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Universe, Volume 7, Issue 10 (October 2021) – 39 articles

Cover Story (view full-size image): In this work, we have analyzed, in detail, dark matter (DM) in a model (the aνMSM) that features sterile neutrinos and a QCD axion sector. This is well-motivated because it can account for all experimentally confirmed evidence of BSM physics (neutrino masses, dark matter, and baryon asymmetry), and it solves the strong CP problem, features a stable electroweak vacuum, and realizes a viable version of Higgs inflation. To date, DM was explained exclusively through the axion; however, as shown in this work, the lightest sterile neutrino can also contribute to DM, and one can have a multi-component (axion–sterile–neutrino). This scenario relates the axion parameters with the sterile neutrino parameters and allows enlarging the region of parameter space with absolute electroweak vacuum stability. View this paper
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Article
Higher Dimensional Lie Algebroid Sigma Model with WZ Term
Universe 2021, 7(10), 391; https://doi.org/10.3390/universe7100391 - 19 Oct 2021
Cited by 1 | Viewed by 184
Abstract
We generalize the (n+1)-dimensional twisted R-Poisson topological sigma model with flux on a target Poisson manifold to a Lie algebroid. Analyzing the consistency of constraints in the Hamiltonian formalism and the gauge symmetry in the Lagrangian formalism, [...] Read more.
We generalize the (n+1)-dimensional twisted R-Poisson topological sigma model with flux on a target Poisson manifold to a Lie algebroid. Analyzing the consistency of constraints in the Hamiltonian formalism and the gauge symmetry in the Lagrangian formalism, geometric conditions of the target space to make the topological sigma model consistent are identified. The geometric condition is an universal compatibility condition of a Lie algebroid with a multisymplectic structure. This condition is a generalization of the momentum map theory of a Lie group and is regarded as a generalization of the momentum section condition of the Lie algebroid. Full article
(This article belongs to the Special Issue Dualities and Geometry)
Article
Generalised Asymptotic Solutions for the Inflaton in the Oscillatory Phase of Reheating
Universe 2021, 7(10), 390; https://doi.org/10.3390/universe7100390 - 19 Oct 2021
Viewed by 273
Abstract
We determine generalised asymptotic solutions for the inflaton field, the Hubble parameter, and the equation-of-state parameter valid during the oscillatory phase of reheating for potentials that close to their global minima behave as even monomial potentials. For the quadratic potential, we derive a [...] Read more.
We determine generalised asymptotic solutions for the inflaton field, the Hubble parameter, and the equation-of-state parameter valid during the oscillatory phase of reheating for potentials that close to their global minima behave as even monomial potentials. For the quadratic potential, we derive a generalised asymptotic expansion for the inflaton with respect to the scale set by inverse powers of the cosmic time. For the quartic potential, we derive an explicit, two-term generalised asymptotic solution in terms of Jacobi elliptic functions, with a scale set by inverse powers of the square root of the cosmic time. In the general case, we find similar two-term solutions where the leading order term is defined implicitly in terms of the Gauss hypergeometric function. The relation between the leading terms of the instantaneous equation-of-state parameter and different averaged values is discussed in the general case. Finally, we discuss the physical significance of the generalised asymptotic solutions in the oscillatory regime and their matching to the appropriate solutions in the thermalization regime. Full article
(This article belongs to the Special Issue Studying the Universe from Spain)
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Article
Asymptotic Safety: Swampland or Wonderland?
Universe 2021, 7(10), 389; https://doi.org/10.3390/universe7100389 - 18 Oct 2021
Cited by 3 | Viewed by 340
Abstract
We investigate the consequences of combining swampland conjectures with the requirement of asymptotic safety. To this end, we explore the infrared regime of asymptotically safe gravity in the quadratic one-loop approximation, and we identify the hypersurface spanned by the endpoints of asymptotically safe [...] Read more.
We investigate the consequences of combining swampland conjectures with the requirement of asymptotic safety. To this end, we explore the infrared regime of asymptotically safe gravity in the quadratic one-loop approximation, and we identify the hypersurface spanned by the endpoints of asymptotically safe renormalization group trajectories. These comprise the allowed values of higher-derivative couplings, as well as standard logarithmic form factors. We determine the intersection of this hypersurface with the regions of parameter space allowed by the weak-gravity conjecture, the swampland de Sitter conjecture, and the trans-Planckian censorship conjecture. The latter two depend on some order-one constants, for generic values of which we show that the overlap region is a proper subspace of the asymptotically safe hypersurface. Moreover, the latter lies inside the region allowed by the weak gravity conjecture assuming electromagnetic duality. Our results suggest a non-trivial interplay between the consistency conditions stemming from ultraviolet completeness of the renormalization group flow, black hole physics, and cosmology. Full article
(This article belongs to the Special Issue Asymptotic Safety in Quantum Gravity)
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Article
Frame-Dragging: Meaning, Myths, and Misconceptions
Universe 2021, 7(10), 388; https://doi.org/10.3390/universe7100388 (registering DOI) - 18 Oct 2021
Cited by 3 | Viewed by 319
Abstract
Originally introduced in connection with general relativistic Coriolis forces, the term frame-dragging is associated today with a plethora of effects related to the off-diagonal element of the metric tensor. It is also frequently the subject of misconceptions leading to incorrect predictions, even of [...] Read more.
