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14 pages, 259 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

by João Magueijo and Lee Smolin

Universe 2019, 5(3), 84; https://doi.org/10.3390/universe5030084 - 21 Mar 2019

Cited by 23 | Viewed by 4896

In this paper, we propose that cosmological time is a quantum observable that does not commute with other quantum operators essential for the definition of cosmological states, notably the cosmological constant. This is inspired by properties of a measure of time—the Chern–Simons time—and [...] Read more.

In this paper, we propose that cosmological time is a quantum observable that does not commute with other quantum operators essential for the definition of cosmological states, notably the cosmological constant. This is inspired by properties of a measure of time—the Chern–Simons time—and the fact that in some theories it appears as a conjugate to the cosmological constant, with the two promoted to non-commuting quantum operators. Thus, the Universe may be “delocalised” in time: it does not know the time, a property which opens up new cosmological scenarios, as well as invalidating several paradoxes, such as the timelike tower of turtles associated with an omnipresent time line. Alternatively, a Universe with a sharply defined clock time must have an indeterminate cosmological constant. The challenge then is to explain how islands of localized time may emerge, and give rise to localized histories. In some scenarios, this is achieved by backward transitions in quantum time, cycling the Universe in something akin to a time machine cycle, with classical flow and quantum ebbing. The emergence of matter in a sea of Lambda probably provides the ballast behind classical behaviour. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology)

15 pages, 325 KiB

Open AccessArticle

Effective Field Theory of Loop Quantum Cosmology

by Martin Bojowald

Universe 2019, 5(2), 44; https://doi.org/10.3390/universe5020044 - 23 Jan 2019

Cited by 14 | Viewed by 3294

Abstract

Quantum cosmology is traditionally formulated in a minisuperspace setting, implicitly averaging fields over space to obtain homogeneous models. For universal reasons related to the uncertainty principle, quantum corrections then depend on the size of the averaging volume. In minisuperspace truncations, the value of [...] Read more.

Quantum cosmology is traditionally formulated in a minisuperspace setting, implicitly averaging fields over space to obtain homogeneous models. For universal reasons related to the uncertainty principle, quantum corrections then depend on the size of the averaging volume. In minisuperspace truncations, the value of this volume remains an arbitrary parameter devoid of physical meaning, but in an effective field theory it is identified with the infrared scale of inhomogeneous modes. Moreover, the infrared scale is running during gravitational collapse, when regions in which homogeneity remains approximately valid shrink to increasingly smaller co-moving sizes. Conceptual implications of this infrared renormalization for perturbative inhomogeneity in quantum cosmology are presented here, mainly for the example of loop quantum cosmology. Several claims made in this framework are altered by infrared renormalization. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology)

10 pages, 762 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Small Black/White Hole Stability and Dark Matter

by Carlo Rovelli and Francesca Vidotto

Universe 2018, 4(11), 127; https://doi.org/10.3390/universe4110127 - 17 Nov 2018

Cited by 59 | Viewed by 8897

Abstract

We show that the expected lifetime of white holes formed as remnants of evaporated black holes is consistent with their production at reheating. We give a simple quantum description of these objects and argue that a quantum superposition of black and white holes [...] Read more.

We show that the expected lifetime of white holes formed as remnants of evaporated black holes is consistent with their production at reheating. We give a simple quantum description of these objects and argue that a quantum superposition of black and white holes with large interiors is stable, because it is protected by the existence of a minimal eigenvalue of the area, predicted by Loop Quantum Gravity. These two results support the hypothesis that a component of dark matter could be formed by small black hole remnants. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology)

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16 pages, 927 KiB

Open AccessFeature PaperArticle

Quantum Gravity at the Corner

by Laurent Freidel and Alejandro Perez

Universe 2018, 4(10), 107; https://doi.org/10.3390/universe4100107 - 15 Oct 2018

Cited by 29 | Viewed by 3189

Abstract

We investigate the quantum geometry of a 2d surface S bounding the Cauchy slices of a 4d gravitational system. We investigate in detail for the first time the boundary symplectic current that naturally arises in the first-order formulation of general relativity in terms [...] Read more.

We investigate the quantum geometry of a 2d surface S bounding the Cauchy slices of a 4d gravitational system. We investigate in detail for the first time the boundary symplectic current that naturally arises in the first-order formulation of general relativity in terms of the Ashtekar–Barbero connection. This current is proportional to the simplest quadratic form constructed out of the pull back to S of the triad field. We show that the would-be-gauge degrees of freedo arising from

S U (2)

gauge transformations plus diffeomorphisms tangent to the boundary are entirely described by the boundary 2-dimensional symplectic form, and give rise to a representation at each point of S of

S L (2, R) \times S U (2)

. Independently of the connection with gravity, this system is very simple and rich at the quantum level, with possible connections with conformal field theory in 2d. A direct application of the quantum theory is modelling of the black horizons in quantum gravity. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology)

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17 pages, 325 KiB

Open AccessArticle

Group Field Theory and Its Cosmology in a Matter Reference Frame

by Steffen Gielen

Universe 2018, 4(10), 103; https://doi.org/10.3390/universe4100103 - 2 Oct 2018

Cited by 33 | Viewed by 3031

Abstract

While the equations of general relativity take the same form in any coordinate system, choosing a suitable set of coordinates is essential in any practical application. This poses a challenge in background-independent quantum gravity, where coordinates are not a priori available and need [...] Read more.

