117 research outputs found
Observational issues in loop quantum cosmology
Quantum gravity is sometimes considered as a kind of metaphysical
speculation. In this review, we show that, although still extremely difficult
to reach, observational signatures can in fact be expected. The early universe
is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop
Quantum Gravity as one of the best candidate for a non-perturbative and
background-independant quantization of gravity, we detail some expected
features.Comment: 75 pages, invited topical review for Classical and Quantum Gravit
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Loop Quantum Cosmology yields two kinds of quantum corrections to the
effective equations of motion for cosmological perturbations. Here we focus on
the holonomy kind and we study the problem of the closure of the resulting
algebra of constraints. Up to now, tensor, vector and scalar perturbations were
studied independently, leading to different algebras of constraints. The
structures of the related algebras were imposed by the requirement of anomaly
freedom. In this article we show that the algebra can be modified by a very
simple quantum correction, holding for all types of perturbations. This
demonstrates the consistency of the theory and shows that lessons from the
study of scalar perturbations should be taken into account when studying tensor
modes. The Mukhanov-Sasaki equations of motion are similarly modified by a
simple term.Comment: 5 page
Quantum gravity, space-time structure, and cosmology
A set of diverse but mutually consistent results obtained in different
settings has spawned a new view of loop quantum gravity and its physical
implications, based on the interplay of operator calculations and effective
theory: Quantum corrections modify, but do not destroy, space-time and the
notion of covariance. Potentially observable effects much more promising than
those of higher-curvature effective actions result; loop quantum gravity has
turned into a falsifiable framework, with interesting ingredients for new
cosmic world views. At Planckian densities, space-time disappears and is
replaced by 4-dimensional space without evolution.Comment: 8 pages, 7 figures, Plenary talk at CosGrav12, held at Indian
Statistical Institute, Kolkat
Probing Loop Quantum Gravity with Evaporating Black Holes
This letter aims at showing that the observation of evaporating black holes
should allow distinguishing between the usual Hawking behavior and Loop Quantum
Gravity (LQG) expectations. We present a full Monte-Carlo simulation of the
evaporation in LQG and statistical tests that discriminate between competing
models. We conclude that contrarily to what was commonly thought, the
discreteness of the area in LQG leads to characteristic features that qualify
evaporating black holes as objects that could reveal quantum gravity
footprints.Comment: 5 pages, 3 figures. Version accpeted by Phys. Rev. Let
A no-singularity scenario in loop quantum gravity
Canonical methods allow the derivation of effective gravitational actions
from the behavior of space-time deformations reflecting general covariance.
With quantum effects, the deformations and correspondingly the effective
actions change, revealing dynamical implications of quantum corrections. A new
systematic way of expanding these actions is introduced showing as a first
result that inverse-triad corrections of loop quantum gravity simplify the
asymptotic dynamics near a spacelike collapse singularity. By generic quantum
effects, the singularity is removed.Comment: 10 page
Lattice loop quantum cosmology: scalar perturbations
We study the scalar modes of linear perturbations in loop quantum cosmology.
This is done on a lattice where each cell is taken to be homogeneous and
isotropic and can be quantized via standard homogeneous loop quantum cosmology
techniques. The appropriate interactions between nearby cells are included in
the Hamiltonian in order to obtain the correct physics. It is shown that the
quantum theory is anomaly-free: the scalar and diffeomorphism constraint
operators weakly commute with the Hamiltonian. Finally, the effective theory
encoding the leading order quantum gravity corrections is derived and is shown
to give the same holonomy-corrected effective equations that have been obtained
in previous studies.Comment: 32 pages, v2: Minor change
Regularizing cosmological singularities by varying physical constants
Varying physical constant cosmologies were claimed to solve standard
cosmological problems such as the horizon, the flatness and the
-problem. In this paper, we suggest yet another possible application
of these theories: solving the singularity problem. By specifying some examples
we show that various cosmological singularities may be regularized provided the
physical constants evolve in time in an appropriate way.Comment: 9 pages, 6 figures, Revtex4-1, an improved version to appear in JCA
Spherically symmetric Einstein-Maxwell theory and loop quantum gravity corrections
Effects of inverse triad corrections and (point) holonomy corrections,
occuring in loop quantum gravity, are considered on the properties of
Reissner-Nordstr\"om black holes. The version of inverse triad corrections with
unmodified constraint algebra reveals the possibility of occurrence of three
horizons (over a finite range of mass) and also shows a mass threshold beyond
which the inner horizon disappears. For the version with modified constraint
algebra, coordinate transformations are no longer a good symmetry. The
covariance property of spacetime is regained by using a \emph{quantum} notion
of mapping from phase space to spacetime. The resulting quantum effects in both
versions of these corrections can be associated with renormalization of either
mass, charge or wave function. In neither of the versions, Newton's constant is
renormalized. (Point) Holonomy corrections are shown to preclude the undeformed
version of constraint algebra as also a static solution, though
time-independent solutions exist. A possible reason for difficulty in
constructing a covariant metric for these corrections is highlighted.
Furthermore, the deformed algebra with holonomy corrections is shown to imply
signature change.Comment: 38 pages, 9 figures, matches published versio
Anomaly-free vector perturbations with holonomy corrections in loop quantum cosmology
We investigate vector perturbations with holonomy corrections in the
framework of loop quantum cosmology. Conditions to achieve anomaly freedom for
these perturbations are found at all orders. This requires the introduction of
counter-terms in the hamiltonian constraint. We also show that anomaly freedom
requires the diffeomorphism constraint to hold its classical form when scalar
matter is added although the issue of a vector matter source, required for full
consistency, remains to be investigated. The gauge-invariant variable and the
corresponding equation of motion are derived. The propagation of vector modes
through the bounce is finally discussed.Comment: 16 pages, 1 figure. Matches version published in Class. Quantum Gra
Loop quantum gravity: the first twenty five years
This is a review paper invited by the journal "Classical ad Quantum Gravity"
for a "Cluster Issue" on approaches to quantum gravity. I give a synthetic
presentation of loop gravity. I spell-out the aims of the theory and compare
the results obtained with the initial hopes that motivated the early interest
in this research direction. I give my own perspective on the status of the
program and attempt of a critical evaluation of its successes and limits.Comment: 24 pages, 3 figure
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