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Universe, Volume 3, Issue 3 (September 2017)

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Cover Story Comparison of lensing by point mass in vacuum (top) and in plasma (bottom left and bottom right). [...] Read more.
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Research

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Open AccessArticle Etherington’s Distance Duality with Birefringence
Universe 2017, 3(3), 52; doi:10.3390/universe3030052
Received: 18 May 2017 / Revised: 30 June 2017 / Accepted: 3 July 2017 / Published: 6 July 2017
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Abstract
We consider light propagation in a spacetime whose kinematics allow weak birefringence, and whose dynamics have recently been derived by gravitational closure. Revisiting the definitions of luminosity and angular diameter distances in this setting, we present a modification of the Etherington distance duality
[...] Read more.
We consider light propagation in a spacetime whose kinematics allow weak birefringence, and whose dynamics have recently been derived by gravitational closure. Revisiting the definitions of luminosity and angular diameter distances in this setting, we present a modification of the Etherington distance duality relation in a weak gravitational field around a point mass. This provides the first concrete example of how the non-metricities implied by gravitational closure of birefringent electrodynamics affect observationally testable relations. Full article
(This article belongs to the Special Issue Gravitational Lensing and Astrometry)
Open AccessArticle Non-Linear Stationary Solutions in Realistic Models for Analog Black-Hole Lasers
Universe 2017, 3(3), 54; doi:10.3390/universe3030054
Received: 9 March 2017 / Revised: 25 June 2017 / Accepted: 4 July 2017 / Published: 10 July 2017
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Abstract
From both a theoretical and an experimental point of view, Bose–Einstein condensates are good candidates for studying gravitational analogues of black holes and black-hole lasers. In particular, a recent experiment has shown that a black-hole laser configuration can be created in the laboratory.
[...] Read more.
From both a theoretical and an experimental point of view, Bose–Einstein condensates are good candidates for studying gravitational analogues of black holes and black-hole lasers. In particular, a recent experiment has shown that a black-hole laser configuration can be created in the laboratory. However, the most considered theoretical models for analog black-hole lasers are quite difficult to implement experimentally. In order to fill this gap, we devote this work to present more realistic models for black-hole lasers. For that purpose, we first prove that, by symmetrically extending every black-hole configuration, one can obtain a black-hole laser configuration with an arbitrarily large supersonic region. Based on this result, we propose the use of an attractive square well and a double delta-barrier, which can be implemented using standard experimental tools, for studying black-hole lasers. We also compute the different stationary states of these setups, identifying the true ground state of the system and discussing the relation between the obtained solutions and the appearance of dynamical instabilities. Full article
(This article belongs to the collection Open Questions in Black Hole Physics)
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Open AccessArticle On the Near-Horizon Canonical Quantum Microstates from AdS2/CFT1 and Conformal Weyl Gravity
Universe 2017, 3(3), 56; doi:10.3390/universe3030056
Received: 22 March 2017 / Revised: 18 June 2017 / Accepted: 13 July 2017 / Published: 17 July 2017
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Abstract
We compute the full asymptotic symmetry group of black holes belonging to the same equivalence class of solutions within the conformal Weyl gravity formalism. We do this within an AdS2/CFT1 correspondence and by performing a
[...] Read more.
We compute the full asymptotic symmetry group of black holes belonging to the same equivalence class of solutions within the conformal Weyl gravity formalism. We do this within an A d S 2 / C F T 1 correspondence and by performing a Robinson–Wilczek two-dimensional reduction, thus enabling the construction of effective quantum theory of the remaining field content. The resulting energy momentum tensors generate asymptotic Virasoro algebras to s-waves, with calculable central extensions. These centers, in conjunction with their proper regularized lowest Virasoro eigenmodes, yield the Bekenstein–Hawking black hole entropy via the statistical Cardy formula. We also analyze quantum holomorphic fluxes of the dual conformal field theories (CFTs) in the near horizon, giving rise to finite Hawking temperatures weighted by the central charges of the respective black hole spacetimes. We conclude with a discussion and outlook for future work. Full article
(This article belongs to the collection Open Questions in Black Hole Physics)
Open AccessArticle Gravitational Lensing in Presence of Plasma: Strong Lens Systems, Black Hole Lensing and Shadow
Universe 2017, 3(3), 57; doi:10.3390/universe3030057
Received: 26 April 2017 / Revised: 5 July 2017 / Accepted: 13 July 2017 / Published: 17 July 2017
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Abstract
In this article, we present an overview of the new developments in problems of the plasma influence on the effects of gravitational lensing, complemented by pieces of new material and relevant discussions. Deflection of light in the presence of gravity and plasma is
[...] Read more.
