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Universe, Volume 7, Issue 6 (June 2021) – 45 articles

Cover Story (view full-size image): Thousands of exoplanets are known to orbit nearby stars, and the search for habitable worlds continues in earnest. We present the possibility for sub-Neptune-sized exoplanets of the right temperature to host life in the atmosphere. Sub-Neptunes are much more favorable for discovery and atmosphere study than rocky exoplanets; yet, sub-Neptunes have deep atmospheres that extend far into the interior, such that passive microbial-like life particles must persist aloft in a region with liquid water clouds for long enough to metabolize and reproduce before downward transport to lower altitudes that may be too hot for life of any kind to survive. Meteoritic material could provide nutrients, though life would also need to reuse and recycle metals. Regardless of the uncertainties, the search for biosignature gases can be a part of general observational studies of sub-Neptune exoplanets. View [...] Read more.
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Article
Transport Coefficients of Hyperonic Neutron Star Cores
Universe 2021, 7(6), 203; https://doi.org/10.3390/universe7060203 - 20 Jun 2021
Viewed by 283
Abstract
We consider transport properties of the hypernuclear matter in neutron star cores. In particular, we calculate the thermal conductivity, the shear viscosity, and the momentum transfer rates for npΣΛeμ composition of dense matter in β–equilibrium for baryon number densities in the range 0.1–1 fm3. The calculations are based on baryon interactions treated within the framework of the non-relativistic Brueckner-Hartree-Fock theory. Bare nucleon-nucleon (NN) interactions are described by the Argonne v18 phenomenological potential supplemented with the Urbana IX three-nucleon force. Nucleon-hyperon (NY) and hyperon-hyperon (YY) interactions are based on the NSC97e and NSC97a models of the Nijmegen group. We find that the baryon contribution to transport coefficients is dominated by the neutron one as in the case of neutron star cores containing only nucleons. In particular, we find that neutrons dominate the total thermal conductivity over the whole range of densities explored and that, due to the onset of Σ which leads to the deleptonization of the neutron star core, they dominate also the shear viscosity in the high density region, in contrast with the pure nucleonic case where the lepton contribution is always the dominant one. Full article
(This article belongs to the Special Issue Fundamental Processes in Neutron Stars and Supernovae)
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Communication
Determination of the Magnetic Field Strength and Geometry in the Accretion Disks of AGNs by Optical Spectropolarimetry
Universe 2021, 7(6), 202; https://doi.org/10.3390/universe7060202 - 18 Jun 2021
Viewed by 307
Abstract
Based on the spectropolarimetric data of 33 Seyfert type 1 galaxies observed with the BTA-6m telescope of the Special Astrophysical Observatory, we estimated the magnetic field values at the event horizon of the supermassive black hole BH and the exponents of the power-law dependence s of the magnetic field on the radius. We used the model of optically thick geometrically thin Shakura–Sunyaev accretion disk. The average value of logBH[G] was found to be ∼4, which is in good agreement with the results obtained by other methods. The average value of s is s1.7, and its distribution maximum span is in the range od 1.85<s<2.0. This is a rather interesting result, since s=5/4 is usually adopted in calculations for Shakura–Sunyaev accretion disks. In addition, for two objects PG 1545+210 and 2MASX J06021107+2828382, the measured degree of polarization is greater than the maximum possible value at the angle between the line of sight and the axis of the accretion disk i=45°. It was concluded that for these objects the angle should be closer to i=60°. Full article
(This article belongs to the Special Issue Advances in the Physics of Stars - in Memory of Prof. Yuri N. Gnedin)
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Review
Superinsulators: An Emergent Realisation of Confinement
Universe 2021, 7(6), 201; https://doi.org/10.3390/universe7060201 - 17 Jun 2021
Viewed by 315
Abstract
Superinsulators (SI) are a new topological state of matter, predicted by our collaboration and experimentally observed in the critical vicinity of the superconductor-insulator transition (SIT). SI are dual to superconductors and realise electric-magnetic (S)-duality. The effective field theory that describes this topological phase [...] Read more.
Superinsulators (SI) are a new topological state of matter, predicted by our collaboration and experimentally observed in the critical vicinity of the superconductor-insulator transition (SIT). SI are dual to superconductors and realise electric-magnetic (S)-duality. The effective field theory that describes this topological phase of matter is governed by a compact Chern-Simons in (2+1) dimensions and a compact BF term in (3+1) dimensions. While in a superconductor the condensate of Cooper pairs generates the Meissner effect, which constricts the magnetic field lines penetrating a type II superconductor into Abrikosov vortices, in superinsulators Cooper pairs are linearly bound by electric fields squeezed into strings (dual Meissner effect) by a monopole condensate. Magnetic monopoles, while elusive as elementary particles, exist in certain materials in the form of emergent quasiparticle excitations. We demonstrate that at low temperatures magnetic monopoles can form a quantum Bose condensate (plasma in (2+1) dimensions) dual to the charge condensate in superconductors. The monopole Bose condensate manifests as a superinsulating state with infinite resistance, dual to superconductivity. The monopole supercurrents result in the electric analogue of the Meissner effect and lead to linear confinement of the Cooper pairs by Polyakov electric strings in analogy to quarks in hadrons. Superinsulators realise thus one of the mechanism proposed to explain confinement in QCD. Moreover, the string mechanism of confinement implies asymptotic freedom at the IR fixed point. We predict thus for superinsulators a metallic-like low temperature behaviour when samples are smaller than the string scale. This has been experimentally confirmed. We predict that an oblique version of SI is realised as the pseudogap state of high-TC superconductors. Full article
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Communication
First Results of the 140Ce(n,γ)141Ce Cross-Section Measurement at n_TOF
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Universe 2021, 7(6), 200; https://doi.org/10.3390/universe7060200 - 17 Jun 2021
Viewed by 448
Abstract
An accurate measurement of the 140Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the 140Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in 140Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the 140Ce Maxwellian-averaged cross-section. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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Review
Space Photometry with Brite-Constellation
Universe 2021, 7(6), 199; https://doi.org/10.3390/universe7060199 - 16 Jun 2021
Viewed by 230
Abstract
BRITE-Constellation is devoted to high-precision optical photometric monitoring of bright stars, distributed all over the Milky Way, in red and/or blue passbands. Photometry from space avoids the turbulent and absorbing terrestrial atmosphere and allows for very long and continuous observing runs with high [...] Read more.
