Universe

12 pages, 277 KiB

Open AccessArticle

Semiheaps and Ternary Algebras in Quantum Mechanics Revisited

by Andrew James Bruce

Universe 2022, 8(1), 56; https://doi.org/10.3390/universe8010056 - 17 Jan 2022

Cited by 6 | Viewed by 2747

We re-examine the appearance of semiheaps and (para-associative) ternary algebras in quantum mechanics. In particular, we review the construction of a semiheap on a Hilbert space and the set of bounded operators on a Hilbert space. The new aspect of this work is [...] Read more.

We re-examine the appearance of semiheaps and (para-associative) ternary algebras in quantum mechanics. In particular, we review the construction of a semiheap on a Hilbert space and the set of bounded operators on a Hilbert space. The new aspect of this work is a discussion of how symmetries of a quantum system induce homomorphisms of the relevant semiheaps and ternary algebras. Full article

(This article belongs to the Special Issue Selected Topics in Gravity, Field Theory and Quantum Mechanics)

10 pages, 1177 KiB

Open AccessCommunication

One-Point Statistics Matter in Extended Cosmologies

by Alex Gough and Cora Uhlemann

Universe 2022, 8(1), 55; https://doi.org/10.3390/universe8010055 - 17 Jan 2022

Cited by 6 | Viewed by 2268

Abstract

The late universe contains a wealth of information about fundamental physics and gravity, wrapped up in non-Gaussian fields. To make use of as much information as possible, it is necessary to go beyond two-point statistics. Rather than going to higher-order N-point correlation [...] Read more.

The late universe contains a wealth of information about fundamental physics and gravity, wrapped up in non-Gaussian fields. To make use of as much information as possible, it is necessary to go beyond two-point statistics. Rather than going to higher-order N-point correlation functions, we demonstrate that the probability distribution function (PDF) of spheres in the matter field (a one-point function) already contains a significant amount of this non-Gaussian information. The matter PDF dissects different density environments which are lumped together in two-point statistics, making it particularly useful for probing modifications of gravity or expansion history. Our approach in Cataneo et al. 2021 extends the success of Large Deviation Theory for predicting the matter PDF in

Λ

CDM in these “extended” cosmologies. A Fisher forecast demonstrates the information content in the matter PDF via constraints for a Euclid-like survey volume combining the 3D matter PDF with the 3D matter power spectrum. Adding the matter PDF halves the uncertainties on parameters in an evolving dark energy model, relative to the power spectrum alone. Additionally, the matter PDF contains enough non-linear information to substantially increase the detection significance of departures from General Relativity, with improvements up to six times the power spectrum alone. This analysis demonstrates that the matter PDF is a promising non-Gaussian statistic for extracting cosmological information, particularly for beyond

Λ

CDM models. Full article

(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)

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18 pages, 1262 KiB

Open AccessArticle

Constraints on the Production of Phosphine by Venusian Volcanoes

by William Bains, Oliver Shorttle, Sukrit Ranjan, Paul B. Rimmer, Janusz J. Petkowski, Jane S. Greaves and Sara Seager

Universe 2022, 8(1), 54; https://doi.org/10.3390/universe8010054 - 17 Jan 2022

Cited by 12 | Viewed by 2493

Abstract

The initial reports of the presence of phosphine in the cloud decks of Venus have led to the suggestion that volcanism is the source of phosphine, through volcanic phosphides ejected into the clouds. Here, we examine the idea that mantle plume volcanism, bringing [...] Read more.

The initial reports of the presence of phosphine in the cloud decks of Venus have led to the suggestion that volcanism is the source of phosphine, through volcanic phosphides ejected into the clouds. Here, we examine the idea that mantle plume volcanism, bringing material from the deep mantle to the surface, could generate observed amounts of phosphine through the interaction of explosively erupted phosphide with sulfuric acid clouds. The direct eruption of deep mantle phosphide is unphysical, but a shallower material could contain traces of phosphide, and could be erupted to the surface. The explosive eruption that efficiently transports material to the clouds would require ocean:magma interactions or the subduction of a hydrated oceanic crust, neither of which occur on modern Venus. The transport of the erupted material to altitudes coinciding with the observations of phosphine is consequently very inefficient. Using the model proposed by Truong and Lunine as a base case, we estimate that an eruption volume of at least 21,600 km³/year would be required to explain the presence of 1 ppb phosphine in the clouds. This is greater than any historical terrestrial eruption rate, and would have several detectable consequences for remote and in situ observations to confirm. More realistic lithospheric mineralogy, volcano mechanics or atmospheric photochemistry require even more volcanism. Full article

