42 research outputs found
Barriers in Quantum Gravity
I discuss recent progress in our understanding of two barriers in quantum
gravity: in the case of 2d quantum gravity and in the case of
Euclidean Einstein-Hilbert gravity formulated in space-time dimensions .Comment: standard latex, 10 pages. (one year old contribution to Trieste
workshop, but continued demand for preprints has motivated me to put it on
the bulletin board), NBI-HE-93-3
Electroweak Magnetism, W-condensation and Anti-Screening
We review how external magnetic fields act as perfect probes of the
non-abelian nature of the electroweak theory.Comment: Latex 12p. NBI-HE-93-1
Scaling behavior of regularized bosonic strings
We implement a proper-time UV regularisation of the Nambu-Goto string,
introducing an independent metric tensor and the corresponding Lagrange
multiplier, and treating them in the mean-field approximation justified for
long strings and/or when the dimensions of space-time is large. We compute the
regularised determinant of the 2d Laplacian for the closed string winding
around a compact dimension, obtaining in this way the effective action, whose
minimisation determines the energy of the string ground state in the mean-field
approximation. We discuss the existence of two scaling limits when the cutoff
is taken to infinity. One scaling limit reproduces the results obtained by the
hypercubic regularisation of the Nambu-Goto string as well as by the use of the
dynamical triangulation regularisation of the Polyakov string. The other
scaling limit reproduces the results obtained by canonical quantisation of the
Nambu-Goto string.Comment: 35 page
Multi-point functions of weighted cubic maps
We study the geodesic two- and three-point functions of random weighted cubic
maps, which are obtained by assigning random edge lengths to random cubic
planar maps. Explicit expressions are obtained by taking limits of recently
established bivariate multi-point functions of general planar maps. We give an
alternative interpretation of the two-point function in terms of an Eden model
exploration process on a random planar triangulation. Finally, the scaling
limits of the multi-point functions are studied, showing in particular that the
two- and three-point functions of the Brownian map are recovered as the number
of faces is taken to infinity.Comment: 28 pages, 7 figures, several details and clarifications adde
Geodesic distances in Liouville quantum gravity
In order to study the quantum geometry of random surfaces in Liouville
gravity, we propose a definition of geodesic distance associated to a Gaussian
free field on a regular lattice. This geodesic distance is used to numerically
determine the Hausdorff dimension associated to shortest cycles of 2d quantum
gravity on the torus coupled to conformal matter fields, showing agreement with
a conjectured formula by Y. Watabiki. Finally, the numerical tools are put to
test by quantitatively comparing the distribution of lengths of shortest cycles
to the corresponding distribution in large random triangulations.Comment: 21 pages, 8 figure
String theory as a Lilliputian world
Lattice regularizations of the bosonic string allow no tachyons. This has
often been viewed as the reason why these theories have never managed to make
any contact to standard continuum string theories when the dimension of
spacetime is larger than two. We study the continuum string theory in large
spacetime dimensions where simple mean field theory is reliable. By keeping
carefully the cutoff we show that precisely the existence of a tachyon makes it
possible to take a scaling limit which reproduces the lattice-string results.
We compare this scaling limit with another scaling limit which reproduces
standard continuum-string results. If the people working with lattice
regularizations of string theories are akin to Gulliver they will view the
standard string-world as a Lilliputian world no larger than a few lattice
spacings.Comment: 11 pages, 1 figur
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.Comment: 23 pages, Latex, NBI-HE-93-6
The matrix model for hypergeometric Hurwitz numbers
We present the multi-matrix models that are the generating functions for
branched covers of the complex projective line ramified over fixed points
, , (generalized Grotendieck's dessins d'enfants) of fixed
genus, degree, and the ramification profiles at two points, and . We
take a sum over all possible ramifications at other points with the fixed
length of the profile at and with the fixed total length of profiles at
the remaining points. All these models belong to a class of
hypergeometric Hurwitz models thus being tau functions of the
Kadomtsev--Petviashvili (KP) hierarchy. In the case described above, we can
present the obtained model as a chain of matrices with a (nonstandard)
nearest-neighbor interaction of the type \tr M_iM_{i+1}^{-1}. We describe the
technique for evaluating spectral curves of such models, which opens the
possibility of applying the topological recursion for developing
-expansions of these model. These spectral curves turn out to be of an
algebraic type.Comment: 12 pages, 2 figures in LaTeX, contribution to the volume of TMPh
celebrating the 75th birthday of A A Slavno
Semi-classical Dynamical Triangulations
For non-critical string theory the partition function reduces to an integral
over moduli space after integrating over matter fields. The moduli integrand is
known analytically for genus one surfaces. The formalism of dynamical
triangulations provides us with a regularization of non-critical string theory
and we show that even for very small triangulations it reproduces very well the
continuum integrand when the central charge of the matter fields is large
negative, thus providing a striking example of how the quantum fluctuations of
geometry disappear when .Comment: 11 pages, 5 figure