52 research outputs found
Graviton propagator as a tool to test spinfoam models
I briefly review the advancements in the construction of the graviton
propagator in the context of LQG and Spinfoam Models. In particular the
problems of the Barrett-Crane vertex in giving the correct long-distance limit
and the introduction of the new corrected models. This kind of calculation
applied to an alternative vertex with given asymptotic can give the correct
propagator and is then able to help selecting spinfoam models. In particular
the study of the asymptotic properties of the new models shows the predicted
behavior able to overcome the BC difficulties and to give the correct
propagatorComment: to appear in the Proceedings of the 3rd Stueckelberg Workshop on
Relativistic Field Theories, July 2008, Pescara - Ital
Phenomenology for an extra-dimension from gravitational waves propagation on a Kaluza-Klein space-time
In the present work we analize the behavior of 5-dimensional gravitational
waves propagating on a Kaluza-Klein background and we face separately the two
cases in which respectively the waves are generated before and after the
process of dimensional compactification. We show that if the waves are
originated on a 5-d space-time which fulfills the principle of general
relativity, then the process of compactification can not reduce the dynamics to
the pure 4-dimensional scalar, vector and tensor degrees of freedom. In
particular, while the electromagnetic waves evolve independently, the scalar
and tensor fields couple to each other; this feature appears because, when the
gauge conditions are splitted, the presence of the scalar ripple prevents that
the 4-d gravitational waves are traceless. The phenomenological issue of this
scheme consists of an anomalous relative amplitude of the two independent
polarizations which characterize the 4-d gravitational waves. Such profile of
polarization amplitudes, if detected, would outline the extra-dimension in a
very reliable way, because a wave with non-zero trace can not arise from
ordinary matter sources. We discuss the above mentioned phenomenon either in
the case of a unit constant value of the background scalar component (when the
geodesic deviation is treated with precise outputs), and assuming such
background field as a dynamical degree (only qualitative conclusion are
provided here, because the details of the polarization amplitudes depend on the
choice of specific metric forms). Finally we perturb a real Kaluza-Klein theory
showing that in this context, while the electromagnetic waves propagate
independently, the 4-d gravitational waves preserve their ordinary structure,
while the scalar plays for them the role of source.Comment: 21 pages, 2 figures, to appear on Int. Journ. Mod. Phys.
Tensorial Structure of the LQG graviton propagator
We review the construction of the tensorial structure of the graviton
propagator in the context of loop quantum gravity and spinfoam formalism. The
main result of this analysis is that applying the same strategy used to compute
the diagonal terms, the Barrett-Crane vertex is unable to yield the correct
propagator in the long distance limit. The problem is in the
intertwiner-independence of the Barrett-Crane vertex. We also review the
asymptotic behavior of an alternative vertex that is able to give the correct
propagator.Comment: 4 pages,; to appear in the proceedings of the II Stueckelberg
Workshop, Int.J.Mod.Phys.
Spin-Foam Models and the Physical Scalar Product
This paper aims at clarifying the link between Loop Quantum Gravity and
Spin-Foam models in four dimensions. Starting from the canonical framework, we
construct an operator P acting on the space of cylindrical functions
Cyl(), where is the 4-simplex graph, such that its ma- trix
elements are, up to some normalization factors, the vertex amplitude of
Spin-Foam models. The Spin-Foam models we are considering are the topological
model, the Barrett-Crane model and the Engle-Pereira-Rovelli model. The
operator P is usually called the "projector" into physical states and its
matrix elements gives the physical scalar product. Therefore, we relate the
physical scalar product of Loop Quantum Gravity to vertex amplitudes of some
Spin-Foam models. We discuss the possibility to extend the action of P to any
cylindrical functions on the space manifold.Comment: 24 page
Quantum reduced loop gravity effective Hamiltonians from a statistical regularization scheme
We introduce a new regularization scheme for Quantum Cosmology in Loop
Quantum Gravity (LQG) using the tools of Quantum Reduced Loop Gravity (QRLG).
It is obtained considering density matrices for superposition of graphs based
on statistical countings of microstates compatible with macroscopic
configurations. We call this procedure statistical regularization scheme. In
particular, we show how the and schemes introduced in Loop
Quantum Cosmology (LQC) emerge with specific choices of density matrices.
Within this new scheme we compute effective Hamiltonians suitable to describe
quantum corrected Friedmann and Bianchi I universes and their leading orders
coincide with the corresponding effective LQC Hamiltonians in the
scheme. We compute the next to the leading orders corrections and numerical
investigation of the resulting dynamics shows evidence for the
emergent-bouncing universe scenario to be a general property of the isotropic
sector of QRLG.Comment: 22 pages, 4 figures. Two small typos fixed. Conclusions unchange
Quantum Reduced Loop Gravity
Quantum Reduced Loop Gravity provides a promising framework for a consistent characterization of the early Universe dynamics. Inspired by BKL conjecture, a flat Universe is described as a collection of Bianchi I homogeneous patches. The resulting quantum dynamics is described by the scalar constraint operator, whose matrix elements can be analytically computed. The effective semiclassical dynamics is discussed, and the differences with Loop Quantum Cosmology are emphasized
- …