Research

Jump to: Review

11 pages, 1680 KiB

Open AccessCommunication

by Olamide Odutola and Arundhati Dasgupta

Universe 2024, 10(2), 65; https://doi.org/10.3390/universe10020065 - 31 Jan 2024

Viewed by 922

In this paper, we discuss the semi-classical gravitational wave corrections to Gauss’s law and obtain an explicit solution for the electromagnetic potential. The gravitational wave perturbs the Coulomb potential with a function that propagates it to the asymptotics. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

► Show Figures

Figure 1

10 pages, 287 KiB

Open AccessArticle

A New Perspective on Doubly Special Relativity

by J. M. Carmona, J. L. Cortés, J. J. Relancio and M. A. Reyes

Universe 2023, 9(3), 150; https://doi.org/10.3390/universe9030150 - 15 Mar 2023

Cited by 1 | Viewed by 1369

Abstract

Doubly special relativity considers a deformation of the special relativistic kinematics parametrized by a high-energy scale, in such a way that it preserves a relativity principle. When this deformation is assumed to be applied to any interaction between particles, one faces some inconsistencies. [...] Read more.

Doubly special relativity considers a deformation of the special relativistic kinematics parametrized by a high-energy scale, in such a way that it preserves a relativity principle. When this deformation is assumed to be applied to any interaction between particles, one faces some inconsistencies. In order to avoid them, we propose a new perspective where the deformation affects only the interactions between elementary particles. A consequence of this proposal is that the deformation cannot modify the special relativistic energy–momentum relation of a particle. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

► Show Figures

Figure 1

18 pages, 809 KiB

Open AccessArticle

Æther as an Inevitable Consequence of Quantum Gravity

by Sergey Cherkas and Vladimir Kalashnikov

Universe 2022, 8(12), 626; https://doi.org/10.3390/universe8120626 - 28 Nov 2022

Cited by 3 | Viewed by 1542

Abstract

The fact that quantum gravity does not admit an invariant vacuum state has far-reaching consequences for all physics. It points out that space could not be empty, and we return to the notion of an æther. Such a concept requires a preferred reference [...] Read more.

The fact that quantum gravity does not admit an invariant vacuum state has far-reaching consequences for all physics. It points out that space could not be empty, and we return to the notion of an æther. Such a concept requires a preferred reference frame for describing universe expansion and black holes. Here, we intend to find a reference system or class of metrics that could be attributed to “æther”. We discuss a vacuum and quantum gravity from three essential viewpoints: universe expansion, black hole existence, and quantum decoherence. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

► Show Figures

Figure 1

16 pages, 328 KiB

Open AccessArticle

Equivalence Principle in Classical and Quantum Gravity

by Nikola Paunković and Marko Vojinović

Universe 2022, 8(11), 598; https://doi.org/10.3390/universe8110598 - 12 Nov 2022

Cited by 2 | Viewed by 1247

Abstract

We give a general overview of various flavours of the equivalence principle in classical and quantum physics, with special emphasis on the so-called weak equivalence principle, and contrast its validity in mechanics versus field theory. We also discuss its generalisation to a theory [...] Read more.

We give a general overview of various flavours of the equivalence principle in classical and quantum physics, with special emphasis on the so-called weak equivalence principle, and contrast its validity in mechanics versus field theory. We also discuss its generalisation to a theory of quantum gravity. Our analysis suggests that only the strong equivalence principle can be considered fundamental enough to be generalised to a quantum gravity context since all other flavours of equivalence principle hold only approximately already at the classical level. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

20 pages, 874 KiB

Open AccessArticle

Vacuum Polarization Instead of “Dark Matter” in a Galaxy

by Sergey L. Cherkas and Vladimir L. Kalashnikov

Universe 2022, 8(9), 456; https://doi.org/10.3390/universe8090456 - 01 Sep 2022

Cited by 3 | Viewed by 1273

Abstract

We considered a vacuum polarization inside a galaxy in the eikonal approximation and found that two possible types of polarization exist. The first type is described by the equation of state

p = ρ / 3

, similar to radiation. Using the conformally [...] Read more.

We considered a vacuum polarization inside a galaxy in the eikonal approximation and found that two possible types of polarization exist. The first type is described by the equation of state

p = ρ / 3

, similar to radiation. Using the conformally unimodular metric allows us to construct a non-singular solution for this vacuum “substance” if a compact astrophysical object exists in the galaxy’s center. As a result, a “dark” galactical halo appears that increases the rotation velocity of a test particle as a function of the distance from a galactic center. The second type of vacuum polarization has a more complicated equation of state. As a static physical effect, it produces the renormalization of the gravitational constant, thus, causing no static halo. However, a non-stationary polarization of the second type, resulting from an exponential increase (or decrease) of the galactic nuclei mass with time in some hypothetical time-dependent process, produces a gravitational potential, appearing similar to a dark matter halo. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

► Show Figures

Figure 1

10 pages, 286 KiB

Open AccessArticle

Path Integral Action for a Resonant Detector of Gravitational Waves in the Generalized Uncertainty Principle Framework

by Soham Sen, Sukanta Bhattacharyya and Sunandan Gangopadhyay

Universe 2022, 8(9), 450; https://doi.org/10.3390/universe8090450 - 28 Aug 2022

Cited by 4 | Viewed by 1041

Abstract

The Heisenberg uncertainty principle is modified by the introduction of an observer-independent minimal length. In this work, we have considered the resonant gravitational wave detector in the modified uncertainty principle framework, where we have used the position momentum uncertainty relation with a quadratic [...] Read more.

