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Universe, Volume 4, Issue 3 (March 2018) – 15 articles

Cover Story (view full-size image): The paper "Investigating the Poor Match among Different Precessing Gravitational Waveforms" sheds light on the differences among gravitational waveforms generated by the same black hole binary coalescence. It also identifies some of the key factors generating these differences. The figure below shows the match values ℳ for two waveforms in the restricted parameter spaces [m, ν], [m, χ1] and [ν, χ1] from left to right (the three rows representing SpinTaylorT4-SpinDominatedWf, SpinDominatedWf-SEOBNRv3, SpinTaylorT4-SEOBNRv3 matches). View this paper.
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Article
PHENIX Results of Three-Particle Bose-Einstein Correlations in \({\sqrt{s_{NN}}}\) = 200 GeV Au+Au Collisions
Universe 2018, 4(3), 57; https://doi.org/10.3390/universe4030057 - 19 Mar 2018
Cited by 2 | Viewed by 1101
Abstract
Bose-Einstein correlations (BECs) of identical hadrons reveal information about hadron creation from the strongly interacting matter formed in ultrarelativistic heavy-ion collisions. The measurement of three-particle correlations may in particular shed light on hadron creation mechanisms beyond thermal/chaotic emission. In this paper, we show [...] Read more.
Bose-Einstein correlations (BECs) of identical hadrons reveal information about hadron creation from the strongly interacting matter formed in ultrarelativistic heavy-ion collisions. The measurement of three-particle correlations may in particular shed light on hadron creation mechanisms beyond thermal/chaotic emission. In this paper, we show the status of PHENIX measurements of three-pion correlations as a function of momentum differences within the triplets. We analyze the shape of the correlation functions through the assumption of Lévy sources and a proper treatment of the Coulomb interaction within the triplets. We measure the three-particle correlation strength ( λ 3 ), which, together with the two-particle correlation strength λ 2 , encodes information about hadron creation mechanisms. From a consistent analysis of two- and three-particle correlation strengths, we establish a new experimental measure of thermalization and coherence in the source. Full article
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Article
Investigating the Poor Match among Different Precessing Gravitational Waveforms
Universe 2018, 4(3), 56; https://doi.org/10.3390/universe4030056 - 16 Mar 2018
Viewed by 1283
Abstract
The sixfold direct detection of gravitational waves opened the era of gravitational wave astronomy. All of these gravitational waves were emitted by black hole or neutron star binaries. The determination of the parameters characterizing compact binaries requires the accurate knowledge of waveforms. Three [...] Read more.
The sixfold direct detection of gravitational waves opened the era of gravitational wave astronomy. All of these gravitational waves were emitted by black hole or neutron star binaries. The determination of the parameters characterizing compact binaries requires the accurate knowledge of waveforms. Three different waveforms (Spin Dominated, SpinTaylorT4 and Spinning Effective One Body fitted to Numerical Relativity, SEOBNR) are considered in the spin-aligned and precessing cases, in the parameter ranges where the larger spin dominates over the orbital angular momentum. The degeneracy in the parameter space of each waveform is analyzed, then the matches among the waveforms are investigated. Our results show that in the spin-aligned case only the inspiral Spin-dominated and SpinTaylorT4 waveforms agree well with each other. The highest matches of these with SEOBNR are at different parameters as compared to where SEOBNR shows the best match with itself, reflecting SEOBNR being full inspiral-merger-ringdown waveform, with coefficients fitted to numerical relativity, rather than arising from post-Newtonian (PN) calculations. In the precessing case, the matches between the pairs of all waveforms are significantly lower. We identify possible causes of this in (1) the implementation of the angular dynamics carried out at different levels of accuracy for different waveforms; (2) differences in the inclusiveness of the merger process and in the PN coefficients of the inspiral waveforms (Spin-Dominated, SpinTaylorT4) and the full SEOBNR waveform. Full article
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Article
Circular Geodesics, Paczyński-Witta Potential and QNMs in the Eikonal Limit for Ayón-Beato-García Black Hole
Universe 2018, 4(3), 55; https://doi.org/10.3390/universe4030055 - 13 Mar 2018
Cited by 5 | Viewed by 1460
Abstract
We investigate the comprehensive geodesic structure of a spherically symmetric, static charged regular Ayón-Beato and García black hole (BH). We derive the equation of innermost stable circular orbit (ISCO), marginally bound circular orbit (MBCO) and circular photon orbit (CPO) of said BH, which [...] Read more.
