2D Quantum Gravity Research Articles

A twisted FZZ-like dual for the 2D black hole

We study the duality between string theory formulated on a curved exact background (the two-dimensional black hole) and string theory in flat space with a tachyon-like potential. We generalize previous results on this subject by discussing a twisted version of the Fateev-Zamolodchikov-Zamolodchikov conjecture (FZZ). This duality is shown to hold at the level of N -point correlation functions on the sphere topology, and connects tree-level string amplitudes in the Euclidean version of the 2D black hole to correlation functions in a nonlinear sigma-model in flat space but in presence of a tachyon wall potential and a linear dilaton. The dual CFT we propose here corresponds to the perturbed 2D quantum gravity coupled to c n = 2 momentum mode perturbation, while the usual sine-Liouville potential involved in the FZZ duality would correspond to the vortex sector n = 1. We give a precise prescription for computing correlation functions in the twisted model.

New spinfoam vertex for quantum gravity

We introduce a new spinfoam vertex to be used in models of 4d quantum gravity based on SU(2) and SO(4) BF theory plus constraints. It can be seen as the conventional vertex of SU(2) BF theory, the 15j symbol, in a particular basis constructed using SU(2) coherent states. This basis makes the geometric interpretation of the variables transparent: they are the vectors normal to the triangles within each tetrahedron. We study the condition under which these states can be considered semiclassical, and we show that the semiclassical ones dominate the evaluation of quantum correlations. Finally, we describe how the constraints reducing BF to gravity can be directly written in terms of the new variables, and how the semiclassicality of the states might improve understanding the correct way to implement the constraints.

3D spinfoam quantum gravity: matter as a phase of the group field theory

An effective field theory for matter coupled to three-dimensional quantum gravity was recently derived in the context of spinfoam models (Freidel and Livine 2006 Phys. Rev. Lett. 96 221301 (Preprint hep-th/0512113)). In this paper, we show how this relates to group field theories and generalized matrix models. In the first part, we realize that the effective field theory can be recast as a matrix model where couplings between matrices of different sizes can occur. In a second part, we provide a family of classical solutions to the three-dimensional group field theory. By studying perturbations around these solutions, we generate the dynamics of the effective field theory. We identify a particular case which leads to the action of Freidel and Livine (2006 Phys. Rev. Lett. 96 221301 (Preprint hep-th/0512113)) for a massive field living in a flat non-commutative spacetime. The most general solutions lead to field theories with nonlinear redefinitions of the momentum which we propose to interpret as living on curved spacetimes. We conclude by discussing the possible extension to four-dimensional spinfoam models.

Coupling gauge theory to spinfoam 3D quantum gravity

We construct a spinfoam model for Yang–Mills theory coupled to quantum gravity in three-dimensional Riemannian spacetime. We define the partition function of the coupled system as a power series in g02G that can be evaluated order by order using grasping rules and the recoupling theory. With respect to previous attempts in the literature, this model assigns the dynamical variables of gravity and Yang–Mills theory to the same simplices of the spinfoam, and it thus provides transition amplitudes for the spin network states of the canonical theory. For SU(2) Yang–Mills theory we show explicitly that the partition function has a semiclassical limit given by the Regge discretization of the classical Yang–Mills action.

Boundary loop models and 2D quantum gravity

We study the O(n) loop model on a dynamically triangulated disc, with a new type of boundary condition,discovered recently by Jacobsen and Saleur. The partition function of the model is that of agas of self-and mutually avoiding loops covering the disc. The Jacobsen–Saleur (JS)boundary condition prescribes that the loops that do not touch the boundary have fugacity, while the loops touching the boundary at least once are given different fugacityy. The class of JS boundary conditions, labeled by the real numbery, contains theNeumann (y = n)and Dirichlet (y = 1) boundary conditions as particular cases. Here we consider the dense phase ofthe loop gas, where we compute the two-point boundary correlators of theL-leg operators with mixed Neumann–JS boundary condition. The resultcoincides with the boundary two-point function in Liouville theory, derivedby Fateev, Zamolodchikov and Zamolodchikov. The Liouville charge ofthe boundary operators match, by the KPZ correspondence, with theL-leg boundary exponents conjectured by JS.

Grasping rules and semiclassical limit of the geometry in the Ponzano–Regge model

We show how the expectation values of geometrical quantities in 3D quantum gravity can be explicitly computed using grasping rules. We compute the volume of a labelled tetrahedron using the triple grasping. We show that the large spin expansion of this value is dominated by the classical expression, and we study the next to leading order quantum corrections.

