Special Issue "Symmetry and Quantum Gravity"

A special issue of Symmetry (ISSN 2073-8994).

Deadline for manuscript submissions: 31 March 2020.

Special Issue Editor

Dr. Giovanni Modanese
E-Mail Website
Guest Editor
Free University of Bozen-Bolzano, Faculty of Science and Technology, Bolzano, Italy
Interests: Applied mathematics; complex systems; theoretical physics

Special Issue Information

Dear Colleagues,

In quantum field models of electro-weak and strong interactions, "internal" symmetries have played a fundamental role, even before the epoch of gauge theories: Just think of the isospin symmetry, current algebra, Cabibbo angle, etc. For effective theories of quantum gravity (QG), one expects spacetime symmetries to play an equally important role, not only in the weak-field expansion on a fixed background, but also when strong fluctuations of the spacetime geometry are admitted (with the ensuing conceptual/interpretation problems). The most natural approaches for describing these situations are based on functional integrals, either in the metric formalism or with Regge calculus or dynamical triangulations. Typical quantities of interest are the analogues of Wilson loops or other invariant vacuum fluctuations. We thus invite to consider such approaches, summarize their status and propose advances, also at the computational/simulation level, with special attention for the role of symmetries and their phenomenological consequences. The underlying action should be Einstein's action, possibly with addition of vacuum energy terms and with a discussion of its non-positivity properties and relations between gravitational waves and gravitons. In this context, theories "beyond spacetime" like loop QG or group QG should be considered only with regard to their "emergent spacetime" properties.

Dr. Giovanni Modanese
Guest Editor

Manuscript Submission Information

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Keywords

  • Quantum gravity
  • effective quantum gravity
  • path integrals, functional integrals
  • Regge calculus
  • general covariance
  • diffeomorphisms
  • metric formulation
  • Einstein action
  • gravitons

Published Papers (2 papers)

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Research

Open AccessArticle
Metrics with Zero and Almost-Zero Einstein Action in Quantum Gravity
Symmetry 2019, 11(10), 1288; https://doi.org/10.3390/sym11101288 - 14 Oct 2019
Abstract
We generate numerically on a lattice an ensemble of stationary metrics, with spherical symmetry, which have Einstein action SE « ћ. This is obtained through a Metropolis algorithm with weight exp(−β2S2E) and β » ћ−1. [...] Read more.
We generate numerically on a lattice an ensemble of stationary metrics, with spherical symmetry, which have Einstein action SE « ћ. This is obtained through a Metropolis algorithm with weight exp(−β2S2E) and β » ћ−1. The squared action in the exponential allows to circumvene the problem of the non-positivity of SE. The discretized metrics obtained exhibit a spontaneous polarization in regions of positive and negative scalar curvature. We compare this ensemble with a class of continuous metrics previously found, which satisfy the condition SE = 0 exactly, or in certain cases even the stronger condition R(x) = 0 for any x. All these gravitational field configurations are of considerable interest in quantum gravity, because they represent possible vacuum fluctuations and are markedly different from Wheeler’s “spacetime foam”. Full article
(This article belongs to the Special Issue Symmetry and Quantum Gravity)
Open AccessArticle
Vacuum Condensate Picture of Quantum Gravity
Symmetry 2019, 11(1), 87; https://doi.org/10.3390/sym11010087 - 14 Jan 2019
Cited by 1
Abstract
In quantum gravity perturbation theory in Newton’s constant G is known to be badly divergent, and as a result not very useful. Nevertheless, some of the most interesting phenomena in physics are often associated with non-analytic behavior in the coupling constant and the [...] Read more.
In quantum gravity perturbation theory in Newton’s constant G is known to be badly divergent, and as a result not very useful. Nevertheless, some of the most interesting phenomena in physics are often associated with non-analytic behavior in the coupling constant and the existence of nontrivial quantum condensates. It is therefore possible that pathologies encountered in the case of gravity are more likely the result of inadequate analytical treatment, and not necessarily a reflection of some intrinsic insurmountable problem. The nonperturbative treatment of quantum gravity via the Regge–Wheeler lattice path integral formulation reveals the existence of a new phase involving a nontrivial gravitational vacuum condensate, and a new set of scaling exponents characterizing both the running of G and the long-distance behavior of invariant correlation functions. The appearance of such a gravitational condensate is viewed as analogous to the (equally nonperturbative) gluon and chiral condensates known to describe the physical vacuum of QCD. The resulting quantum theory of gravity is highly constrained, and its physical predictions are found to depend only on one adjustable parameter, a genuinely nonperturbative scale ξ in many ways analogous to the scaling violation parameter Λ M ¯ S of QCD. Recent results point to significant deviations from classical gravity on distance scales approaching the effective infrared cutoff set by the observed cosmological constant. Such subtle quantum effects are expected to be initially small on current cosmological scales, but could become detectable in future high precision satellite experiments. Full article
(This article belongs to the Special Issue Symmetry and Quantum Gravity)
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