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Jan Ambjorn

Publications and source records attributed to Jan Ambjorn.

52 records · Page 3Linked to original sources
arXiv2004

Quantum rolling tachyon

We consider the quantum treatment of the rolling tachyon background describing the decay of D-branes in the limit of weak string coupling. We focus on the propagation of an open string in the fluctuating background and show how the boundary string action is modified by quantum effects. A bilocal term in the boundary action is generated which, however, does not spoil the vanishing of the $β$ function at one loop. The propagation of an open string for large times is found to be very strongly suppressed.

hep-th↗
arXiv2004

Non-Commutativity of the Zero Chemical Potential Limit and the Thermodynamic Limit in Finite Density Systems

Monte Carlo simulations of finite density systems are often plagued by the complex action problem. We point out that there exists certain non-commutativity in the zero chemical potential limit and the thermodynamic limit when one tries to study such systems by reweighting techniques. This is demonstrated by explicit calculations in a Random Matrix Theory, which is thought to be a simple qualitative model for finite density QCD. The factorization method allows us to understand how the non-commutativity, which appears at the intermediate steps, cancels in the end results for physical observables.

hep-lat↗
arXiv2003

The decay of quantum D-branes

We study the {\em quantum} decay of D0-branes in two-dimensional 0B string theory. The quantum nature of the branes provides a natural cut-off for the closed string emission rate. We find exact quantum mechanical wavefunctions for the decaying branes and show how one can include the effects of the Fermi sea for any string coupling (Fermi energy).

hep-th↗
arXiv2003

The Factorization Method for Simulating Systems With a Complex Action

We propose a method for Monte Carlo simulations of systems with a complex action. The method has the advantages of being in principle applicable to any such system and provides a solution to the overlap problem. We apply it in random matrix theory of finite density QCD where we compare with analytic results. In this model we find non--commutativity of the limits $μ\to 0$ and $N\to\infty$ which could be of relevance in QCD at finite density.

hep-lat↗
arXiv1999

On the phase diagram of 2d Lorentzian Quantum Gravity

The phase diagram of 2d Lorentzian quantum gravity (LQG) coupled to conformal matter is studied. A phase transition is observed at $c=c_{\rm crit}$ ($1/2<c_{\rm crit}<4$) which can be thought of as the analogue of the $c=1$ barrier of Euclidean quantum gravity (EQG). The non--trivial properties of the quantum geometry are discussed.

hep-lat↗
arXiv1997

U(1) lattice gauge theory and N=2 supersymmetric Yang-Mills theory

We discuss the physics of four-dimensional compact U(1) lattice gauge theory from the point of view of softly broken N=2 supersymmetric SU(2) Yang-Mills theory. We provide arguments in favor of (pseudo-)critical mass exponents 1/3, 5/11 and 1/2, in agreement with the values observed in the computer simulations. We also show that the J^{CP} assignment of some of the lowest lying states can be naturally explained.

hep-th↗
arXiv1996

RG flow in an exactly solvable model with fluctuating geometry

A recently proposed renormalization group technique, based on the hierarchical structures present in theories with fluctuating geometry, is implemented in the model of branched polymers. The renormalization group equations can be solved analytically, and the flow in coupling constant space can be determined.

hep-lat↗
arXiv1995

Quantization of Geometry

Contents: 1. Introduction 2. Bosonic propagators and random paths 3. Random surfaces and strings 4. Matrix models and two-dimensional quantum gravity 5. The mystery of $c > 1$ 6. Euclidean quantum gravity in $d > 2$ 7. Discussion

hep-th↗
arXiv1995

Scaling in four dimensional quantum gravity

We discuss scaling relations in four dimensional simplicial quantum gravity. Using numerical results obtained with a new algorithm called ``baby universe surgery'' we study the critical region of the theory. The position of the phase transition is given with high accuracy and some critical exponents are measured. Their values prove that the transition is continuous. We discuss the properties of two distinct phases of the theory. For large values of the bare gravitational coupling constant the internal Hausdorff dimension is {\em two} (the elongated phase), and the continuum theory is that of so called branched polymers. For small values of the bare gravitational coupling constant the internal Hausdorff dimension seems to be {\em infinite} (the crumpled phase). We conjecture that this phase corresponds to a theory of topological gravity. {\em At} the transition point the Hausdorff dimension might be finite and larger than two. This transition point is a potential candidate for a non-perturbative theory of quantum gravity.

hep-th↗
arXiv1994

Recent progress in the theory of random surfaces and simplicial quantum gravity

Some of the recent developments in the theory of random surfaces and simplicial quantum gravity is reviewed. For 2d quantum gravity this includes the failure of Regge calculus, our improved understanding of the $c>1$ regime, some surprises for q-state Potts models with $q > 4$, attempts to use renormalization group techniques, new critical behavior of random surface models with extrinsic curvature and improved algorithms. For simplicial quantum gravity in higher dimensions it includes a discussion of the exponential entropy bound needed for the models to be well defined, the question of ``computational ergodicity'' and the question of how to extract continuum behavior from the lattice simulations.

hep-lat↗
arXiv1994

New critical phenomena in 2d quantum gravity

We study $q=10$ and $q=200$ state Potts models on dynamical triangulated lattices and demonstrate that these models exhibit continuous phase transitions, contrary to the first order transition present on regular lattices. For $q=10$ the transition seems to be of 2nd order, while it seems to be of 3rd order for $q=200$. For $q=200$ the phase transition also induces a transition between typical fractal structures of the piecewise linear surfaces corresponding to the triangulations. The typical surface changes from having a tree-like structure to a fractal structure characterizing pure gravity when the temperature drops below the critical temperature. An investigation of the alignment of spin clusters shows that they are strongly correlated to the underlying fractal structure of the triangulated surfaces.

hep-lat↗
arXiv1994

Barriers in Quantum Gravity

I discuss recent progress in our understanding of two barriers in quantum gravity: $c > 1$ in the case of 2d quantum gravity and $D > 2$ in the case of Euclidean Einstein-Hilbert gravity formulated in space-time dimensions $D >2$.

hep-th↗
arXiv1993

2D Quantum Gravity Coupled to Renormalizable Matter Fields

We consider two-dimensional quantum gravity coupled to matter fields which are renormalizable, but not conformal invariant. Questions concerning the $\b$ function and the effective action are addressed, and the effective action and the dressed renormalization group equations are determined for various matter potentials.

hep-th↗
arXiv1993

From 1-matrix model to Kontsevich model

Loop equations of matrix models express the invariance of the models under field redefinitions. We use loop equations to prove that it is possible to define continuum times for the generic hermitian {1-matrix} model such that all correlation functions in the double scaling limit agree with the corresponding correlation functions of the Kontsevich model expressed in terms of kdV times. In addition the double scaling limit of the partition function of the hermitian matrix model agree with the $τ$-function of the kdV hierarchy corresponding to the Kontsevich model (and not the square of the $τ$-function) except for some complications at genus zero.

hep-th↗