In this article, we present an overview of the new developments in problems of the plasma influence on the effects of gravitational lensing, complemented by pieces of new material and relevant discussions. Deflection of light in the presence of gravity and plasma is determined by a complex combination of various physical phenomena: gravity, dispersion, refraction. In particular, the gravitational deflection itself, in a homogeneous plasma without refraction, differs from the vacuum one and depends on the frequency of the photon. In an inhomogeneous plasma, chromatic refraction also takes place. We describe chromatic effects in strong lens systems including a shift of angular position of image and a change in magnification. We also investigate high-order images that arise when lensing on a black hole surrounded by homogeneous plasma. The recent results of analytical studies of the effect of plasma on the shadow of the Schwarzschild and Kerr black holes are presented. Full article
(This article belongs to the Special Issue Gravitational Lensing and Astrometry)
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Open AccessArticle Entropy Budget for Hawking Evaporation
Universe 2017, 3(3), 58; doi:10.3390/universe3030058
Received: 30 January 2017 / Revised: 17 July 2017 / Accepted: 18 July 2017 / Published: 22 July 2017
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Abstract
Blackbody radiation, emitted from a furnace and described by a Planck spectrum, contains (on average) an entropy of 3.9±2.5 bits per photon. Since normal physical burning is a unitary process, this amount of entropy is compensated by
[...] Read more.
Blackbody radiation, emitted from a furnace and described by a Planck spectrum, contains (on average) an entropy of 3 . 9 ± 2 . 5 bits per photon. Since normal physical burning is a unitary process, this amount of entropy is compensated by the same amount of “hidden information” in correlations between the photons. The importance of this result lies in the posterior extension of this argument to the Hawking radiation from black holes, demonstrating that the assumption of unitarity leads to a perfectly reasonable entropy/information budget for the evaporation process. In order to carry out this calculation, we adopt a variant of the “average subsystem” approach, but consider a tripartite pure system that includes the influence of the rest of the universe, and which allows “young” black holes to still have a non-zero entropy; which we identify with the standard Bekenstein entropy. Full article
(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)
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Open AccessArticle The First Detection of Gravitational Waves
Universe 2017, 3(3), 59; doi:10.3390/universe3030059
Received: 15 June 2017 / Revised: 23 July 2017 / Accepted: 25 July 2017 / Published: 31 July 2017
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Abstract
This article deals with the first detection of gravitational waves by the advanced Laser Interferometer Gravitational Wave Observatory (LIGO) detectors on 14 September 2015, where the signal was generated by two stellar mass black holes with masses 36 M and 29 M
[...] Read more.
This article deals with the first detection of gravitational waves by the advanced Laser Interferometer Gravitational Wave Observatory (LIGO) detectors on 14 September 2015, where the signal was generated by two stellar mass black holes with masses 36 M and 29 M that merged to form a 62 M black hole, releasing 3 M energy in gravitational waves, almost 1.3 billion years ago. We begin by providing a brief overview of gravitational waves, their sources and the gravitational wave detectors. We then describe in detail the first detection of gravitational waves from a binary black hole merger. We then comment on the electromagnetic follow up of the detection event with various telescopes. Finally, we conclude with the discussion on the tests of gravity and fundamental physics with the first gravitational wave detection event. Full article
(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)
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Open AccessArticle Do We Live in the Best of All Possible Worlds? The Fine-Tuning of the Constants of Nature
Universe 2017, 3(3), 60; doi:10.3390/universe3030060
Received: 14 June 2017 / Revised: 20 July 2017 / Accepted: 26 July 2017 / Published: 1 August 2017
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Abstract
Our existence depends on a variety of constants which appear to be extremely fine-tuned to allow for the existence of life as we know it. These include the number of spatial dimensions, the strengths of the forces, the masses of the particles, the
[...] Read more.
Our existence depends on a variety of constants which appear to be extremely fine-tuned to allow for the existence of life as we know it. These include the number of spatial dimensions, the strengths of the forces, the masses of the particles, the composition of the Universe, and others. On the occasion of the 300th anniversary of the death of G.W. Leibniz, we discuss the question of whether we live in the “Best of all possible Worlds”. The hypothesis of a multiverse could explain the mysterious fine tuning of so many fundamental quantities. Anthropic arguments are critically reviewed. Full article
(This article belongs to the Special Issue Varying Constants and Fundamental Cosmology)
Open AccessArticle AdS/CFT in Fractional Dimension and Higher-Spins at One Loop
Universe 2017, 3(3), 61; doi:10.3390/universe3030061
Received: 6 July 2017 / Revised: 31 July 2017 / Accepted: 3 August 2017 / Published: 14 August 2017
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Abstract
Large-N, ϵ-expansion or the conformal bootstrap allow one to make sense of some of conformal field theories in non-integer dimension, which suggests that AdS/CFT may also extend to fractional dimensions. It was shown recently that the sphere free energy and
[...] Read more.
