Quantum Gravity

A special issue of Symmetry (ISSN 2073-8994). This special issue belongs to the section "Physics".

Deadline for manuscript submissions: closed (31 December 2021) | Viewed by 9980

Special Issue Editor


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Guest Editor
Departmnet of Physics, University of Aberdeen, Scotland, UK

Special Issue Information

Dear colleagues,

Quantum gravity is the ultimate goal for modern research and has been widely regarded as the Holy Grail of physics. Its mathematical complexity and lack of experimental guidance have contributed to great challenges to progress. Nevertheless, tremendous conceptual and technical advances have recently been achieved through a burst of efforts and strategies.

A key factor for the success of many theories in modern physics is the principle of symmetry. This is clearly demonstrated in the crucial roles of gauge invariance in the standard model of particle physics and the frame independence of general relativity. Indeed, many recent advances in quantum gravity relate fundamentally to symmetries (or their breaking) including Lorentz invariance, the equivalence principle, background independence, gauge symmetry, supersymmetry, and scale invariance.

In this Special Issue we aim to highlight recent research in quantum gravity, that may feature such symmetries. The scope covers nonexclusively some of the latest topics on loop quantum gravity and cosmology, quantum black holes, gravitational decoherence, supergravity, gravitational S-duality, experimental and observational quantum gravity, and analogue gravity.

Dr. Charles Wang
Guest Editor

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Keywords

  • loop quantum gravity
  • spin foam
  • spin network
  • supergravity
  • gravitational S-duality
  • gravitational decoherence
  • quantum gravity phenomenology
  • quantum black hole
  • Hawking/Unruh radiation
  • analogue gravity

Published Papers (5 papers)

