Special Issue "Modified Theories of Gravity"

A special issue of Symmetry (ISSN 2073-8994).

Deadline for manuscript submissions: closed (31 January 2020).

Special Issue Editor

Prof. Dr. Stephon Alexander
Website
Guest Editor
Department of Physics & Astronomy, HB 6127, Wilder Lab, Dartmouth College, Hanover, New Hampshire 03755, USA
Interests: theoretical cosmology; cosmological constant problem; baryogenesis; cosmic inflation; structure formation; singularity resolution; quantum fields in curved spaces; string cosmology; quantum gravity; string theory; modified theories of gravity; loop quantum gravity; spin foams; geometry and math of music; instrument acoustic modelling
Special Issues and Collections in MDPI journals

Special Issue Information

Dear Colleagues,

Einstein’s theory of general relativity has seen tremendous experimental success over the last century, most recently with the observation of gravitational waves. However, a handful of observations, such as dark matter, dark energy and black holes motivate the construction of theories that go beyond general relativity. Also, we still lack a complete theory of the early universe free of singularities, which also motivate modified theories of gravity that address these issues. Finally, modified theories of gravity arise naturally in the semiclassical limit of some quantum theories of gravity such as string theory and loop quantum gravity. Conversely, modified theories of gravity can motivate the correct starting point for quantizing gravity. This Special Issue will explore all of these dimensions of modified theories of gravity.

Prof. Stephon Alexander
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 papers will be 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. Symmetry is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1400 CHF (Swiss Francs). 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

  • modified theories of gravity
  • loop quantum gravity
  • string theory
  • dark matter
  • dark energy
  • black holes

Published Papers (7 papers)

Order results
Result details
Select all
Export citation of selected articles as:

Research

Open AccessArticle
Dark Matter as a Result of Field Oscillations in the Modified Theory of Induced Gravity
Symmetry 2020, 12(1), 41; https://doi.org/10.3390/sym12010041 - 24 Dec 2019
Cited by 1
Abstract
The paper studies the modified theory of induced gravity (MTIG). The solutions of the MTIG equations contain two branches (stages): Einstein (ES) and “restructuring” (RS). Previously, solutions were found that the values of such parameters as the “Hubble parameter”, gravitational and cosmological “constants” [...] Read more.
The paper studies the modified theory of induced gravity (MTIG). The solutions of the MTIG equations contain two branches (stages): Einstein (ES) and “restructuring” (RS). Previously, solutions were found that the values of such parameters as the “Hubble parameter”, gravitational and cosmological “constants” at the RS stage, fluctuate near monotonously developing mean values. This article gives MTIG equations with arbitrary potential. Solutions of the equations of geodesic curves are investigated for the case of centrally symmetric space and quadratic potential at the RS stage. The oscillatory nature of the solutions leads to the appearance of a gravitational potential containing a spectrum of minima, as well as to antigravity, which is expressed by acceleration directed from the center. Such solutions lead to the distribution of the potential of the gravitational field creating an additional mass effect at large distances and are well suited for modeling the effect of dark matter in galaxies. The solutions of the equation of geodesic lines are obtained and analyzed. We found that the transition from flat asymptotics to oscillatory asymptotics at large distances from the center with a combination of the presence of antigravity zones leads to a rich variety of shapes and dynamics of geodesic curves and to the formation of complex structures. Full article
(This article belongs to the Special Issue Modified Theories of Gravity)
Show Figures

Figure 1

Open AccessArticle
Constraints on the String T-Duality Propagator from the Hydrogen Atom
Symmetry 2019, 11(12), 1478; https://doi.org/10.3390/sym11121478 - 04 Dec 2019
Cited by 1
Abstract
We investigated the implications of string theory in the high-precision regime of quantum mechanics. In particular, we examined a quantum field theoretical propagator which was derived from string theory when compactified at the T-duality self-dual radius and which is closely related to the [...] Read more.
We investigated the implications of string theory in the high-precision regime of quantum mechanics. In particular, we examined a quantum field theoretical propagator which was derived from string theory when compactified at the T-duality self-dual radius and which is closely related to the path integral duality. Our focus was on the hydrogen ground state energy and the 1 S 1 / 2 2 S 1 / 2 transition frequency, as they are the most precisely explored properties of the hydrogen atom. The T-duality propagator alters the photon field dynamics leading to a modified Coulomb potential. Thus, our study is complementary to investigations where the electron evolution is modified, as in studies of a minimal length in the context of the generalized uncertainty principle. The first manifestation of the T-duality propagator arises at fourth order in the fine-structure constant, including a logarithmic term. For the first time, constraints on the underlying parameter, the zero-point length, are presented. They reach down to 3.9 × 10 19 m and are in full agreement with previous studies on black holes. Full article
(This article belongs to the Special Issue Modified Theories of Gravity)
Show Figures