(This article belongs to the Section Planetary Sciences)

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12 pages, 394 KiB

Open AccessArticle

Entropy of Artificial Intelligence

by Tamás Sándor Biró and Antal Jakovác

Universe 2022, 8(1), 53; https://doi.org/10.3390/universe8010053 - 16 Jan 2022

Cited by 1 | Viewed by 2688

Abstract

We describe a model of artificial intelligence systems based on the dimension of the probability space of the input set available for recognition. In this scenario, we can understand a subset, which means that we can decide whether an object is an element [...] Read more.

We describe a model of artificial intelligence systems based on the dimension of the probability space of the input set available for recognition. In this scenario, we can understand a subset, which means that we can decide whether an object is an element of a given subset or not in an efficient way. In the machine learning (ML) process we define appropriate features, in this way shrinking the defining bit-length of classified sets during the learning process. This can also be described in the language of entropy: while natural processes tend to increase the disorder, that is, increase the entropy, learning creates order, and we expect that it decreases a properly defined entropy. Full article

(This article belongs to the Section Mathematical Physics)

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17 pages, 2366 KiB

Open AccessArticle

Estimating the Cosmological Constant from Shadows of Kerr–de Sitter Black Holes

by Misba Afrin and Sushant G. Ghosh

Universe 2022, 8(1), 52; https://doi.org/10.3390/universe8010052 - 16 Jan 2022

Cited by 19 | Viewed by 2612

Abstract

The Event Horizon Telescope collaboration has revealed the first direct image of a black hole, as per the shadow of a Kerr black hole of general relativity. However, other Kerr-like rotating black holes of modified gravity theories cannot be ignored, and they are [...] Read more.

The Event Horizon Telescope collaboration has revealed the first direct image of a black hole, as per the shadow of a Kerr black hole of general relativity. However, other Kerr-like rotating black holes of modified gravity theories cannot be ignored, and they are essential as they offer an arena in which these theories can be tested through astrophysical observation. This motivates us to investigate asymptotically de Sitter rotating black holes wherein interpreting the cosmological constant

Λ

as the vacuum energy leads to a deformation in the vicinity of a black hole—new Kerr–de Sitter solution, which has a richer geometric structure than the original one. We derive an analytical formula necessary for the shadow of the new Kerr–de Sitter black holes and then visualize the shadow of black holes for various parameters for an observer at given coordinates

(r_{0}, θ_{0})

in the domain

(r_{0}, r_{c})

and estimate the cosmological constant

Λ

from its shadow observables. The shadow observables of the new Kerr–de Sitter black holes significantly deviate from the corresponding observables of the Kerr–de Sitter black hole over an appreciable range of the parameter space. Interestingly, we find a finite parameter space for (

Λ

, a) where the observables of the two black holes are indistinguishable. Full article

(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)

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8 pages, 252 KiB

Open AccessCommunication

A Note on Singularity Avoidance in Fourth-Order Gravity

by Luca Fabbri

Universe 2022, 8(1), 51; https://doi.org/10.3390/universe8010051 - 13 Jan 2022

Cited by 1 | Viewed by 1730

Abstract

We consider the fourth-order differential theory of gravitation to treat the problem of singularity avoidance: studying the short-distance behaviour in the case of black-holes and the big-bang we are going to see a way to attack the issue from a general perspective. Full article

(This article belongs to the Special Issue Higher-Derivative Theories of Gravity)

13 pages, 312 KiB

Open AccessReview

Quantum Gravity Phenomenology from the Thermodynamics of Spacetime

by Ana Alonso-Serrano and Marek Liška

Universe 2022, 8(1), 50; https://doi.org/10.3390/universe8010050 - 13 Jan 2022

Cited by 1 | Viewed by 2273

Abstract

This work is based on the formalism developed in the study of the thermodynamics of spacetime used to derive Einstein equations from the proportionality of entropy within an area. When low-energy quantum gravity effects are considered, an extra logarithmic term in the area [...] Read more.

