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Quantum Physics including Gravity: Highlights and Novelties

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

Deadline for manuscript submissions: 30 June 2024 | Viewed by 7244

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Prof. Dr. Norma G. Sanchez

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CNRS. International School D. Chalonge—Hector de Vega, 75014 Paris, France
Interests: cosmology; quantum physics; quantum space–time black holes; gravitational physics; fundamental physics; theoretical astrophysics; theoretical physics; new quantum physical systems; new quantum information systems
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Special Issue Information

Dear Colleagues,

This Special Issue aims to provide a collection of both research papers and review papers on the fascinating problems of quantum physics including gravity, from quantum fields in curved space times to Planckian and trans-Planckian physics and quantum space-time, at the cutting edge of these domains and their most recent advances, whether being of foundational and conceptual nature or of implications and implementations; in cosmology, black holes, non-trivial vaccua, topologies, and geometries, being within the known approaches and frameworks, or with new research avenues in these fields, as well as experimental and observational tests and results. Topics include:

Quantum fields and strings in curved space times, theory and applications.
Quantum physics and black holes, semiclassical and quantum black holes.
Quantum fields in cosmology, inflation and dark energy.
Planckian and trans-Planckian physics.
Quantum space-time.

Papers can be submitted at any time before the deadline. Accepted papers are published as soon they are accepted. Early Submission is encouraged.

Prof. Dr. Norma G. Sanchez
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 physics
curved space-times
cosmology
black holes
semi-classical gravity
quantum gravity
trans-Planckian physics
quantum space-time

Published Papers (5 papers)

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Research

Jump to: Review

16 pages, 381 KiB

Open AccessArticle

Quantum-Spacetime Symmetries: A Principle of Minimum Group Representation

by Diego J. Cirilo-Lombardo and Norma G. Sanchez

Universe 2024, 10(1), 22; https://doi.org/10.3390/universe10010022 - 04 Jan 2024

Cited by 1 | Viewed by 1046

Abstract

We show that, as in the case of the principle of minimum action in classical and quantum mechanics, there exists an even more general principle in the very fundamental structure of quantum spacetime: this is the principle of minimal group representation, which allows us to consistently and simultaneously obtain a natural description of spacetime’s dynamics and the physical states admissible in it. The theoretical construction is based on the physical states that are average values of the generators of the metaplectic group

M p (n)

, the double covering of

S L (2 C)

in a vector representation, with respect to the coherent states carrying the spin weight. Our main results here are: (i) There exists a connection between the dynamics given by the metaplectic-group symmetry generators and the physical states (the mappings of the generators through bilinear combinations of the basic states). (ii) The ground states are coherent states of the Perelomov–Klauder type defined by the action of the metaplectic group that divides the Hilbert space into even and odd states that are mutually orthogonal. They carry spin weight of 1/4 and 3/4, respectively, from which two other basic states can be formed. (iii) The physical states, mapped bilinearly with the basic 1/4- and 3/4-spin-weight states, plus their symmetric and antisymmetric combinations, have spin contents

s = 0, 1 / 2, 1, 3 / 2

and 2. (iv) The generators realized with the bosonic variables of the harmonic oscillator introduce a natural supersymmetry and a superspace whose line element is the geometrical Lagrangian of our model. (v) From the line element as operator level, a coherent physical state of spin 2 can be obtained and naturally related to the metric tensor. (vi) The metric tensor is naturally discretized by taking the discrete series given by the basic states (coherent states) in the n number representation, reaching the classical (continuous) spacetime for n

\to \infty

. (vii) There emerges a relation between the eigenvalue

α

of our coherent-state metric solution and the black-hole area (entropy) as

A_{b h} / 4 l_{p}^{2} = |α|

, relating the phase space of the metric found,

g_{a b}

, and the black hole area,

A_{b h}

, through the Planck length

l_{p}^{2}

and the eigenvalue

|α|

of the coherent states. As a consequence of the lowest level of the quantum-discrete-spacetime spectrum—e.g., the ground state associated to

n = 0

and its characteristic length—there exists a minimum entropy related to the black-hole history. Full article

(This article belongs to the Special Issue Quantum Physics including Gravity: Highlights and Novelties)

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16 pages, 668 KiB

Open AccessFeature PaperArticle

Classical Mechanics with Inequality Constraints and Gravity Models with Limiting Curvature

by Andrei V. Frolov and Valeri P. Frolov

Universe 2023, 9(6), 284; https://doi.org/10.3390/universe9060284 - 10 Jun 2023

Viewed by 1204

Abstract

In this paper, we discuss mechanical systems with inequality constraints

Φ (q, \dot{q}, . . .) \leq 0

. We demonstrate how such constraints can be taken into account by proper modification of the action which describes the [...] Read more.

In this paper, we discuss mechanical systems with inequality constraints

Φ (q, \dot{q}, . . .) \leq 0

. We demonstrate how such constraints can be taken into account by proper modification of the action which describes the original unconstrained dynamics. To illustrate this approach, we consider a harmonic oscillator in the model with limiting velocity. We compare the behavior of such an oscillator with the behavior of a relativistic oscillator and demonstrate that when the amplitude of the oscillator is large, the properties of both types of oscillators are quite similar. We also discuss inequality constraints, which contain higher derivatives. At the end of the paper, we briefly discuss possible applications of the developed approach to gravity models with limiting curvature. Full article

(This article belongs to the Special Issue Quantum Physics including Gravity: Highlights and Novelties)