We investigate the comprehensive geodesic structure of a spherically symmetric, static charged regular Ayón-Beato and García black hole (BH). We derive the equation of innermost stable circular orbit (ISCO), marginally bound circular orbit (MBCO) and circular photon orbit (CPO) of said BH, which are most relevant to BH accretion disk theory. Using time-like geodesic properties, we derive Paczyński-Witta potential form for this BH which are very relevant to determine the general relativistic effects on the accretion disk. We show that at a certain radius (For example in case of Reissner-Nordstrøm (RN) BH, r = Q 2 M ), there exists zero angular momentum (ZAM) orbits due to the repulsive gravity. We also show that in the eikonal approximation, the real and imaginary parts of the quasi normal modes (QNM) of the regular BHs can be expressed as in terms of the frequency of the BH and the instability time scale of the unstable null circular geodesics. Moreover, we study the Bañados, Silk and West effect for this BH. We show that the center-of-mass (CM) energy of colliding neutral test particles near the infinite red-shift surface of the regular BHs have the finite energy. In the Appendix section, we have discussed the possibility of a regular ABG BH can act as particle accelerators when two charged test particles of different energies are colliding and approaching to the horizon of the BH provided that one of charged test particle has a critical value of charge. Full article
(This article belongs to the Special Issue Black Hole Thermodynamics)
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Article
Anomalous Electromagnetic Transport in Compact Stars
Universe 2018, 4(3), 54; https://doi.org/10.3390/universe4030054 - 12 Mar 2018
Cited by 7 | Viewed by 1205
Abstract
We study the anomalous electromagnetic transport properties of a quark-matter phase that can be realized in the presence of a magnetic field in the low-temperature/moderate-high-density region of the Quantum Chromodynamics (QCD) phase map. In this so-called Magnetic Dual Chiral Density Wave phase, an [...] Read more.
We study the anomalous electromagnetic transport properties of a quark-matter phase that can be realized in the presence of a magnetic field in the low-temperature/moderate-high-density region of the Quantum Chromodynamics (QCD) phase map. In this so-called Magnetic Dual Chiral Density Wave phase, an inhomogeneous condensate is dynamically induced producing a nontrivial topology, a consequence of the asymmetry of the lowest Landau level modes of the quasiparticles in this phase. The nontrivial topology manifests in the electromagnetic effective action via a chiral anomaly term θ F μ ν F ˜ μ ν , with an axion field θ given by the phase of the Dual Chiral Density Wave condensate. The coupling of the axion with the electromagnetic field leads to several macroscopic effects that include, among others, an anomalous, nondissipative Hall current, an anomalous electric charge, magnetoelectricity, and the formation of a hybridized propagating mode known as an axion polariton. The possible existence of this phase in the inner core of neutron stars opens a window to search for signals of its anomalous transport properties. Full article
(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)
Article
Non-Radial Oscillation Modes of Superfluid Neutron Stars Modeled with CompOSE
Universe 2018, 4(3), 53; https://doi.org/10.3390/universe4030053 - 09 Mar 2018
Viewed by 1527
Abstract
We compute the principal non-radial oscillation mode frequencies of Neutron Stars described with a Skyrme-like Equation of State (EoS), taking into account the possibility of neutron and proton superfluidity. Using the CompOSE database and interpolation routines to obtain the needed thermodynamic quantities, we [...] Read more.
We compute the principal non-radial oscillation mode frequencies of Neutron Stars described with a Skyrme-like Equation of State (EoS), taking into account the possibility of neutron and proton superfluidity. Using the CompOSE database and interpolation routines to obtain the needed thermodynamic quantities, we solve the fluid oscillation equations numerically in the background of a fully relativistic star, and identify imprints of the superfluid state. Though these modes cannot be observed with current technology, increased sensitivity of future Gravitational-Wave Observatories could allow us to observe these oscillations and potentially constrain or refine models of dense matter relevant to the interior of neutron stars. Full article
(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)
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Article
Looking for the Phase Transition—Recent NA61/SHINE Results
Universe 2018, 4(3), 52; https://doi.org/10.3390/universe4030052 - 09 Mar 2018
Cited by 15 | Viewed by 1729
Abstract
The fixed-target NA61/SHINE experiment at the CERN Super Proton Synchrotron (SPS) seeks to find the critical point (CR) of strongly interacting matter as well as the properties of the onset of deconfinement. The experiment provides a scan of measurements of particle spectra and [...] Read more.
