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We consider circular Wilson loops in a defect version of $\mathcal{N}=4$ super-Yang-Mills theory which is dual to the D3-D5 brane system with $k$ units of flux. When the loops are parallel to the defect, we can construct both BPS and non-BPS operators, depending on the orientation of the scalar couplings in the R-symmetry directions. At strong 't Hooft coupling we observe, in the non supersymmetric case, a Gross-Ooguri-like phase transition in the dual gravitational theory: the familiar disk solution dominates, as expected, when the operator is far from the defect while a cylindrical string worldsheet, connecting the boundary loop with the probe D5-brane, is favourite below a certain distance (or equivalently for large radii of the circles). In the BPS case, instead, the cylindrical solution does not exist for any choice of the physical parameters, suggesting that the exchange of light supergravity modes always saturate the expectation value at strong coupling. We study the double-scaling limit for large $k$ and large 't Hooft coupling, finding full consistency in the non-BPS case between the string solution and the one-loop perturbative result. Finally we discuss, in the BPS case, the failure of the double-scaling limit and the OPE expansion of the Wilson loop, finding consistency with the known results for the one-point functions of scalar composite operators., Comment: 51 pages, 17 Figures

We study the strong coupling behaviour of $1/4$-BPS circular Wilson loops (a family of "latitudes") in ${\cal N}=4$ Super Yang-Mills theory, computing the one-loop corrections to the relevant classical string solutions in AdS$_5\times$S$^5$. Supersymmetric localization provides an exact result that, in the large 't Hooft coupling limit, should be reproduced by the sigma-model approach. To avoid ambiguities due to the absolute normalization of the string partition function, we compare the $ratio$ between the generic latitude and the maximal 1/2-BPS circle: Any measure-related ambiguity should simply cancel in this way. We use Gel'fand-Yaglom method to calculate the relevant functional determinants, that present some complications with respect to the standard circular case. After a careful numerical evaluation of our final expression we still find disagreement with the localization answer: The difference is encoded into a precise "remainder function". We comment on the possible origin and resolution of this discordance., Comment: References added, comments in introduction and conclusions added, it matches published version

We discuss some geometrical aspects of the semiclassical quantization of string solutions in Type IIB Green-Schwarz action on $\ads_5\times \sphere^5$. We concentrate on quadratic fluctuations around classical configurations, expressing the relevant differential operators in terms of (intrinsic and extrinsic) invariants of the background geometry. The aim of our exercises is to present some compact expressions encoding the spectral properties of bosonic and fermionic fluctuations. The appearing of non-trivial structures on the relevant bundles and their role in concrete computations are also considered. We corroborate the presentation of general formulas by working out explicitly a couple of relevant examples, namely the spinning string and the latitude BPS Wilson loop., Comment: 34 pages. v3: one reference added

We show that in D=4 AdS, $s\ge 3/2$ partially massless (PM) fermions retain the duality invariances of their flat space massless counterparts. They have tuned ratios $ {m^2}/{M^2}\ne 0$ that turn them into sums of effectively massless unconstrained helicity $\pm(s,\cdots,\frac{3}{2})$ excitations, shorn of the lowest (non-dual)\helicity $\pm \frac{1}{2}$-rung and -more generally- of succeeding higher rung as well. Each helicity mode is separately duality invariant, like its flat space counterpart., Comment: 3 pages

We present two new families of Wilson loop operators in N= 6 supersymmetric Chern-Simons theory. The first one is defined for an arbitrary contour on the three dimensional space and it resembles the Zarembo's construction in N=4 SYM. The second one involves arbitrary curves on the two dimensional sphere. In both cases one can add certain scalar and fermionic couplings to the Wilson loop so it preserves at least two supercharges. Some previously known loops, notably the 1/2 BPS circle, belong to this class, but we point out more special cases which were not known before. They could provide further tests of the gauge/gravity correspondence in the ABJ(M) case and interesting observables, exactly computable by localization, Comment: 9 pages, no figure. arXiv admin note: text overlap with arXiv:0912.3006 by other authors

