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

Deadline for manuscript submissions: closed (29 February 2024) | Viewed by 6382

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Special Issue Editor

Prof. Dr. Giovanni Montani

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Guest Editor

1. ENEA, Fusion and Nuclear Safety Department, C.R. Frascati, Via E. Fermi 45, 00044 Frascati, Italy
2. Physics Department, “Sapienza” University of Rome, P.le Aldo Moro 5, 00185 Roma, Italy
Interests: primordial cosmology; quantum gravity; modified theories of gravity; plasma astrophysics; cosmological plasmas; turbulent transport in plasmas

Special Issue Information

Dear Colleagues,

The dynamics of the quantum universe is an excellent arena to test the predictivity of different proposals for a quantum theory of gravity, and in some cases, the mini-superspace formulation is rich with physical insight.

In the last two decades, quantum cosmology has been characterized by a new impulse, mainly due to the formulation of Big-Bounce dynamics in Loop Quantum Cosmology. Further, other formulations based on a covariant description of the gravitational field dynamics have been successful in offering physical intuitions on the emergence of a classical Universe from the Planckian era.

Despite this improvement of our understanding about the notion and the meaning of a quantum universe, many relevant open questions remain in the field and call attention for further discussion and investigation. For this reason, the goal of this issue is to put together material coming from different approaches and try to find those common features which could constitute a valuable horizon for future studies.

This Special Issue aims at focusing attention on the most recent and promising developments in quantum cosmology, both in the canonical formulation, as well as in covariant approaches (e.g., path integral or covariant loop quantum gravity studies). Three main aspects are considered: (i) proposals for suitable clocks in quantum cosmologies; (ii) the implementation of fundamental symmetries in the mini-superspace; (iii) the emergence of Big-Bounce dynamics. Implications on universe quantum dynamics, coming from extended theories of gravity, are also considered. We invite the submission of original papers in the form of regular articles or short letters, but also review articles dedicated to selected topics in quantum cosmology.

Prof. Dr. Giovanni Montani
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

Big-Bounce scenarios
time in quantum cosmology
symmetries of the quantum universe
canonical quantum cosmology
covariant approaches to quantum cosmology
quantum cosmology from extended gravity

Published Papers (6 papers)

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Research

15 pages, 4439 KiB

Open AccessArticle

Non-Commutative Classical and Quantum Fractionary Cosmology: FRW Case

by J. Socorro, J. Juan Rosales and Leonel Toledo-Sesma

Universe 2024, 10(5), 192; https://doi.org/10.3390/universe10050192 - 25 Apr 2024

Viewed by 271

Abstract

In this work, we will explore the effects of non-commutativity in fractional classical and quantum schemes using the flat Friedmann–Robertson–Walker (FRW) cosmological model coupled to a scalar field in the K-essence formalism. In previous work, we have obtained the commutative solutions in both [...] Read more.

q_{n c}^{i}

do not commute, so the symplectic structure was modified. In the quantum regime, the probability density presents a new structure in the scalar field corresponding to the value of the non-commutative parameter, in the sense that this probability density undergoes a shift back to the direction of the scale factor, causing classical evolution to arise earlier than in the commutative world. Full article

(This article belongs to the Special Issue Recent Advances in Quantum Cosmology)

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Figure 1

5 pages, 215 KiB

Open AccessCommunication

Upper Bound of Barrow Entropy Index from Black Hole Fragmentation

by Jiayi Xia and Yen Chin Ong

Universe 2024, 10(4), 177; https://doi.org/10.3390/universe10040177 - 11 Apr 2024

Viewed by 460

Abstract

Both classical and quantum arguments suggest that if Barrow entropy is correct, its index

δ

Both classical and quantum arguments suggest that if Barrow entropy is correct, its index

δ

must be energy-dependent, which would affect the very early universe. Based on thermodynamic stability that sufficiently large black holes should not fragment, we argue that Barrow entropy correction must be small, except possibly at the Planckian regime. Furthermore, the fact that a solar mass black hole does not fragment implies an upper bound

δ ≲ O (10^{- 3})

, which surprisingly lies in the same range as the bound obtained from some cosmological considerations assuming fixed

δ

. This indicates that allowing

δ

to run does not raise its allowed value. We briefly comment on the case of Kaniadakis entropy. Full article

(This article belongs to the Special Issue Recent Advances in Quantum Cosmology)

31 pages, 1248 KiB

Open AccessArticle

A Loop Quantum-Corrected Family of Chiral Cosmology Models

by Luis Rey Díaz-Barrón, Abraham Espinoza-García, Sinuhé Alejandro Pérez-Payán and J. Socorro

Universe 2024, 10(2), 88; https://doi.org/10.3390/universe10020088 - 12 Feb 2024

Viewed by 1041

Abstract

We construct and examine a holonomy-corrected chiral fields model of cosmological relevance. Specifically, we holonomize the Hamiltonian corresponding to a quintom field scenario with additional kinetic interaction (governed by the constant chiral metric,

m_{a b}

) on a flat FLRW background and [...] Read more.

