Special Issue "Gravity, Black Holes and Cosmology XXI"

A special issue of Universe (ISSN 2218-1997).

Deadline for manuscript submissions: closed (30 June 2018).

Special Issue Editors

Prof. Dr. Gianluca Calcagni
E-Mail Website
Guest Editor
Instituto de Estructura de la Materia – CSIC, Madrid, Spain
Interests: nonlocal quantum gravity; nonlocal cosmology; dimensional flow in quantum gravity; multifractal spacetimes
Prof. Dr. Cosimo Bambi
E-Mail Website
Guest Editor
1. Center for Field Theory and Particle Physics and Department of Physics, Fudan University, Shanghai, China
2. Theoretical Astrophysics, Eberhard Karls Universität Tübingen, Tübingen, Germany
Interests: tests of general relativity and of alternative theories of gravity; black holes; quantum gravity phenomenology; high-energy astrophysics; physics of the early Universe
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Leonardo Modesto
E-Mail Website
Guest Editor
Department of Physics, Southern University of Science and Technology (SUSTech), Shenzhen, China
Interests: Classical and Quantum Gravity; Quantum Field Theory; Classical and Quantum Black Holes; Physics beyond the Standard Model of Particle Physics; Cosmology; Theoretical Astrophysics

Special Issue Information

Dear Colleagues,

It is a pleasure to announce the Special Issue “Gravity, black holes and cosmology XXI”. Since its first applications in the XX century, and the crowning with the formulation of the Standard Model of electroweak and strong interactions, quantum field theory has evolved, both in meaning and intention. Its perturbative and nonperturbative incarnations have been playing an undisputed role in modern theoretical physics, not only in the refinement of the Standard Model and of its supersymmetric extensions, but also in the quest for a quantum theory of gravitation, carried on in the programs of string theory, supergravity, and many other quantum gravities (nonlocal quantum gravity, asymptotic safety, group field theory, and so on), and in our understanding of astrophysical and cosmological processes, ranging from the Big Bang to stellar evolution, and from the cosmological constant to gravitational waves.

This Special Issue aims to recapitulate part of these achievements and offer a perspective on near- and far-future applications of quantum field theory and gravity in the present century, including nonlocal quantum gravity, general gravitational theories beyond Einstein's gravity, black holes at the classical and quantum level, cosmology, and observational tests.

Review and perspective papers are especially welcome. All papers in this special issue will be published *free of charge*.

Prof. Dr. Gianluca Calcagni
Prof. Dr. Cosimo Bambi
Prof. Dr. Leonardo Modesto
Guest Editors

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. 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

  • Perturbative quantum field theory
  • Nonperturbative quantum field theory
  • Nonlocal quantum gravity
  • Super-renormalizable quantum gravity theories
  • String theory
  • Supergravity
  • Loop quantum gravity
  • Theories beyond Einstein gravity
  • Spacetime singularities
  • Classical and quantum black holes
  • Cosmology
  • Big bang
  • Inflation
  • Cosmological constant and dark energy
  • Astroparticle physics
  • Gravitational waves

Published Papers (15 papers)