While the equations of general relativity take the same form in any coordinate system, choosing a suitable set of coordinates is essential in any practical application. This poses a challenge in background-independent quantum gravity, where coordinates are not a priori available and need to be reconstructed from physical degrees of freedom. We review the general idea of coupling free scalar fields to gravity and using these scalars as a “matter reference frame”. The resulting coordinate system is harmonic, i.e., it satisfies the harmonic (de Donder) gauge. We then show how to introduce such matter reference frames in the group field theory approach to quantum gravity, where spacetime is emergent from a “condensate” of fundamental quantum degrees of freedom of geometry, and how to use matter coordinates to extract physics. We review recent results in homogeneous and inhomogeneous cosmology, and give a new application to the case of spherical symmetry. We find tentative evidence that spherically-symmetric group field theory condensates defined in this setting can reproduce the near-horizon geometry of a Schwarzschild black hole. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology)

26 pages, 853 KiB

Open AccessEditor’s ChoiceArticle

A Status Report on the Phenomenology of Black Holes in Loop Quantum Gravity: Evaporation, Tunneling to White Holes, Dark Matter and Gravitational Waves

by Aurélien Barrau, Killian Martineau and Flora Moulin

Universe 2018, 4(10), 102; https://doi.org/10.3390/universe4100102 - 2 Oct 2018

Cited by 38 | Viewed by 9241

Abstract

The understanding of black holes in loop quantum gravity is becoming increasingly accurate. This review focuses on the possible experimental or observational consequences of the underlying spinfoam structure of space-time. It addresses both the aspects associated with the Hawking evaporation and the ones [...] Read more.

The understanding of black holes in loop quantum gravity is becoming increasingly accurate. This review focuses on the possible experimental or observational consequences of the underlying spinfoam structure of space-time. It addresses both the aspects associated with the Hawking evaporation and the ones due to the possible existence of a bounce. Finally, consequences for dark matter and gravitational waves are considered. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology)

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15 pages, 912 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Inconsistencies of the New No-Boundary Proposal

by Job Feldbrugge, Jean-Luc Lehners and Neil Turok

Universe 2018, 4(10), 100; https://doi.org/10.3390/universe4100100 - 29 Sep 2018

Cited by 61 | Viewed by 4637

Abstract

In previous works, we have demonstrated that the path integral for real, Lorentzian four-geometries in Einstein gravity yields sensible results in well-understood physical situations, but leads to uncontrolled fluctuations when the “no boundary” condition proposed by Hartle and Hawking is imposed. In order [...] Read more.

In previous works, we have demonstrated that the path integral for real, Lorentzian four-geometries in Einstein gravity yields sensible results in well-understood physical situations, but leads to uncontrolled fluctuations when the “no boundary” condition proposed by Hartle and Hawking is imposed. In order to circumvent our result, new definitions for the gravitational path integral have been sought, involving specific choices for a class of complex four-geometries to be included. In their latest proposal, Diaz Dorronsoro et al. advocate for integrating the lapse over a complex circular contour enclosing the origin. In this note, we show that, like their earlier proposal, this leads to mathematical and physical inconsistencies and thus cannot be regarded as a basis for quantum cosmology. We also comment on Vilenkin and Yamada’s recent modification of the “tunneling" proposal, made in order to avoid the same problems. We show that it leads to the breakdown of perturbation theory in a strong coupling regime. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology)

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34 pages, 485 KiB

Open AccessEditor’s ChoiceArticle

Cosmological Constant from Condensation of Defect Excitations

by Bianca Dittrich

Universe 2018, 4(7), 81; https://doi.org/10.3390/universe4070081 - 19 Jul 2018

Cited by 12 | Viewed by 3424

Abstract

A key challenge for many quantum gravity approaches is to construct states that describe smooth geometries on large scales. Here we define a family of

(2 + 1)

-dimensional quantum gravity states which arise from curvature excitations concentrated at point like [...] Read more.

A key challenge for many quantum gravity approaches is to construct states that describe smooth geometries on large scales. Here we define a family of

(2 + 1)

-dimensional quantum gravity states which arise from curvature excitations concentrated at point like defects and describe homogeneously curved geometries on large scales. These states represent therefore vacua for three-dimensional gravity with different values of the cosmological constant. They can be described by an anomaly-free first class constraint algebra quantized on one and the same Hilbert space for different values of the cosmological constant. A similar construction is possible in four dimensions, in this case the curvature is concentrated along string-like defects and the states are vacua of the Crane-Yetter model. We will sketch applications for quantum cosmology and condensed matter. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology)

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