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Universe, Volume 4, Issue 8 (August 2018) – 9 articles

Cover Story (view full-size image): The detection of gravitational waves (GWs) by LIGO/Virgo collaboration provides a new probe into the nature of gravity. In General Relativity, GWs have two polarizations. In a generic metric theory of gravity, there are up to six polarizations. Therefore, the measurement of the polarizations is the best way to probe the nature of gravity. We analyzed the polarizations of the Horndeski theory and f(R) gravity. We found out that, besides the plus and cross polarizations, the massless Horndeski theory predicts the breathing polarization, and the longitudinal polarization always accompanies the breathing one as a mixed single state in the massive Horndeski theory and f(R) gravity. The extra polarizations may be detected by pulsar timing arrays. It is also shown that the classification of polarizations using Newman-Penrose variables cannot be applied to massive modes or the local Lorentz violating theories. [...] Read more.
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6 pages, 270 KiB  
Conference Report
Dark Matter with Genuine Spin-2 Fields
by Federico R. Urban
Universe 2018, 4(8), 90; https://doi.org/10.3390/universe4080090 - 18 Aug 2018
Viewed by 2463
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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7 pages, 4318 KiB  
Conference Report
Using Trajectories in Quantum Cosmology
by Patrick Peter
Universe 2018, 4(8), 89; https://doi.org/10.3390/universe4080089 - 15 Aug 2018
Cited by 3 | Viewed by 2960
Abstract
Quantum cosmology based on the Wheeler De Witt equation represents a simple way to implement plausible quantum effects in a gravitational setup. In its minisuperspace version wherein one restricts attention to FLRW metrics with a single scale factor and only a few degrees [...] Read more.
Quantum cosmology based on the Wheeler De Witt equation represents a simple way to implement plausible quantum effects in a gravitational setup. In its minisuperspace version wherein one restricts attention to FLRW metrics with a single scale factor and only a few degrees of freedom describing matter, one can obtain exact solutions and thus acquire full knowledge of the wave function. Although this is the usual way to treat a quantum mechanical system, it turns out however to be essentially meaningless in a cosmological framework. Turning to a trajectory approach then provides an effective means of deriving physical consequences. Full article
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18 pages, 326 KiB  
Conference Report
The Case for Nonlocal Modifications of Gravity
by Richard P. Woodard
Universe 2018, 4(8), 88; https://doi.org/10.3390/universe4080088 - 10 Aug 2018
Cited by 23 | Viewed by 2483
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)
8 pages, 757 KiB  
Conference Report
Preinflationary Dynamics of Power-Law Potential in Loop Quantum Cosmology
by M. Shahalam
Universe 2018, 4(8), 87; https://doi.org/10.3390/universe4080087 - 10 Aug 2018
Cited by 9 | Viewed by 2364
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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12 pages, 8553 KiB  
Review
Seeing Black Holes: From the Computer to the Telescope
by Jean-Pierre Luminet
Universe 2018, 4(8), 86; https://doi.org/10.3390/universe4080086 - 09 Aug 2018
Cited by 8 | Viewed by 8968
Abstract
Astronomical observations are about to deliver the very first telescopic image of the massive black hole lurking at the Galactic Center. The mass of data collected in one night by the Event Horizon Telescope network, exceeding everything that has ever been done in [...] Read more.
Astronomical observations are about to deliver the very first telescopic image of the massive black hole lurking at the Galactic Center. The mass of data collected in one night by the Event Horizon Telescope network, exceeding everything that has ever been done in any scientific field, should provide a recomposed image in 2018. All this, forty years after the first numerical simulations performed by the present author. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2018 - Gravitational Physics)
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14 pages, 395 KiB  
Conference Report
The Polarizations of Gravitational Waves
by Yungui Gong and Shaoqi Hou
Universe 2018, 4(8), 85; https://doi.org/10.3390/universe4080085 - 06 Aug 2018
Cited by 41 | Viewed by 3196
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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6 pages, 377 KiB  
Conference Report
Gravitational Waves in Einstein-Æther Theory and Generalized TeVeS Theory after GW170817
by Shaoqi Hou and Yungui Gong
Universe 2018, 4(8), 84; https://doi.org/10.3390/universe4080084 - 01 Aug 2018
Cited by 24 | Viewed by 2774
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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19 pages, 501 KiB  
Article
A Generalized Solution of Bianchi Type-V Models with Time-Dependent G and Λ
by Alnadhief H. A. Alfedeel, Amare Abebe and Hussam M. Gubara
Universe 2018, 4(8), 83; https://doi.org/10.3390/universe4080083 - 27 Jul 2018
Cited by 9 | Viewed by 2447
Abstract
We study the homogeneous but anisotropic Bianchi type-V cosmological model with time-dependent gravitational and cosmological “constants”. Exact solutions of the Einstein field equations (EFEs) are presented in terms of adjustable parameters of quantum field theory in a spatially curved and expanding background. [...] Read more.
We study the homogeneous but anisotropic Bianchi type-V cosmological model with time-dependent gravitational and cosmological “constants”. Exact solutions of the Einstein field equations (EFEs) are presented in terms of adjustable parameters of quantum field theory in a spatially curved and expanding background. It has been found that the general solution of the average scale factor a as a function of time involved the hypergeometric function. Two cosmological models are obtained from the general solution of the hypergeometric function and the Emden–Fowler equation. The analysis of the models shows that, for a particular choice of parameters in our first model, the cosmological “constant” decreases whereas the Newtonian gravitational “constant” increases with time, and for another choice of parameters, the opposite behaviour is observed. The models become isotropic at late times for all parameter choices of the first model. In the second model of the general solution, both the cosmological and gravitational “constants” decrease while the model becomes more anisotropic over time. The exact dynamical and kinematical quantities have been calculated analytically for each model. Full article
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8 pages, 232 KiB  
Conference Report
Non-Locality and Late-Time Cosmic Acceleration from an Ultraviolet Complete Theory
by Gaurav Narain and Tianjun Li
Universe 2018, 4(8), 82; https://doi.org/10.3390/universe4080082 - 26 Jul 2018
Cited by 9 | Viewed by 2496
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)
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