Large-N, ϵ -expansion or the conformal bootstrap allow one to make sense of some of conformal field theories in non-integer dimension, which suggests that AdS/CFT may also extend to fractional dimensions. It was shown recently that the sphere free energy and the a-anomaly coefficient of the free scalar field can be reproduced as a one-loop effect in the dual higher-spin theory in a number of integer dimensions. We extend this result to all integer and also to fractional dimensions. Upon changing the boundary conditions in the higher-spin theory the sphere free energy of the large-N Wilson-Fisher CFT can also be reproduced from the higher-spin side. Full article
(This article belongs to the Special Issue Higher Spin Gauge Theories)
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Open AccessArticle Fractal Structure of Hadrons: Experimental and Theoretical Signatures
Universe 2017, 3(3), 62; doi:10.3390/universe3030062
Received: 28 June 2017 / Revised: 7 August 2017 / Accepted: 14 August 2017 / Published: 26 August 2017
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Abstract
One important ingredient in the study of cosmological evolution is the equation of state of the primordial matter formed in the first stages of the Universe. It is believed that the first matter produced was of hadronic nature, probably the quark–gluon plasma which
[...] Read more.
One important ingredient in the study of cosmological evolution is the equation of state of the primordial matter formed in the first stages of the Universe. It is believed that the first matter produced was of hadronic nature, probably the quark–gluon plasma which has been studied in high-energy collisions. There are several experimental indications of self-similarity in hadronic systems—in particular in multiparticle production at high energies. Theoretically, this property was associated with the dynamics of particle production, but it is also possible to relate self-similarity to the hadron structure—in particular to a fractal structure of this system. In doing so, it is found that the thermodynamics of hadron systems at equilibrium must present specific properties that are indeed supported by data. In particular, the well-known self-consistence principle proposed by Hagedorn 50 years ago is shown to be valid, and can correctly describe experimental distributions, mass spectrum of observed particles, and other properties of the hadronic matter. In the present work, a review of the theoretical developments related to the thermodynamical properties of hadronic matter and its applications in other fields is presented. Full article
Open AccessArticle Infinite Spin Fields in d = 3 and Beyond
Universe 2017, 3(3), 63; doi:10.3390/universe3030063
Received: 27 July 2017 / Revised: 17 August 2017 / Accepted: 18 August 2017 / Published: 30 August 2017
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Abstract
In this paper, we consider the frame-like formulation for the so-called infinite (continuous) spin representations of the Poincare algebra. In the three-dimensional case, we give explicit Lagrangian formulation for bosonic and fermionic infinite spin fields (including the complete sets of the gauge-invariant objects
[...] Read more.
In this paper, we consider the frame-like formulation for the so-called infinite (continuous) spin representations of the Poincare algebra. In the three-dimensional case, we give explicit Lagrangian formulation for bosonic and fermionic infinite spin fields (including the complete sets of the gauge-invariant objects and all the necessary extra fields). Moreover, we find the supertransformations for the supermultiplet containing one bosonic and one fermionic field, leaving the sum of their Lagrangians invariant. Properties of such fields and supermultiplets in four and higher dimensions are also briefly discussed. Full article
(This article belongs to the Special Issue Higher Spin Gauge Theories)
Open AccessArticle Induced Action for Conformal Higher Spins from Worldline Path Integrals
Universe 2017, 3(3), 64; doi:10.3390/universe3030064
Received: 7 August 2017 / Revised: 28 August 2017 / Accepted: 29 August 2017 / Published: 4 September 2017
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Abstract
Conformal higher spin (CHS) fields, yet being non unitary, provide a remarkable example of a consistent interacting higher spin theory in flat space background, that is local to all orders. The non-linear action is defined as the logarithmically UV divergent part of a
[...] Read more.
Conformal higher spin (CHS) fields, yet being non unitary, provide a remarkable example of a consistent interacting higher spin theory in flat space background, that is local to all orders. The non-linear action is defined as the logarithmically UV divergent part of a one-loop scalar effective action. In this paper we take a particle model, that describes the interaction of a scalar particle to the CHS background, and compute its path integral on the circle. We thus provide a worldline representation for the CHS action, and rederive its quadratic part. We plan to come back to the subject, to compute cubic and higher vertices, in a future work. Full article
(This article belongs to the Special Issue Higher Spin Gauge Theories)
Open AccessArticle Extra-Dimensional “Metamaterials”: A Model of Inflation Due to a Metric Signature Transition
Universe 2017, 3(3), 66; doi:10.3390/universe3030066
Received: 2 June 2017 / Revised: 13 September 2017 / Accepted: 18 September 2017 / Published: 20 September 2017
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Abstract
Lattices of topological defects, such as Abrikosov lattices and domain wall lattices, often arise as metastable ground states in higher-dimensional field theoretical models. We demonstrate that such lattice states may be described as extra-dimensional “metamaterials” via higher-dimensional effective medium theory. A 4 +
[...] Read more.
