Gravitational Singularities and Their Quantum Fates

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Gravitation".

Deadline for manuscript submissions: closed (20 August 2022) | Viewed by 14379

Special Issue Editor

Department of Fundamental Research, National Centre for Nuclear Research, Warsaw, Poland
Interests: gravitational singularities; quantum gravity; problem of time; quantum entanglement; decoherence; interpretations of quantum mechanics

Special Issue Information

Dear Colleagues,

Commonly used spacetime in cosmology and astrophysics, the FLRW model, is isotropic and homogeneous. However, Lifshitz found (1946) that isotropy is unstable in the evolution towards the singularity. This important discovery initiated an extensive examination of the dynamics of anisotropic but homogeneous models, i.e., Bianchi-type. The result of these investigations, carried out by Belinski, Khalatnikov and Lifshitz, led to the conclusion that general relativity includes a generic solution with singularity.

The existence of generic singularity in solutions to Einstein’s equations signals the existence of a limit of validity of general relativity and means that this classical theory is incomplete. It is commonly believed that the imposition of quantum rules onto general relativity may lead to quantum theory devoid of singularities. Thus, the obtained regular quantum general relativity could be used to address the issues of cosmological and black hole singularities.

This Special Issue is devoted to gathering developments addressing the above problem within available and new quantization schemes. Approaches suggesting overcoming the singularity problem via some departure from general relativity might be considered as well. Observational data with any support for quantum gravity are highly welcomed. Topics of interest include the attached keywords. We invite you to submit original and review papers.

I look forward to your contributions.


Prof. Dr. Włodzimierz Piechocki
Guest Editor

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Keywords

  • Gravitational singularities
  • Quantum gravity
  • Coherent states quantization
  • Loop quantum gravity quantization
  • Quantization via causal dynamical triangulations
  • Problem of time
  • Quantum bounce
  • Decoherence
  • Observational frontier of quantum gravity

Published Papers (7 papers)

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Research

Jump to: Review

8 pages, 256 KiB  
Communication
Singularities in Inflationary Cosmological Models
by Leonardo Fernández-Jambrina
Universe 2021, 7(12), 491; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7120491 - 13 Dec 2021
Cited by 1 | Viewed by 1586
Abstract
Due to the accelerated expansion of the universe, the possibilities for the formation of singularities has changed from the classical Big Bang and Big Crunch singularities to include a number of new scenarios. In recent papers it has been shown that such singularities [...] Read more.
Due to the accelerated expansion of the universe, the possibilities for the formation of singularities has changed from the classical Big Bang and Big Crunch singularities to include a number of new scenarios. In recent papers it has been shown that such singularities may appear in inflationary cosmological models with a fractional power scalar field potential. In this paper we enlarge the analysis of singularities in scalar field cosmological models by the use of generalised power expansions of their Hubble scalars and their scalar fields in order to describe all possible models leading to a singularity, finding other possible cases. Unless a negative scalar field potential is considered, all singularities are weak and of type IV. Full article
(This article belongs to the Special Issue Gravitational Singularities and Their Quantum Fates)
13 pages, 429 KiB  
Article
Vacuum Semiclassical Gravity Does Not Leave Space for Safe Singularities
by Julio Arrechea, Carlos Barceló, Valentin Boyanov and Luis J. Garay
Universe 2021, 7(8), 281; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7080281 - 01 Aug 2021
Cited by 8 | Viewed by 1582
Abstract
General relativity predicts its own demise at singularities but also appears to conveniently shield itself from the catastrophic consequences of such singularities, making them safe. For instance, if strong cosmic censorship were ultimately satisfied, spacetime singularities, although present, would not pose any practical [...] Read more.
General relativity predicts its own demise at singularities but also appears to conveniently shield itself from the catastrophic consequences of such singularities, making them safe. For instance, if strong cosmic censorship were ultimately satisfied, spacetime singularities, although present, would not pose any practical problems to predictability. Here, we argue that under semiclassical effects, the situation should be rather different: the potential singularities which could appear in the theory will generically affect predictability, and so one will be forced to analyse whether there is a way to regularise them. For these possible regularisations, the presence and behaviour of matter during gravitational collapse and stabilisation into new structures will play a key role. First, we show that the static semiclassical counterparts to the Schwarzschild and Reissner–Nordström geometries have singularities which are no longer hidden behind horizons. Then, we argue that in dynamical scenarios of formation and evaporation of black holes, we are left with only three possible outcomes which could avoid singularities and eventual predictability issues. We briefly analyse the viability of each one of them within semiclassical gravity and discuss the expected characteristic timescales of their evolution. Full article
(This article belongs to the Special Issue Gravitational Singularities and Their Quantum Fates)
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34 pages, 942 KiB  
Article
The de Broglie–Bohm Quantum Theory and Its Application to Quantum Cosmology
by Nelson Pinto-Neto
Universe 2021, 7(5), 134; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7050134 - 07 May 2021
Cited by 7 | Viewed by 2249
Abstract
We review the de Broglie–Bohm quantum theory. It is an alternative description of quantum phenomena in accordance with all the quantum experiments already performed. Essentially, it is a dynamical theory about objectively real trajectories in the configuration space of the physical system under [...] Read more.
We review the de Broglie–Bohm quantum theory. It is an alternative description of quantum phenomena in accordance with all the quantum experiments already performed. Essentially, it is a dynamical theory about objectively real trajectories in the configuration space of the physical system under investigation. Hence, it is not necessarily probabilistic, and it dispenses with the collapse postulate, making it suitable to be applied to cosmology. The emerging cosmological models are usually free of singularities, with a bounce connecting a contracting era with an expanding phase, which we are now observing. A theory of cosmological perturbations can also be constructed under this framework, which can be successfully confronted with current observations, and can complement inflation or even be an alternative to it. Full article
(This article belongs to the Special Issue Gravitational Singularities and Their Quantum Fates)
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26 pages, 1269 KiB  
Article
Hunting for Gravitational Quantum Spikes
by Andrzej Góźdź, Włodzimierz Piechocki, Grzegorz Plewa and Tomasz Trześniewski
Universe 2021, 7(3), 49; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7030049 - 28 Feb 2021
Cited by 2 | Viewed by 1332
Abstract
We present the result of our examination of quantum structures called quantum spikes. The classical spikes that are known in gravitational systems, occur in the evolution of the inhomogeneous spacetimes. A different kind of spikes, which we name strange spikes, can be seen [...] Read more.
We present the result of our examination of quantum structures called quantum spikes. The classical spikes that are known in gravitational systems, occur in the evolution of the inhomogeneous spacetimes. A different kind of spikes, which we name strange spikes, can be seen in the dynamics of the homogeneous sector of the Belinski–Khalatnikov–Lifshitz scenario. They can be made visible if the so-called inhomogeneous initial data are used. The question to be explored is whether the strange spikes may survive quantization. The answer is in the affirmative. However, this is rather a subtle effect that needs further examination using sophisticated analytical and numerical tools. The spikes seem to be of fundamental importance, both at classical and quantum levels, as they may serve as seeds of real structures in the universe. Full article
(This article belongs to the Special Issue Gravitational Singularities and Their Quantum Fates)
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Review