BRITE-Constellation is devoted to high-precision optical photometric monitoring of bright stars, distributed all over the Milky Way, in red and/or blue passbands. Photometry from space avoids the turbulent and absorbing terrestrial atmosphere and allows for very long and continuous observing runs with high time resolution and thus provides the data necessary for understanding various processes inside stars (e.g., asteroseismology) and in their immediate environment. While the first astronomical observations from space focused on the spectral regions not accessible from the ground it soon became obvious around 1970 that avoiding the turbulent terrestrial atmosphere significantly improved the accuracy of photometry and satellites explicitly dedicated to high-quality photometry were launched. A perfect example is BRITE-Constellation, which is the result of a very successful cooperation between Austria, Canada and Poland. Research highlights for targets distributed nearly over the entire HRD are presented, but focus primarily on massive and hot stars. Full article
(This article belongs to the Special Issue Variable Stars as Seen with Photometric Space Telescopes)
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Article
Axial Anomaly in Galaxies and the Dark Universe
Universe 2021, 7(6), 198; https://doi.org/10.3390/universe7060198 - 13 Jun 2021
Viewed by 556
Abstract
Motivated by the SU(2)CMB modification of the cosmological model ΛCDM, we consider isolated fuzzy-dark-matter lumps, made of ultralight axion particles whose masses arise due to distinct SU(2) Yang–Mills scales and the Planck mass MP. In contrast to SU(2)CMB, these Yang–Mills theories are in confining phases (zero temperature) throughout most of the Universe’s history and associate with the three lepton flavours of the Standard Model of particle physics. As the Universe expands, axionic fuzzy dark matter comprises a three-component fluid which undergoes certain depercolation transitions when dark energy (a global axion condensate) is converted into dark matter. We extract the lightest axion mass ma,e=0.675×1023 eV from well motivated model fits to observed rotation curves in low-surface-brightness galaxies (SPARC catalogue). Since the virial mass of an isolated lump solely depends on MP and the associated Yang–Mills scale the properties of an e-lump predict those of μ- and τ-lumps. As a result, a typical e-lump virial mass ∼6.3×1010M suggests that massive compact objects in galactic centers such as Sagittarius A* in the Milky Way are (merged) μ- and τ-lumps. In addition, τ-lumps may constitute globular clusters. SU(2)CMB is always thermalised, and its axion condensate never has depercolated. If the axial anomaly indeed would link leptons with dark matter and the CMB with dark energy then this would demystify the dark Universe through a firmly established feature of particle physics. Full article
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Article
Constraining CP4 3HDM with Top Quark Decays
Universe 2021, 7(6), 197; https://doi.org/10.3390/universe7060197 - 10 Jun 2021
Cited by 2 | Viewed by 400
Abstract
CP4 3HDM is a unique three-Higgs-doublet model equipped with a higher-order CP-symmetry in the scalar and Yukawa sector. Based on a single assumption (the minimal model with a CP-symmetry of order 4 and no accidental symmetry), it leads to a remarkable correlation between its scalar and Yukawa sectors, which echoes in its phenomenology. A recent scan of the parameter space of CP4 3HDM under the assumption of scalar alignment identified a few dozens of points which passed many flavor constraints. In the present work, however, we show that almost all of these points are now ruled out by the recent LHC searches of tH+b with subsequent hadronic decays of H+. Apart from a few points with charged Higgses heavier than the top quark, only one point survives all the checks, the model with an exotic, non-2HDM-like generation pattern of H+ couplings with quarks. One can expect many more points with exotic H+ couplings to quarks if the scalar alignment assumption is relaxed. Full article
(This article belongs to the Special Issue Innovative Detection Strategies for New Physics Searches)
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Article
Applications of pT-xR Variables in Describing Inclusive Cross Sections at the LHC
Universe 2021, 7(6), 196; https://doi.org/10.3390/universe7060196 - 09 Jun 2021
Viewed by 324
Abstract
Invariant inclusive single-particle/jet cross sections in p–p collisions can be factorized in terms of two separable pT dependences, a [pTs] sector and an [xRpTs] sector. Here, we extend our earlier work by analyzing more extensive data to explore various s-dependent attributes and other systematics of inclusive jet, photon and single particle reactions. Approximate power laws in s, pT and xR are found. Physical arguments are given which relate observations to the underlying physics of parton–parton hard scattering and the parton distribution functions in the proton. We show that the A(s,pT) function, introduced in our earlier publication to describe the pT dependence of the inclusive cross section, is directly related to the underlying hard parton–parton scattering for jet production, with little influence from soft physics. In addition to the a function, we introduce another function, the F(s,xR) function that obeys radial scaling for inclusive jets and offers another test of the underlying parton physics. An application to heavy ion physics is given, where we use our variables to determine the transparency of cold nuclear matter to penetrating heavy mesons through the lead nucleus. Full article
(This article belongs to the Special Issue Analysis Techniques and Algorithms for QCD Studies)
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Article
Green Functions Scattering in the Casimir Effect
Universe 2021, 7(6), 195; https://doi.org/10.3390/universe7060195 - 09 Jun 2021
Viewed by 496
Abstract
We propose Green functions scattering method to obtain the Casimir–Polder potential between anisotropic atom and one or two planar parallel plates. Lifshitz formula for pressure between two dielectric half-spaces separated by a vacuum slit is derived within the same method. The method is [...] Read more.