Originally introduced in connection with general relativistic Coriolis forces, the term frame-dragging is associated today with a plethora of effects related to the off-diagonal element of the metric tensor. It is also frequently the subject of misconceptions leading to incorrect predictions, even of nonexistent effects. We show that there are three different levels of frame-dragging corresponding to three distinct gravitomagnetic objects: gravitomagnetic potential 1-form, field, and tidal tensor, whose effects are independent, and sometimes opposing. It is seen that, from the two analogies commonly employed, the analogy with magnetism holds strong where it applies, whereas the fluid-dragging analogy (albeit of some use, qualitatively, in the first level) is, in general, misleading. Common misconceptions (such as viscous-type “body-dragging”) are debunked. Applications considered include rotating cylinders (Lewis–Weyl metrics), Kerr, Kerr–Newman and Kerr–dS spacetimes, black holes surrounded by disks/rings, and binary systems. Full article
(This article belongs to the Special Issue Frame-Dragging and Gravitomagnetism)
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Article
Relativistic Fractional-Dimension Gravity
Universe 2021, 7(10), 387; https://doi.org/10.3390/universe7100387 - 18 Oct 2021
Viewed by 415
Abstract
This paper presents a relativistic version of Newtonian Fractional-Dimension Gravity (NFDG), an alternative gravitational model recently introduced and based on the theory of fractional-dimension spaces. This extended version—Relativistic Fractional-Dimension Gravity (RFDG)—is based on other existing theories in the literature and might be useful [...] Read more.
This paper presents a relativistic version of Newtonian Fractional-Dimension Gravity (NFDG), an alternative gravitational model recently introduced and based on the theory of fractional-dimension spaces. This extended version—Relativistic Fractional-Dimension Gravity (RFDG)—is based on other existing theories in the literature and might be useful for astrophysical and cosmological applications. In particular, in this work, we review the mathematical theory for spaces with non-integer dimensions and its connections with the non-relativistic NFDG. The Euler–Lagrange equations for scalar fields can also be extended to spaces with fractional dimensions, by adding an appropriate weight factor, and then can be used to generalize the Laplacian operator for rectangular, spherical, and cylindrical coordinates. In addition, the same weight factor can be added to the standard Hilbert action in order to obtain the field equations, following methods used for scalar-tensor models of gravity, multi-scale spacetimes, and fractional gravity theories. We then apply the field equations to standard cosmology and to the Friedmann-Lemaître-Robertson-Walker metric. Using a suitable weight vtt, depending on the synchronous time t and on a single time-dimension parameter αt, we extend the Friedmann equations to the RFDG case. This allows for the computation of the scale factor at for different values of the fractional time-dimension αt and the comparison with standard cosmology results. Future additional work on the subject, including studies of the cosmological late-time acceleration, type Ia supernovae data, and related dark energy theory will be needed to establish this model as a relativistic alternative theory of gravity. Full article
(This article belongs to the Special Issue Modified Theories of Gravity and Cosmological Applications)
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Article
Present and Future of 0ν2β Searches with Germanium
Universe 2021, 7(10), 386; https://doi.org/10.3390/universe7100386 - 18 Oct 2021
Viewed by 475
Abstract
Among the several experiments and techniques conceived of to search for neutrinoless double β decay (0ν2β) in a handful of isotopes, presently the best lower limit on the half-life for this rare process, is provided by those using [...] Read more.
Among the several experiments and techniques conceived of to search for neutrinoless double β decay (0ν2β) in a handful of isotopes, presently the best lower limit on the half-life for this rare process, is provided by those using 76Ge, a rare isotope of germanium. Such a lower limit is of 1.8 × 1026 y. Building from such a successful achievement of the GERDA and Majorana Demonstrator experiments, the baton with 76Ge passes now to the LEGEND experiment. Using a two-stage approach with about 200 kg and then 1 t of germanium, LEGEND aims to attain a sensitivity of around 1028 y, which will enable it to probe the standard inverted-ordering neutrino mass scenario. We touch upon the past generation of experiments to illustrate their strong and weak points, review the general concept and design of LEGEND, and describe the LEGEND-200 detector and its preliminary performance. We also illustrate how the backgrounds can have a dramatic effect on the search and in which way the latter can be mitigated. Full article
(This article belongs to the Special Issue Neutrinoless Double Beta Decay)
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Article
Weak Deflection Angle and Shadow by Tidal Charged Black Hole
Universe 2021, 7(10), 385; https://doi.org/10.3390/universe7100385 - 18 Oct 2021
Viewed by 350
Abstract
In this article, we calculate the deflection angle of a tidal charged black hole (TCBH) in weak field limits. First, we obtain the Gaussian optical curvature and then apply the Gauss–Bonnet theorem on it. With the help of Gibbons–Werner method, we are able [...] Read more.