We study the correlators of a recently discovered family of BPS Wilson loops in ${\cal N}=4$ supersymmetric U(N) Yang-Mills theory. When the contours lie on a two-sphere in the space-time, we propose a closed expression that is valid for all values of the coupling constant $g$ and for any rank $N$, by exploiting the suspected relation with two-dimensional gauge theories. We check this formula perturbatively at order ${\cal O}(g^4)$ for two latitude Wilson loops and we show that, in the limit where one of the loops shrinks to a point, logarithmic corrections in the shrinking radius are absent at ${\cal O}(g^6)$. This last result strongly supports the validity of our general expression and suggests the existence of a peculiar protected local operator arising in the OPE of the Wilson loop. At strong coupling we compare our result to the string dual of the ${\cal N}=4$ SYM correlator in the limit of large separation, presenting some preliminary evidence for the agreement., Comment: 20 page, 8 figures

We study the relations between two-dimensional Yang-Mills theory on the torus, topological string theory on a Calabi-Yau threefold whose local geometry is the sum of two line bundles over the torus, and Chern-Simons theory on torus bundles. The chiral partition function of the Yang-Mills gauge theory in the large N limit is shown to coincide with the topological string amplitude computed by topological vertex techniques. We use Yang-Mills theory as an efficient tool for the computation of Gromov-Witten invariants and derive explicitly their relation with Hurwitz numbers of the torus. We calculate the Gopakumar-Vafa invariants, whose integrality gives a non-trivial confirmation of the conjectured nonperturbative relation between two-dimensional Yang-Mills theory and topological string theory. We also demonstrate how the gauge theory leads to a simple combinatorial solution for the Donaldson-Thomas theory of the Calabi-Yau background. We match the instanton representation of Yang-Mills theory on the torus with the nonabelian localization of Chern-Simons gauge theory on torus bundles over the circle. We also comment on how these results can be applied to the computation of exact degeneracies of BPS black holes in the local Calabi-Yau background., Comment: 62 pages, 5 figures

The counting of microstates of BPS black-holes on local Calabi-Yau of the form ${\mathcal O}(p-2)\oplus{\mathcal O}(-p) \longrightarrow S^2$ is explored by computing the partition function of q-deformed Yang-Mills theory on $S^2$. We obtain, at finite $N$, the instanton expansion of the gauge theory. It can be written exactly as the partition function for U(N) Chern-Simons gauge theory on a Lens space, summed over all non-trivial vacua, plus a tower of non-perturbative instanton contributions. In the large $N$ limit we find a peculiar phase structure in the model. At weak string coupling the theory reduces to the trivial sector and the topological string partition function on the resolved conifold is reproduced in this regime. At a certain critical point, instantons are enhanced and the theory undergoes a phase transition into a strong coupling regime. The transition from the strong coupling phase to the weak coupling phase is of third order., Comment: 16 pages, 3 figures; Invited talk given at QG05, Cala Gonone (Italy), September 2005

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. D15

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 Pages

We give a simple general extension to all free bosonic and fermionic massless gauge fields of a recent proof that spin 2 is duality invariant in flat space. We also discuss its validity in (A)dS backgrounds and the relevance of supersymmetry., Comment: 3 pages

We describe a novel double scaling limit of large N Yang-Mills theory on a two-dimensional torus and its relation to the geometry of the principal moduli spaces of holomorphic differentials., Comment: 7 pages; Based on invited talk given by R.J.S. at ``Recent Developments in String/M-Theory and Field Theory'', 37th International Symposium Ahrenshoop on the Theory of Elementary Particles, Berlin-Schmoeckwitz, Germany, August 23--27 2004; to be published in Fortschritte der Physik

We describe and solve a double scaling limit of large N Yang-Mills theory on a two-dimensional torus. We find the exact strong-coupling expansion in this limit and describe its relation to the conventional Gross-Taylor series. The limit retains only the chiral sector of the full gauge theory and the coefficients of the expansion determine the asymptotic Hurwitz numbers, in the limit of infinite winding number, for simple branched coverings of a torus. These numbers are computed exactly from the gauge theory vacuum amplitude and shown to coincide with the volumes of the principal moduli spaces of holomorphic differentials. The string theory interpretation of the double scaling limit is also described., Comment: 14 pages; v2: references added