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Research

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Article
The Vacuum State of Primordial Fluctuations in Hybrid Loop Quantum Cosmology
Universe 2018, 4(10), 98; https://doi.org/10.3390/universe4100098 - 22 Sep 2018
Cited by 16 | Viewed by 1465
Abstract
We investigate the role played by the vacuum of the primordial fluctuations in hybrid Loop Quantum Cosmology. We consider scenarios where the inflaton potential is a mass term and the unperturbed quantum geometry is governed by the effective dynamics of Loop Quantum Cosmology. [...] Read more.
We investigate the role played by the vacuum of the primordial fluctuations in hybrid Loop Quantum Cosmology. We consider scenarios where the inflaton potential is a mass term and the unperturbed quantum geometry is governed by the effective dynamics of Loop Quantum Cosmology. In this situation, the phenomenologically interesting solutions have a preinflationary regime where the kinetic energy of the inflaton dominates over the potential. For these kind of solutions, we show that the primordial power spectra depend strongly on the choice of vacuum. We study in detail the case of adiabatic states of low order and the non-oscillating vacuum introduced by Martín de Blas and Olmedo, all imposed at the bounce. The adiabatic spectra are typically suppressed at large scales, and display rapid oscillations with an increase of power at intermediate scales. In the non-oscillating vacuum, there is power suppression for large scales, but the rapid oscillations are absent. We argue that the oscillations are due to the imposition of initial adiabatic conditions in the region of kinetic dominance, and that they would also be present in General Relativity. Finally, we discuss the sensitivity of our results to changes of the initial time and other data of the model. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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Article
Taming the Beast: Diffusion Method in Nonlocal Gravity
Universe 2018, 4(9), 95; https://doi.org/10.3390/universe4090095 - 12 Sep 2018
Cited by 7 | Viewed by 1337 | Correction
Abstract
We present a method to solve the nonlinear dynamical equations of motion in gravitational theories with fundamental nonlocalities of a certain type. For these specific form factors, which appear in some renormalizable theories, the number of field degrees of freedom and of initial [...] Read more.
We present a method to solve the nonlinear dynamical equations of motion in gravitational theories with fundamental nonlocalities of a certain type. For these specific form factors, which appear in some renormalizable theories, the number of field degrees of freedom and of initial conditions is finite. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
Article
Recent Progress in Fighting Ghosts in Quantum Gravity
Universe 2018, 4(9), 91; https://doi.org/10.3390/universe4090091 - 26 Aug 2018
Cited by 6 | Viewed by 1426
Abstract
We review some of the recent results which can be useful for better understanding of the problem of stability of vacuum and in general classical solutions in higher derivative quantum gravity. The fourth derivative terms in the purely gravitational vacuum sector are requested [...] Read more.
We review some of the recent results which can be useful for better understanding of the problem of stability of vacuum and in general classical solutions in higher derivative quantum gravity. The fourth derivative terms in the purely gravitational vacuum sector are requested by renormalizability already in both semiclassical and complete quantum gravity theories. However, because of these terms, the spectrum of the theory has unphysical ghost states which jeopardize the stability of classical solutions. At the quantum level, ghosts violate unitarity, and thus ghosts look incompatible with the consistency of the theory. The “dominating” or “standard” approach is to treat higher derivative terms as small perturbations at low energies. Such an effective theory is supposed to glue with an unknown fundamental theory in the high energy limit. We argue that the perspectives for such a scenario are not clear, to say the least. On the other hand, recently, there was certain progress in understanding physical conditions which can make ghosts not offensive. We survey these results and discuss the properties of the unknown fundamental theory which can provide these conditions satisfied. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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Article
A Scale at 10 MeV, Gravitational Topological Vacuum, and Large Extra Dimensions
Universe 2018, 4(7), 80; https://doi.org/10.3390/universe4070080 - 18 Jul 2018
Cited by 2 | Viewed by 1402
Abstract
We discuss a possible scale of gravitational origin at around 10 MeV, or 1012 cm, which arises in the MacDowell–Mansouri formalism of gravity due to the topological Gauss–Bonnet term in the action, as pointed out by Bjorken several years ago. A [...] Read more.
We discuss a possible scale of gravitational origin at around 10 MeV, or 1012 cm, which arises in the MacDowell–Mansouri formalism of gravity due to the topological Gauss–Bonnet term in the action, as pointed out by Bjorken several years ago. A length scale of the same size emerges also in the Kodama solution in gravity, which is known to be closely related to the MacDowell–Mansouri formulation. We particularly draw attention to the intriguing incident that the existence of six compact extra dimensions originated from TeV-scale quantum gravity as well points to a length scale of 1012 cm, as the compactification scale. The presence of six such extra dimensions is also in remarkable consistency with the MacDowell–Mansouri formalism; it provides a possible explanation for the factor of ∼10120 multiplying the Gauss–Bonnet term in the action. We also comment on the relevant implications of such a scale regarding the thermal history of the universe motivated by the fact that it is considerably close to 1–2 MeV below which the weak interactions freeze out, leading to Big Bang Nucleosynthesis. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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Article
Near-Horizon Geodesics for Astrophysical and Idealised Black Holes: Coordinate Velocity and Coordinate Acceleration
Universe 2018, 4(6), 68; https://doi.org/10.3390/universe4060068 - 28 May 2018
Cited by 1 | Viewed by 1912
Abstract
Geodesics (by definition) have an intrinsic 4-acceleration zero. However, when expressed in terms of coordinates, the coordinate acceleration d 2 x i / d t 2 can very easily be non-zero, and the coordinate velocity d x i / d t can behave [...] Read more.
Geodesics (by definition) have an intrinsic 4-acceleration zero. However, when expressed in terms of coordinates, the coordinate acceleration d 2 x i / d t 2 can very easily be non-zero, and the coordinate velocity d x i / d t can behave unexpectedly. The situation becomes extremely delicate in the near-horizon limit—for both astrophysical and idealised black holes—where an inappropriate choice of coordinates can quite easily lead to significant confusion. We shall carefully explore the relative merits of horizon-penetrating versus horizon-non-penetrating coordinates, arguing that in the near-horizon limit the coordinate acceleration d 2 x i / d t 2 is best interpreted in terms of horizon-penetrating coordinates. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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Review