In this article, we calculate the deflection angle of a tidal charged black hole (TCBH) in weak field limits. First, we obtain the Gaussian optical curvature and then apply the Gauss–Bonnet theorem on it. With the help of Gibbons–Werner method, we are able to calculate the light’s deflection angle by TCBH in weak field limits. After calculating the deflection angle of light, we check the graphical behavior of TCBH. Moreover, we further find the light’s deflection angle in the presence of the plasma medium and also check the graphical behavior in the presence of the plasma medium. Moreover, we investigate the shadow of TCBH. For calculating the shadow, we first find the null geodesics around the TCBH and then find its shadow radius. We also obtain TCBH’s shadow in the plasma medium. Hence, we discuss the shadow of the TCBH, using the M87* parameters announced by the event horizon telescope. Full article
(This article belongs to the Section Gravitation)
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Article
Multi-Modal Clustering Events Observed by Horizon-10T and Axion Quark Nuggets
Universe 2021, 7(10), 384; https://doi.org/10.3390/universe7100384 - 15 Oct 2021
Viewed by 276
Abstract
The Horizon-10T collaboration have reported observation of Multi-Modal Events (MME) containing multiple peaks suggesting their clustering origin. These events are proven to be hard to explain in terms of conventional cosmic rays (CR). We propose that these MMEs might be result of the [...] Read more.
The Horizon-10T collaboration have reported observation of Multi-Modal Events (MME) containing multiple peaks suggesting their clustering origin. These events are proven to be hard to explain in terms of conventional cosmic rays (CR). We propose that these MMEs might be result of the dark matter annihilation events within the so-called axion quark nugget (AQN) dark matter model, which was originally invented for completely different purpose to explain the observed similarity between the dark and the visible components in the Universe, i.e., ΩDMΩvisible without any fitting parameters. We support this proposal by demonstrating that the observations, including the frequency of appearance, intensity, the spatial distribution, the time duration, the clustering features, and many other properties nicely match the emission characteristics of the AQN annihilation events in atmosphere. We list a number of features of the AQN events which are very distinct from conventional CR air showers. The observation (non-observation) of these features may substantiate (refute) our proposal. Full article
(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy)
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Communication
Solving Linear Tensor Equations
Universe 2021, 7(10), 383; https://doi.org/10.3390/universe7100383 - 15 Oct 2021
Viewed by 157
Abstract
We develop a systematic way to solve linear equations involving tensors of arbitrary rank. We start off with the case of a rank 3 tensor, which appears in many applications, and after finding the condition for a unique solution we derive this solution. [...] Read more.
We develop a systematic way to solve linear equations involving tensors of arbitrary rank. We start off with the case of a rank 3 tensor, which appears in many applications, and after finding the condition for a unique solution we derive this solution. Subsequently, we generalize our result to tensors of arbitrary rank. Finally, we consider a generalized version of the former case of rank 3 tensors and extend the result when the tensor traces are also included. Full article
Article
Equation of State and Composition of Proto-Neutron Stars and Merger Remnants with Hyperons
Universe 2021, 7(10), 382; https://doi.org/10.3390/universe7100382 - 15 Oct 2021
Viewed by 199
Abstract
Finite-temperature equation of state (EoS) and the composition of dense nuclear and hypernuclear matter under conditions characteristic of neutron star binary merger remnants and supernovas are discussed. We consider both neutrino free-streaming and trapped regimes which are separated by a temperature of a [...] Read more.
Finite-temperature equation of state (EoS) and the composition of dense nuclear and hypernuclear matter under conditions characteristic of neutron star binary merger remnants and supernovas are discussed. We consider both neutrino free-streaming and trapped regimes which are separated by a temperature of a few MeV. The formalism is based on covariant density functional (CDF) theory for the full baryon octet with density-dependent couplings, suitably adjusted in the hypernuclear sector. The softening of the EoS with the introduction of the hyperons is quantified under various conditions of lepton fractions and temperatures. We find that Λ, Ξ, and Ξ0 hyperons appear in the given order with a sharp density increase at zero temperature at the threshold being replaced by an extended increment over a wide density range at high temperatures. The Λ hyperon survives in the deep subnuclear regime. The triplet of Σs is suppressed in cold hypernuclear matter up to around seven times the nuclear saturation density, but appears in significant fractions at higher temperatures, T20 MeV, in both supernova and merger remnant matter. We point out that a special isospin degeneracy point exists where the baryon abundances within each of the three isospin multiplets are equal to each other as a result of (approximate) isospin symmetry. At that point, the charge chemical potential of the system vanishes. We find that under the merger remnant conditions, the fractions of electron and μ-on neutrinos are close and are about 1%, whereas in the supernova case, we only find a significant fraction (∼10%) of electron neutrinos, given that in this case, the μ-on lepton number is zero. Full article
(This article belongs to the Special Issue Properties and Dynamics of Neutron Stars and Proto-Neutron Stars)
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Communication
The Stochastic Gravitational Wave Background from Magnetars
Universe 2021, 7(10), 381; https://doi.org/10.3390/universe7100381 - 14 Oct 2021
Viewed by 234
Abstract
Magnetars have already been a potential candidate as gravitational wave sources that could be detected by current and future terrestrial as well as ground-based gravitational wave detectors. In this article, we focus on the gravitational wave emission from the distorted rotating neutron stars. [...] Read more.