We propose Green functions scattering method to obtain the Casimir–Polder potential between anisotropic atom and one or two planar parallel plates. Lifshitz formula for pressure between two dielectric half-spaces separated by a vacuum slit is derived within the same method. The method is also applied to known conducting systems including graphene which are overviewed. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
Review
Localization of Dirac Fermions in Finite-Temperature Gauge Theory
Universe 2021, 7(6), 194; https://doi.org/10.3390/universe7060194 - 08 Jun 2021
Viewed by 349
Abstract
It is by now well established that Dirac fermions coupled to non-Abelian gauge theories can undergo an Anderson-type localization transition. This transition affects eigenmodes in the lowest part of the Dirac spectrum, the ones most relevant to the low-energy physics of these models. [...] Read more.
It is by now well established that Dirac fermions coupled to non-Abelian gauge theories can undergo an Anderson-type localization transition. This transition affects eigenmodes in the lowest part of the Dirac spectrum, the ones most relevant to the low-energy physics of these models. Here we review several aspects of this phenomenon, mostly using the tools of lattice gauge theory. In particular, we discuss how the transition is related to the finite-temperature transitions leading to the deconfinement of fermions, as well as to the restoration of chiral symmetry that is spontaneously broken at low temperature. Other topics we touch upon are the universality of the transition, and its connection to topological excitations (instantons) of the gauge field and the associated fermionic zero modes. While the main focus is on Quantum Chromodynamics, we also discuss how the localization transition appears in other related models with different fermionic contents (including the quenched approximation), gauge groups, and in different space-time dimensions. Finally, we offer some speculations about the physical relevance of the localization transition in these models. Full article
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Article
Heating in Magnetar Crusts from Electron Captures
Universe 2021, 7(6), 193; https://doi.org/10.3390/universe7060193 - 08 Jun 2021
Viewed by 404
Abstract
The persistent thermal luminosity of magnetars and their outbursts suggest the existence of some internal heat sources located in their outer crust. The compression of matter accompanying the decay of the magnetic field may trigger exothermic electron captures and, possibly, pycnonuclear fusions of [...] Read more.
The persistent thermal luminosity of magnetars and their outbursts suggest the existence of some internal heat sources located in their outer crust. The compression of matter accompanying the decay of the magnetic field may trigger exothermic electron captures and, possibly, pycnonuclear fusions of light elements that may have been accreted onto the surface from the fallback of supernova debris, from a disk or from the interstellar medium. This scenario bears some resemblance to deep crustal heating in accreting neutron stars, although the matter composition and the thermodynamic conditions are very different. The maximum possible amount of heat that can be released by each reaction and their locations are determined analytically taking into account the Landau–Rabi quantization of electron motion. Numerical results are also presented using experimental, as well as theoretical nuclear data. Whereas the heat deposited is mainly determined by atomic masses, the locations of the sources are found to be very sensitive to the magnetic field strength, thus providing a new way of probing the internal magnetic field of magnetars. Most sources are found to be concentrated at densities 10101011 g cm−3 with heat power W10351036 erg/s, as found empirically by comparing cooling simulations with observed thermal luminosity. The change of magnetic field required to trigger the reactions is shown to be consistent with the age of known magnetars. This suggests that electron captures and pycnonuclear fusion reactions may be a viable heating mechanism in magnetars. The present results provide consistent microscopic inputs for neutron star cooling simulations, based on the same model as that underlying the Brussels-Montreal unified equations of state. Full article
(This article belongs to the Special Issue Fundamental Processes in Neutron Stars and Supernovae)
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Article
Testing Gravitational Theories in the Field of the Earth with the SaToR-G Experiment
Universe 2021, 7(6), 192; https://doi.org/10.3390/universe7060192 - 08 Jun 2021
Viewed by 353
Abstract
A new satellite-based experiment in the field of gravitation, SaToR-G, is presented. It aims to compare the predictions of different theories of gravitation in the limit of weak-field and slow-motion. The ultimate goal of SaToR-G is searching for possible “new physics” beyond General [...] Read more.