The fixed-target NA61/SHINE experiment at the CERN Super Proton Synchrotron (SPS) seeks to find the critical point (CR) of strongly interacting matter as well as the properties of the onset of deconfinement. The experiment provides a scan of measurements of particle spectra and fluctuations in proton–proton, proton–nucleus, and nucleus–nucleus interactions as functions of collision energy and system size, corresponding to a two-dimensional phase diagram (T- μ B ). New NA61/SHINE results are shown here, including transverse momentum and multiplicity fluctuations in Ar+Sc collisions as compared to NA61 p+p and Be+Be data, as well earlier NA49 A+A results. Recently, a preliminary effect of change in the system size dependence, labelled as the “percolation threshold” or the “onset of fireball”, was observed in NA61/SHINE data. This effect is closely related to the vicinity of the hadronic phase space transition region and will be discussed in the text. Full article
(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)
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Communication
Hadron–Quark Combustion as a Nonlinear, Dynamical System
Universe 2018, 4(3), 51; https://doi.org/10.3390/universe4030051 - 07 Mar 2018
Cited by 2 | Viewed by 1252
Abstract
The hadron–quark combustion front is a system that couples various processes, such as chemical reactions, hydrodynamics, diffusion, and neutrino transport. Previous numerical work has shown that this system is very nonlinear, and can be very sensitive to some of these processes. In these [...] Read more.
The hadron–quark combustion front is a system that couples various processes, such as chemical reactions, hydrodynamics, diffusion, and neutrino transport. Previous numerical work has shown that this system is very nonlinear, and can be very sensitive to some of these processes. In these proceedings, we contextualize the hadron–quark combustion as a nonlinear system, subject to dramatic feedback triggered by leptonic weak decays and neutrino transport. Full article
(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)
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Review
The Merger of Two Compact Stars: A Tool for Dense Matter Nuclear Physics
Universe 2018, 4(3), 50; https://doi.org/10.3390/universe4030050 - 07 Mar 2018
Cited by 11 | Viewed by 1542
Abstract
We discuss the different signals, in gravitational and electromagnetic waves, emitted during the merger of two compact stars. We will focus in particular on the possible contraints that those signals can provide on the equation of state of dense matter. Indeed, the stiffness [...] Read more.
We discuss the different signals, in gravitational and electromagnetic waves, emitted during the merger of two compact stars. We will focus in particular on the possible contraints that those signals can provide on the equation of state of dense matter. Indeed, the stiffness of the equation of state and the particle composition of the merging compact stars strongly affect, e.g., the life time of the post-merger remnant and its gravitational wave signal, the emission of the short gamma-ray-burst, the amount of ejected mass and the related kilonova. The first detection of gravitational waves from the merger of two compact stars in August 2017, GW170817, and the subsequent detections of its electromagnetic counterparts, GRB170817A and AT2017gfo, is the first example of the era of “multi-messenger astronomy”: we discuss what we have learned from this detection on the equation of state of compact stars and we provide a tentative interpretation of this event, within the two families scenario, as being due to the merger of a hadronic star with a quark star. Full article
(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)
Article
On Maximal Homogeneous 3-Geometries—A Polyhedron Algorithm for Space Tilings
Universe 2018, 4(3), 49; https://doi.org/10.3390/universe4030049 - 06 Mar 2018
Cited by 2 | Viewed by 1474
Abstract
In this paper we introduce a polyhedron algorithm that has been developed for finding space groups. In order to demonstrate the problem and the main steps of the algorithm, we consider some regular plane tilings in our examples, and then we generalize the [...] Read more.
In this paper we introduce a polyhedron algorithm that has been developed for finding space groups. In order to demonstrate the problem and the main steps of the algorithm, we consider some regular plane tilings in our examples, and then we generalize the method for 3-dimensional spaces. Applying the algorithm and its computer implementation we investigate periodic, face-to-face, regular polyhedron tilings in 3-spaces of constant curvature and of the other homogeneous 3-geometries, too. We summarize and visualize the most important results, emphasizing the fixed-point-free space groups which determine 3-dimensional manifolds. Full article
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Article
Rotating Quark Stars in General Relativity
Universe 2018, 4(3), 48; https://doi.org/10.3390/universe4030048 - 05 Mar 2018
Cited by 2 | Viewed by 1479
Abstract
We have built quasi-equilibrium models for uniformly rotating quark stars in general relativity. The conformal flatness approximation is employed and the Compact Object CALculator (cocal) code is extended to treat rotating stars with surface density discontinuity. In addition to the widely [...] Read more.