This work is based on the formalism developed in the study of the thermodynamics of spacetime used to derive Einstein equations from the proportionality of entropy within an area. When low-energy quantum gravity effects are considered, an extra logarithmic term in the area is added to the entropy expression. Here, we present the derivation of the quantum modified gravitational dynamics from this modified entropy expression and discuss its main features. Furthermore, we outline the application of the modified dynamics to cosmology, suggesting the replacement of the Big Bang singularity with a regular bounce. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

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4 pages, 172 KiB

Open AccessEditorial

Modern Approaches to Non-Perturbative QCD and Other Confining Gauge Theories

by Dmitry Antonov

Universe 2022, 8(1), 49; https://doi.org/10.3390/universe8010049 - 13 Jan 2022

Cited by 1 | Viewed by 1240

Abstract

The primary goal of this Special Issue was to create a collection of reviews on the modern approaches to the problem of quark confinement in QCD [...] Full article

(This article belongs to the Special Issue Modern Approaches to Non-Perturbative QCD and other Confining Gauge Theories)

16 pages, 719 KiB

Open AccessArticle

Equations of State for Hadronic Matter and Mass-Radius Relations of Neutron Stars with Strong Magnetic Fields

by Chinatsu Watanabe, Naotaka Yoshinaga and Shuichiro Ebata

Universe 2022, 8(1), 48; https://doi.org/10.3390/universe8010048 - 12 Jan 2022

Cited by 4 | Viewed by 1834

Abstract

Neutron star is an important object for us to verify the equation of state of hadronic matter. For a specific choice of equations of state, mass and radius of a neutron star are determined, for which there are constraints from observations. According to [...] Read more.

Neutron star is an important object for us to verify the equation of state of hadronic matter. For a specific choice of equations of state, mass and radius of a neutron star are determined, for which there are constraints from observations. According to some previous studies, since the strong magnetic field acts as a repulsive force, there is a possibility that neutron stars with strong magnetic fields may have relatively heavier masses than other non-magnetized neutron stars. In this paper, the structure of a neutron star with a strong internal magnetic field is investigated by changing its internal functional form to see how much the neutron star can be massive and also how radius of a neutron star can be within a certain range. Full article

(This article belongs to the Special Issue Nuclear Physics and Multimessenger Astrophysics)

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16 pages, 1043 KiB

Open AccessArticle

Scalar Perturbations of Black Holes in the

f (R) = R - 2 α \sqrt{R}

Model

by Ping Li, Rui Jiang, Jian Lv and Xianghua Zhai

Universe 2022, 8(1), 47; https://doi.org/10.3390/universe8010047 - 12 Jan 2022

Cited by 2 | Viewed by 1757

Abstract

In this paper, we study the perturbations of the charged static spherically symmetric black holes in the

f (R) = R - 2 α \sqrt{R}

model by a scalar field. We analyze the quasinormal modes spectrum, superradiant modes, and superradiant instability [...] Read more.

In this paper, we study the perturbations of the charged static spherically symmetric black holes in the

f (R) = R - 2 α \sqrt{R}

model by a scalar field. We analyze the quasinormal modes spectrum, superradiant modes, and superradiant instability of the black holes. The frequency of the quasinormal modes is calculated in the frequency domain by the third-order WKB method, and in the time domain by the finite difference method. The results by the two methods are consistent and show that the black hole stabilizes quicker for larger

α

satisfying the horizon condition. We then analyze the superradiant modes when the massive charged scalar field is scattered by the black hole. The frequency of the superradiant wave satisfies

ω \in (\frac{μ}{\sqrt{2}}, ω_{c})

, where

μ

is the mass of the scalar field, and

ω_{c}

is the critical frequency of the superradiance. The amplification factor is also calculated by numerical method. Furthermore, the superradiant instability of the black hole is studied analytically, and the results show that there is no superradiant instability for such a system. Full article

