arXiv subjects

Ioannis Bakas

Publications and source records attributed to Ioannis Bakas.

30 records · Page 2Linked to original sources

arXiv2008

Energy-momentum/Cotton tensor duality for AdS4 black holes

We consider the theory of gravitational quasi-normal modes for general linear perturbations of AdS4 black holes. Special emphasis is placed on the effective Schrodinger problems for axial and polar perturbations that realize supersymmetric partner potential barriers on the half-line. Using the holographic renormalization method, we compute the energy-momentum tensor for perturbations satisfying arbitrary boundary conditions at spatial infinity and discuss some aspects of the problem in the hydrodynamic representation. It is also observed in this general framework that the energy-momentum tensor of black hole perturbations and the energy momentum tensor of the gravitational Chern-Simons action (known as Cotton tensor) exhibit an axial-polar duality with respect to appropriately chosen supersymmetric partner boundary conditions on the effective Schrodinger wave-functions. This correspondence applies to perturbations of very large AdS4 black holes with shear viscosity to entropy density ratio equal to 1/4π, thus providing a dual graviton description of their hydrodynamic modes. We also entertain the idea that the purely dissipative modes of black hole hydrodynamics may admit Ricci flow description in the non-linear regime.

hep-th↗

arXiv2008

Ricci flows and expansion in axion-dilaton cosmology

We study renormalization-group flows by deforming a class of conformal sigma-models. We consider overall scale factor perturbation of Einstein spaces as well as more general anisotropic deformations of three-spheres. At leading order in alpha, renormalization-group equations turn out to be Ricci flows. In the three-sphere background, the latter is the Halphen system, which is exactly solvable in terms of modular forms. We also analyze time-dependent deformations of these systems supplemented with an extra time coordinate and time-dependent dilaton. In some regimes time evolution is identified with renormalization-group flow and time coordinate can appear as Liouville field. The resulting space-time interpretation is that of a homogeneous isotropic Friedmann-Robertson-Walker universe in axion-dilaton cosmology. We find as general behaviour the superposition of a big-bang (polynomial) expansion with a finite number of oscillations at early times. Any initial anisotropy disappears during the evolution.

hep-th↗

arXiv2007

Renormalization group equations and geometric flows

The quantum field theory of two-dimensional sigma models with bulk and boundary couplings provides a natural framework to realize and unite different species of geometric flows that are of current interest in mathematics. In particular, the bulk renormalization group equation gives rise to the Ricci flow of target space metrics, to lowest order in perturbation theory, whereas the boundary renormalization group equation gives rise to the mean curvature flow for embedded branes. Together they form a coupled system of parabolic non-linear second order differential equations that can be further generalized to include non-trivial fluxes. Some closely related higher order curvature flows, such as the Calabi and Willmore flows associated to quadratic curvature functionals, are also briefly discussed as they arise in physics and mathematics. However, there is no known interpretation for them, as yet, in the context of quantum field theory.

hep-th↗

arXiv2005

On the integrability of spherical gravitational waves in vacuum

The general class of Robinson-Trautman metrics that describe gravitational radiation in the exterior of bounded sources in four space-time dimensions is shown to admit zero curvature formulation in terms of appropriately chosen two-dimensional gauge connections. The result, which is valid for either type II or III metrics, implies that the gravitational analogue of the Lienard-Wiechert fields of Maxwell equations form a new integrable sector of Einstein equations for any value of the cosmological constant. The method of investigation is similar to that used for integrating the Ricci flow in two dimensions. The zero modes of the gauge symmetry (factored by the center) generate Kac's K_2 simple Lie algebra with infinite growth.

gr-qc↗

arXiv2004

Ricci flows and their integrability in two dimensions

We review the main aspects of Ricci flows as they arise in physics and mathematics. In field theory they describe the renormalization group equations of the target space metric of two dimensional sigma models to lowest order in the perturbative expansion. As such they provide an off-shell approach to the problem of tachyon condensation and vacuum selection in closed string theory in the weak gravitational regime. In differential geometry they introduce a systematic framework to find canonical metrics on Riemannian manifolds and make advances towards their classification by proving the geometrization conjecture. We focus attention to geometric deformations in low dimensions and find that they also exhibit a rich algebraic structure. The Ricci flow in two dimensions is shown to be integrable using an infinite dimensional algebra with antisymmetric Cartan kernel that incorporates the deformation variable into its root system. The deformations of two dimensional surfaces also control the Ricci flow on 3-manifolds and their decomposition into prime factors by applying surgery prior to the formation of singularities along shrinking cycles. A few simple examples are briefly discussed including the notion of Ricci solitons. Other applications to physical systems are also listed at the end.

hep-th↗

arXiv2003

Renormalization group flows and continual Lie algebras

We study the renormalization group flows of two-dimensional metrics in sigma models and demonstrate that they provide a continual analogue of the Toda field equations based on the infinite dimensional algebra G(d/dt;1). The resulting Toda field equation is a non-linear generalization of the heat equation, which is integrable in target space and shares the same dissipative properties in time. We provide the general solution of the renormalization group flows in terms of free fields, via Backlund transformations, and present some simple examples that illustrate the validity of their formal power series expansion in terms of algebraic data. We study in detail the sausage model that arises as geometric deformation of the O(3) sigma model, and give a new interpretation to its ultra-violet limit by gluing together two copies of Witten's two-dimensional black hole in the asymptotic region. We also provide some new solutions that describe the renormalization group flow of negatively curved spaces in different patches, which look like a cane in the infra-red region. Finally, we revisit the transition of a flat cone C/Z_n to the plane, as another special solution, and note that tachyon condensation in closed string theory exhibits a hidden relation to the infinite dimensional algebra G(d/dt;1) in the regime of gravity. Its exponential growth holds the key for the construction of conserved currents and their systematic interpretation in string theory, but they still remain unknown.

