1,093 research outputs found
Relic gravitons as the observable for Loop Quantum Cosmology
In this paper we investigate tensor modes of perturbations in the universe
governed by Loop Quantum Cosmology. We derive the equation for tensor modes and
investigate numerically effects of quantum corrections. This investigation
reveals that the region of super-adiabatic amplification of tensor modes is
smaller in comparison with the classical case. Neglecting quantum corrections
to the equation for tensor modes and holding underlying loop dynamics we study
analytically the creation of gravitons. We calculate the power spectrum of
tensor perturbations during the super-inflationary phase induced by Loop
Quantum Gravity. The main result obtained is the spectrum of gravitons,
produced in the transition from the quantum to classical regime of the
Universe. Obtained spectrum is characterized by a hard branch. The numerical
investigation shows the strong dependence of frequency with
respect to quantum numbers. The parameter
approaches even to for
highest frequencies. We compare our results with recent constraints from the
Laser Interferometer Gravitational-wave Observatory (LIGO) and find that it is
possible to test the quantum effects in the early Universe.Comment: RevTeX4, 10 pages, 4 figures; v2: extended, more clarification on
quantum correction
Quantum gravity in the very early universe
General relativity describes the gravitational field geometrically and in a
self-interacting way because it couples to all forms of energy, including its
own. Both features make finding a quantum theory difficult, yet it is important
in the high-energy regime of the very early universe. This review article
introduces some of the results for the quantum nature of space-time which
indicate that there is a discrete, atomic picture not just for matter but also
for space and time. At high energy scales, such deviations from the continuum
affect the propagation of matter, the expansion of the universe, and perhaps
even the form of symmetries such as Lorentz or CP transformations. All these
effects may leave traces detectable by sensitive measurements, as pointed out
here by examples.Comment: 10 pages, plenary talk at "6th International Conference on Physics
and Astrophysics of Quark Gluon Plasma" (ICPAQGP 2010), Goa, Indi
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
Loop Quantum Cosmology, Boundary Proposals, and Inflation
Loop quantum cosmology of the closed isotropic model is studied with a
special emphasis on a comparison with traditional results obtained in the
Wheeler-DeWitt approach. This includes the relation of the dynamical initial
conditions with boundary conditions such as the no-boundary or the tunneling
proposal and a discussion of inflation from quantum cosmology.Comment: 20 pages, 6 figure
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
Initial Conditions for a Universe
In physical theories, boundary or initial conditions play the role of
selecting special situations which can be described by a theory with its
general laws. Cosmology has long been suspected to be different in that its
fundamental theory should explain the fact that we can observe only one
particular realization. This is not realized, however, in the classical
formulation and in its conventional quantization; the situation is even worse
due to the singularity problem. In recent years, a new formulation of quantum
cosmology has been developed which is based on quantum geometry, a candidate
for a theory of quantum gravity. Here, the dynamical law and initial conditions
turn out to be linked intimately, in combination with a solution of the
singularity problem.Comment: 7 pages, this essay was awarded First Prize in the Gravity Research
Foundation Essay Contest 200
Loop Quantum Cosmology: Recent Progress
Aspects of the full theory of loop quantum gravity can be studied in a
simpler context by reducing to symmetric models like cosmological ones. This
leads to several applications where loop effects play a significant role when
one is sensitive to the quantum regime. As a consequence, the structure of and
the approach to classical singularities are very different from general
relativity: The quantum theory is free of singularities, and there are new
phenomenological scenarios for the evolution of the very early universe
including inflation. We give an overview of the main effects, focussing on
recent results obtained by several different groups.Comment: 17 pages, 2 figures, Plenary talk at ICGC 04, Cochin, Indi
Homogeneous Loop Quantum Cosmology: The Role of the Spin Connection
Homogeneous cosmological models with non-vanishing intrinsic curvature
require a special treatment when they are quantized with loop quantum
cosmological methods. Guidance from the full theory which is lost in this
context can be replaced by two criteria for an acceptable quantization,
admissibility of a continuum approximation and local stability. A quantization
of the corresponding Hamiltonian constraints is presented and shown to lead to
a locally stable, non-singular evolution compatible with almost classical
behavior at large volume. As an application, the Bianchi IX model and its
modified behavior close to its classical singularity is explored.Comment: revtex4, 36 pages, 10 figures. In version 2 the introduction is
expanded, section III E is added and a paragraph on relevance of results is
added in the conclusions. Refs updated, results unchanged. To appear in
Class. Quant. Gravit
Non-standard loop quantum cosmology
We present results concerning the nature of the cosmological big bounce(BB)
transition within the loop geometry underlying loop quantum cosmology (LQC).
Our canonical quantization method is an alternative to the standard LQC. An
evolution parameter we use has clear interpretation both at classical and
quantum levels. The physical volume operator has discrete spectrum which is
bounded from below. The minimum gap in the spectrum defines a quantum of the
volume. The spectra of operators are parametrized by a free parameter to be
determined.Comment: 4 pages, prepared for the proceedings of the Grassmannian Conference
in Fundamental Cosmology (Grasscosmofun'09), 14-19 September 2009, Szczecin,
Polan
Homogeneous Loop Quantum Cosmology
Loop quantum cosmological methods are extended to homogeneous models in
diagonalized form. It is shown that the diagonalization leads to a
simplification of the volume operator such that its spectrum can be determined
explicitly. This allows the calculation of composite operators, most
importantly the Hamiltonian constraint. As an application the dynamics of the
Bianchi I model is studied and it is shown that its loop quantization is free
of singularities.Comment: 25 pages, 3 figure
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