Non-rational 2D quantum gravity: I. World sheet CFT

We address the problem of computing the tachyon correlation functions in Liouville gravity with generic (non-rational) matter central charge c < 1 . We consider two variants of the theory. The first is the conventional one in which the effective matter interaction is given by the two matter screening charges. In the second variant the interaction is defined by the Liouville dressings of the non-trivial vertex operator of zero dimension. This particular deformation, referred to as “diagonal”, is motivated by the comparison with the discrete approach, which is the subject of a subsequent paper. In both theories we determine the ground ring of ghost zero physical operators by computing its OPE action on the tachyons and derive recurrence relations for the tachyon bulk correlation functions. We find 3- and 4-point solutions to these functional equations for various matter spectra. In particular, we find a closed expression for the 4-point function of order operators in the diagonal theory.

Quantum gravity with matter and group field theory

A generalization of the matrix model idea to quantum gravity in three and higher dimensions is known as group field theory (GFT). In this paper we study generalized GFT models that can be used to describe 3D quantum gravity coupled to point particles. The generalization considered is that of replacing the group leading to pure quantum gravity by the twisted product of the group with its dual—the so-called Drinfeld double of the group. The Drinfeld double is a quantum group in that it is an algebra that is both non-commutative and non-cocommutative, and special care is needed to define group field theory for it. We show how this is done, and study the resulting GFT models. Of special interest is a new topological model that is the ‘Ponzano–Regge’ model for the Drinfeld double. However, as we show, this model does not describe point particles. Motivated by the GFT considerations, we consider a more general class of models that are defined not using GFT, but the so-called chain mail techniques. A general model of this class does not produce 3-manifold invariants, but has an interpretation in terms of point particle Feynman diagrams.

Non-rational 2D quantum gravity II. Target space CFT

We explore the formulation of non-rational 2D quantum gravity in terms of a chiral CFT on a Riemann surface associated with the target space. The CFT in question is constructed as the collective theory for a matrix chain, which is dual to a statistical height model on dynamical triangulations. The heights are associated with the sheets of the Riemann surface, which represents an infinite branched cover of the spectral plane. We consider two examples of height models: the SOS model and the semi-restricted SOS (SRSOS) model, in which the heights are restricted from below. Both models are described in the continuum limit by theories of 2D quantum gravity with conformal matter, perturbed by a thermal operator ( 1 , 3 ) . We give a compact operator expression for the n-loop amplitudes as a collection of target space Feynman rules. The n-point functions of local fields are obtained by shrinking the loops. In particular, we show that the 4-point function of order operators in the SRSOS model coincides with the 4-point function of the “diagonal” world sheet CFT studied in [I. Kostov, V. Petkova, Non-rational 2D quantum gravity I. World sheet CFT, hep-th/0512346].

S-duality in 3D gravity with torsion

The deformation of the connection in three spacetime dimensions by the kinematically equivalent coframe is shown to induce a duality between the (Lorentz-) rotational and translational field momenta, for which the coupling to the deformation parameter is inverted. This new kind of strong/weak duality, pertinent to 3D, is instrumental for studying exact solutions of the 3D Poincaré gauge field equations and the particle content of propagating modes on a background of constant curvature. For a topological Chern–Simons model of gravity, the propagating modes ‘living’ on an Anti-de Sitter (AdS) background correspond to real massive particles. Yang–Mills type generalizations and new cubic Lagrangians are found and completely classified in 3D. AdS or black hole type solutions with constant axial torsion emerge, also for these higher-order Lagrangians with new ‘exotic’ torsion-curvature couplings. Their pattern complies with our S-duality, with new repercussions for the field redefinition of the metric in 3D quantum gravity and the cosmological constant problem.

Causality and matter propagation in 3D spin foam quantum gravity

In this paper we tackle the issue of causality in quantum gravity, in the context of 3d spin foam models. We identify the correct procedure for implementing the causality/orientation dependence restriction that reduces the path integral for BF theory to that of quantum gravity in first order form. We construct explicitly the resulting causal spin foam model. We then add matter degrees of freedom to it and construct a causal spin foam model for 3d quantum gravity coupled to matter fields. Finally, we show that the corresponding spin foam amplitudes admit a natural approximation as the Feynman amplitudes of a noncommutative quantum field theory, with the appropriate Feynman propagators weighting the lines of propagation, and that this effective field theory reduces to the usual quantum field theory in flat space in the no-gravity limit.

Group field theory formulation of 3D quantum gravity coupled to matter fields

We present a new group field theory describing 3D Riemannian quantum gravity coupled to matter fields for any choice of spin and mass. The perturbative expansion of the partition function produces fat graphs coloured with SU(2) algebraic data, from which one can reconstruct at once a three-dimensional simplicial complex representing spacetime and its geometry, like in the Ponzano–Regge formulation of pure 3D quantum gravity, and the Feynman graphs for the matter fields. The model then assigns quantum amplitudes to these fat graphs given by spin foam models for gravity coupled to interacting massive spinning point particles, whose properties we discuss.