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Review
Conformal Symmetry in Field Theory and in Quantum Gravity
Universe 2018, 4(11), 125; https://doi.org/10.3390/universe4110125 - 15 Nov 2018
Cited by 13 | Viewed by 1339
Abstract
Conformal symmetry always played an important role in field theory (both quantum and classical) and in gravity. We present construction of quantum conformal gravity and discuss its features regarding scattering amplitudes and quantum effective action. First, the long and complicated story of UV-divergences [...] Read more.
Conformal symmetry always played an important role in field theory (both quantum and classical) and in gravity. We present construction of quantum conformal gravity and discuss its features regarding scattering amplitudes and quantum effective action. First, the long and complicated story of UV-divergences is recalled. With the development of UV-finite higher derivative (or non-local) gravitational theory, all problems with infinities and spacetime singularities might be completely solved. Moreover, the non-local quantum conformal theory reveals itself to be ghost-free, so the unitarity of the theory should be safe. After the construction of UV-finite theory, we focused on making it manifestly conformally invariant using the dilaton trick. We also argue that in this class of theories conformal anomaly can be taken to vanish by fine-tuning the couplings. As applications of this theory, the constraints of the conformal symmetry on the form of the effective action and on the scattering amplitudes are shown. We also remark about the preservation of the unitarity bound for scattering. Finally, the old model of conformal supergravity by Fradkin and Tseytlin is briefly presented. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)