Magnetars have already been a potential candidate as gravitational wave sources that could be detected by current and future terrestrial as well as ground-based gravitational wave detectors. In this article, we focus on the gravitational wave emission from the distorted rotating neutron stars. The deformation is assumed to be symmetric around an axis that is perpendicular to the rotation axis. The form is applied in the context of a neutron star whose magnetic field has been deformed on its own. By introducing the effects from all magnetars in the Universe, based on various proposed magnetic field configurations, such as poloidal and toroidal, the stochastic gravitational wave background can be generated. We choose to figure out exactly how the observations of the stochastic gravitational wave background should be used to understand much more about physics correlated with the magnetar behavior, based on the restriction on the ellipticity of the magnetar. Full article
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Article
Analysis of Birefringence and Dispersion Effects from Spacetime-Symmetry Breaking in Gravitational Waves
Universe 2021, 7(10), 380; https://doi.org/10.3390/universe7100380 - 14 Oct 2021
Cited by 3 | Viewed by 256
Abstract
In this work, we review the effective field theory framework to search for Lorentz and CPT symmetry breaking during the propagation of gravitational waves. The article is written so as to bridge the gap between the theory of spacetime-symmetry breaking and the analysis [...] Read more.
In this work, we review the effective field theory framework to search for Lorentz and CPT symmetry breaking during the propagation of gravitational waves. The article is written so as to bridge the gap between the theory of spacetime-symmetry breaking and the analysis of gravitational-wave signals detected by ground-based interferometers. The primary physical effects beyond General Relativity that we explore here are dispersion and birefringence of gravitational waves. We discuss their implementation in the open-source LIGO-Virgo algorithm library suite, and we discuss the statistical method used to perform a Bayesian inference of the posterior probability of the coefficients for symmetry-breaking. We present preliminary results of this work in the form of simulations of modified gravitational waveforms, together with sensitivity studies of the measurements of the coefficients for Lorentz and CPT violation. The findings show the high potential of gravitational wave sources across the sky to sensitively probe for these signals of new physics. Full article
(This article belongs to the Special Issue Exotic Phenomenon with Gravitational Waves)
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Article
Higher Regularity, Inverse and Polyadic Semigroups
Universe 2021, 7(10), 379; https://doi.org/10.3390/universe7100379 - 13 Oct 2021
Viewed by 308
Abstract
We generalize the regularity concept for semigroups in two ways simultaneously: to higher regularity and to higher arity. We show that the one-relational and multi-relational formulations of higher regularity do not coincide, and each element has several inverses. The higher idempotents are introduced, [...] Read more.
We generalize the regularity concept for semigroups in two ways simultaneously: to higher regularity and to higher arity. We show that the one-relational and multi-relational formulations of higher regularity do not coincide, and each element has several inverses. The higher idempotents are introduced, and their commutation leads to unique inverses in the multi-relational formulation, and then further to the higher inverse semigroups. For polyadic semigroups we introduce several types of higher regularity which satisfy the arity invariance principle as introduced: the expressions should not depend of the numerical arity values, which allows us to provide natural and correct binary limits. In the first definition no idempotents can be defined, analogously to the binary semigroups, and therefore the uniqueness of inverses can be governed by shifts. In the second definition called sandwich higher regularity, we are able to introduce the higher polyadic idempotents, but their commutation does not provide uniqueness of inverses, because of the middle terms in the higher polyadic regularity conditions. Finally, we introduce the sandwich higher polyadic regularity with generalized idempotents. Full article
(This article belongs to the Special Issue Gauge Theory, Strings and Supergravity)
Article
The Early Evolution of Solar Flaring Plasma Loops
Universe 2021, 7(10), 378; https://doi.org/10.3390/universe7100378 - 11 Oct 2021
Viewed by 258
Abstract
Plasma loops are the elementary structures of solar flaring active regions and dominate the whole process of flaring eruptions. Standard flare models explain evolution and eruption after magnetic reconnection around the hot cusp-structure above the top of plasma loops very well; however, the [...] Read more.
Plasma loops are the elementary structures of solar flaring active regions and dominate the whole process of flaring eruptions. Standard flare models explain evolution and eruption after magnetic reconnection around the hot cusp-structure above the top of plasma loops very well; however, the early evolution of plasma loops before the onset of magnetic reconnection is poorly understood. Considering that magnetic gradients are ubiquitous in solar plasma loops, this work applies the magnetic-gradient pumping (MGP) mechanism to study the early evolution of flaring plasma loops. The results indicate that early evolution depends on the magnetic field distribution and the geometry of the plasma loops, which dominate the balance between the accumulation and dissipation of the energy around loop tops. Driven by MGP process, both of the density and temperature as well as the plasma β value around the looptop will increase in the early phase of the plasma loop’s evolution. In fact, the solar plasma loops will have two distinct evolutionary results: low, initially dense plasma loops with relatively strong magnetic fields tend to be stable for their maximum β value, which is always smaller than the critical value β<βc, while the higher, initially diluted solar plasma loops with relatively weak magnetic fields tend to be unstable for their β values, exceeding the critical value β>βc at a time of about one hour after the formation of the solar-magnetized plasma loop. The latter may produce ballooning instability and may finally trigger the following magnetic reconnection and eruptions. These physical scenarios may provide us with a new viewpoint to understand the nature and origin of solar flares. Full article
(This article belongs to the Special Issue Solar Coronal Loop Dynamics)
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Article
Analytical Solution and Quasi-Periodic Behavior of a Charged Dilaton Black Hole
Universe 2021, 7(10), 377; https://doi.org/10.3390/universe7100377 - 09 Oct 2021
Viewed by 381
Abstract
With the vast breakthrough brought by the Event Horizon Telescope, the theoretical analysis of various black holes has become more critical than ever. In this paper, the second-order asymptotic analytical solution of the charged dilaton black hole flow in the spinodal region is [...] Read more.