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Research

34 pages, 562 KiB  
Article
Comparing Quantum Gravity Models: String Theory, Loop Quantum Gravity, and Entanglement Gravity versus SU(∞)-QGR
by Houri Ziaeepour
Symmetry 2022, 14(1), 58; https://doi.org/10.3390/sym14010058 - 02 Jan 2022
Cited by 1 | Viewed by 1967
Abstract
In a previous article we proposed a new model for quantum gravity (QGR) and cosmology, dubbed SU()-QGR. One of the axioms of this model is that Hilbert spaces of the Universe and its subsystems represent the [...] Read more.
In a previous article we proposed a new model for quantum gravity (QGR) and cosmology, dubbed SU()-QGR. One of the axioms of this model is that Hilbert spaces of the Universe and its subsystems represent the SU() symmetry group. In this framework, the classical spacetime is interpreted as being the parameter space characterizing states of the SU() representing Hilbert spaces. Using quantum uncertainty relations, it is shown that the parameter space—the spacetime—has a 3+1 dimensional Lorentzian geometry. Here, after a review of SU()-QGR, including a demonstration that its classical limit is Einstein gravity, we compare it with several QGR proposals, including: string and M-theories, loop quantum gravity and related models, and QGR proposals inspired by the holographic principle and quantum entanglement. The purpose is to find their common and analogous features, even if they apparently seem to have different roles and interpretations. The hope is that this exercise provides a better understanding of gravity as a universal quantum force and clarifies the physical nature of the spacetime. We identify several common features among the studied models: the importance of 2D structures; the algebraic decomposition to tensor products; the special role of the SU(2) group in their formulation; the necessity of a quantum time as a relational observable. We discuss how these features can be considered as analogous in different models. We also show that they arise in SU()-QGR without fine-tuning, additional assumptions, or restrictions. Full article
(This article belongs to the Special Issue Quantum Gravity)
12 pages, 588 KiB  
Article
Schrödinger–Newton Model with a Background
by José Tito Mendonça
Symmetry 2021, 13(6), 1007; https://doi.org/10.3390/sym13061007 - 04 Jun 2021
Cited by 4 | Viewed by 1708
Abstract
This paper considers the Schrödinger–Newton (SN) equation with a Yukawa potential, introducing the effect of locality. We also include the interaction of the self-gravitating quantum matter with a radiation background, describing the effects due to the environment. Matter and radiation are coupled by [...] Read more.
This paper considers the Schrödinger–Newton (SN) equation with a Yukawa potential, introducing the effect of locality. We also include the interaction of the self-gravitating quantum matter with a radiation background, describing the effects due to the environment. Matter and radiation are coupled by photon scattering processes and radiation pressure. We apply this extended SN model to the study of Jeans instability and gravitational collapse. We show that the instability thresholds and growth rates are modified by the presence of an environment. The Yukawa scale length is more relevant for large-scale density perturbations, while the quantum effects become more relevant at small scales. Furthermore, coupling with the radiation environment modifies the character of the instability and leads to the appearance of two distinct instability regimes: one, where both matter and radiation collapse together, and others where regions of larger radiation intensity coincide with regions of lower matter density. This could explain the formation of radiation bubbles and voids of matter. The present work extends the SN model in new directions and could be relevant to astrophysical and cosmological phenomena, as well as to laboratory experiments simulating quantum gravity. Full article
(This article belongs to the Special Issue Quantum Gravity)
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16 pages, 949 KiB  
Article
Bremsstrahlung of Light through Spontaneous Emission of Gravitational Waves
by Charles H.-T. Wang and Melania Mieczkowska
Symmetry 2021, 13(5), 852; https://doi.org/10.3390/sym13050852 - 11 May 2021
Viewed by 2725
Abstract
Zero-point fluctuations are a universal consequence of quantum theory. Vacuum fluctuations of electromagnetic field have provided crucial evidence and guidance for QED as a successful quantum field theory with a defining gauge symmetry through the Lamb shift, Casimir effect, and spontaneous emission. In [...] Read more.
Zero-point fluctuations are a universal consequence of quantum theory. Vacuum fluctuations of electromagnetic field have provided crucial evidence and guidance for QED as a successful quantum field theory with a defining gauge symmetry through the Lamb shift, Casimir effect, and spontaneous emission. In an accelerated frame, the thermalisation of the zero-point electromagnetic field gives rise to the Unruh effect linked to the Hawking effect of a black hole via the equivalence principle. This principle is the basis of general covariance, the symmetry of general relativity as the classical theory of gravity. If quantum gravity exists, the quantum vacuum fluctuations of the gravitational field should also lead to the quantum decoherence and dissertation of general forms of energy and matter. Here we present a novel theoretical effect involving the spontaneous emission of soft gravitons by photons as they bend around a heavy mass and discuss its observational prospects. Our analytic and numerical investigations suggest that the gravitational bending of starlight predicted by classical general relativity should also be accompanied by the emission of gravitational waves. This in turn redshifts the light causing a loss of its energy somewhat analogous to the bremsstrahlung of electrons by a heavier charged particle. It is suggested that this new effect may be important for a combined astronomical source of intense gravity and high-frequency radiation such as X-ray binaries and that the proposed LISA mission may be potentially sensitive to the resulting sub-Hz stochastic gravitational waves. Full article
(This article belongs to the Special Issue Quantum Gravity)
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7 pages, 241 KiB  
Article
Emergent Space-Time in a Bubble Universe
by James Moffat
Symmetry 2021, 13(4), 729; https://doi.org/10.3390/sym13040729 - 20 Apr 2021
Viewed by 1528
Abstract
I exploit the close connection between the tessellation of space-time in the Regge calculus and an Eilenberg homology to investigate the deep quantum nature of space-time in a simple bubble universe of a size consistent with the Planck regime. Following the mathematics allows [...] Read more.
I exploit the close connection between the tessellation of space-time in the Regge calculus and an Eilenberg homology to investigate the deep quantum nature of space-time in a simple bubble universe of a size consistent with the Planck regime. Following the mathematics allows me to define this granulated space-time as the embedding space of the skeleton of a computational spin network inside a quantum computer. This approach can be regarded as a quantum simulation of the equivalent physics. I can, therefore, define a fundamental characterisation of any high-energy physical process at the Planck scale as equivalent to a quantum simulation inside a quantum computer. Full article
(This article belongs to the Special Issue Quantum Gravity)
13 pages, 878 KiB  
Article
Embedding Gauss–Bonnet Scalarization Models in Higher Dimensional Topological Theories
by Carlos Herdeiro, Eugen Radu and D. H. Tchrakian
Symmetry 2021, 13(4), 590; https://doi.org/10.3390/sym13040590 - 02 Apr 2021
Cited by 2 | Viewed by 1199
Abstract
In the presence of appropriate non-minimal couplings between a scalar field and the curvature squared Gauss–Bonnet (GB) term, compact objects such as neutron stars and black holes (BHs) can spontaneously scalarize, becoming a preferred vacuum. Such strong gravity phase transitions have attracted considerable [...] Read more.
In the presence of appropriate non-minimal couplings between a scalar field and the curvature squared Gauss–Bonnet (GB) term, compact objects such as neutron stars and black holes (BHs) can spontaneously scalarize, becoming a preferred vacuum. Such strong gravity phase transitions have attracted considerable attention recently. The non-minimal coupling functions that allow this mechanism are, however, always postulated ad hoc. Here, we point out that families of such functions naturally emerge in the context of Higgs–Chern–Simons gravity models, which are found as dimensionally descents of higher dimensional, purely topological, Chern–Pontryagin non-Abelian densities. As a proof of concept, we study spherically symmetric scalarized BH solutions in a particular Einstein-GB-scalar field model, whose coupling is obtained from this construction, pointing out novel features and caveats thereof. The possibility of vectorization is also discussed, since this construction also originates vector fields non-minimally coupled to the GB invariant. Full article
(This article belongs to the Special Issue Quantum Gravity)
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