Figure 1

Open AccessArticle
Renormalizable and Unitary Model of Quantum Gravity
Symmetry 2019, 11(11), 1334; https://doi.org/10.3390/sym11111334 - 25 Oct 2019
Cited by 1
Abstract
We consider R + R 2 relativistic quantum gravity with the action where all possible terms quadratic in the curvature tensor are added to the Einstein-Hilbert term. This model was shown to be renormalizable in the work by K.S. Stelle. In this paper, [...] Read more.
We consider R + R 2 relativistic quantum gravity with the action where all possible terms quadratic in the curvature tensor are added to the Einstein-Hilbert term. This model was shown to be renormalizable in the work by K.S. Stelle. In this paper, we demonstrate that the R + R 2 model is also unitary contrary to the statements made in the literature, in particular in the work by Stelle. New expressions for the R + R 2 Lagrangian within dimensional regularization and the graviton propagator are derived. We demonstrate that the R + R 2 model is a good candidate for the fundamental quantum theory of gravity. Full article
(This article belongs to the Special Issue Modified Theories of Gravity)
Open AccessArticle
Quantum-Gravitational Trans-Planckian Energy of a Time-Dependent Black Hole
Symmetry 2019, 11(10), 1303; https://doi.org/10.3390/sym11101303 - 16 Oct 2019
Cited by 1
Abstract
We continue our recent endeavor in which a time-dependent black hole solution of a one-loop quantum-corrected Einstein-scalar system was obtained and its near-horizon behavior was analyzed. The energy analysis led to a trans-Planckian scaling behavior near the event horizon. In the present work, [...] Read more.
We continue our recent endeavor in which a time-dependent black hole solution of a one-loop quantum-corrected Einstein-scalar system was obtained and its near-horizon behavior was analyzed. The energy analysis led to a trans-Planckian scaling behavior near the event horizon. In the present work, the analysis is extended to a rotating black hole solution of an Einstein–Maxwell-scalar system with a Higgs potential. Although the analysis becomes much more complex compared to that of the previous, we observe the same basic features, including the quantum-gravitational trans-Planckian energy near the horizon. Full article
(This article belongs to the Special Issue Modified Theories of Gravity)
Open AccessArticle
Classical Limit for Dirac Fermions with Modified Action in the Presence of a Black Hole
Symmetry 2019, 11(10), 1294; https://doi.org/10.3390/sym11101294 - 15 Oct 2019
Abstract
We consider the model of Dirac fermions coupled to gravity as proposed, in which superluminal velocities of particles are admitted. In this model an extra term is added to the conventional Hamiltonian that originates from Planck physics. Due to this term, a closed [...] Read more.
We consider the model of Dirac fermions coupled to gravity as proposed, in which superluminal velocities of particles are admitted. In this model an extra term is added to the conventional Hamiltonian that originates from Planck physics. Due to this term, a closed Fermi surface is formed in equilibrium inside the black hole. In this paper we propose the covariant formulation of this model and analyse its classical limit. We consider the dynamics of gravitational collapse. It appears that the Einstein equations admit a solution identical to that of ordinary general relativity. Next, we consider the motion of particles in the presence of a black hole. Numerical solutions of the equations of motion are found which demonstrate that the particles are able to escape from the black hole. Full article
(This article belongs to the Special Issue Modified Theories of Gravity)
Show Figures

Figure 1

Open AccessArticle
The Quantum Cosmological Constant
Symmetry 2019, 11(9), 1130; https://doi.org/10.3390/sym11091130 - 05 Sep 2019
Cited by 7
Abstract
We present an extension of general relativity in which the cosmological constant becomes dynamical and turns out to be conjugate to the Chern–Simons invariant of the Ashtekar connection on a spatial slicing. The latter has been proposed Soo and Smolin as a time [...] Read more.
We present an extension of general relativity in which the cosmological constant becomes dynamical and turns out to be conjugate to the Chern–Simons invariant of the Ashtekar connection on a spatial slicing. The latter has been proposed Soo and Smolin as a time variable for quantum gravity: the Chern–Simons time. In the quantum theory, the inverse cosmological constant and Chern–Simons time will then become conjugate operators. The “Kodama state” gets a new interpretation as a family of transition functions. These results imply an uncertainty relation between Λ and Chern–Simons time; the consequences of which will be discussed elsewhere. Full article
(This article belongs to the Special Issue Modified Theories of Gravity)
Open AccessArticle
Applications of Canonical Quantum Gravity to Cosmology
Symmetry 2019, 11(8), 1005; https://doi.org/10.3390/sym11081005 - 03 Aug 2019
Abstract
We applied quantum gravitational results to spatially unbounded Friedmann universes and tried to answer some questions related to dark energy, dark matter, inflation, and the missing antimatter. Full article
(This article belongs to the Special Issue Modified Theories of Gravity)
Back to TopTop