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Universe
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Probing the Quantum Space-Time
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Probing the Quantum Space-Time

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Foundations of Quantum Mechanics and Quantum Gravity".

Deadline for manuscript submissions: closed (31 May 2023) | Viewed by 6114

Special Issue Editors

Dr. Lisa Glaser

E-Mail Website
Guest Editor
Mathematical Physics, Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Wien, Austria
Interests: quantum gravity; non-commutative geometry; spectral triples; causal set theory; computer simulations
Dr. Alessia Platania

E-Mail Website
Guest Editor
Perimeter Institute for Theoretical Physics, 31 Caroline St. N, Waterloo, ON N2L 2Y5, Canada
Interests: quantum gravity; asymptotic safety; renormalization group; quantum-gravity phenomenology; cosmology; black holes
Dr. Sebastian Steinhaus

E-Mail Website
Guest Editor
Theoretisch-Physikalisches Institut, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany
Interests: quantum gravity; spin foam models; numerical techniques; renormalization

Special Issue Information

Dear Colleagues,

More than one hundred years after the inception of general relativity, the quantization of the gravitational interaction remains a major open challenge in theoretical physics. Over the last decades, many promising approaches to quantum gravity were developed, each of them starting from different key principles and assumptions. As a result, their ideas on quantum geometry and thus quantum space-time differ significantly. This makes it imperative to understand their predictions for properties of space-time at the smallest scales to unlock their implications. Only then can we hope to develop observables that allow us to compare them across approaches.

It is the goal of this Special Issue to collect recent results probing the nature of quantum space-time, independent of which approach to quantum gravity they arise from. We especially welcome contributions that propose new observables in quantum gravity, and can thus facilitate connections and comparisons across approaches, and tools that help us unlock and examine new features of space-time at the smallest scales.

Dr. Lisa Glaser
Dr. Alessia Platania
Dr. Sebastian Steinhaus
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • quantum gravity
  • quantum space-time
  • quantum theory
  • general relativity
  • observables
  • perturbative gravity
  • non-perturbative gravity
  • space-time at small scales

Published Papers (4 papers)

