Konstadinos Sfetsos

The Black hole and FRW geometries of non-relativistic gravity

Abstract We consider the recently proposed non-relativistic Hořava–Lifshitz four-dimensional theory of gravity. We study a particular limit of the theory which admits flat Minkowski vacuum and we discuss thoroughly the quadratic fluctuations around it. We find that there are two propagating polarizations of the metric. We then explicitly construct a spherically symmetric, asymptotically flat, black hole solution that represents the analog of the Schwarzschild solution of GR. We show that this theory has the same Newtonian and post-Newtonian limits as GR and thus, it passes the classical tests. We also consider homogeneous and isotropic cosmological solutions and we show that although the equations are identical with GR cosmology, the couplings are constrained by the observed primordial abundance of 4 He.

Spinors and the AdS/CFT correspondence

We consider a free massive spinor field in Euclidean Anti-de Sitter space. The usual Dirac action in bulk is supplemented by a certain boundary term. The boundary conditions of the field are parametrized by a spinor on the boundary, subject to a projection. We calculate the dependence of the partition function on this boundary spinor. The result agrees with the generating functional of the correlation functions of a quasi-primary spinor operator, of a certain scaling dimension, in a free conformal field theory on the boundary.

Microscopic derivation of the Bekenstein-Hawking entropy formula for non-extremal black holes

Abstract We derive the Bekenstein-Hawking entropy formula for four- and five-dimensional non-supersymmetric black holes (which include the Schwarzchild ones) by counting microscopic states. This is achieved by first showing that these black holes are U -dual to the three-dimensional black hole of Banados-Teitelboim-Zanelli and then counting microscopic states of the latter following Carlips approach. Black holes higher than five dimensional are also considered. We discuss the connection of our approach to the D-brane picture.

On non-abelian T-dual geometries with Ramond fluxes

Abstract We show how to implement T-duality along non-abelian isometries in backgrounds with non-vanishing Ramond fields. When the dimension of the isometry group is odd (even) the duality swaps (preserves) the chirality of the theory. In certain cases a non-abelian duality can result in a massive type-IIA background. We provide two examples by dualising SU ( 2 ) isometry subgroups in AdS 5 × S 5 and AdS 3 × S 3 × T 4 . The resultant dual geometries inherit the original AdS factors but have transverse spaces with reduced isometry and preserve only half of the original supersymmetry. The non-abelian dual of AdS 5 × S 5 has an M-theory lift which is related to the gravity duals of N = 2 superconformal theories. We comment on a possible interpretation of this as a high spin limit.

Relativistic quantum gravity at a Lifshitz point

We show that the Hořava theory for the completion of General Relativity at UV scales can be interpreted as a gauge fixed Tensor-Vector theory, and it can be extended to an invariant theory under the full group of four-dimensional diffeomorphisms. In this respect, although being fully relativistic, it results to be locally anisotropic in the time-like and space-like directions defined by a family of irrotational observers. We show that this theory propagates generically three degrees of freedom: two of them are related to the four-dimensional diffeomorphism invariant graviton (the metric) and one is related to a propagating scalar mode of the vector. Finally, we note that in the present formulation, matter can be consistently coupled to gravity.

Non-standard compactifications with mass gaps and Newton's law

The four-dimensional Minkowski space-time is considered as a three-brane embedded in five dimensions, using solutions of five-dimensional supergravity. These backgrounds have a string theoretical interpretation in terms of D3-brane distributions. By studying linear fluctuations of the graviton we find a zero-mode representing the massless graviton in four-dimensional space-time. The novelty of our models is that the graviton spectrum has a genuine mass gap (independent of the position of the world-brane) above the zero-mode or it is discrete. Hence, an effective four-dimensional theory on a brane that includes the massless graviton mode is well defined. The gravitational force between point particles deviates from the Newton law by Yukawa-type corrections, which we compute explicitly. We show that the parameters of our solutions can be chosen such that these corrections lie within experimental bounds.

On running couplings in gauge theories from type-IIB supergravity

Abstract We construct an explicit solution of type-IIB supergravity describing the strong coupling regime of a non-supersymmetric gauge theory. The latter has a running coupling with an ultraviolet stable fixed point corresponding to the N =4 SU(N) super-Yang–Mills theory at large N. The running coupling has a power law behaviour, argued to be universal, that is consistent with holography. Around the critical point, our solution defines an asymptotic expansion for the gauge coupling beta-function. We also calculate the first correction to the Coulombic quark–antiquark potential.

Supergravity and the jet quenching parameter in the presence of R-charge densities

Following a recent proposal, we employ the AdS/CFT correspondence to compute the jet quenching parameter for = 4 Yang-Mills theory at nonzero R-charge densities. Using as dual supergravity backgrounds non-extremal rotating branes, we find that the presence of the R-charges generically enhances the jet quenching phenomenon. However, at fixed temperature, this enhancement might or might not be a monotonically increasing function of the R-charge density and depends on the number of independent angular momenta describing the solution. We perform our analysis for the canonical as well as for the grand canonical ensemble which give qualitatively similar results.

Gauged WZW models and non-Abelian duality.

We consider WZW models based on the non-semi-simple algebras that were recently constructed as contractions of corresponding algebras for semisimple groups. We give the explicit expression for the action of these models, as well as for a generalization of them, and discuss their general properties. Furthermore we consider gauged WZW models based on these non-semi-simple algebras and we show that they are equivalent to non-Abelian duality transformations on WZW actions. We also show that a general non-Abelian duality transformation can be thought of as a limiting case of the non-Abelian quotient theory of the direct product of the original action and the WZW action for the symmetry gauge group [ital H]. In this action there is no Lagrange multiplier term that constrains the gauge field strength to vanish. A particular result is that the gauged WZW action for the coset ([ital G][sub [ital k]][direct product][ital H][sub [ital l]])/[ital H][sub [ital k]+[ital l]] becomes in a certain limit, involving [ital l][r arrow][infinity], the dualized WZW action for [ital G][sub [ital k]] with respect to the subgroup [ital H].

Supersymmetric AdS(6) via T Duality

We present a new supersymmetric AdS(6) solution of type IIB supergravity with SU(2) isometry. Through the AdS/CFT correspondence, this has potentially very interesting implications for 5D fixed point theories. This solution is the result of a non-Abelian T duality on the known supersymmetric AdS(6) solution of massive IIA. The SU(2) R symmetry is untouched, leading to sixteen supercharges and preserved supersymmetry.