedure for extracting the propagators in gauge theories in presence of a sharp gauge fixing and we apply it to derive the propagators in quantum gravity in the radial gauge, both in the first and in the second order formalism in any space-time dimension. In the three dimensional case such propagators vanish except for singular collinear contributions, in agreement with the absence of propagating gravitons.Comment: 38 pages, 1 fig. not available, LATEX, IFUP-TH-30/9

Definition of Chern-Simons Terms in Thermal QED_3 Revisited

We present two compact derivations of the correct definition of the Chern-Simons term in the topologically non trivial context of thermal $QED_3$. One is based on a transgression descent from a D=4 background connection, the other on embedding the abelian model in SU(2). The results agree with earlier cohomology conclusions and can be also used to justify a recent simple heuristic approach. The correction to the naive Chern-Simons term, and its behavior under large gauge transformations are displayed.Comment: 9 pages, RevTex, no figures, new derivation from non abelian embedding adde