(This article belongs to the Collection Nobel Prize 2020: Selected Articles on Black Hole and General Relativity)

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20 pages, 21570 KiB

Open AccessArticle

EAS Observation Conditions in the SPHERE-2 Balloon Experiment

by Elena Bonvech, Dmitry Chernov, Miroslav Finger, Michael Finger, Vladimir Galkin, Dmitry Podgrudkov, Tatiana Roganova and Igor Vaiman

Universe 2022, 8(1), 46; https://doi.org/10.3390/universe8010046 - 11 Jan 2022

Cited by 4 | Viewed by 1606

Abstract

The SPHERE project studies primary cosmic rays by detection of the Cherenkov light of extensive air showers reflected from the snow covered surface of the earth. Measurements with the aerial-based detector SPHERE-2 were performed in 2011–2013. The detector was lifted by a balloon [...] Read more.

The SPHERE project studies primary cosmic rays by detection of the Cherenkov light of extensive air showers reflected from the snow covered surface of the earth. Measurements with the aerial-based detector SPHERE-2 were performed in 2011–2013. The detector was lifted by a balloon to altitudes of up to 900 m above the snow covered surface of Lake Baikal, Russia. The results of the experiment are summarized now in a series of papers that opens with this article. An overview of the SPHERE-2 detector telemetry monitoring systems is presented along with the analysis of the measurements conditions including atmosphere profile. The analysis of the detector state and environment atmosphere conditions monitoring provided various cross-checks of detector calibration, positioning, and performance. Full article

(This article belongs to the Special Issue Astroparticle Physics)

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19 pages, 1494 KiB

Open AccessArticle

Nucleosynthesis, Mixing Processes, and Gas Pollution from AGB Stars

by Paolo Ventura, Flavia Dell’Agli, Marco Tailo, Marco Castellani, Ester Marini, Silvia Tosi and Marcella Di Criscienzo

Universe 2022, 8(1), 45; https://doi.org/10.3390/universe8010045 - 11 Jan 2022

Cited by 6 | Viewed by 1789

Abstract

We discuss the evolution of stars through the asymptotic giant branch, focusing on the physical mechanisms potentially able to alter the surface chemical composition and on how changes in the chemistry of the external regions affect the physical properties of the star and [...] Read more.

We discuss the evolution of stars through the asymptotic giant branch, focusing on the physical mechanisms potentially able to alter the surface chemical composition and on how changes in the chemistry of the external regions affect the physical properties of the star and the duration of this evolutionary phase. We focus on the differences between the evolution of low-mass stars, driven by the growth of the core mass and by the surface carbon enrichment, and that of their higher mass counterparts, which experience hot bottom burning. In the latter sources, the variation of the surface chemical composition reflects the equilibria of the proton capture nucleosynthesis experienced at the base of the convective envelope. The pollution expected from this class of stars is discussed, outlining the role of mass and metallicity on the chemical composition of the ejecta. To this aim, we considered evolutionary models of 0.7–8

M_{⊙}

stars in a wide range of metallicities, extending from the ultra-metal-poor domain to super-solar chemistries. Full article

(This article belongs to the Special Issue AGB Stars—In Honor of Professor Maurizio Busso on the Occasion of His 70th Birthday)

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17 pages, 917 KiB

Open AccessArticle

Extremely Metal-Poor Asymptotic Giant Branch Stars

by Mario Cirillo, Luciano Piersanti and Oscar Straniero

Universe 2022, 8(1), 44; https://doi.org/10.3390/universe8010044 - 11 Jan 2022

Cited by 1 | Viewed by 1593

Abstract

Little is known about the first stars, but hints on this stellar population can be derived from the peculiar chemical composition of the most metal-poor objects in the Milky Way and in resolved stellar populations of nearby galaxies. In this paper, we review [...] Read more.