With the vast breakthrough brought by the Event Horizon Telescope, the theoretical analysis of various black holes has become more critical than ever. In this paper, the second-order asymptotic analytical solution of the charged dilaton black hole flow in the spinodal region is constructed from the perspective of dynamics by using the two-timing scale method. Through a numerical comparison with the original charged dilaton black hole system, it is found that the constructed analytical solution is highly consistent with the numerical solution. In addition, several quasi-periodic motions of the charged dilaton black hole flow are numerically obtained under different groups of irrational frequency ratios, and the phase portraits of the black hole flow with sufficiently small thermal parameter perturbation display good stability. Finally, the final evolution state of black hole flow over time is studied according to the obtained analytical solution. The results show that the smaller the integral constant of the system, the greater the periodicity of the black hole flow. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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Review
Hyperons in Finite and Infinite Nuclear Systems
Universe 2021, 7(10), 376; https://doi.org/10.3390/universe7100376 - 09 Oct 2021
Viewed by 238
Abstract
In this work, we shortly review the role and properties of hyperons in finite and infinite nuclear systems such as hypernuclei and neutron stars. Particularly, we describe different production mechanisms of hypernuclei, discuss some aspects of their γ-ray spectroscopy and their weak [...] Read more.
In this work, we shortly review the role and properties of hyperons in finite and infinite nuclear systems such as hypernuclei and neutron stars. Particularly, we describe different production mechanisms of hypernuclei, discuss some aspects of their γ-ray spectroscopy and their weak decay modes, and give a few strokes on their theoretical description. We reexamine also the role played by hyperons on the properties of neutron and proto-neutron stars with a special emphasis on the well-known “hyperon puzzle”, of which we discuss some of the solutions that have been proposed to tackle this problem. Finally, we review the role of hyperons on the cooling properties of newly born neutron stars and on the so-called r-mode instability. Full article
(This article belongs to the Special Issue Nuclear Physics and Multimessenger Astrophysics)
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Article
Lomonosov GRB Catalogue: The First Experience of Prompt Emission Multi-Wavelength Observations
Universe 2021, 7(10), 375; https://doi.org/10.3390/universe7100375 - 09 Oct 2021
Viewed by 258
Abstract
This paper presents a catalogue of gamma-ray bursts (GRBs) that were detected by the instruments onboard the Lomonosov space observatory. The Lomonosov mission gave the first experience of not only multi-wavelength (from optical to gamma) observations of GRBs but also multi-messenger observations of [...] Read more.
This paper presents a catalogue of gamma-ray bursts (GRBs) that were detected by the instruments onboard the Lomonosov space observatory. The Lomonosov mission gave the first experience of not only multi-wavelength (from optical to gamma) observations of GRBs but also multi-messenger observations of extreme phenomena and GRBs. The detailed light curves and energy spectra of the detected GRBs are presented. The results of the prompt, early an afterglow optical observations of several GRBs are discussed. Full article
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Review
Evolution of Data Formats in Very-High-Energy Gamma-Ray Astronomy
Universe 2021, 7(10), 374; https://doi.org/10.3390/universe7100374 - 08 Oct 2021
Viewed by 372
Abstract
Most major scientific results produced by ground-based gamma-ray telescopes in the last 30 years have been obtained by expert members of the collaborations operating these instruments. This is due to the proprietary data and software policies adopted by these collaborations. However, the advent [...] Read more.
Most major scientific results produced by ground-based gamma-ray telescopes in the last 30 years have been obtained by expert members of the collaborations operating these instruments. This is due to the proprietary data and software policies adopted by these collaborations. However, the advent of the next generation of telescopes and their operation as observatories open to the astronomical community, along with a generally increasing demand for open science, confront gamma-ray astronomers with the challenge of sharing their data and analysis tools. As a consequence, in the last few years, the development of open-source science tools has progressed in parallel with the endeavour to define a standardised data format for astronomical gamma-ray data. The latter constitutes the main topic of this review. Common data specifications provide equally important benefits to the current and future generation of gamma-ray instruments: they allow the data from different instruments, including legacy data from decommissioned telescopes, to be easily combined and analysed within the same software framework. In addition, standardised data accessible to the public, and analysable with open-source software, grant fully-reproducible results. In this article, we provide an overview of the evolution of the data format for gamma-ray astronomical data, focusing on its progression from private and diverse specifications to prototypical open and standardised ones. The latter have already been successfully employed in a number of publications paving the way to the analysis of data from the next generation of gamma-ray instruments, and to an open and reproducible way of conducting gamma-ray astronomy. Full article
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Article
The Nuclear Matter Density Functional under the Nucleonic Hypothesis
Universe 2021, 7(10), 373; https://doi.org/10.3390/universe7100373 - 06 Oct 2021
Cited by 4 | Viewed by 352
Abstract
A Bayesian analysis of the possible behaviors of the dense matter equation of state informed by recent LIGO-Virgo as well as NICER measurements reveals that all the present observations are compatible with a fully nucleonic hypothesis for the composition of dense matter, even [...] Read more.