The Heisenberg uncertainty principle is modified by the introduction of an observer-independent minimal length. In this work, we have considered the resonant gravitational wave detector in the modified uncertainty principle framework, where we have used the position momentum uncertainty relation with a quadratic order correction only. We have then used the path integral approach to calculate an action for the bar detector in the presence of a gravitational wave and then derived the Lagrangian of the system, leading to the equation of motion for the configuration-space position coordinate in one dimension. We then find a perturbative solution for the coordinate of the detector for a circularly polarized gravitational wave, leading to a classical solution of the same for the given initial conditions. Using this classical form of the coordinate of the detector, we finally obtain the classical form of the on-shell action describing the harmonic oscillator–gravitational wave system. Finally, we have obtained the free particle propagator containing the quantum fluctuation term considering gravitational wave interaction. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

6 pages, 330 KiB

Open AccessArticle

Constraints on the Duration of Inflation from Entanglement Entropy Bounds

by Suddhasattwa Brahma

Universe 2022, 8(9), 438; https://doi.org/10.3390/universe8090438 - 24 Aug 2022

Cited by 3 | Viewed by 995

Abstract

Using the fact that we only observe those modes that exit the Hubble horizon during inflation, one can calculate the entanglement entropy of such long-wavelength perturbations by tracing out the unobservable sub-Hubble fluctuations they are coupled with. On requiring that this perturbative entanglement [...] Read more.

Using the fact that we only observe those modes that exit the Hubble horizon during inflation, one can calculate the entanglement entropy of such long-wavelength perturbations by tracing out the unobservable sub-Hubble fluctuations they are coupled with. On requiring that this perturbative entanglement entropy, which increases with time, obey the covariant entropy bound for an accelerating background, we find an upper bound on the duration of inflation. This presents a new perspective on the (meta-)stability of de Sitter spacetime and an associated lifetime for it. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

► Show Figures

Figure 1

16 pages, 2342 KiB

Open AccessArticle

Phenomenological Inflationary Model in Supersymmetric Quantum Cosmology

by Nephtalí E. Martínez-Pérez, Cupatitzio Ramírez-Romero and Víctor M. Vázquez-Báez

Universe 2022, 8(8), 414; https://doi.org/10.3390/universe8080414 - 06 Aug 2022

Cited by 2 | Viewed by 1055

Abstract

We consider the effective evolution of a phenomenological model from FLRW supersymmetric quantum cosmology with a scalar field. The scalar field acts as a clock and inflaton. We examine a family of simple superpotentials that produce an inflation whose virtual effect on inhomogeneous [...] Read more.

We consider the effective evolution of a phenomenological model from FLRW supersymmetric quantum cosmology with a scalar field. The scalar field acts as a clock and inflaton. We examine a family of simple superpotentials that produce an inflation whose virtual effect on inhomogeneous fluctuations shows very good agreement with PLANCK observational evidence for the tensor-to-scalar ratio and the scalar spectral index. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

► Show Figures

Figure 1

12 pages, 419 KiB

Open AccessArticle

Massive Neutron Stars and White Dwarfs as Noncommutative Fuzzy Spheres

by Surajit Kalita and Banibrata Mukhopadhyay

Universe 2022, 8(8), 388; https://doi.org/10.3390/universe8080388 - 22 Jul 2022

Viewed by 1008

Abstract

Over the last couple of decades, there have been direct and indirect evidences for massive compact objects than their conventional counterparts. A couple of such examples are super-Chandrasekhar white dwarfs and massive neutron stars. The observations of more than a dozen peculiar over-luminous [...] Read more.

Over the last couple of decades, there have been direct and indirect evidences for massive compact objects than their conventional counterparts. A couple of such examples are super-Chandrasekhar white dwarfs and massive neutron stars. The observations of more than a dozen peculiar over-luminous type Ia supernovae predict their origins from super-Chandrasekhar white dwarf progenitors. On the other hand, recent gravitational wave detection and some pulsar observations provide arguments for massive neutron stars, lying in the famous mass-gap between lowest astrophysical black hole and conventional highest neutron star masses. We show that the idea of a squashed fuzzy sphere, which brings in noncommutative geometry, can self-consistently explain either of the massive objects as if they are actually fuzzy or squashed fuzzy spheres. Noncommutative geometry is a branch of quantum gravity. If the above proposal is correct, it will provide observational evidences for noncommutativity. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

► Show Figures

Figure 1

Review

Jump to: Research

24 pages, 516 KiB

Open AccessReview

Aspects of Quantum Gravity Phenomenology and Astrophysics

by Arundhati Dasgupta and José Fajardo-Montenegro

Universe 2023, 9(3), 128; https://doi.org/10.3390/universe9030128 - 01 Mar 2023

Cited by 2 | Viewed by 1438

Abstract

With the discovery of gravitational waves, the search for the quantum of gravity, the graviton, is imminent. We discuss the current status of the bounds on graviton mass from experiments as well as the theoretical understanding of these particles. We provide an overview [...] Read more.