Little is known about the first stars, but hints on this stellar population can be derived from the peculiar chemical composition of the most metal-poor objects in the Milky Way and in resolved stellar populations of nearby galaxies. In this paper, we review the evolution and nucleosynthesis of metal-poor and extremely metal-poor (EMP) stars with low and intermediate masses. In particular, new models of 6 M

_{⊙}

with three different levels of metallicity, namely

Z = 10^{- 4}

,

10^{- 6}

and

10^{- 10}

, are presented. In addition, we illustrate the results obtained for a 2 M

_{⊙}

,

Z = 10^{- 5}

model. All these models have been computed by means of the latest version of the FuNS code. We adopted a fully coupled scheme of solutions for the complete set of differential equations describing the evolution of the physical structure and the chemical abundances, as modified by nuclear processes and convective mixing. The scarcity of CNO in the material from which these stars formed significantly affects their evolution, their final fate and their contribution to the chemical pollution of the ISM in primordial galaxies. We show the potential of these models for the interpretation of the composition of EMP stars, with particular emphasis on CEMP stars. Full article

(This article belongs to the Special Issue AGB Stars—In Honor of Professor Maurizio Busso on the Occasion of His 70th Birthday)

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19 pages, 412 KiB

Open AccessArticle

Higher-Dimensional Regular Reissner–Nordström Black Holes Associated with Linear Electrodynamics

by Yu-Mei Wu and Yan-Gang Miao

Universe 2022, 8(1), 43; https://doi.org/10.3390/universe8010043 - 11 Jan 2022

Cited by 1 | Viewed by 1408

Abstract

Following the interpretation of matter source that the energy-momentum tensor of anisotropic fluid can be dealt with effectively as the energy-momentum tensor of perfect fluid plus linear (Maxwell) electromagnetic field, we obtain the regular higher-dimensional Reissner–Nordström (Tangherlini–RN) solution by starting with the noncommutative [...] Read more.

Following the interpretation of matter source that the energy-momentum tensor of anisotropic fluid can be dealt with effectively as the energy-momentum tensor of perfect fluid plus linear (Maxwell) electromagnetic field, we obtain the regular higher-dimensional Reissner–Nordström (Tangherlini–RN) solution by starting with the noncommutative geometry-inspired Schwarzschild solution. Using the boundary conditions that connect the noncommutative Schwarzschild solution in the interior of the charged perfect fluid sphere to the Tangherlini–RN solution in the exterior of the sphere, we find that the interior structure can be reflected by an exterior parameter, the charge-to-mass ratio. Moreover, we investigate the stability of the boundary under mass perturbation and indicate that the new interpretation imposes a rigid restriction upon the charge-to-mass ratio. This restriction, in turn, permits a stable noncommutative black hole only in the 4-dimensional spacetime. Full article

(This article belongs to the Section Gravitation)

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17 pages, 421 KiB

Open AccessReview

Searches for Violation of CPT Symmetry and Lorentz Invariance with Astrophysical Neutrinos

by Celio A. Moura and Fernando Rossi-Torres

Universe 2022, 8(1), 42; https://doi.org/10.3390/universe8010042 - 11 Jan 2022

Cited by 5 | Viewed by 1800

Abstract

Neutrinos are a powerful tool for searching physics beyond the standard model of elementary particles. In this review, we present the status of the research on charge-parity-time (CPT) symmetry and Lorentz invariance violations using neutrinos emitted from the collapse of stars such as [...] Read more.

Neutrinos are a powerful tool for searching physics beyond the standard model of elementary particles. In this review, we present the status of the research on charge-parity-time (CPT) symmetry and Lorentz invariance violations using neutrinos emitted from the collapse of stars such as supernovae and other astrophysical environments, such as gamma-ray bursts. Particularly, supernova neutrino fluxes may provide precious information because all neutrino and antineutrino flavors are emitted during a burst of tens of seconds. Models of quantum gravity may allow the violation of Lorentz invariance and possibly of CPT symmetry. Violation of Lorentz invariance may cause a modification of the dispersion relation and, therefore, in the neutrino group velocity as well in the neutrino wave packet. These changes can affect the arrival time signal registered in astrophysical neutrino detectors. Direction or time-dependent oscillation probabilities and anisotropy of the neutrino velocity are manifestations of the same kind of new physics. CPT violation, on the other hand, may be responsible for different oscillation patterns for neutrino and antineutrino and unconventional energy dependency of the oscillation phase or of the mixing angles. Future perspectives for possible CPT and Lorentz violating systems are also presented. Full article