We analyze large gauge invariance in combined nonabelian and thermal QFT and their physical consequences for D=3 effective actions. After briefly reviewing the structure of bundles and large gauge transformations that arise in non-simply connected 3-manifolds and gauge groups, we discuss their connections to Chern-Simons terms and Wilson-Polyakov loops. We then provide an invariant characterization of the ``abelian'' fluxes encountered in explicit computations of finite temperature effective actions. In particular we relate, and provide explicit realizations of, these fluxes to a topological index that measures the obstruction to global diagonalization of the loops around compactified time. We also explore the fate of, and exhibit some everywhere smooth, large transformations for non-vanishing index in various topologies., Comment: 21 pages, Latex, no figure

After briefly reviewing the hamiltonian approach to 2+1 dimensional gravity in absence of matter on closed universes, we consider 2+1 dimensional gravity coupled to point particles in an open universe. We show that the hamiltonian structure of the theory is the result of a conjecture put forward by Polyakov in a different context. A proof is given of such a conjecture. Finally we give the exact quantization of the two particle problem in open space., Comment: Talk by P. Menotti at the 25th Johns Hopkins Workshop, Florence September 3-5, 2001, 14 pages LaTex

A proof is given of Polyakov conjecture about the auxiliary parameters of the SU(1,1) Riemann-Hilbert problem for general elliptic singularities. Such a result is related to the uniformization of the the sphere punctured by n conical defects. Its relevance to the hamiltonian structure of 2+1 dimensional gravity in the maximally slicing gauge is stressed., Comment: Talk by P. Menotti at Int. Europhysics Conference on High Energy Physics, Budapest 12-18 July 2001, 5 pages latex

In this paper we shall investigate the possibility of solving U(1) theories on the non-commutative (NC) plane for arbitrary values of $\theta$ by exploiting Morita equivalence. This duality maps the NC U(1) on the two-torus with a rational parameter $\theta$ to the standard U(N) theory in the presence of a 't Hooft flux, whose solution is completely known. Thus, assuming a smooth dependence on $\theta$, we are able to construct a series rational approximants of the original theory, which is finally reached by taking the large $N-$limit at fixed 't Hooft flux. As we shall see, this procedure hides some subletities since the approach of $N$ to infinity is linked to the shrinking of the commutative two-torus to zero-size. The volume of NC torus instead diverges and it provides a natural cut-off for some intermediate steps of our computation. In this limit, we shall compute both the partition function and the correlator of two Wilson lines. A remarkable fact is that the configurations, providing a finite action in this limit, are in correspondence with the non-commutative solitons (fluxons) found independently by Polychronakos and by Gross and Nekrasov, through a direct computation on the plane., Comment: 21 pages, JHEP3 preprint tex-format

We obtain the induced Lorentz- and CPT-violating term in QED at finite temperature using imaginary-time formalism and dimensional regularization. Its form resembles a Chern-Simons-like structure, but, unexpectedly, it does not depend on the temporal component of the fixed $b_\mu$ constant vector that is coupled to the axial current. Nevertheless Ward identities are respected and its coefficient vanishes at T=0, consistently with previous computations with the same regularization procedure, and it is a non-trivial function of temperature. We argue that at finite $T$ a Chern-Simons-like Lorentz- and CPT-violating term is generically present, the value of its coefficient being unambiguously determined up to a $T-$independent constant, related to the zero-temperature renormalization conditions., Comment: 15 pages, Latex, 1 figure in eps-format (included)

We examine the ultraviolet behaviour of supergravity theories as a function of dimension and number of supercharges. We do so by the computation of one and two-loop physical on-shell four point amplitudes. For maximal supergravity, our computations prove the non-renomalisability of supergravity for $D \geq 6$ (including the maximal D=11 case) and give strong evidence for the existance of a five-loop counterterm in D=4. For type I supergravity our results indicate similar patterns. e shall also explore a remarkable relationship between gravity amplitudes and those of Yang-Mills theories. In many ways gravity calculations discover features which relate to the equivalent Yang-Mills features by a squaring proceedure., Comment: Talk presented by D. Dunbar at "Non-perturbative Quantum effects 2000"