With the discovery of gravitational waves, the search for the quantum of gravity, the graviton, is imminent. We discuss the current status of the bounds on graviton mass from experiments as well as the theoretical understanding of these particles. We provide an overview of current experiments in astrophysics such as the search for Hawking radiation in gamma-ray observations and neutrino detectors, which will also shed light on the existence of primordial black holes. Finally, the semiclassical corrections to the image of the event horizon are discussed. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

► Show Figures

Figure 1

39 pages, 1172 KiB

Open AccessReview

WKB Approaches to Restore Time in Quantum Cosmology: Predictions and Shortcomings

by Giulia Maniccia, Mariaveronica De Angelis and Giovanni Montani

Universe 2022, 8(11), 556; https://doi.org/10.3390/universe8110556 - 25 Oct 2022

Cited by 13 | Viewed by 1372

Abstract

In this review, we analyse different aspects concerning the possibility to separate a gravity-matter system into a part which lives close to a quasi-classical state and a “small” quantum subset. The considered approaches are all relying on a WKB expansion of the dynamics [...] Read more.

In this review, we analyse different aspects concerning the possibility to separate a gravity-matter system into a part which lives close to a quasi-classical state and a “small” quantum subset. The considered approaches are all relying on a WKB expansion of the dynamics by an order parameter and the natural arena consists of the Bianchi universe minisuperspace. We first discuss how, limiting the WKB expansion to the first order of approximation, it is possible to recover for the quantum subsystem a Schrödinger equation, as written on the classical gravitational background. Then, after having tested the validity of the approximation scheme for the Bianchi I model, we give some applications for the quantum subsystem in the so-called “corner” configuration of the Bianchi IX model. We individualize the quantum variable in the small one of the two anisotropy degrees of freedom. The most surprising result is the possibility to obtain a non-singular Bianchi IX cosmology when the scenario is extrapolated backwards in time. In this respect, we provide some basic hints on the extension of this result to the generic cosmological solution. In the last part of the review, we consider the same scheme to the next order of approximation identifying the quantum subset as made of matter variables only. This way, we are considering the very fundamental problem of non-unitary morphology of the quantum gravity corrections to quantum field theory discussing some proposed reformulations. Instead of constructing the time dependence via that one of the classical gravitational variables on the label time as in previous works, we analyse a recent proposal to construct time by fixing a reference frame. This scheme can be reached both introducing the so-called “kinematical action”, as well as by the well-known Kuchar–Torre formulation. In both cases, the Schrödinger equation, amended for quantum gravity corrections, has the same morphology and we provide a cosmological implementation of the model, to elucidate its possible predictions. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

► Show Figures

Figure 1

21 pages, 349 KiB

Open AccessReview

Space–Time Physics in Background-Independent Theories of Quantum Gravity

by Martin Bojowald

Universe 2021, 7(7), 251; https://doi.org/10.3390/universe7070251 - 20 Jul 2021

Cited by 6 | Viewed by 1757

Abstract

Background independence is often emphasized as an important property of a quantum theory of gravity that takes seriously the geometrical nature of general relativity. In a background-independent formulation, quantum gravity should determine not only the dynamics of space–time but also its geometry, which [...] Read more.

Background independence is often emphasized as an important property of a quantum theory of gravity that takes seriously the geometrical nature of general relativity. In a background-independent formulation, quantum gravity should determine not only the dynamics of space–time but also its geometry, which may have equally important implications for claims of potential physical observations. One of the leading candidates for background-independent quantum gravity is loop quantum gravity. By combining and interpreting several recent results, it is shown here how the canonical nature of this theory makes it possible to perform a complete space–time analysis in various models that have been proposed in this setting. In spite of the background-independent starting point, all these models turned out to be non-geometrical and even inconsistent to varying degrees, unless strong modifications of Riemannian geometry are taken into account. This outcome leads to several implications for potential observations as well as lessons for other background-independent approaches. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

► Show Figures

Figure 1

Journal Menu

Journal Browser

Quantum Gravity Phenomenology II

Share This Special Issue

Special Issue Editors

Special Issue Information

Keywords

Related Special Issue

Published Papers (12 papers)

Research

Review

Further Information

Guidelines

MDPI Initiatives

Follow MDPI