(This article belongs to the Special Issue Investigating the Matter-Antimatter Asymmetry)

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11 pages, 1016 KiB

Open AccessArticle

Dynamics and Merger Rate of Primordial Black Holes in a Cluster

by Viktor D. Stasenko, Alexander A. Kirillov and Konstantin M. Belotsky

Universe 2022, 8(1), 41; https://doi.org/10.3390/universe8010041 - 11 Jan 2022

Cited by 1 | Viewed by 1588

Abstract

The PBH clusters can be sources of gravitational waves, and the merger rate depends on the spatial distribution of PBHs in the cluster which changes over time. It is well known that gravitational collisional systems experience the core collapse that leads to significant [...] Read more.

The PBH clusters can be sources of gravitational waves, and the merger rate depends on the spatial distribution of PBHs in the cluster which changes over time. It is well known that gravitational collisional systems experience the core collapse that leads to significant increase of the central density and shrinking of the core. After core collapse, the cluster expands almost self-similarly (i.e., density profile extends in size without changing its shape). These dynamic processes affect the merger rate of PBHs. In this paper, the dynamics of the PBH cluster is considered using the Fokker–Planck equation. We calculate the merger rate of PBHs on cosmic time scales and show that its time dependence has a unique signature. Namely, it grows by about an order of magnitude at the moment of core collapse which depends on the characteristics of the cluster, and then decreases according to the dependence

R \propto t^{- 1.48}

. It was obtained for monochromatic and power-law PBH mass distributions with some fixed parameters. Obtained results can be used to test the model of the PBH clusters via observation of gravitational waves at high redshift. Full article

(This article belongs to the Special Issue Project of the Virtual Institute of Astroparticle Physics (VIA))

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52 pages, 653 KiB

Open AccessArticle

Fundamental Physics and Computation: The Computer-Theoretic Framework

by Sergio Miguel-Tomé, Ángel L. Sánchez-Lázaro and Luis Alonso-Romero

Universe 2022, 8(1), 40; https://doi.org/10.3390/universe8010040 - 11 Jan 2022

Cited by 3 | Viewed by 6700

Abstract

The central goal of this manuscript is to survey the relationships between fundamental physics and computer science. We begin by providing a short historical review of how different concepts of computer science have entered the field of fundamental physics, highlighting the claim that [...] Read more.

The central goal of this manuscript is to survey the relationships between fundamental physics and computer science. We begin by providing a short historical review of how different concepts of computer science have entered the field of fundamental physics, highlighting the claim that the universe is a computer. Following the review, we explain why computational concepts have been embraced to interpret and describe physical phenomena. We then discuss seven arguments against the claim that the universe is a computational system and show that those arguments are wrong because of a misunderstanding of the extension of the concept of computation. Afterwards, we address a proposal to solve Hempel’s dilemma using the computability theory but conclude that it is incorrect. After that, we discuss the relationship between the proposals that the universe is a computational system and that our minds are a simulation. Analysing these issues leads us to proposing a new physical principle, called the principle of computability, which claims that the universe is a computational system (not restricted to digital computers) and that computational power and the computational complexity hierarchy are two fundamental physical constants. On the basis of this new principle, a scientific paradigm emerges to develop fundamental theories of physics: the computer-theoretic framework (CTF). The CTF brings to light different ideas already implicit in the work of several researchers and provides a new view on the universe based on computer theoretic concepts that expands the current view. We address different issues regarding the development of fundamental theories of physics in the new paradigm. Additionally, we discuss how the CTF brings new perspectives to different issues, such as the unreasonable effectiveness of mathematics and the foundations of cognitive science. Full article

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16 pages, 440 KiB

Open AccessArticle

M-Class Solar Flares in Solar Cycles 23 and 24: Properties and Space Weather Relevance

by Rositsa Miteva and Susan W. Samwel

Universe 2022, 8(1), 39; https://doi.org/10.3390/universe8010039 - 10 Jan 2022

Cited by 12 | Viewed by 2549

Abstract

A comprehensive statistical analysis on the properties and accompanied phenomena of all M-class solar flares (as measured in soft X-rays) in the last two solar cycles (1996–2019) is presented here with a focus on their space weather potential. The information about the parent [...] Read more.