A Bayesian analysis of the possible behaviors of the dense matter equation of state informed by recent LIGO-Virgo as well as NICER measurements reveals that all the present observations are compatible with a fully nucleonic hypothesis for the composition of dense matter, even in the core of the most massive pulsar PSR J0740+6620. Under the hypothesis of a nucleonic composition, we extract the most general behavior of the energy per particle of symmetric matter and density dependence of the symmetry energy, compatible with the astrophysical observations as well as our present knowledge of low-energy nuclear physics from effective field theory predictions and experimental nuclear mass data. These results can be used as a null hypothesis to be confronted with future constraints on dense matter to search for possible exotic degrees of freedom. Full article
(This article belongs to the Special Issue Properties and Dynamics of Neutron Stars and Proto-Neutron Stars)
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Article
A New Sample of Gamma-Ray Emitting Jetted Active Galactic Nuclei—Preliminary Results
Universe 2021, 7(10), 372; https://doi.org/10.3390/universe7100372 - 05 Oct 2021
Viewed by 687
Abstract
We are compiling a new list of gamma-ray jetted active galactic nuclei (AGN), starting from the fourth catalog of point sources of the Fermi Large Area Telescope (LAT). Our aim is to prepare a list of jetted AGN with known redshifts and classifications [...] Read more.
We are compiling a new list of gamma-ray jetted active galactic nuclei (AGN), starting from the fourth catalog of point sources of the Fermi Large Area Telescope (LAT). Our aim is to prepare a list of jetted AGN with known redshifts and classifications to be used to calibrate jet power. We searched in the available literature for all the published optical spectra and multiwavelength studies useful to characterize the sources. We found new, missed, or even forgotten information leading to a substantial change in the redshift values and classification of many sources. We present here the preliminary results of this analysis and some statistics based on the gamma-ray sources with right ascension within the interval 0h--12h (J2000). Although flat-spectrum radio quasars and BL Lac objects are still the dominant populations, there is a significant increase in the number of other objects, such as misaligned AGN, narrow-line Seyfert 1 galaxies, and Seyfert galaxies. We also introduced two new classes of objects: changing-look AGN and ambiguous sources. About one third of the sources remain unclassified. Full article
(This article belongs to the Special Issue Panchromatic View of the Life-Cycle of AGN)
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Article
Three-Dimensional MHD Modeling of Interplanetary Solar Wind Using Self-Consistent Boundary Condition Obtained from Multiple Observations and Machine Learning
Universe 2021, 7(10), 371; https://doi.org/10.3390/universe7100371 - 04 Oct 2021
Viewed by 289
Abstract
Three-dimensional (3-d) magnetohydrodynamics (MHD) modeling is a key method for studying the interplanetary solar wind. In this paper, we introduce a new 3-d MHD solar wind model driven by the self-consistent boundary condition obtained from multiple observations and the Artificial Neural Network (ANN) [...] Read more.
Three-dimensional (3-d) magnetohydrodynamics (MHD) modeling is a key method for studying the interplanetary solar wind. In this paper, we introduce a new 3-d MHD solar wind model driven by the self-consistent boundary condition obtained from multiple observations and the Artificial Neural Network (ANN) machine learning technique. At the inner boundary, the magnetic field is derived using the magnetogram and potential field source surface extrapolation; the electron density is derived from the polarized brightness (pB) observations, the velocity can be deduced by an ANN using both the magnetogram and pB observations, and the temperature is derived from the magnetic field and electron density by a self-consistent method. Then, the 3-d interplanetary solar wind from CR2057 to CR2062 is modeled by the new model with the self-consistent boundary conditions. The modeling results present various observational characteristics at different latitudes, and are in better agreement with both the OMNI and Ulysses observations compared to our previous MHD model based only on photospheric magnetic field observations. Full article
(This article belongs to the Special Issue Plasma Physics in Astronomy)
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Article
Hybrid Stars with Color Superconducting Cores in an Extended FCM Model
Universe 2021, 7(10), 370; https://doi.org/10.3390/universe7100370 - 01 Oct 2021
Viewed by 400
Abstract
We investigate the influence of repulsive vector interactions and color superconductivity on the structure of neutron stars using an extended version of the field correlator method (FCM) for the description of quark matter. The hybrid equation of state is constructed using the Maxwell [...] Read more.
We investigate the influence of repulsive vector interactions and color superconductivity on the structure of neutron stars using an extended version of the field correlator method (FCM) for the description of quark matter. The hybrid equation of state is constructed using the Maxwell description, which assumes a sharp hadron-quark phase transition. The equation of state of hadronic matter is computed for a density-dependent relativistic lagrangian treated in the mean-field approximation, with parameters given by the SW4L nuclear model. This model described the interactions among baryons in terms of σ, ω, ρ, σ*, and ϕ mesons. Quark matter is assumed to be in either the CFL or the 2SC+s color superconducting phase. The possibility of sequential (hadron-quark, quark-quark) transitions in ultra-dense matter is investigated. Observed data related to massive pulsars, gravitational-wave events, and NICER are used to constrain the parameters of the extended FCM model. The successful equations of state are used to explore the mass-radius relationship, radii, and tidal deformabilities of hybrid stars. A special focus lies on investigating consequences that slow or fast conversions of quark-hadron matter have on the stability and the mass-radius relationship of hybrid stars. We find that if slow conversion should occur, a new branch of stable massive stars would exist whose members have radii that are up to 1.5 km smaller than those of conventional neutron stars of the same mass. Such objects could be possible candidates for the stellar high-mass object of the GW190425 binary system. Full article
(This article belongs to the Special Issue Properties and Dynamics of Neutron Stars and Proto-Neutron Stars)
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Review
Space-Based Photometry of Binary Stars: From Voyager to TESS
Universe 2021, 7(10), 369; https://doi.org/10.3390/universe7100369 - 30 Sep 2021
Viewed by 282
Abstract
Binary stars are crucial laboratories for stellar physics, so have been photometric targets for space missions beginning with the very first orbiting telescope (OAO-2) launched in 1968. This review traces the binary stars observed and the scientific results obtained from the early days [...] Read more.