A new satellite-based experiment in the field of gravitation, SaToR-G, is presented. It aims to compare the predictions of different theories of gravitation in the limit of weak-field and slow-motion. The ultimate goal of SaToR-G is searching for possible “new physics” beyond General Relativity, which represent the state-of-the-art of our current knowledge of gravitational physics. A key role in the above perspective is the theoretical and experimental framework that confines our work. To this end, we will exploit as much as possible the classical framework suggested by R.H. Dicke over fifty years ago. Full article
Article
Analysis of Observations near the Fourth Electron Gyrofrequency Heating Experiment in EISCAT
Universe 2021, 7(6), 191; https://doi.org/10.3390/universe7060191 - 08 Jun 2021
Viewed by 350
Abstract
We present the observations of the artificial ionospheric heating experiment of EISCAT (European Incoherent Scatter Scientific Association) on 22 February 2012 in Tromsø, Norway. When the pump is operating near the fourth electron gyrofrequency, the UHF radar observation shows some strong enhancements in [...] Read more.
We present the observations of the artificial ionospheric heating experiment of EISCAT (European Incoherent Scatter Scientific Association) on 22 February 2012 in Tromsø, Norway. When the pump is operating near the fourth electron gyrofrequency, the UHF radar observation shows some strong enhancements in electron temperature, electron density, ion line, and the outshifted plasma lines. Based on some existing theories, we find the following: first, Langmuir waves scattering off lower hybrid density fluctuations and strong Langmuir turbulence (SLT) in the Zakharov model cannot completely explain the outshifted plasma lines, but the data suggest that this phenomenon is related to the cascade of the pump wave and should be researched further; second, the spatiotemporal consistency between the enhancement in electron density/electron temperature reaches up to three to four times that of the undisturbed state and HF-enhanced ion lines (HFILs) suggest that SLT excited by parametric instability plays a significant role in superthermal electron formation and electron acceleration; third, some enhancements in HFILs and HF-induced plasma lines (HFPLs) are generated by parametric decay instability (PDI) during underdense heating in the third cycle, we suggest that this is due to the existence of a second cut-off in the upper hybrid dispersion relation as derived from a kinetic description. Full article
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Article
On the Coupling of Generalized Proca Fields to Degenerate Scalar-Tensor Theories
Universe 2021, 7(6), 190; https://doi.org/10.3390/universe7060190 - 07 Jun 2021
Viewed by 159
Abstract
We prove that vector fields described by the generalized Proca class of theories do not admit consistent coupling with a gravitational sector defined by a scalar–tensor theory of the degenerate type. Under the assumption that there exists a frame in which the Proca [...] Read more.
We prove that vector fields described by the generalized Proca class of theories do not admit consistent coupling with a gravitational sector defined by a scalar–tensor theory of the degenerate type. Under the assumption that there exists a frame in which the Proca field interacts with gravity only through the metric tensor, our analysis shows that at least one of the constraints associated with the degeneracy of the scalar–tensor sector is inevitably lost whenever the vector theory includes coupling with the Christoffel connection. Full article
Article
Shaping Dynamical Casimir Photons
Universe 2021, 7(6), 189; https://doi.org/10.3390/universe7060189 - 06 Jun 2021
Viewed by 760
Abstract
Temporal modulation of the quantum vacuum through fast motion of a neutral body or fast changes of its optical properties is known to promote virtual into real photons, the so-called dynamical Casimir effect. Empowering modulation protocols with spatial control could enable the shaping [...] Read more.
Temporal modulation of the quantum vacuum through fast motion of a neutral body or fast changes of its optical properties is known to promote virtual into real photons, the so-called dynamical Casimir effect. Empowering modulation protocols with spatial control could enable the shaping of spectral, spatial, spin, and entanglement properties of the emitted photon pairs. Space–time quantum metasurfaces have been proposed as a platform to realize this physics via modulation of their optical properties. Here, we report the mechanical analog of this phenomenon by considering systems in which the lattice structure undergoes modulation in space and in time. We develop a microscopic theory that applies both to moving mirrors with a modulated surface profile and atomic array meta-mirrors with perturbed lattice configuration. Spatiotemporal modulation enables motion-induced generation of co- and cross-polarized photon pairs that feature frequency-linear momentum entanglement as well as vortex photon pairs featuring frequency-angular momentum entanglement. The proposed space–time dynamical Casimir effect can be interpreted as induced dynamical asymmetry in the quantum vacuum. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
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Article
Jet-Induced Feedback in the [O III] Lines of Early Evolution Stage Active Galactic Nuclei
Universe 2021, 7(6), 188; https://doi.org/10.3390/universe7060188 - 06 Jun 2021
Viewed by 367
Abstract
It is well known that active galactic nuclei (AGN) show various forms of interaction with their host galaxy, in a number of phenomena generally called AGN feedback. In particular, the relativistic plasma jets launched by a fraction of AGN can strongly affect their [...] Read more.
