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Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology, Second...
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Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology, Second Edition

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

Deadline for manuscript submissions: 20 June 2025 | Viewed by 2266

Special Issue Editor

Prof. Dr. Gerald B. Cleaver

E-Mail Website
Guest Editor
Early Universe Cosmology and Strings (EUCOS) Group, Center for Astrophysics, Space Physics and Engineering Research (CASPER), Baylor University, Waco, TX 76798, USA
Interests: quantum field theory; quantum gravity; quantum cosmology; traversable wormholes; Casimir effect; quantum information theory; quantum thermodynamics; philosophical foundations of quantum mechanics; multiverse concepts
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Quantum gravity continues to be an active and extremely productive field of cosmology and theoretical physics. Advancements towards the unification of general relativity with quantum mechanics continue to emerge through a range of complementary approaches. New insights are being gained, offering additional clues to properties of an ultimate, underlying quantum gravity. Volume 2 of this Special Edition of Universe is devoted to the most recent developments in the field, with a special focus on loop quantum gravity/loop quantum cosmology. All related articles are invited to be submitted to Volume 2.

Topics of interest for Volume Two of this Special Issue include, but are not limited to, the following:

  • Quantum geometry, quantum gravity, and cosmological implications;
  • Non-perturbative approach to quantum gravity (e.g., loop quantum gravity, group field theory, spin foam);
  • String theory and string cosmology;
  • Modified gravity;
  • Emergent gravity.

Prof. Dr. Gerald B. Cleaver
Guest Editor

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
  • non-perturbative approaches
  • planck scale
  • supergravity
  • non-commutative geometry
  • loop quantum gravity
  • loop quantum cosmology
  • string theory
  • emergent gravity
  • Horava-Lifshitz gravity

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Published Papers (3 papers)

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Research

28 pages, 2471 KiB  
Article
Universal Properties of the Evolution of the Universe in Modified Loop Quantum Cosmology
by Jamal Saeed, Rui Pan, Christian Brown, Gerald Cleaver and Anzhong Wang
Universe 2024, 10(10), 397; https://doi.org/10.3390/universe10100397 - 15 Oct 2024
Cited by 3 | Viewed by 625
Abstract
In this paper, we systematically study the evolution of the Universe within the framework of a modified loop quantum cosmological model (mLQC-I) using various inflationary potentials, including chaotic, Starobinsky, generalized Starobinsky, polynomials of the first and second kinds, generalized T-models and natural inflation. [...] Read more.
In this paper, we systematically study the evolution of the Universe within the framework of a modified loop quantum cosmological model (mLQC-I) using various inflationary potentials, including chaotic, Starobinsky, generalized Starobinsky, polynomials of the first and second kinds, generalized T-models and natural inflation. In all these models, the big bang singularity is replaced by a quantum bounce, and the evolution of the Universe, both before and after the bounce, is universal and weakly dependent on the inflationary potentials, as long as the evolution is dominated by the kinetic energy of the inflaton at the bounce. In particular, the pre-bounce evolution can be universally divided into three different phases: pre-bouncing, pre-transition, and pre-de Sitter. The pre-bouncing phase occurs immediately before the quantum bounce, during which the evolution of the Universe is dominated by the kinetic energy of the inflaton. Thus, the equation of state of the inflaton is about one, w(ϕ)≃1. Soon, the inflation potential takes over, so w(ϕ) rapidly falls from one to negative one. This pre-transition phase is very short and quickly turns into the pre-de Sitter phase, whereby the effective cosmological constant of Planck size takes over and dominates the rest of the contracting phase. Throughout the entire pre-bounce regime, the evolution of both the expansion factor and the inflaton can be approximated by universal analytical solutions, independent of the specific inflation potentials. Full article
(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology, Second Edition)
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14 pages, 4193 KiB  
Article
Decoding Quantum Gravity Information with Black Hole Accretion Disk
by Lei You, Yu-Hang Feng, Rui-Bo Wang, Xian-Ru Hu and Jian-Bo Deng
Universe 2024, 10(10), 393; https://doi.org/10.3390/universe10100393 - 11 Oct 2024
Cited by 3 | Viewed by 624
Abstract
Integrating loop quantum gravity with classical gravitational collapse models offers an effective solution to the black hole singularity problem and predicts the formation of a white hole in the later stages of collapse. Furthermore, the quantum extension of Kruskal spacetime indicates that white [...] Read more.
Integrating loop quantum gravity with classical gravitational collapse models offers an effective solution to the black hole singularity problem and predicts the formation of a white hole in the later stages of collapse. Furthermore, the quantum extension of Kruskal spacetime indicates that white holes may convey information about earlier companion black holes. Photons emitted from the accretion disks of these companion black holes enter the black hole, traverse the highly quantum region, and then re-emerge from white holes in our universe. This process enables us to observe images of the companion black holes’ accretion disks, providing insights into quantum gravity. In our study, we successfully obtained these accretion disk images. Our results indicate that these accretion disk images are confined within a circle with a radius equal to the critical impact parameter, while traditional accretion disk images are typically located outside this circle. As the observational angle increases, the accretion disk images transition from a ring shape to a shell-like shape. Furthermore, the positional and width characteristics of these accretion disk images are opposite to those of traditional accretion disk images. These findings provide valuable references for astronomical observations aimed at validating the investigated quantum gravity model. Full article
(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology, Second Edition)
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28 pages, 1121 KiB  
Article
Comparing Analytic and Numerical Studies of Tensor Perturbations in Loop Quantum Cosmology
by Guillermo A. Mena Marugán, Antonio Vicente-Becerril and Jesús Yébana Carrilero
Universe 2024, 10(9), 365; https://doi.org/10.3390/universe10090365 - 11 Sep 2024
Viewed by 603
Abstract
We investigate the implications of different quantization approaches in Loop Quantum Cosmology for the primordial power spectrum of tensor modes. Specifically, we consider the hybrid and dressed metric approaches to derive the effective mass that governs the evolution of the tensor modes. Our [...] Read more.
We investigate the implications of different quantization approaches in Loop Quantum Cosmology for the primordial power spectrum of tensor modes. Specifically, we consider the hybrid and dressed metric approaches to derive the effective mass that governs the evolution of the tensor modes. Our study comprehensively examines the two resulting effective masses and how to estimate them in order to obtain approximated analytic solutions to the tensor perturbation equations. Since Loop Quantum Cosmology incorporates preinflationary effects in the dynamics of the perturbations, we do not have at our disposal a standard choice of privileged vacuum, like the Bunch–Davies state in quasi-de Sitter inflation. We then select the vacuum state by a recently proposed criterion which removes unwanted oscillations in the power spectrum and guarantees an asymptotic diagonalization of the Hamiltonian in the ultraviolet. This vacuum is usually called the NO-AHD (from the initials of Non-Oscillating with Asymptotic Hamiltonian Diagonalization) vacuum. Consequently, we compute the power spectrum by using our analytic approximations and by introducing a suitable numerical procedure, adopting in both cases an NO-AHD vacuum. With this information, we compare the different spectra obtained from the hybrid and the dressed metric approaches, as well as from the analytic and numerical procedures. In particular, this proves the remarkable accuracy of our approximations. Full article
(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology, Second Edition)
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