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30 pages, 493 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Dark Energy Is the Cosmological Quantum Vacuum Energy of Light Particles—The Axion and the Lightest Neutrino

by Héctor J. de Vega and Norma G. Sanchez

Universe 2023, 9(4), 167; https://doi.org/10.3390/universe9040167 - 30 Mar 2023

Cited by 2 | Viewed by 1625

Abstract

We uncover the general mechanism and the nature of today’s dark energy (DE). This is only based on well-known quantum physics and cosmology. We show that the observed DE today originates from the cosmological quantum vacuum of light particles, which provides a continuous energy distribution able to reproduce the data. Bosons give positive contributions to the DE, while fermions yield negative contributions. As usual in field theory, ultraviolet divergences are subtracted from the physical quantities. The subtractions respect the symmetries of the theory, and we normalize the physical quantities to be zero for the Minkowski vacuum. The resulting finite contributions to the energy density and the pressure from the quantum vacuum grow as

log a (t)

, where

a (t)

is the scale factor, while the particle contributions dilute as

1 / a^{3} (t)

, as it must be for massive particles. We find the explicit dark energy equation of state of today to be

P = w (z) H

: it turns to be slightly

w (z) < - 1

with

w (z)

asymptotically reaching the value

- 1

from below. A scalar particle can produce the observed dark energy through its quantum cosmological vacuum provided that (i) its mass is of the order of

10^{- 3}

eV = 1 meV, (ii) it is very weakly coupled, and (iii) it is stable on the time scale of the age of the universe. The axion vacuum thus appears as a natural candidate. The neutrino vacuum (especially the lightest mass eigenstate) can give negative contributions to the dark energy. We find that

w (z = 0)

is slightly below

- 1

by an amount ranging from

(- 1.5 \times 10^{- 3})

(- 8 \times 10^{- 3})

and we predict the axion mass to be in the range between 4 and 5

meV

. We find that the universe will expand in the future faster than the de Sitter universe as an exponential in the square of the cosmic time. Dark energy today arises from the quantum vacuum of light particles in FRW cosmological space-time in an analogous way to the Casimir vacuum effect of quantum fields in Minkowski space-time with non-trivial boundary conditions. Full article

(This article belongs to the Special Issue Quantum Physics including Gravity: Highlights and Novelties)

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11 pages, 277 KiB

Open AccessArticle

Quantum Scalar-Field Propagator in a Stochastic Gravitational-Plane Wave

by Zbigniew Haba

Universe 2022, 8(12), 648; https://doi.org/10.3390/universe8120648 - 05 Dec 2022

Cited by 2 | Viewed by 1655

Abstract

A stochastic metric can appear in classical as well as in quantum gravity. We show that if the linearized stochastic Gaussian gravitational-plane wave has the frequency spectrum

ω^{4 γ - 1}

(

0 \leq γ < 1

), then the equal-time propagator [...] Read more.

A stochastic metric can appear in classical as well as in quantum gravity. We show that if the linearized stochastic Gaussian gravitational-plane wave has the frequency spectrum

ω^{4 γ - 1}

(

0 \leq γ < 1

), then the equal-time propagator of the scalar field behaves as

p^{- \frac{1}{1 - γ}}

for large momenta. We discuss models of quantum-field theory where such anomalous behavior can appear. Full article

(This article belongs to the Special Issue Quantum Physics including Gravity: Highlights and Novelties)

Review

Jump to: Research

15 pages, 324 KiB

Open AccessReview

Phenomenologies in Hypersphere Soliton and Stringy Photon Models

by Soon-Tae Hong

Universe 2023, 9(9), 378; https://doi.org/10.3390/universe9090378 - 23 Aug 2023

Viewed by 621

Abstract

We consider the Dirac quantization in the first-class formalism to investigate the hypersphere soliton model (HSM) defined on the

S^{3}

hypersphere. To do this, we construct the first-class Hamiltonian possessing the Weyl ordering correction. In the HSM, we evaluate the baryon physical [...] Read more.

We consider the Dirac quantization in the first-class formalism to investigate the hypersphere soliton model (HSM) defined on the

S^{3}

hypersphere. To do this, we construct the first-class Hamiltonian possessing the Weyl ordering correction. In the HSM, we evaluate the baryon physical quantities such as the baryon masses, magnetic moments, axial coupling constant and charge radii, most predicted values of which are in good agreement with the corresponding experimental data. Moreover, shuffling the baryon and transition magnetic moments, we find the model independent sum rules. In the HSM we also evaluate the baryon intrinsic frequencies such as

ω_{N} = 0.87 \times 10^{23} s^{- 1}

and

ω_{Δ} = 1.74 \times 10^{23} s^{- 1}

of the nucleon and delta baryon, respectively, to yield the identity

ω_{Δ} = 2 ω_{N}

. Next, making use of the Nambu-Goto string action and its extended rotating bosonic string theory, we formulate the stringy photon model to obtain the energy of the string configuration, which consists of the rotational and vibrational energies of the open string. Exploiting this total string energy, we evaluate the photon intrinsic frequency

ω_{γ} = 9.00 \times 10^{23} s^{- 1}

, which is comparable to the corresponding baryon intrinsic frequencies. We also predict the photon size

{⟨ r^{2} ⟩}^{1 / 2} (photon) = 0.17 fm

, which is approximately 21% of the proton magnetic charge radius. Full article

(This article belongs to the Special Issue Quantum Physics including Gravity: Highlights and Novelties)

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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Bei Lok Bernard Hu
Norma Sanchez
Maurizio Gasperini
James B. Hartle
Valeri P. Frolov
John Ashmead
Jerzy Lukierski
Vyacheslav Dokuchaev
John Klauder
Lucas Chibebe Céleri

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