It is well known that active galactic nuclei (AGN) show various forms of interaction with their host galaxy, in a number of phenomena generally called AGN feedback. In particular, the relativistic plasma jets launched by a fraction of AGN can strongly affect their environment. We present here a study of the [O III] λλ4959,5007 lines in a diverse sample of early evolution stage AGN–specifically narrow-line Seyfert 1 galaxies. Radio imaging observations of all of the sources enable a division to jetted and non-jetted sources, and exploiting this we show that the ionized gas properties are significantly influenced by the presence of the jets, as we often find the [O III] lines (blue-)shifted with respect to their restframe wavelength. We also show how the radio morphology and the radio spectral index do not seem to play a role in the origin of the [O III] shifts, thus suggesting that the source inclination is not relevant to the lines displacement. We do not find a strong relation between the [O III] line properties and the bolometric luminosity, suggesting that within our sample radiatively driven outflows do not seem to have a significant contribution to the [O III] line kinematics. We finally suggest that [O III] shifts may be a good proxy to identify the presence of relativistic jets. Additional studies, especially with integral-field spectroscopy, will provide a deeper insight into the relation between jets and their environment in early evolution stage AGN. Full article
(This article belongs to the Special Issue Panchromatic View of the Life-Cycle of AGN)
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Article
New Black Hole Solutions in N = 2 and N = 8 Gauged Supergravity
Universe 2021, 7(6), 187; https://doi.org/10.3390/universe7060187 - 06 Jun 2021
Viewed by 458
Abstract
We review a special class of N=2 supergravity model that interpolates all the single-dilaton truncations of the maximal SO(8) gauged supergravity. We also provide explicit non-extremal, charged black hole solutions and their supersymmetric limits, asymptotic charges, thermodynamics and boundary conditions. We also discuss a suitable Hamilton–Jacobi formulation and related BPS flow equations for the supersymmetric configurations, with an explicit form for the superpotential function. Finally, we briefly analyze certain models within the class under consideration as consistent truncations of the maximal, N=8 gauged supergravity in four dimensions. Full article
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Article
Cosmic Tangle: Loop Quantum Cosmology and CMB Anomalies
Universe 2021, 7(6), 186; https://doi.org/10.3390/universe7060186 - 06 Jun 2021
Viewed by 446
Abstract
Loop quantum cosmology is a conflicted field in which exuberant claims of observability coexist with serious objections against the conceptual and physical viability of its current formulations. This contribution presents a non-technical case study of the recent claim that loop quantum cosmology might [...] Read more.
Loop quantum cosmology is a conflicted field in which exuberant claims of observability coexist with serious objections against the conceptual and physical viability of its current formulations. This contribution presents a non-technical case study of the recent claim that loop quantum cosmology might alleviate anomalies in the observations of the cosmic microwave background. Full article
(This article belongs to the Special Issue Quantum Cosmology)
Review
Axion-Like Particle Searches with IACTs
Universe 2021, 7(6), 185; https://doi.org/10.3390/universe7060185 - 05 Jun 2021
Viewed by 434
Abstract
The growing interest in axion-like particles (ALPs) stems from the fact that they provide successful theoretical explanations of physics phenomena, from the anomaly of the CP-symmetry conservation in strong interactions to the observation of an unexpectedly large TeV photon flux from astrophysical sources, at distances where the strong absorption by the intergalactic medium should make the signal very dim. In this latter condition, which is the focus of this review, a possible explanation is that TeV photons convert to ALPs in the presence of strong and/or extended magnetic fields, such as those in the core of galaxy clusters or around compact objects, or even those in the intergalactic space. This mixing affects the observed γ-ray spectrum of distant sources, either by signal recovery or the production of irregularities in the spectrum, called ‘wiggles’, according to the specific microscopic realization of the ALP and the ambient magnetic field at the source, and in the Milky Way, where ALPs may be converted back to γ rays. ALPs are also proposed as candidate particles for the Dark Matter. Imaging Atmospheric Cherenkov telescopes (IACTs) have the potential to detect the imprint of ALPs in the TeV spectrum from several classes of sources. In this contribution, we present the ALP case and review the past decade of searches for ALPs with this class of instruments. Full article
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Article
Introducing Quantum and Statistical Physics in the Footsteps of Einstein: A Proposal
Universe 2021, 7(6), 184; https://doi.org/10.3390/universe7060184 - 04 Jun 2021
Viewed by 397
Abstract
Introducing some fundamental concepts of quantum physics to high school students, and to their teachers, is a timely challenge. In this paper we describe ongoing research, in which a teaching–learning sequence for teaching quantum physics, whose inspiration comes from some of the fundamental [...] Read more.
Introducing some fundamental concepts of quantum physics to high school students, and to their teachers, is a timely challenge. In this paper we describe ongoing research, in which a teaching–learning sequence for teaching quantum physics, whose inspiration comes from some of the fundamental papers about the quantum theory of radiation by Albert Einstein, is being developed. The reason for this choice goes back essentially to the fact that the roots of many subtle physical concepts, namely quanta, wave–particle duality and probability, were introduced for the first time in one of these papers, hence their study may represent a useful intermediate step towards tackling the final incarnation of these concepts in the full theory of quantum mechanics. An extended discussion of some elementary tools of statistical physics, mainly Boltzmann’s formula for entropy and statistical distributions, which are necessary but may be unfamiliar to the students, is included. This discussion can also be used independently to introduce some rudiments of statistical physics. In this case, part of the inspiration came from some of Einstein’s papers. We present preliminary, qualitative results obtained with both teachers and selected pupils from various high schools in southern Italy, in the course of several outreach activities. Although the proposal was only tested in this limited context for now, the preliminary results are very promising and they indicate that this proposal can be fruitfully employed for the task. Full article
Communication
Beryllium Radioactive Isotopes as a Probe to Measure the Residence Time of Cosmic Rays in the Galaxy and Halo Thickness: A “Data-Driven” Approach
Universe 2021, 7(6), 183; https://doi.org/10.3390/universe7060183 - 04 Jun 2021
Viewed by 420
Abstract
Cosmic rays are a powerful tool for the investigation of the structure of the magnetic fields in the Galactic halo and the properties of the inter-stellar medium. Two parameters of the cosmic ray propagation models, the Galactic halo (half) thickness, H, and [...] Read more.