Lattices of topological defects, such as Abrikosov lattices and domain wall lattices, often arise as metastable ground states in higher-dimensional field theoretical models. We demonstrate that such lattice states may be described as extra-dimensional “metamaterials” via higher-dimensional effective medium theory. A 4 + 1 dimensional extension of Maxwell electrodynamics with a compactified time-like dimension is considered as an example. It is demonstrated that from the point of view of macroscopic electrodynamics an Abrikosov lattice state in such a 4 + 1 dimensional spacetime may be described as a uniaxial hyperbolic medium. Extraordinary photons perceive this medium as a 3 + 1 dimensional Minkowski spacetime in which one of the original spatial dimensions plays the role of a new time-like coordinate. Since the metric signature of this effective spacetime depends on the Abrikosov lattice periodicity, the described model may be useful in studying metric signature transitions. Full article
(This article belongs to the Special Issue Inflationary Universe Models: Predictions and Observations)
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Review

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Open AccessReview Microlensing and Its Degeneracy Breakers: Parallax, Finite Source, High-Resolution Imaging, and Astrometry
Universe 2017, 3(3), 53; doi:10.3390/universe3030053
Received: 28 April 2017 / Revised: 20 June 2017 / Accepted: 4 July 2017 / Published: 7 July 2017
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Abstract
First proposed by Paczynski in 1986, microlensing has been instrumental in the search for compact dark matter as well as discovery and characterization of exoplanets. In this article, we provide a brief history of microlensing, especially on the discoveries of compact objects and
[...] Read more.
First proposed by Paczynski in 1986, microlensing has been instrumental in the search for compact dark matter as well as discovery and characterization of exoplanets. In this article, we provide a brief history of microlensing, especially on the discoveries of compact objects and exoplanets. We then review the basics of microlensing and how astrometry can help break the degeneracy, providing a more robust determination of the nature of the microlensing events. We also outline prospects that will be made by on-going and forth-coming experiments/observatories. Full article
(This article belongs to the Special Issue Gravitational Lensing and Astrometry)
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Open AccessReview Black Holes: Eliminating Information or Illuminating New Physics?
Universe 2017, 3(3), 55; doi:10.3390/universe3030055
Received: 6 March 2017 / Revised: 3 July 2017 / Accepted: 7 July 2017 / Published: 13 July 2017
Cited by 2 | PDF Full-text (700 KB) | HTML Full-text | XML Full-text
Abstract
Black holes, initially thought of as very interesting mathematical and geometric solutions of general relativity, over time, have come up with surprises and challenges for modern physics. In modern times, they have started to test our confidence in the fundamental understanding of nature.
[...] Read more.
Black holes, initially thought of as very interesting mathematical and geometric solutions of general relativity, over time, have come up with surprises and challenges for modern physics. In modern times, they have started to test our confidence in the fundamental understanding of nature. The most serious charge on the black holes is that they eat up information, never to release and subsequently erase it. This goes absolutely against the sacred principles of all other branches of fundamental sciences. This realization has shaken the very base of foundational concepts, both in quantum theory and gravity, which we always took for granted. Attempts to get rid of of this charge, have led us to crossroads with concepts, hold dearly in quantum theory. The sphere of black hole’s tussle with quantum theory has readily and steadily grown, from the advent of the Hawking radiation some four decades back, into domain of quantum information theory in modern times, most aptly, recently put in the form of the firewall puzzle. Do black holes really indicate something sinister about their existence or do they really point towards the troubles of ignoring the fundamental issues, our modern theories are seemingly plagued with? In this review, we focus on issues pertaining to black hole evaporation, the development of the information loss paradox, its recent formulation, the leading debates and promising directions in the community. Full article
(This article belongs to the collection Open Questions in Black Hole Physics)
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Other

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Open AccessComment Oscillating Stars and the Evidence of Dark Matter. A Comment on “Can the Periodic Spectral Modulations Observed in 236 Sloan Sky Survey Stars Be Due To Dark Matter Effects?” by F. Tamburini and I. Licata
Universe 2017, 3(3), 65; doi:10.3390/universe3030065
Received: 1 August 2017 / Revised: 12 September 2017 / Accepted: 12 September 2017 / Published: 13 September 2017
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Abstract
Dark matter is probably the most fascinating enigma of modern physics [...]
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