Binary stars are crucial laboratories for stellar physics, so have been photometric targets for space missions beginning with the very first orbiting telescope (OAO-2) launched in 1968. This review traces the binary stars observed and the scientific results obtained from the early days of ultraviolet missions (OAO-2, Voyager, ANS, IUE), through a period of diversification (Hipparcos, WIRE, MOST, BRITE), to the current era of large planetary transit surveys (CoRoT, Kepler, TESS). In this time observations have been obtained of detached, semi-detached and contact binaries containing dwarfs, sub-giants, giants, supergiants, white dwarfs, planets, neutron stars and accretion discs. Recent missions have found a huge variety of objects such as pulsating stars in eclipsing binaries, multi-eclipsers, heartbeat stars and binaries hosting transiting planets. Particular attention is paid to eclipsing binaries, because they are staggeringly useful, and to the NASA Transiting Exoplanet Survey Satellite (TESS) because its huge sky coverage enables a wide range of scientific investigations with unprecedented ease. These results are placed into context, future missions are discussed, and a list of important science goals is presented. Full article
(This article belongs to the Special Issue Variable Stars as Seen with Photometric Space Telescopes)
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Article
High-Order Multipole and Binary Love Number Universal Relations
Universe 2021, 7(10), 368; https://doi.org/10.3390/universe7100368 - 30 Sep 2021
Cited by 1 | Viewed by 302 | Correction
Abstract
Using a data set of approximately 2 million phenomenological equations of state consistent with observational constraints, we construct new equation-of-state-insensitive universal relations that exist between the multipolar tidal deformability parameters of neutron stars, Λl, for several high-order multipoles ( [...] Read more.
Using a data set of approximately 2 million phenomenological equations of state consistent with observational constraints, we construct new equation-of-state-insensitive universal relations that exist between the multipolar tidal deformability parameters of neutron stars, Λl, for several high-order multipoles (l=5,6,7,8), and we consider finite-size effects of these high-order multipoles in waveform modeling. We also confirm the existence of a universal relation between the radius of the 1.4M NS, R1.4 and the reduced tidal parameter of the binary, Λ˜, and the chirp mass. We extend this relation to a large number of chirp masses and to the radii of isolated NSs of different mass M, RM. We find that there is an optimal value of M for every M such that the uncertainty in the estimate of RM is minimized when using the relation. We discuss the utility and implications of these relations for the upcoming LIGO O4 run and third-generation detectors. Full article
(This article belongs to the Special Issue Properties and Dynamics of Neutron Stars and Proto-Neutron Stars)
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Review
Fuzzy Instantons in Landscape and Swampland: Review of the Hartle–Hawking Wave Function and Several Applications
Universe 2021, 7(10), 367; https://doi.org/10.3390/universe7100367 - 30 Sep 2021
Viewed by 383
Abstract
The Euclidean path integral is well approximated by instantons. If instantons are dynamical, they will necessarily be complexified. Fuzzy instantons can have multiple physical applications. In slow-roll inflation models, fuzzy instantons can explain the probability distribution of the initial conditions of the universe. [...] Read more.
The Euclidean path integral is well approximated by instantons. If instantons are dynamical, they will necessarily be complexified. Fuzzy instantons can have multiple physical applications. In slow-roll inflation models, fuzzy instantons can explain the probability distribution of the initial conditions of the universe. Although the potential shape does not satisfy the slow-roll conditions due to the swampland criteria, the fuzzy instantons can still explain the origin of the universe. If we extend the Euclidean path integral beyond the Hartle–Hawking no-boundary proposal, it becomes possible to examine fuzzy Euclidean wormholes that have multiple physical applications in cosmology and black hole physics. Full article
(This article belongs to the Special Issue Quantum Cosmology)
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Article
Hints for a Gravitational Transition in Tully–Fisher Data
Universe 2021, 7(10), 366; https://doi.org/10.3390/universe7100366 - 29 Sep 2021
Cited by 5 | Viewed by 322
Abstract
We use an up-to-date compilation of Tully–Fisher data to search for transitions in the evolution of the Tully–Fisher relation. Using an up-to-date data compilation, we find hints at 3σ level for a transition at critical distances Dc9 Mpc [...] Read more.