Cosmic rays are a powerful tool for the investigation of the structure of the magnetic fields in the Galactic halo and the properties of the inter-stellar medium. Two parameters of the cosmic ray propagation models, the Galactic halo (half) thickness, H, and the diffusion coefficient, D, are loosely constrained by current cosmic ray flux measurements; in particular, a large degeneracy exists, with only H/D being well measured. The 10Be/9Be isotopic flux ratio (thanks to the 2 My lifetime of 10Be) can be used as a radioactive clock providing the measurement of cosmic ray residence time in a galaxy. This is an important probe with which to solve the H/D degeneracy. Past measurements of 10Be/9Be isotopic flux ratios in cosmic rays are scarce, and were limited to low energy and affected by large uncertainties. Here a new technique to measure 10Be/9Be isotopic flux ratio, with a data-driven approach in magnetic spectrometers is presented. As an example, by applying the method to beryllium events published via PAMELA experiment, it is now possible to determine the important 10Be/9Be measurement while avoiding the prohibitive uncertainties coming from Monte Carlo simulations. It is shown how the accuracy of PAMELA data strengthens the experimental indication for the relativistic time dilation of 10Be decay in cosmic rays; this should improve the knowledge of the H parameter. Full article
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Review
Progress in Constraining Nuclear Symmetry Energy Using Neutron Star Observables Since GW170817
Universe 2021, 7(6), 182; https://doi.org/10.3390/universe7060182 - 04 Jun 2021
Cited by 9 | Viewed by 584
Abstract
The density dependence of nuclear symmetry energy is among the most uncertain parts of the Equation of State (EOS) of dense neutron-rich nuclear matter. It is currently poorly known especially at suprasaturation densities partially because of our poor knowledge about isovector nuclear interactions [...] Read more.
The density dependence of nuclear symmetry energy is among the most uncertain parts of the Equation of State (EOS) of dense neutron-rich nuclear matter. It is currently poorly known especially at suprasaturation densities partially because of our poor knowledge about isovector nuclear interactions at short distances. Because of its broad impacts on many interesting issues, pinning down the density dependence of nuclear symmetry energy has been a longstanding and shared goal of both astrophysics and nuclear physics. New observational data of neutron stars including their masses, radii, and tidal deformations since GW170817 have helped improve our knowledge about nuclear symmetry energy, especially at high densities. Based on various model analyses of these new data by many people in the nuclear astrophysics community, while our brief review might be incomplete and biased unintentionally, we learned in particular the following: (1) The slope parameter L of nuclear symmetry energy at saturation density ρ0 of nuclear matter from 24 new analyses of neutron star observables was about L57.7±19 MeV at a 68% confidence level, consistent with its fiducial value from surveys of over 50 earlier analyses of both terrestrial and astrophysical data within error bars. (2) The curvature Ksym of nuclear symmetry energy at ρ0 from 16 new analyses of neutron star observables was about Ksym107±88 MeV at a 68% confidence level, in very good agreement with the systematics of earlier analyses. (3) The magnitude of nuclear symmetry energy at 2ρ0, i.e., Esym(2ρ0)51±13 MeV at a 68% confidence level, was extracted from nine new analyses of neutron star observables, consistent with the results from earlier analyses of heavy-ion reactions and the latest predictions of the state-of-the-art nuclear many-body theories. (4) While the available data from canonical neutron stars did not provide tight constraints on nuclear symmetry energy at densities above about 2ρ0, the lower radius boundary R2.01=12.2 km from NICER’s very recent observation of PSR J0740+6620 of mass 2.08±0.07M and radius R=12.216.3 km at a 68% confidence level set a tight lower limit for nuclear symmetry energy at densities above 2ρ0. (5) Bayesian inferences of nuclear symmetry energy using models encapsulating a first-order hadron–quark phase transition from observables of canonical neutron stars indicated that the phase transition shifted appreciably both L and Ksym to higher values, but with larger uncertainties compared to analyses assuming no such phase transition. (6) The high-density behavior of nuclear symmetry energy significantly affected the minimum frequency necessary to rotationally support GW190814’s secondary component of mass (2.50–2.67) M as the fastest and most massive pulsar discovered so far. Overall, thanks to the hard work of many people in the astrophysics and nuclear physics community, new data of neutron star observations since the discovery of GW170817 have significantly enriched our knowledge about the symmetry energy of dense neutron-rich nuclear matter. Full article
(This article belongs to the Special Issue Neutron Stars and Gravitational Wave Observations)
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Communication
Deviation from Slow-Roll Regime in the EGB Inflationary Models with rNe1
Universe 2021, 7(6), 181; https://doi.org/10.3390/universe7060181 - 04 Jun 2021
Cited by 3 | Viewed by 382
Abstract
We consider Einstein–Gauss–Bonnet (EGB) inflationary models using the effective potential approach. We present evolution equations in the slow-roll regime using the effective potential and the tensor-to-scalar ratio. The choice of the effective potential is related to an expression of the spectral index in [...] Read more.