Jump to: Research

37 pages, 2097 KiB  
Review
Probing the Interior of the Schwarzschild Black Hole Using Congruences: LQG vs. GUP
by Saeed Rastgoo and Saurya Das
Universe 2022, 8(7), 349; https://0-doi-org.brum.beds.ac.uk/10.3390/universe8070349 - 24 Jun 2022
Cited by 5 | Viewed by 1103
Abstract
We review, as well as provide some new results regarding the study of the structure of spacetime and the singularity in the interior of the Schwarzschild black hole in both loop quantum gravity and generalized uncertainty principle approaches, using congruences and their associated [...] Read more.
We review, as well as provide some new results regarding the study of the structure of spacetime and the singularity in the interior of the Schwarzschild black hole in both loop quantum gravity and generalized uncertainty principle approaches, using congruences and their associated expansion scalar and the Raychaudhuri equation. We reaffirm previous results that in loop quantum gravity, in all three major schemes of polymer quantization, the expansion scalar, Raychaudhuri equation and the Kretschmann scalar remain finite everywhere in the interior. In the context of the eneralized uncertainty principle, we show that only two of the four models we study lead to similar results. These two models have the property that their algebra is modified by configuration variables rather than the momenta. Full article
(This article belongs to the Special Issue Gravitational Singularities and Their Quantum Fates)
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16 pages, 912 KiB  
Review
Recent Observations of Gravitational Waves by LIGO and Virgo Detectors
by Andrzej Królak and Paritosh Verma
Universe 2021, 7(5), 137; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7050137 - 09 May 2021
Cited by 4 | Viewed by 3354
Abstract
In this paper we present the most recent observations of gravitational waves (GWs) by LIGO and Virgo detectors. We also discuss contributions of the recent Nobel prize winner, Sir Roger Penrose to understanding gravitational radiation and black holes (BHs). We make a short [...] Read more.
In this paper we present the most recent observations of gravitational waves (GWs) by LIGO and Virgo detectors. We also discuss contributions of the recent Nobel prize winner, Sir Roger Penrose to understanding gravitational radiation and black holes (BHs). We make a short introduction to GW phenomenon in general relativity (GR) and we present main sources of detectable GW signals. We describe the laser interferometric detectors that made the first observations of GWs. We briefly discuss the first direct detection of GW signal that originated from a merger of two BHs and the first detection of GW signal form merger of two neutron stars (NSs). Finally we present in more detail the observations of GW signals made during the first half of the most recent observing run of the LIGO and Virgo projects. Finally we present prospects for future GW observations. Full article
(This article belongs to the Special Issue Gravitational Singularities and Their Quantum Fates)
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26 pages, 3165 KiB  
Review
CDT Quantum Toroidal Spacetimes: An Overview
by Jan Ambjorn, Zbigniew Drogosz, Jakub Gizbert-Studnicki, Andrzej Görlich, Jerzy Jurkiewicz and Dániel Németh
Universe 2021, 7(4), 79; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7040079 - 26 Mar 2021
Cited by 16 | Viewed by 2073
Abstract
Lattice formulations of gravity can be used to study non-perturbative aspects of quantum gravity. Causal Dynamical Triangulations (CDT) is a lattice model of gravity that has been used in this way. It has a built-in time foliation but is coordinate-independent in the spatial [...] Read more.
Lattice formulations of gravity can be used to study non-perturbative aspects of quantum gravity. Causal Dynamical Triangulations (CDT) is a lattice model of gravity that has been used in this way. It has a built-in time foliation but is coordinate-independent in the spatial directions. The higher-order phase transitions observed in the model may be used to define a continuum limit of the lattice theory. Some aspects of the transitions are better studied when the topology of space is toroidal rather than spherical. In addition, a toroidal spatial topology allows us to understand more easily the nature of typical quantum fluctuations of the geometry. In particular, this topology makes it possible to use massless scalar fields that are solutions to Laplace’s equation with special boundary conditions as coordinates that capture the fractal structure of the quantum geometry. When such scalar fields are included as dynamical fields in the path integral, they can have a dramatic effect on the geometry. Full article
(This article belongs to the Special Issue Gravitational Singularities and Their Quantum Fates)
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