Other

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Correction
Correction: Calcagni, G. Taming the Beast: Diffusion Method in Nonlocal Gravity. Universe 2018, 4, 95
Universe 2019, 5(5), 121; https://doi.org/10.3390/universe5050121 - 21 May 2019
Cited by 3 | Viewed by 880
Abstract
The author did not realize that some of the initial conditions in [...] Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
Conference Report
Black Hole Bounces on the Road to Quantum Gravity
Universe 2018, 4(9), 92; https://doi.org/10.3390/universe4090092 - 28 Aug 2018
Cited by 5 | Viewed by 1622
Abstract
Quantum resolutions of the space-time singularity at the end of gravitational collapse may provide hints towards the properties of a final theory of Quantum-Gravity. The mechanism by which the singularity is avoided and replaced by a bounce depends on the specific behaviour of [...] Read more.
Quantum resolutions of the space-time singularity at the end of gravitational collapse may provide hints towards the properties of a final theory of Quantum-Gravity. The mechanism by which the singularity is avoided and replaced by a bounce depends on the specific behaviour of gravity in the strong field and may have implications for the geometry of the space-time also in the weak field. In the last few decades, several scenarios for black hole bounces have been proposed and I shall argue that the times are now mature to ask the question of whether such bounces can be observed in astrophysical phenomena. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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Conference Report
Dark Matter with Genuine Spin-2 Fields
Universe 2018, 4(8), 90; https://doi.org/10.3390/universe4080090 - 18 Aug 2018
Viewed by 1287
Abstract
Gravity is the only force which is telling us about the existence of Dark Matter. I will review the idea that this must be the case because Dark Matter is nothing else than a manifestation of Gravity itself, in the guise of an [...] Read more.
Gravity is the only force which is telling us about the existence of Dark Matter. I will review the idea that this must be the case because Dark Matter is nothing else than a manifestation of Gravity itself, in the guise of an additional, massive, spin-2 particle. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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Conference Report
The Case for Nonlocal Modifications of Gravity
Universe 2018, 4(8), 88; https://doi.org/10.3390/universe4080088 - 10 Aug 2018
Cited by 18 | Viewed by 1310
Abstract
The huge amounts of undetected and exotic dark matter and dark energy needed to make general relativity work on large scales argue that we should investigate modifications of gravity. The only stable, metric-based and invariant alternative to general relativity is f(R [...] Read more.
The huge amounts of undetected and exotic dark matter and dark energy needed to make general relativity work on large scales argue that we should investigate modifications of gravity. The only stable, metric-based and invariant alternative to general relativity is f(R) models. These models can explain primordial inflation, but they cannot dispense with either dark matter or dark energy. I advocate nonlocal modifications of gravity, not as new fundamental theories but rather as the gravitational vacuum polarization engendered by infrared quanta produced during primordial inflation. I also discuss some of the many objections which have been raised to this idea. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
Conference Report
Preinflationary Dynamics of Power-Law Potential in Loop Quantum Cosmology
Universe 2018, 4(8), 87; https://doi.org/10.3390/universe4080087 - 10 Aug 2018
Cited by 7 | Viewed by 1090
Abstract
In this article, I mainly discuss the dynamics of the pre-inflationary Universe for the potential V ( ϕ ) ϕ n with n = 5 / 3 in the context of loop quantum cosmology, in which the big bang singularity is resolved [...] Read more.
In this article, I mainly discuss the dynamics of the pre-inflationary Universe for the potential V ( ϕ ) ϕ n with n = 5 / 3 in the context of loop quantum cosmology, in which the big bang singularity is resolved by a non-singular quantum bounce. In the case of the kinetic energy-dominated initial conditions of the scalar field at the bounce, the numerical evolution of the Universe can be split up into three regimes: bouncing, transition, and slow-roll inflation. In the bouncing regime, the numerical evolution of the scale factor does not depend on a wide range of initial values, or on the inflationary potentials. I calculate the number of e-folds in the slow-roll regime, by which observationally identified initial conditions are obtained. Additionally, I display the phase portrait for the model under consideration. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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The Polarizations of Gravitational Waves
Universe 2018, 4(8), 85; https://doi.org/10.3390/universe4080085 - 06 Aug 2018
Cited by 28 | Viewed by 1466
Abstract
The gravitational wave provides a new method to examine General Relativity and its alternatives in the high speed, strong field regime. Alternative theories of gravity generally predict more polarizations than General Relativity, so it is important to study the polarization contents of theories [...] Read more.