m_{a b}

) on a flat FLRW background and contrast the resulting model with the corresponding purely classical system. In particular, it is shown that the single LQC bouncing stage is ensured to be realized, provided the full chiral kinetic energy function does not change sign during evolution. (As preparation, a particularly simple k-essence field is examined within the effective LQC scheme; some exact solutions are obtained in the process.) Additionally, under the said assumption, it is established that the landmark bouncing mechanism of standard (effective) LQC is still guaranteed to be featured even when taking any finite number of fields

ϕ^{1}, \dots ϕ^{m}

and

m_{a b}

to be dependent on such fields (the particular zero-potential case corresponding to a family of simple purely kinetic k-essence multi-field cosmology models). Full article

(This article belongs to the Special Issue Recent Advances in Quantum Cosmology)

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15 pages, 463 KiB

Open AccessArticle

Scalar Product for a Version of Minisuperspace Model with Grassmann Variables

by Sergey L. Cherkas and Vladimir L. Kalashnikov

Universe 2023, 9(12), 508; https://doi.org/10.3390/universe9120508 - 07 Dec 2023

Viewed by 1039

Abstract

Grassmann variables are used to formally transform a system with constraints into an unconstrained system. As a result, the Schrödinger equation arises instead of the Wheeler–DeWitt one. The Schrödinger equation describes a system’s evolution, but a definition of the scalar product is needed to calculate the mean values of the operators. We suggest an explicit formula for the scalar product related to the Klein–Gordon scalar product. The calculation of the mean values is compared with an etalon method in which a redundant degree of freedom is excluded. Nevertheless, we note that a complete correspondence with the etalon picture is not found. Apparently, the picture with Grassmann variables requires a further understanding of the underlying Hilbert space. Full article

(This article belongs to the Special Issue Recent Advances in Quantum Cosmology)

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23 pages, 414 KiB

Open AccessArticle

Study of the Inflationary Spectrum in the Presence of Quantum Gravity Corrections

by Giulia Maniccia, Giovanni Montani and Leonardo Torcellini

Universe 2023, 9(4), 169; https://doi.org/10.3390/universe9040169 - 30 Mar 2023

Cited by 2 | Viewed by 980

Abstract

After a brief review of the different approaches to predicting the possible quantum gravity corrections to quantum field theory, we discuss in some detail the formulation based on a Gaussian reference frame fixing. Then, we utilize this scenario in the determination of the inflationary spectrum of primordial perturbations. We consider the quantization of an inhomogeneous, free, massless scalar field in a quasi-classical isotropic Universe by developing a WKB expansion of the dynamics of the next order in the Planckian parameter, with respect to the one at which standard QFT emerges. The quantum gravity corrections to the scale-invariant spectrum are discussed in a specific primordial cosmological setting and then in a general minisuperspace formalism, showing that there is no mode-dependent effect, and thus the scale invariant inflationary spectrum is preserved. This result is discussed in connection to the absence of a matter backreaction on the gravitational background in the considered paradigm. Full article

(This article belongs to the Special Issue Recent Advances in Quantum Cosmology)

21 pages, 2007 KiB

Open AccessArticle

The Phase Space Analysis of Interacting K-Essence Dark Energy Models in Loop Quantum Cosmology

by Bohai Chen, Yabo Wu, Jianan Chi, Wenzhong Liu and Yiliang Hu

Universe 2022, 8(10), 520; https://doi.org/10.3390/universe8100520 - 06 Oct 2022

Cited by 2 | Viewed by 1738

Abstract

The present work deals with two kinds of k-essence dark energy models within the framework of loop quantum cosmology (LQC). The two kinds of k-essence models originates from two forms of Lagrangians, i.e., [...] Read more.

L_{1} = F (X) V (ϕ)

and

L_{2} = F (X) - V (ϕ)

, where

F (X)

and

V (ϕ)

stand for the kinetic term and potential of the scalar field

ϕ

, respectively. Two models are based on different phase variables settings, and the general form of autonomous dynamical system is deduced for each Lagrangian. Then, the dynamical stabilities of the critical points in each model are analysed in different forms of

F (X)

and

V (ϕ)

. Model I is a 3-dim system with four stable points, and Model II is a 4-dim system but reduced to a 3-dim system using the symmetry analysis, which has five stable points. Moreover, the corresponding cosmological quantities, such as

Ω_{ϕ}

w_{ϕ}

and q, are calculated at each critical point. To compare these with the case of the classical Einstein cosmology (EC), the dynamical evolutionary trajectories in the phase space and evolutionary curves of the cosmological quantities are drawn for both EC and LQC cases, which shows that the loop quantum gravity effects diminish in the late-time universe but are significant in the early time. Further, the effects of interaction

Q = α H ρ_{m}

on the evolutions of the universe are discussed. With the loop quantum gravity effects, bouncing universe is achieved in both models for different initial values of

ϕ_{0}

{\dot{ϕ}}_{0}

H_{0}

ρ_{0}

and coupling parameter

α

, which helps to avoid singularities. However, the interaction has little effect on bounce, although it is important to the stability of some critical points. Full article

(This article belongs to the Special Issue Recent Advances in Quantum Cosmology)

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