It is shown that the reduced particle dynamics of 2+1 dimensional gravity in the maximally slicing gauge is of hamiltonian nature. We give the exact diffeomorphism which transforms the expression of the spinning cone geometry in the Deser, Jackiw, 't Hooft gauge to the maximally slicing gauge. It is explicitly shown that the boundary term in the action, written in hamiltonian form gives the Hamiltonian for the reduced particle dynamics. The quantum mechanical translation of the two particle Hamiltonian is given and the Green function computed., Comment: Latex 10 pages; contribution to the Proceedings of the Marcel Grossmann Meeting (MG9) (electronic version)

We compute the tree level 4-particle bosonic scattering amplitudes in D=11 supergravity. By construction, they are part of a linearized supersymmetry-, coordinate- and 3-form gauge-invariant. While this on-shell invariant is nonlocal, suitable SUSY-preserving differentiations turn it into a local one with correct dimension to provide a natural lowest (two-loop) order counterterm candidate. Its existence shows explicitly that no symmetries protect this ultimate supergravity from the nonrenormalizability of its lower-dimensional counterparts., Comment: 14 pages

Motivated by recent work involving the graviton-graviton tree scattering amplitude, and its twin descriptions as the square of the Bel-Robinson tensor, $B_{\m\n\a\b}$, and as the "current-current interaction" square of gravitational energy pseudo-tensors $t_{\a\b}$,we find an exact tensor-square root equality $B_{\mn\a\b} = \pa^2_\mn t_{\a\b}$, for a combination of Einstein and Landau-Lifschitz $t_\ab$, in Riemann normal coordinates. In the process, we relate, on-shell, the usual superpotential basis for classifying pseudo-tensors with one spanned by polynomials in the curvature., Comment: 7 pages

We settle the ``apparent'' paradox present in thermal QED_3 that the perturbative series is not invariant, as manifested by the temperature dependence of the induced Chern-Simons term, by showing that large (unlike small) transformations and hence their Ward identities, are not perturbative order-preserving. Instead the thermal effective gauge field actions induced by charged fermions in QED_3 can be made invariant under both small and large gauge transformations by suitable regularization of the Dirac operator determinant, at the usual price of parity anomalies. Our result is illustrated by a concrete example., Comment: 5 pages sprocl.tex, Talks presented at Pascos98

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 RevTeX

Two different families of abelian chiral gauge theories on the torus are investigated: the aim is to test the consistency of two-dimensional anomalous gauge theories in the presence of global degrees of freedom for the gauge field. An explicit computation of the partition functions shows that unitarity is recovered in particular regions of parameter space and that the effective dynamics is described in terms of fermionic interacting models. For the first family, this connection with fermionic models uncovers an exact duality which is conjectured to hold in the nonabelian case as well., Comment: RevTex, 13 pages, references added

We point out that the claims made in the paper ``Non-thermalizability of a Quantum Field Theory'' (hep-th/9802008) by C. R. Hagen are irrelevant to our recent results concerning large gauge invariance of the effective action in thermal QED., Comment: 3 pages RevTex; no figures

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 added

We analyze and give explicit representations for the effective abelian vector gauge field actions generated by charged fermions with particular attention to the thermal regime in odd dimensions, where spectral asymmetry can be present. We show, through $\zeta-$function regularization, that both small and large gauge invariances are preserved at any temperature and for any number of fermions at the usual price of anomalies: helicity/parity invariance will be lost in even/odd dimensions, and in the latter even at zero mass. Gauge invariance dictates a very general ``Fourier'' representation of the action in terms of the holonomies that carry the novel, large gauge invariant, information. We show that large (unlike small) transformations and hence their Ward identities, are not perturbative order-preserving, and clarify the role of (properly redefined) Chern-Simons terms in this context. From a powerful representation of the action in terms of massless heat kernels, we are able to obtain rigorous gauge invariant expansions, for both small and large fermion masses, of its separate parity even and odd parts in arbitrary dimension. The representation also displays both the nonperturbative origin of a finite renormalization ambiguity, and its physical resolution by requiring decoupling at infinite mass. Finally, we illustrate these general results by explicit computation of the effective action for some physical examples of field configurations in the three dimensional case, where our conclusions on finite temperature effects may have physical relevance. Nonabelian results will be presented separately., Comment: 36 pages, RevTeX, no figures