We consider Einstein–Gauss–Bonnet (EGB) inflationary models using the effective potential approach. We present evolution equations in the slow-roll regime using the effective potential and the tensor-to-scalar ratio. The choice of the effective potential is related to an expression of the spectral index in terms of e-folding number Ne. The satisfaction of the slow-roll regime is mostly related to the form of the tensor-to-scalar ratio r. The case of r1/Ne2 leads to a generalization of α-attractors inflationary parameters to Einstein–Gauss–Bonnet gravity with exponential effective potential. Moreover, the cosmological attractors include models with r1/Ne. And we check the satisfaction of the slow-roll regime during inflation for models with r1/Ne. Full article
(This article belongs to the Special Issue Cosmological Models, Quantum Theories and Astrophysical Observations)
Communication
Optimal Perturbation Technique within the Asymptotic Iteration Method for Heavy-Light Meson Mass Splittings
Universe 2021, 7(6), 180; https://doi.org/10.3390/universe7060180 - 04 Jun 2021
Viewed by 454
Abstract
For further insight into the perturbation technique within the framework of the asymptotic iteration method (PAIM), we suggest this method to be used as an alternative method to the traditional well-known perturbation techniques. We show by means of very simple algebraic manipulations that [...] Read more.
For further insight into the perturbation technique within the framework of the asymptotic iteration method (PAIM), we suggest this method to be used as an alternative method to the traditional well-known perturbation techniques. We show by means of very simple algebraic manipulations that PAIM can be directly applied to obtain the symbolic expectation value of any perturbed potential piece without using the eigenfunction of the unperturbed problem. One of the fundamental advantages of PAIM is its ability to extract a reference unperturbed potential piece or pieces from the total Hamiltonian which can be solved exactly within AIM. After all, one can easily compute the symbolic expectation values of the remaining potential pieces. As an example, the present scheme is applied to the semi-relativistic wave equation with the harmonic-oscillator potential implemented with the Fermi–Breit potential terms. In particular, the non-trivial symbolic expectation values of the Dirac delta function, and the momentum-dependent orbit–orbit coupling terms are successfully calculated. Results are then used, as an illustration, to compute the semi-relativistic s-wave heavy-light meson masses. We obtain good agreement with experimental data for the meson mass splittings cu¯, cd¯, cs¯, bu¯, bd¯, bs¯. Full article
(This article belongs to the Section High Energy Nuclear and Particle Physics)
Article
Global Portraits of Nonminimal Teleparallel Inflation
Universe 2021, 7(6), 179; https://doi.org/10.3390/universe7060179 - 03 Jun 2021
Cited by 3 | Viewed by 426
Abstract
We construct global phase portraits of inflationary dynamics in teleparallel gravity models with a scalar field nonminimally coupled to torsion scalar. The adopted set of variables can clearly distinguish between different asymptotic states as fixed points, including the kinetic and inflationary regimes. The [...] Read more.
We construct global phase portraits of inflationary dynamics in teleparallel gravity models with a scalar field nonminimally coupled to torsion scalar. The adopted set of variables can clearly distinguish between different asymptotic states as fixed points, including the kinetic and inflationary regimes. The key role in the description of inflation is played by the heteroclinic orbits that run from the asymptotic saddle points to the late time attractor point and are approximated by nonminimal slow roll conditions. To seek the asymptotic fixed points, we outline a heuristic method in terms of the “effective potential” and “effective mass”, which can be applied for any nonminimally coupled theories. As particular examples, we study positive quadratic nonminimal couplings with quadratic and quartic potentials and note how the portraits differ qualitatively from the known scalar-curvature counterparts. For quadratic models, inflation can only occur at small nonminimal coupling to torsion, as for larger coupling, the asymptotic de Sitter saddle point disappears from the physical phase space. Teleparallel models with quartic potentials are not viable for inflation at all, since for small nonminimal coupling, the asymptotic saddle point exhibits weaker than exponential expansion, and for larger coupling, it also disappears. Full article
(This article belongs to the Special Issue Teleparallel Gravity: Foundations and Observational Constraints)
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Article
Possible Effects of the Fractal Distribution of Relic Wormholes
Universe 2021, 7(6), 178; https://doi.org/10.3390/universe7060178 - 03 Jun 2021
Viewed by 489
Abstract
We discuss the possibility that the distribution of relic wormholes may possess fractal properties. Relic wormholes and their fractal distributions are predicted in a natural way by lattice quantum gravity models. This provides a new approach to some long standing problems. That is [...] Read more.