hep-th↗

arXiv2003

On integrable models from pp-wave string backgrounds

We construct solutions of type IIB supergravity with non-trivial Ramond-Ramond 5-form in ten dimensions by replacing the transverse flat space of pp-wave backgrounds with exact $N=(4,4)$ $c=4$ superconformal field theory blocks. These solutions, which also include a dilaton and (in some cases) an anti-symmetric tensor field, lead to integrable models on the world-sheet in the light-cone gauge of string theory. In one instance we demonstrate explicitly the emergence of the complex sine-Gordon model, which coincides with integrable perturbations of the corresponding superconformal building blocks in the transverse space. In other cases we arrive at the supersymmetric Liouville theory or at the complex sine-Liouville model. For axionic instantons in the transverse space, as for the (semi)-wormhole geometry, we obtain an entire class of supersymmetric pp-wave backgrounds by solving the Killing spinor equations as in flat space, supplemented by the appropriate chiral projections; as such, they generalize the usual Neveu-Schwarz five-brane solution of type IIB supergravity in the presence of a Ramond-Ramond 5-form. We also present some further examples of interacting light-cone models and we briefly discuss the role of dualities in the resulting string theory backgrounds.

hep-th↗

arXiv1999

Remarks on the Atiyah-Hitchin metric

We outline the construction of the Atiyah-Hitchin metric on the moduli space of SU(2) BPS monopoles with charge 2, first as an algebraic curve in C^3 following Donaldson and then as a solution of the Toda field equations in the continual large N limit. We adopt twistor methods to solve the underlying uniformization problem, which by the generalized Legendre transformation yield the Kahler coordinates and the Kahler potential of the metric. We also comment on the connection between twistors and the Seiberg-Witten construction of quantum moduli spaces, as they arise in three dimensional supersymmetric gauge theories, and briefly address the uniformization of algebraic curves in C^3 in the context of large N Toda theory. (Based on talks delivered in September 1998 at the 32nd International Symposium Ahrenshoop on the Theory of Elementary Particles, Buckow; the 21st Triangular Meeting on Quantum Field Theory, Crete and the TMR meeting on Quantum Aspects of Gauge Theories, Supersymmetry and Unification, Corfu; to be published in the proceedings in Fortschritte der Physik.)

hep-th↗

arXiv1997

Supersymmetric Solutions in Three-Dimensional Heterotic String Theory

We consider the low-energy effective field theory of heterotic string theory compactified on a seven-torus, and we construct electrically charged as well as more general solitonic solutions. These solutions preserve 1/2, 1/4 and 1/8 of N=8, D=3 supersymmetry and have Killing spinors which exist due to cancellation of holonomies. The associated space-time line elements do not exhibit the conical structure that often arises in 2+1 dimensional gravity theories.

hep-th↗

arXiv1996

Gravitational dressing of massive soliton theories

The massive soliton theories describe integrable perturbations of WZW cosets as generalized multi-component sine-Gordon models. We study their coupling to 2-dim gravity in the conformal gauge and show that the resulting models can be interpreted as conformal non-Abelian Toda theories when a certain algebraic condition is satisfied. These models, however, do not provide quantum mechanically consistent string backgrounds in the case the underlying WZW constraints are first solved classically.

hep-th↗

arXiv1994

String Dualities and the Geroch Group

We examine the properties and symmetries of the lowest order effective theory of 4-dim string backgrounds with axion and dilaton fields and zero cosmological constant. The dimensional reduction yields an O(2,2) current group of transformations in the presence of two commuting Killing symmetries. Special emphasis is placed on the identification of the T and S string duality symmetries, and their intertwining relations. (Contributed to the Proceedings of the Satellite Colloquium "Topology, Strings and Integrable Models" to the XIth International Congress of Mathematical Physics, Paris, 25--28 July 1994; Diderot Editeur.)

hep-th↗

arXiv1994

Space Time Interpretation of S-Duality and Supersymmetry Violations of T-Duality

The S--duality transformations of the lowest order string effective theory admit a space time interpretation for 4-dim backgrounds with one Killing symmetry. Starting from pure gravity and performing a sequence of intertwined T-S-T duality transformations we obtain new solutions which are always pure gravitational. In this fashion, S-duality induces an $SL(2,R)$ transformation in the space of target space metrics which coincides with the action of the Ehlers-Geroch group and interchanges the electric with the magnetic aspects of gravity. Specializing to gravitational instanton backgrounds we show that ALE instantons are mapped to (multi) Taub-NUT backgrounds and vice-versa. We find, however, that the self-duality of the metric is not generically preserved, unless the corresponding Killing vector field has self-dual covariant derivatives. Thus, the T-S-T transformations are not always compatible with the world-sheet supersymmetry of $N=4$ superconformal string vacua. We also provide an algebraic characterization of the corresponding obstruction and associate it with a breakdown of space time supersymmetry under rotational T-duality transformations.

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