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Research

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24 pages, 524 KiB  
Article
Gauge-Invariant Perturbations at a Quantum Gravity Bounce
by Steffen Gielen and Lisa Mickel
Universe 2023, 9(1), 29; https://0-doi-org.brum.beds.ac.uk/10.3390/universe9010029 - 31 Dec 2022
Cited by 3 | Viewed by 1102
Abstract
We study the dynamics of gauge-invariant scalar perturbations in cosmological scenarios with a modified Friedmann equation, such as quantum gravity bouncing cosmologies. We work within a separate universe approximation which captures wavelengths larger than the cosmological horizon; this approximation has been successfully applied [...] Read more.
We study the dynamics of gauge-invariant scalar perturbations in cosmological scenarios with a modified Friedmann equation, such as quantum gravity bouncing cosmologies. We work within a separate universe approximation which captures wavelengths larger than the cosmological horizon; this approximation has been successfully applied to loop quantum cosmology and group field theory. We consider two variables commonly used to characterise scalar perturbations: the curvature perturbation on uniform-density hypersurfaces ζ and the comoving curvature perturbation R. For standard cosmological models in general relativity as well as in loop quantum cosmology, these quantities are conserved and equal on super-horizon scales for adiabatic perturbations. Here we show that while these statements can be extended to a more general form of modified Friedmann equations similar to that of loop quantum cosmology, in other cases, such as the simplest group field theory bounce scenario, ζ is conserved across the bounce whereas R is not. We relate our results to approaches based on a second-order equation for a single perturbation variable, such as the Mukhanov–Sasaki equation. Full article
(This article belongs to the Special Issue Probing the Quantum Space-Time)
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19 pages, 384 KiB  
Article
Revisiting Dudas-Mourad Compactifications
by Ivano Basile, Salvatore Raucci and Sylvain Thomée
Universe 2022, 8(10), 544; https://0-doi-org.brum.beds.ac.uk/10.3390/universe8100544 - 19 Oct 2022
Cited by 8 | Viewed by 1002
Abstract
Superstring theories in ten dimensions allow spacetime supersymmetry breaking at the string scale at the expense of controlled Minkowski backgrounds. The next-to-maximally symmetric backgrounds, found by Dudas and Mourad, involve a warped compactification on an interval associated with codimension-one defects. We generalize these [...] Read more.
Superstring theories in ten dimensions allow spacetime supersymmetry breaking at the string scale at the expense of controlled Minkowski backgrounds. The next-to-maximally symmetric backgrounds, found by Dudas and Mourad, involve a warped compactification on an interval associated with codimension-one defects. We generalize these solutions by varying the effective field theory parameters, and we discuss the dimensional reduction on the interval. In particular, we show that scalars and form fields decouple in a certain range of dimensions, yielding Einstein-Yang-Mills theory. Moreover, we find that the breakdown of this effective description due to light Kaluza-Klein modes reflects the swampland distance conjecture, supporting the consistency of the picture at least qualitatively. Full article
(This article belongs to the Special Issue Probing the Quantum Space-Time)
31 pages, 548 KiB  
Article
Leading Singularities in Higher-Derivative Yang–Mills Theory and Quadratic Gravity
by Gabriel Menezes
Universe 2022, 8(6), 326; https://0-doi-org.brum.beds.ac.uk/10.3390/universe8060326 - 10 Jun 2022
Cited by 4 | Viewed by 1189
Abstract
In this work, we explore general leading singularities of one-loop amplitudes in higher-derivative Yang–Mills and quadratic gravity. These theories are known to possess propagators which contain quadratic and quartic momentum dependence, which leads to the presence of an unstable ghostlike resonance. However, unitarity [...] Read more.
In this work, we explore general leading singularities of one-loop amplitudes in higher-derivative Yang–Mills and quadratic gravity. These theories are known to possess propagators which contain quadratic and quartic momentum dependence, which leads to the presence of an unstable ghostlike resonance. However, unitarity cuts are not to be taken through unstable particles and therefore unitarity is still satisfied. On the other hand, this could engender issues when calculating leading singularities which are generalizations of unitarity cuts. Nevertheless, we will show with explicit examples how leading singularities are still well defined and accordingly they are able to capture relevant information on the analytic structure of amplitudes in such higher-derivative theories. We discuss some simple one-loop amplitudes which clarify these features. Full article
(This article belongs to the Special Issue Probing the Quantum Space-Time)
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Other

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31 pages, 1521 KiB  
Tutorial
How-to Compute EPRL Spin Foam Amplitudes
by Pietro Donà and Pietropaolo Frisoni
Universe 2022, 8(4), 208; https://0-doi-org.brum.beds.ac.uk/10.3390/universe8040208 - 26 Mar 2022
Cited by 15 | Viewed by 1789
Abstract
Spin foam theory is a concrete framework for quantum gravity where numerical calculations of transition amplitudes are possible. Recently, the field became very active, but the entry barrier is steep, mainly because of its unusual language and notions scattered around the literature. This [...] Read more.
Spin foam theory is a concrete framework for quantum gravity where numerical calculations of transition amplitudes are possible. Recently, the field became very active, but the entry barrier is steep, mainly because of its unusual language and notions scattered around the literature. This paper is a pedagogical guide to spin foam transition amplitude calculations. We show how to write an EPRL-FK transition amplitude, from the definition of the 2-complex to its numerical implementation using sl2cfoam-next. We guide the reader using an explicit example balancing mathematical rigor with a practical approach. We discuss the advantages and disadvantages of our strategy and provide a novel look at a recently proposed approximation scheme. Full article
(This article belongs to the Special Issue Probing the Quantum Space-Time)
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