320 research outputs found
Harmonic cosmology: How much can we know about a universe before the big bang?
Quantum gravity may remove classical space-time singularities and thus reveal
what a universe at and before the big bang could be like. In loop quantum
cosmology, an exactly solvable model is available which allows one to address
precise dynamical coherent states and their evolution in such a setting. It is
shown here that quantum fluctuations before the big bang are generically
unrelated to those after the big bang. A reliable determination of pre-big bang
quantum fluctuations would require exceedingly precise observations.Comment: 16 page
Singularities and Quantum Gravity
Although there is general agreement that a removal of classical gravitational
singularities is not only a crucial conceptual test of any approach to quantum
gravity but also a prerequisite for any fundamental theory, the precise
criteria for non-singular behavior are often unclear or controversial. Often,
only special types of singularities such as the curvature singularities found
in isotropic cosmological models are discussed and it is far from clear what
this implies for the very general singularities that arise according to the
singularity theorems of general relativity. In these lectures we present an
overview of the current status of singularities in classical and quantum
gravity, starting with a review and interpretation of the classical singularity
theorems. This suggests possible routes for quantum gravity to evade the
devastating conclusion of the theorems by different means, including modified
dynamics or modified geometrical structures underlying quantum gravity. The
latter is most clearly present in canonical quantizations which are discussed
in more detail. Finally, the results are used to propose a general scheme of
singularity removal, quantum hyperbolicity, to show cases where it is realized
and to derive intuitive semiclassical pictures of cosmological bounces.Comment: 41 pages, lecture course at the XIIth Brazilian School on Cosmology
and Gravitation, September 200
Deformed General Relativity and Effective Actions from Loop Quantum Gravity
Canonical methods can be used to construct effective actions from deformed
covariance algebras, as implied by quantum-geometry corrections of loop quantum
gravity. To this end, classical constructions are extended systematically to
effective constraints of canonical quantum gravity and applied to model systems
as well as general metrics, with the following conclusions: (i) Dispersion
relations of matter and gravitational waves are deformed in related ways,
ensuring a consistent realization of causality. (ii) Inverse-triad corrections
modify the classical action in a way clearly distinguishable from curvature
effects. In particular, these corrections can be significantly larger than
often expected for standard quantum-gravity phenomena. (iii) Finally, holonomy
corrections in high-curvature regimes do not signal the evolution from collapse
to expansion in a "bounce," but rather the emergence of the universe from
Euclidean space at high density. This new version of signature-change cosmology
suggests a natural way of posing initial conditions, and a solution to the
entropy problem.Comment: 44 page
Comment on "Quantum bounce and cosmic recall" [arXiv:0710.4543]
A recently derived inequality on volume fluctuations of a bouncing cosmology,
valid for states which are semiclassical long after the bounce, does not
restrict pre-bounce fluctuations sufficiently strongly to conclude that the
pre-bounce state was semiclassical except in a very weak sense.Comment: 1 pag
Quantum nature of cosmological bounces
Several examples are known where quantum gravity effects resolve the
classical big bang singularity by a bounce. The most detailed analysis has
probably occurred for loop quantum cosmology of isotropic models sourced by a
free, massless scalar. Once a bounce has been realized under fairly general
conditions, the central questions are how strongly quantum it behaves, what
influence quantum effects can have on its appearance, and what quantum
space-time beyond the bounce may look like. This, then, has to be taken into
account for effective equations which describe the evolution properly and can
be used for further phenomenological investigations. Here, we provide the first
analysis with interacting matter with new effective equations valid for weak
self-interactions or small masses. They differ from the free scalar equations
by crucial terms and have an important influence on the bounce and the
space-time around it. Especially the role of squeezed states, which have often
been overlooked in this context, is highlighted. The presence of a bounce is
proven for uncorrelated states, but as squeezing is a dynamical property and
may change in time, further work is required for a general conclusion.Comment: 26 page
Quantum Riemannian Geometry and Black Holes
Black Holes have always played a central role in investigations of quantum
gravity. This includes both conceptual issues such as the role of classical
singularities and information loss, and technical ones to probe the consistency
of candidate theories. Lacking a full theory of quantum gravity, such studies
had long been restricted to black hole models which include some aspects of
quantization. However, it is then not always clear whether the results are
consequences of quantum gravity per se or of the particular steps one had
undertaken to bring the system into a treatable form. Over a little more than
the last decade loop quantum gravity has emerged as a widely studied candidate
for quantum gravity, where it is now possible to introduce black hole models
within a quantum theory of gravity. This makes it possible to use only quantum
effects which are known to arise also in the full theory, but still work in a
rather simple and physically interesting context of black holes. Recent
developments have now led to the first physical results about non-rotating
quantum black holes obtained in this way. Restricting to the interior inside
the Schwarzschild horizon, the resulting quantum model is free of the classical
singularity, which is a consequence of discrete quantum geometry taking over
for the continuous classical space-time picture. This fact results in a change
of paradigm concerning the information loss problem. The horizon itself can
also be studied in the quantum theory by imposing horizon conditions at the
level of states. Thereby one can illustrate the nature of horizon degrees of
freedom and horizon fluctuations. All these developments allow us to study the
quantum dynamics explicitly and in detail which provides a rich ground to test
the consistency of the full theory.Comment: 45 pages, 4 figures, chapter of "Trends in Quantum Gravity Research"
(Nova Science
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