We use an up-to-date compilation of Tully–Fisher data to search for transitions in the evolution of the Tully–Fisher relation. Using an up-to-date data compilation, we find hints at 3σ level for a transition at critical distances Dc9 Mpc and Dc17 Mpc. We split the full sample in two subsamples, according to the measured galaxy distance with respect to splitting distance Dc, and identify the likelihood of the best-fit slope and intercept of one sample with respect to the best-fit corresponding values of the other sample. For Dc9 Mpc and Dc17 Mpc, we find a tension between the two subsamples at a level of Δχ2>17(3.5σ). Using Monte Carlo simulations, we demonstrate that this result is robust with respect to random statistical and systematic variations of the galactic distances and is unlikely in the context of a homogeneous dataset constructed using the Tully–Fisher relation. If the tension is interpreted as being due to a gravitational strength transition, it would imply a shift in the effective gravitational constant to lower values for distances larger than Dc by ΔGG0.1. Such a shift is of the anticipated sign and magnitude but at a somewhat lower distance (redshift) than the gravitational transition recently proposed to address the Hubble and growth tensions (ΔGG0.1 at the transition redshift of zt0.01 (Dc40 Mpc)). Full article
(This article belongs to the Section Cosmology)
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Article
The Epoch of Reionization in Warm Dark Matter Scenarios
Universe 2021, 7(10), 365; https://doi.org/10.3390/universe7100365 - 29 Sep 2021
Cited by 1 | Viewed by 451
Abstract
In this paper we investigate how the Reionization process is affected by early galaxy formation in different cosmological scenarios. We use a semi-analytic model with suppressed initial power spectra to obtain the UV Luminosity Function in thermal Warm Dark Matter and sterile neutrino [...] Read more.
In this paper we investigate how the Reionization process is affected by early galaxy formation in different cosmological scenarios. We use a semi-analytic model with suppressed initial power spectra to obtain the UV Luminosity Function in thermal Warm Dark Matter and sterile neutrino cosmologies. We retrace the ionization history of intergalactic medium with hot stellar emission only, exploiting fixed and variable photons escape fraction models (fesc). For each cosmology, we find an upper limit to fixed fesc, which guarantees the completion of the process at z<6.7. The analysis is tested with two limit hypothesis on high-z ionized hydrogen volume fraction, comparing our predictions with observational results. Full article
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Article
Black Hole Spin and Stellar Flyby Periastron Shift
Universe 2021, 7(10), 364; https://doi.org/10.3390/universe7100364 - 29 Sep 2021
Viewed by 574
Abstract
For a scenario of a close flyby of a compact star near a spinning black hole, we provide analytical and numerical estimates for the shift of trajectory periastron due to relativistic (beyond post-Newtonian) effects. More specifically, we derived a generalized expression (not limited [...] Read more.
For a scenario of a close flyby of a compact star near a spinning black hole, we provide analytical and numerical estimates for the shift of trajectory periastron due to relativistic (beyond post-Newtonian) effects. More specifically, we derived a generalized expression (not limited to quasi-circular or elliptical orbits) directly linking the periastron shift and the spin of the black hole. The expression permits the estimation of black hole spin based on astronomical tracking of locations of stars traveling along highly eccentric (parabolic and hyperbolic) trajectories in close vicinity of a black hole. We also demonstrate how stars traveling on hyperbolic or parabolic trajectories may be (temporarily) captured onto quasi-circular orbits around black holes, and we quantitatively examine conditions for such scenarios. Full article
(This article belongs to the Section Stellar Astronomy)
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Article
Stringy Bubbles Solve de Sitter Troubles
Universe 2021, 7(10), 363; https://doi.org/10.3390/universe7100363 - 28 Sep 2021
Cited by 1 | Viewed by 261
Abstract
Finding four-dimensional de Sitter spacetime solutions in string theory has been a vexing quest ever since the discovery of the accelerating expansion of the universe. Building on a recent analysis of bubble-nucleation in the decay of (false-vacuum) AdS backgrounds where the interfacing bubbles [...] Read more.
Finding four-dimensional de Sitter spacetime solutions in string theory has been a vexing quest ever since the discovery of the accelerating expansion of the universe. Building on a recent analysis of bubble-nucleation in the decay of (false-vacuum) AdS backgrounds where the interfacing bubbles themselves exhibit a de Sitter geometry we show that this resonates strongly with a stringy cosmic brane construction that naturally provides for an exponential mass-hierarchy and the localization of both gravity and matter, in addition to an exponentially suppressed positive cosmological constant. Finally, we argue that these scenarios can be realized in terms of a generalization of a small resolution of a conifold singularity in the context of a (Lorentzian) Calabi–Yau 5-fold, where the isolated (Lorentzian) two complex dimensional Fano variety is a four-dimensional de Sitter spacetime. Full article
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Article
Statefinder and Om Diagnostics for New Generalized Chaplygin Gas Model
Universe 2021, 7(10), 362; https://doi.org/10.3390/universe7100362 - 28 Sep 2021
Cited by 1 | Viewed by 288
Abstract
We explore a unified model of dark matter and dark energy. This new model is a generalization of the generalized Chaplygin gas model and is known as a new generalized Chaplygin gas (NGCG) model. We study the evolutions of the Hubble parameter and [...] Read more.
We explore a unified model of dark matter and dark energy. This new model is a generalization of the generalized Chaplygin gas model and is known as a new generalized Chaplygin gas (NGCG) model. We study the evolutions of the Hubble parameter and the distance modulus for the model under consideration and the standard ΛCDM model and compare that with the observational datasets. Furthermore, we demonstrate two geometric diagnostics analyses including the statefinder (r,s) and Om(z) to the discriminant NGCG model from the standard ΛCDM model. The trajectories of evolution for (r,s) and Om(z) diagnostic planes are shown to understand the geometrical behavior of the NGCG model by using different observational data points. Full article
(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy)
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