Painlevé I asymptotics for orthogonal polynomials with respect to a varying quartic weight

We study polynomials that are orthogonal with respect to a varying quartic weight exp(− N(x2/2 + tx4/4)) for t < 0, where the orthogonality takes place on certain contours in the complex plane. Inspired by developments in 2D quantum gravity, Fokas, Its and Kitaev showed that there exists a critical value for t around which the asymptotics of the recurrence coefficients are described in terms of exactly specified solutions of the Painlevé I equation. In this paper, we present an alternative and more direct proof of this result by means of the Deift/Zhou steepest descent analysis of the Riemann–Hilbert problem associated with the polynomials. Moreover, we extend the analysis to non-symmetric combinations of contours. Special features in the steepest descent analysis are a modified equililbrium problem and the use of Ψ-functions for the Painlevé I equation in the construction of the local parametrix.

3D Quantum Gravity and Effective Noncommutative Quantum Field Theory

We show that the effective dynamics of matter fields coupled to 3D quantum gravity is described after integration over the gravitational degrees of freedom by a braided noncommutative quantum field theory symmetric under a kappa deformation of the Poincaré group.

Bulk Correlation Functions in 2d Quantum Gravity

We compute bulk 3- and 4-point tachyon correlators in the 2d Liouville gravity with non-rational matter central charge c<1, following and comparing two approaches. The continuous CFT approach exploits the action on the tachyons of the ground ring generators deformed by Liouville and matter ``screening charges''. A by-product general formula for the matter 3-point OPE structure constants is derived. We also consider a ``diagonal'' CFT of 2D quantum gravity, in which the degenerate fields are restricted to the diagonal of the semi-infinite Kac table. The discrete formulation of the theory is a generalization of the ADE string theories, in which the target space is the semi-infinite chain of points.

Spinfoam 2D quantum gravity and discrete bundles

In four dimensions (4D), general relativity (GR) can be formulated as a constrained BF theory; we show that the same is true in 2D. We describe a spinfoam quantization of this constrained BF formulation of 2D Riemannian GR, obtained using the Barrett–Crane technique of imposing the constraint as a restriction on the representations summed over. We obtain the expected partition function, thus providing support for the viability of the technique. The result requires the nontrivial topology of the bundle where the gravitational connection is defined to be taken into account. For this purpose, we study the definition of a principal bundle over a simplicial base space. The model also sheds light on several other features of spinfoam quantum gravity: the reality of the partition function, the geometrical interpretation of the Newton constant and the issue of possible finiteness of the partition function of quantum GR.

Ponzano–Regge model revisited: I. Gauge fixing, observables and interacting spinning particles

We show how to properly gauge fix all the symmetries of the Ponzano–Regge model for 3D quantum gravity. This amounts to doing explicit finite computations for transition amplitudes. We give the construction of the transition amplitudes in the presence of interacting quantum spinning particles. We introduce a notion of operators whose expectation value gives rise to either gauge fixing, introduction of time, or insertion of particles, according to the choice. We give the link between the spin foam quantization and the Hamiltonian quantization. We finally show the link between the Ponzano–Regge model and the quantization of Chern–Simons theory based on the double quantum group of SU(2).

Boundary ground ring in 2D string theory

The 2D quantum gravity on a disc, or the non-critical theory of open strings, is known to exhibit an integrable structure, the boundary ground ring, which determines completely the boundary correlation functions. Inspired by the recent progress in boundary Liouville theory, we extend the ground ring relations to the case of non-vanishing boundary Liouville interaction known also as FZZT brane in the context of the 2D string theory. The ring relations yield an over-determined set of functional recurrence equations for the boundary correlation functions. The ring action closes on an infinite array of equally spaced FZZT branes for which we propose a matrix model realization. In this matrix model the boundary ground ring is generated by a pair of complex matrix fields.

Boundary Liouville theory and 2D quantum gravity

We study the boundary correlation functions in Liouville theory and in solvable statistical models of 2D quantum gravity. In Liouville theory we derive functional identities for all fundamental boundary structure constants, similar to the one obtained for the boundary two-point function by Fateev, Zamolodchikov and Zamolodchikov. All these functional identities can be written as difference equations with respect to one of the boundary parameters. Then we switch to the microscopic realization of 2D quantum gravity as a height model on a dynamically triangulated disc and consider the boundary correlation functions of electric, magnetic and twist operators. By cutting open the sum over surfaces along a domain wall, we derive difference equations identical to those obtained in Liouville theory. We conclude that there is a complete agreement between the predictions of Liouville theory and the discrete approach.

Renormalization of 3d quantum gravity from matrix models

Lorentzian simplicial quantum gravity is a non-perturbatively defined theory of quantum gravity which predicts a positive cosmological constant. Since the approach is based on a sum over space–time histories, it is perturbatively non-renormalizable even in three dimensions. By mapping the three-dimensional theory to a two-matrix model with ABAB interaction we show that both the cosmological and the (perturbatively) non-renormalizable gravitational coupling constant undergo additive renormalizations consistent with canonical quantization.