The soliton structure of a gauge theory recently proposed to describe chiral excitations in the Fractional Quantum Hall Effect is investigated. A new type of non-linear derivative Schr\"odinger equation emerges as an effective description of the system that supports novel chiral solitons. We discuss the classical properties of solutions with vanishing and non-vanishing boundary conditions (dark solitons) and we explain their relation to integrable systems. The quantum analysis is also addressed in the framework of a semiclassical approximation improved by Renormalization Group arguments., Comment: 39 page, RevTeX, 6 figures

The effective gauge field actions generated by charged fermions in $QED_3$ and $QCD_3$ can be made invariant under both small and large gauge transformations at any temperature by suitable regularization of the Dirac operator determinant, at the price of parity anomalies. We resolve the paradox that the perturbative expansion is not invariant, as manifested by the temperature dependence of the induced Chern-Simons term, by showing that large (unlike small) transformations and hence their Ward identities, are not perturbative order-preserving. Our results are illustrated through concrete examples of field configurations., Comment: 4 pages, RevTex

We show, in two complementary ways, that D=11 supergravity---in contrast to all its lower dimensional versions---forbids a cosmological extension. First, we linearize the putative model about an Anti de Sitter background and show that it cannot even support a "global" supersymmetry invariance; hence there is no Noether construction that can lead to a local supersymmetry. This is true with the usual 4-form field as well as for a "dual", 7-form, starting point. Second, a cohomology argument, starting from the original full nonlinear theory, establishes the absence of deformations involving spin 3/2 mass and cosmological terms. In both approaches, it is the form field that is responsible for the obstruction. ``Dualizing'' the cosmological constant to an 11-form field also fails.

We investigate the quantum effects of the non-minimal matter-gravity couplings derived by Cangemi and Jackiw in the realm of a specific fermionic theory, namely the abelian Thirring model on a Riemann surface of genus zero and one. The structure and the strength of the new interactions are seen to be highly constrained, when the topology of the underlying manifold is taken into account. As a matter of fact, by requiring to have a well-defined action, we are led both to quantization rules for the coupling constants and to selection rules for the correlation functions. Explicit quantum computations are carried out in genus one (torus). In particular the two-point function and the chiral condensate are carefully derived for this case. Finally the effective gravitational action, coming from integrating out the fermionic degrees of freedoom, is presented. It is different from the standard Liouville one: a new non-local functional of the conformal factor arises and the central charge is improved, depending also on the Thirring coupling constant. This last feature opens the possibility of giving a new explicit representation of the minimal series in terms of a fermionic interacting model., Comment: Latex, 41 Pages

String-inspired 1+1-dimensional gravity is coupled to Yang-Mills fields in the Cangemi-Jackiw gauge-theoretical formulation, based on the extended Poincar\'e group. A family of couplings, which involves metrics obtainable from the physical metric with a conformal rescaling, is considered, and the resulting family of models is investigated both at the classical and the quantum level. In particular, also using a series of Kirillov-Kostant phases, the wave functionals that solve the constraints are identified., Comment: 15 pages, LaTex.

