863 research outputs found

### New multicritical matrix models and multicritical 2d CDT

We define multicritical CDT models of 2d quantum gravity and show that they
are a special case of multicritical generalized CDT models obtained from the
new scaling limit, the so-called "classical" scaling limit, of matrix models.
The multicritical behavior agrees with the multicritical behavior of the
so-called branched polymers.Comment: 16 pages, 4 figures. References adde

### 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 $D >2$.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

### Topology and Confinement in SU(N) Gauge Theories

The large N limit of SU(N) gauge theories in 3+1 dimensions is investigated
on the lattice by extrapolating results obtained for $2 \le N \le 5$. A
numerical determination of the masses of the lowest-lying glueball states and
of the topological susceptibility in the limit $N\to\infty$ is provided. Ratios
of the tensions of stable k-strings over the tension of the fundamental string
are investigated in various regimes and the results are compared with
expectations based on several scenarios -- in particular MQCD and Casimir
scaling. While not conclusive at zero temperature in D=3+1, in the other cases
investigated our data seem to favour the latter.Comment: 3 pages, 2 figures; talk presented by B. Lucini at
Lattice2001(confinement

### Lorentzian and Euclidean Quantum Gravity - Analytical and Numerical Results

We review some recent attempts to extract information about the nature of
quantum gravity, with and without matter, by quantum field theoretical methods.
More specifically, we work within a covariant lattice approach where the
individual space-time geometries are constructed from fundamental simplicial
building blocks, and the path integral over geometries is approximated by
summing over a class of piece-wise linear geometries. This method of
``dynamical triangulations'' is very powerful in 2d, where the regularized
theory can be solved explicitly, and gives us more insights into the quantum
nature of 2d space-time than continuum methods are presently able to provide.
It also allows us to establish an explicit relation between the Lorentzian- and
Euclidean-signature quantum theories. Analogous regularized gravitational
models can be set up in higher dimensions. Some analytic tools exist to study
their state sums, but, unlike in 2d, no complete analytic solutions have yet
been constructed. However, a great advantage of our approach is the fact that
it is well-suited for numerical simulations. In the second part of this review
we describe the relevant Monte Carlo techniques, as well as some of the
physical results that have been obtained from the simulations of Euclidean
gravity. We also explain why the Lorentzian version of dynamical triangulations
is a promising candidate for a non-perturbative theory of quantum gravity.Comment: 69 pages, 16 figures, references adde

### Scaling with a modified Wilson action which suppresses Z_2 artifacts in SU(2) lattice gauge theories

A modified Wilson action which suppresses plaquettes which take negative
values is used to study the scaling behavior of the string tension. The use of
the \b_E scheme gives good agreement with asymptotic two loop results.Comment: Latex (ps figure appended in the end), 7 page

### Center Disorder in the 3D Georgi-Glashow Model

We present a number of arguments relating magnetic disorder to center
disorder, in pure Yang-Mills theory in D=3 and D=4 dimensions. In the case of
the D=3 Georgi-Glashow model, we point out that the abelian field distribution
is not adequatedly represented, at very large scales, by that of a monopole
Coulomb gas. The onset of center disorder is associated with the breakdown of
the Coulomb gas approximation; this scale is pushed off to infinity in the
QED_3 limit of the 3D Georgi-Glashow model, but should approach the
color-screening length in the pure Yang-Mills limit.Comment: 22 pages including 3 figures, Latex2

### A model for emergence of space and time

We study string field theory (third quantization) of the two-dimensional
model of quantum geometry called generalized CDT ("causal dynamical
triangulations"). Like in standard non-critical string theory the so-called
string field Hamiltonian of generalized CDT can be associated with W-algebra
generators through the string mode expansion. This allows us to define an
"absolute" vacuum. "Physical" vacua appear as coherent states created by vertex
operators acting on the absolute vacuum. Each coherent state corresponds to
specific values of the coupling constants of generalized CDT. The cosmological
"time" only exists relatively to a given "physical" vacuum and comes into
existence before space, which is created because the "physical" vacuum is
unstable. Thus each CDT "universe" is created as a "Big Bang" from the absolute
vacuum, its time evolution is governed by the CDT string field Hamiltonian with
given coupling constants, and one can imagine interactions between CDT
universes with different coupling constants ("fourth quantization"

### Stability of the nonperturbative bosonic string vacuum

Quantization of the bosonic string around the classical, perturbative vacuum
is not consistent for spacetime dimensions 2<d<26. Recently we have showed that
at large d there is another so-called mean field vacuum. Here we extend this
mean field calculation to finite d and show that the corresponding mean field
vacuum is stable under quadratic fluctuations for 2<d<26. We point out the
analogy with the two-dimensional O(N)-symmetric sigma-model, where the
1/N-vacuum is very close to the real vacuum state even for finite N, in
contrast to the perturbative vacuum.Comment: v2: 6pp, section about vacuum instability/stability added, to appear
in PL

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