We have built quasi-equilibrium models for uniformly rotating quark stars in general relativity. The conformal flatness approximation is employed and the Compact Object CALculator (cocal) code is extended to treat rotating stars with surface density discontinuity. In addition to the widely used MIT bag model, we have considered a strangeon star equation of state (EoS), suggested by Lai and Xu, that is based on quark clustering and results in a stiff EoS. We have investigated the maximum mass of uniformly rotating axisymmetric quark stars. We have also built triaxially deformed solutions for extremely fast rotating quark stars and studied the possible gravitational wave emission from such configurations. Full article
(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)
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Article
Asymptotic Charges at Null Infinity in Any Dimension
Universe 2018, 4(3), 47; https://doi.org/10.3390/universe4030047 - 02 Mar 2018
Cited by 25 | Viewed by 1393
Abstract
We analyse the conservation laws associated with large gauge transformations of massless fields in Minkowski space. Our aim is to highlight the interplay between boundary conditions and finiteness of the asymptotically conserved charges in any space-time dimension, both even and odd, greater than [...] Read more.
We analyse the conservation laws associated with large gauge transformations of massless fields in Minkowski space. Our aim is to highlight the interplay between boundary conditions and finiteness of the asymptotically conserved charges in any space-time dimension, both even and odd, greater than or equal to three. After discussing nonlinear Yang–Mills theory and revisiting linearised gravity, our investigation extends to cover the infrared behaviour of bosonic massless quanta of any spin. Full article
(This article belongs to the Special Issue Higher Spin Gauge Theories)
Article
Super Bundles
Universe 2018, 4(3), 46; https://doi.org/10.3390/universe4030046 - 01 Mar 2018
Cited by 6 | Viewed by 1217
Abstract
In this paper we give a brief account of the main aspects of the theory of associated and principal super bundles. As an application, we review the Borel-Weil-Bott Theorem in the super setting, and some results on projective embeddings of homogeneous spaces. Full article
(This article belongs to the Special Issue Super Geometry for Super Strings)
Conference Report
Equation of State for Dense Matter with a QCD Phase Transition
Universe 2018, 4(3), 45; https://doi.org/10.3390/universe4030045 - 01 Mar 2018
Viewed by 1518
Abstract
We construct a dense matter equation of state (EoS) starting from a hadronic density dependent relativistic mean-field model with a DD2 parametrization including the excluded volume corrections at low densities. The high density part is given by a Nambu–Jona–Lasinio (NJL) model with multi-quark [...] Read more.
We construct a dense matter equation of state (EoS) starting from a hadronic density dependent relativistic mean-field model with a DD2 parametrization including the excluded volume corrections at low densities. The high density part is given by a Nambu–Jona–Lasinio (NJL) model with multi-quark interactions. This EoS is characterized by increasing speed of sound below and above the phase transition region. The first order transition region has a large latent heat leaving a distinctive signature in the mass-radii relations in terms of twin stars. Full article
(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)
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Article
Light-Like Shockwaves in Scalar-Tensor Theories
Universe 2018, 4(3), 44; https://doi.org/10.3390/universe4030044 - 27 Feb 2018
Cited by 2 | Viewed by 1066
Abstract
Both electromagnetic shock-waves and gravitational waves propagate with the speed of light. If they carry significant energy-momentum, this will change the properties of the space-time they propagate through. This can be described in terms of the junction conditions between space-time regions separated by [...] Read more.
Both electromagnetic shock-waves and gravitational waves propagate with the speed of light. If they carry significant energy-momentum, this will change the properties of the space-time they propagate through. This can be described in terms of the junction conditions between space-time regions separated by a singular, null hypersurface. We derived generic junction conditions for Brans-Dicke theory in the Jordan frame, exploring a formalism based on a transverse vector, rather than normal, which can be applied to any type of hypersurfaces. In the particular case of a non-null hypersurface we obtain a generalised Lanczos equation, in which the jump of the extrinsic curvature is sourced by both the distributional energy-momentum tensor and by the jump in the transverse derivative of the scalar. In the case of null hypersurfaces, the distributional source is decomposed into surface density, current and pressure. The latter, however, ought to vanish by virtue of the scalar junction condition. Full article
Article
Many Aspects of Magnetic Fields in Neutron Stars
Universe 2018, 4(3), 43; https://doi.org/10.3390/universe4030043 - 26 Feb 2018
Cited by 7 | Viewed by 1904
Abstract
In this work, we explore different aspects in which strong magnetic fields play a role in the composition, structure and evolution of neutron stars. More specifically, we discuss (i) how strong magnetic fields change the equation of state of dense matter, alter its [...] Read more.
In this work, we explore different aspects in which strong magnetic fields play a role in the composition, structure and evolution of neutron stars. More specifically, we discuss (i) how strong magnetic fields change the equation of state of dense matter, alter its composition, and create anisotropies, (ii) how they change the structure of neutron stars (such mass and radius) and the formalism necessary to calculate those changes, and (iii) how they can affect neutron stars’ evolution. In particular, we focus on how a time-dependent magnetic field modifies the cooling of a special group known as X-ray dim neutron stars. Full article
(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)
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