We discuss the possibility that the distribution of relic wormholes may possess fractal properties. Relic wormholes and their fractal distributions are predicted in a natural way by lattice quantum gravity models. This provides a new approach to some long standing problems. That is the nature of dark matter phenomena, the origin of Faber-Jackson and Tully-Fisher relations and the observed deficit of baryons. We derive corrections to the Newton’s potential caused by the presence of relic wormholes and show that the analysis of dark matter distribution in galaxies allows us to fix the parameters of the fractal distribution of wormholes. Full article
(This article belongs to the Special Issue Recent Advances in Wormhole Physics)
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Article
Accretion Flow onto Ellis–Bronnikov Wormhole
Universe 2021, 7(6), 177; https://doi.org/10.3390/universe7060177 - 02 Jun 2021
Cited by 1 | Viewed by 531
Abstract
Study of accretion onto wormholes is rather rare compared to that onto black holes. In this paper, we consider accretion flow of cosmological dark energy modeled by barotropic fluid onto the celebrated Ellis–Bronnikov wormhole (EBWH) built by Einstein minimally coupled scalar field ϕ, violating the null energy condition. The accreting fluid is assumed to be phantom, quintessence, dust and stiff matter. We begin by first pointing out a mathematical novelty showing how the EBWH can lead to the Schwarzschild black hole under a complex Wick rotation. Then, we analyze the profiles of fluid radial velocity, density and the rate of mass variation of the EBWH due to accretion and compare the profiles with those of the Schwarzschild black hole. We also analyze accretion to the massless EBWH that has zero ADM mass but has what we call nonzero Wheelerian mass (“mass without mass”), composed of the non-trivial scalar field, that shows gravitational effects. Our conclusion is that the mass of SBH due to phantom accretion decreases consistently with known results, while, in contrast, the mass of EBWH increases. Exactly an opposite behavior emerges for non-phantom accretion to these two objects. Accretion to massless EBWH (i.e., to nonzero Wheelerian mass) shares the same patterns as those of the massive EBWH; hence there is no way to distinguish massive and massless cases by means of accretion flow. The contrasting mass variations due to phantom accretion could be a reflection of the distinct topology of the central objects. Full article
(This article belongs to the Special Issue Recent Advances in Wormhole Physics)
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Article
From the Vector to Scalar Perturbations Addition in the Stark Broadening Theory of Spectral Lines
Universe 2021, 7(6), 176; https://doi.org/10.3390/universe7060176 - 02 Jun 2021
Viewed by 476
Abstract
The effect of plasma Coulomb microfied dynamics on spectral line shapes is under consideration. The analytical solution of the problem is unachievable with famous Chandrasekhar–Von-Neumann results up to the present time. The alternative methods are connected with modeling of a real ion Coulomb [...] Read more.
The effect of plasma Coulomb microfied dynamics on spectral line shapes is under consideration. The analytical solution of the problem is unachievable with famous Chandrasekhar–Von-Neumann results up to the present time. The alternative methods are connected with modeling of a real ion Coulomb field dynamics by approximate models. One of the most accurate theories of ions dynamics effect on line shapes in plasmas is the Frequency Fluctuation Model (FFM) tested by the comparison with plasma microfield numerical simulations. The goal of the present paper is to make a detailed comparison of the FFM results with analytical ones for the linear and quadratic Stark effects in different limiting cases. The main problem is connected with perturbation additions laws known to be vector for small particle velocities (static line shapes) and scalar for large velocities (the impact limit). The general solutions for line shapes known in the frame of scalar perturbation additions are used to test the FFM procedure. The difference between “scalar” and “vector” models is demonstrated both for linear and quadratic Stark effects. It is shown that correct transition from static to impact limits for linear Stark-effect needs in account of the dependence of electric field jumping frequency in FFM on the field strengths. However, the constant jumping frequency is quite satisfactory for description of the quadratic Stark-effect. The detailed numerical comparison for spectral line shapes in the frame of both scalar and vector perturbation additions with and without jumping frequency field dependence for the linear and quadratic Stark effects is presented. Full article
(This article belongs to the Special Issue Advances in Understanding Astrophysical and Atomic Phenomena)
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Article
Group II Oxide Grains: How Massive Are Their AGB Star Progenitors?
Universe 2021, 7(6), 175; https://doi.org/10.3390/universe7060175 - 01 Jun 2021
Viewed by 438
Abstract
Presolar grains and their isotopic compositions provide valuable constraints to AGB star nucleosynthesis. However, there is a sample of O- and Al-rich dust, known as group 2 oxide grains, whose origin is difficult to address. On the one hand, the 17O/16O isotopic ratios shown by those grains are similar to the ones observed in low-mass red giant stars. On the other hand, their large 18O depletion and 26Al enrichment are challenging to account for. Two different classes of AGB stars have been proposed as progenitors of this kind of stellar dust: intermediate mass AGBs with hot bottom burning, or low mass AGBs where deep mixing is at play. Our models of low-mass AGB stars with a bottom-up deep mixing are shown to be likely progenitors of group 2 grains, reproducing together the 17O/16O, 18O/16O and 26Al/27Al values found in those grains and being less sensitive to nuclear physics inputs than our intermediate-mass models with hot bottom burning. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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Article
Geometric Approach to Analytic Marginalisation of the Likelihood Ratio for Continuous Gravitational Wave Searches
Universe 2021, 7(6), 174; https://doi.org/10.3390/universe7060174 - 01 Jun 2021
Cited by 1 | Viewed by 422
Abstract
The likelihood ratio for a continuous gravitational wave signal is viewed geometrically as a function of the orientation of two vectors; one representing the optimal signal-to-noise ratio, and the other representing the maximised likelihood ratio or F-statistic. Analytic marginalisation over the angle between the vectors yields a marginalised likelihood ratio, which is a function of the F-statistic. Further analytic marginalisation over the optimal signal-to-noise ratio is explored using different choices of prior. Monte-Carlo simulations show that the marginalised likelihood ratios had identical detection power to the F-statistic. This approach demonstrates a route to viewing the F-statistic in a Bayesian context, while retaining the advantages of its efficient computation. Full article
(This article belongs to the Special Issue Continuous Gravitational Waves)
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