We show that excitations in a recently proposed gauge theory for anyons on a line in fact do not obey anomalous statistics. On the other hand, the theory supports novel chiral solitons. Also we construct a field-theoretic description of lineal anyons, but gauge fields play no role., Comment: 8 pages, revtex, no figures

The radiation from the black holes of a 1+1-dimensional chiral quantum gravity model is studied. Most notably, a non-trivial dependence on a renormalization parameter that characterizes the anomaly relations is uncovered in an improved semiclassical approximation scheme; this dependence is not present in the naive semiclassical approximation., Comment: 7 pages, LaTex

In the semiclassical analysis of black hole radiation in matter-coupled dilaton gravity, a one-parameter ``$k$"-family of measures for the path integral quantization of the matter fields is considered. The Weyl anomaly is proportional to the parameter $k$, but the black hole radiation seen by minkowskian observers at future null infinity is $k$-independent., Comment: 13 pages, LaTex. [Typographical error in Eq.(36) was corrected. Comments on the relation between our 2D-gravity results and some known results on the chiral Schwinger model were added. Two references were added.]

We derive anomalous Ward identities in two different approaches to the quantization of massless matter-gravity fields in 1+1 dimensions., Comment: 5 pages, LaTex. (Contribution to the Proceedings of the XXXIV Internationale Universit\"atswochen f\"ur Kern- und Teilchenphysik.)

We investigate the gauge-independent Hamiltonian formulation and the anomalous Ward identities of a matter-induced 1+1-dimensional gravity theory invariant under Weyl transformations and area-preserving diffeomorphisms, and compare the results to the ones for the conventional diffeomorphism-invariant theory. We find that, in spite of several technical differences encountered in the analysis, the two theories are essentially equivalent., Comment: 9 pages, LaTex

It is shown that in 2+1 dimensions the Fermi-Walker gauge allows the general solution of the problem of determining the metric from the sources in terms of simple quadratures. This technique is used to solve the problem of the occurrence of closed time like curves (CTC's) in stationary solutions. In fact the Fermi-Walker gauge, due to its physical nature, allows to exploit the weak energy condition and in this connection it is proved that, both for open and closed universes with axial symmetry, the energy condition imply the total absence of closed time like curves. The extension of this theorem to the general stationary problem, in absence of axial symmetry is considered and at present the proof of such generalization is subject to some assumptions on the behavior of the determinant of the dreibeins in this gauge., Comment: Talk presented at Seventh Marcel Grossmann Meeting on General Relativity, July 24-29, REVTEX, 5 pages, MIT-CTP\#2362 IFUP-TH 54/94

We prove a positive energy theorem in 2+1 dimensional gravity for open universes and any matter energy-momentum tensor satisfying the dominant energy condition. We consider on the space-like initial value surface a family of widening Wilson loops and show that the energy-momentum of the enclosed subsystem is a future directed time-like vector whose mass is an increasing function of the loop, until it reaches the value $1/4G$ corresponding to a deficit angle of $2\pi$. At this point the energy-momentum of the system evolves, depending on the nature of a zero norm vector appearing in the evolution equations, either into a time-like vector of a universe which closes kinematically or into a Gott-like universe whose energy momentum vector, as first recognized by Deser, Jackiw and 't Hooft is space-like. This treatment generalizes results obtained by Carroll, Fahri, Guth and Olum for a system of point-like spinless particle, to the most general form of matter whose energy-momentum tensor satisfies the dominant energy condition. The treatment is also given for the anti de Sitter 2+1 dimensional gravity., Comment: (Revtex), 24 pages,MIT-CTP#2324 snd IFUP-TH-33/94

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-49/93

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-43/93

We give the general solution of the stationary problem of 2+1 dimensional gravity in presence of extended sources, also endowed with angular momentum. We solve explicitly the compact support property of the energy momentum tensor and we apply the results to the study of closed time-like curves. In the case of rotational symmetry we prove that the weak energy condition combined with the absence of closed time-like curves at space infinity prevents the existence of closed time-like curves everywhere in an open universe (conical space at infinity)., Comment: 29 pages, REVTEX 3.0, IFUP-TH 18/93

We consider gravity in 2+1 dimensions in presence of extended stationary sources with rotational symmetry. We prove by direct use of Einstein's equations that if i) the energy momentum tensor satisfies the weak energy condition, ii) the universe is open (conical at space infinity), iii) there are no CTC at space infinity, then there are no CTC at all., Comment: 10 pages (REVTEX 3.0), IFUP-60/92

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-30/92