The gravitational wave provides a new method to examine General Relativity and its alternatives in the high speed, strong field regime. Alternative theories of gravity generally predict more polarizations than General Relativity, so it is important to study the polarization contents of theories of gravity to reveal the nature of gravity. In this talk, we analyze the polarization contents of Horndeski theory and f(R) gravity. We find out that in addition to the familiar plus and cross polarizations, a massless Horndeski theory predicts an extra transverse polarization, and there is a mix of pure longitudinal and transverse breathing polarizations in the massive Horndeski theory and f(R) gravity. It is possible to use pulsar timing arrays to detect the extra polarizations in these theories. We also point out that the classification of polarizations using Newman–Penrose variables cannot be applied to massive modes. It cannot be used to classify polarizations in Einstein-æther theory or generalized Tensor-Vector-Scalar (TeVeS) theory, either. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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Conference Report
Gravitational Waves in Einstein-Æther Theory and Generalized TeVeS Theory after GW170817
Universe 2018, 4(8), 84; https://doi.org/10.3390/universe4080084 - 01 Aug 2018
Cited by 18 | Viewed by 1235
Abstract
In this paper , the polarization contents of Einstein-æther theory and the generalized TeVeS theory are studied. The Einstein-æther theory has five polarizations, while the generalized TeVeS theory has six. In particular, transverse and longitudinal breathing polarization are mixed. The possibility of using [...] Read more.
In this paper , the polarization contents of Einstein-æther theory and the generalized TeVeS theory are studied. The Einstein-æther theory has five polarizations, while the generalized TeVeS theory has six. In particular, transverse and longitudinal breathing polarization are mixed. The possibility of using pulsar timing arrays to detect the extra polarizations in Einstein-æther theory was also investigated. The analysis showed that different polarizations cannot be easily distinguished by using pulsar timing arrays in this theory. For generalized TeVeS theory, one of the propagating modes travels much faster than the speed of light due to the speed bound set by GW170817. In some parameter subspaces, the strong coupling does not take place, so this theory is excluded. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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Non-Locality and Late-Time Cosmic Acceleration from an Ultraviolet Complete Theory
Universe 2018, 4(8), 82; https://doi.org/10.3390/universe4080082 - 26 Jul 2018
Cited by 6 | Viewed by 1134
Abstract
A local phenomenological model that reduces to a non-local gravitational theory giving dark energy is proposed. The non-local gravity action is known to fit the data as well as Λ-CDM thereby demanding a more fundamental local treatment. It is seen that the [...] Read more.
A local phenomenological model that reduces to a non-local gravitational theory giving dark energy is proposed. The non-local gravity action is known to fit the data as well as Λ-CDM thereby demanding a more fundamental local treatment. It is seen that the scale-invariant higher-derivative scalar-tensor theory of gravity, which is known to be ultraviolet perturbative renormalizable to all loops and where ghosts become innocuous, generates non-locality at low energies. The local action comprises of two real scalar fields coupled non-minimally with the higher-derivative gravity action. When one of the scalar acquiring the Vacuum Expectation Value (VEV) induces Einstein–Hilbert gravity, generates mass for fields, and gets decoupled from system, it leaves behind a residual theory which in turn leads to a non-local gravity generating dark energy effects. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
Conference Report
relxill_nk: A Relativistic Reflection Model for Testing Einstein’s Gravity
Universe 2018, 4(7), 79; https://doi.org/10.3390/universe4070079 - 11 Jul 2018
Cited by 17 | Viewed by 1448
Abstract
Einstein’s theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in the weak field regime. However, the strong field regime is largely unexplored, and there are many modified and alternative theories that [...] Read more.
Einstein’s theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in the weak field regime. However, the strong field regime is largely unexplored, and there are many modified and alternative theories that have the same predictions as Einstein’s gravity for weak fields and present deviations when gravity becomes strong. relxill_nk is the first relativistic reflection model for probing the spacetime metric in the vicinity of astrophysical black holes and testing Einstein’s gravity in the strong field regime. Here, we present our current constraints on possible deviations from Einstein’s gravity obtained from the black holes in 1H0707–495, Ark 564, GX 339–4, and GS 1354–645. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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