A comprehensive statistical analysis on the properties and accompanied phenomena of all M-class solar flares (as measured in soft X-rays) in the last two solar cycles (1996–2019) is presented here with a focus on their space weather potential. The information about the parent active region and the underlying sunspot (Hale) type is collected for each case, where possible, in order to identify photospheric precondition as precursors for the solar flare eruption or confinement. Associations with coronal mass ejections, solar energetic particles, and interplanetary radio emissions are also evaluated and discussed as possible proxies for flare eruption and subsequent space weather relevance. The results show that the majority (∼80%) of the analyzed M-class flares are of

β

,

β

-

γ

, and

β

-

γ

-

δ

magnetic field configuration. The M-class population of flares is accompanied by CMEs in 41% of the cases and about half of the flare sample has been associated with radio emission from electron beams. A much lower association (≲10%) is obtained with shock wave radio signatures and energetic particles. Furthermore, a parametric scheme is proposed in terms of occurrence rates between M-class flares and a variety of accompanied solar phenomena as a function of flare sub-classes or magnetic type. This study confirms the well-known reduced but inevitable space weather importance of M-class flares. Full article

(This article belongs to the Section Solar and Stellar Physics)

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24 pages, 440 KiB

Open AccessReview

Linear Stability of Black Holes and Naked Singularities

by Gustavo Dotti

Universe 2022, 8(1), 38; https://doi.org/10.3390/universe8010038 - 10 Jan 2022

Cited by 5 | Viewed by 1686

Abstract

A review of the current status of the linear stability of black holes and naked singularities is given. The standard modal approach, that takes advantage of the background symmetries and analyze separately the harmonic components of linear perturbations, is briefly introduced and used [...] Read more.

A review of the current status of the linear stability of black holes and naked singularities is given. The standard modal approach, that takes advantage of the background symmetries and analyze separately the harmonic components of linear perturbations, is briefly introduced and used to prove that the naked singularities in the Kerr–Newman family, as well as the inner black hole regions beyond Cauchy horizons, are unstable and therefore unphysical. The proofs require a treatment of the boundary condition at the timelike boundary, which is given in detail. The nonmodal linear stability concept is then introduced, and used to prove that the domain of outer communications of a Schwarzschild black hole with a non-negative cosmological constant satisfies this stronger stability condition, which rules out transient growths of perturbations, and also to show that the perturbed black hole settles into a slowly rotating Kerr black hole. The encoding of the perturbation fields in gauge invariant curvature scalars and the effects of the perturbation on the geometry of the spacetime is discussed. These notes follow from a course delivered at the V José Plínio Baptista School of Cosmology, held at Guarapari (Espírito Santo) Brazil, from 30 September to 5 October 2021. Full article

(This article belongs to the Special Issue Compact Astrophysical Objects)

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20 pages, 1426 KiB

Open AccessReview

Digging into Axion Physics with (Baby)IAXO

by Theopisti Dafni and Javier Galán

Universe 2022, 8(1), 37; https://doi.org/10.3390/universe8010037 - 8 Jan 2022

Cited by 4 | Viewed by 2554

Abstract

Dark matter searches have been ongoing for three decades; the lack of a positive discovery of the main candidate, the WIMP, after dedicated efforts, has put axions and axion-like particles in the spotlight. The three main techniques employed to search for them complement [...] Read more.

Dark matter searches have been ongoing for three decades; the lack of a positive discovery of the main candidate, the WIMP, after dedicated efforts, has put axions and axion-like particles in the spotlight. The three main techniques employed to search for them complement each other well in covering a wide range in the parameter space defined by the axion decay constant and the axion mass. The International AXion Observatory (IAXO) is an international collaboration planning to build the fourth generation axion helioscope, with an unparalleled expected sensitivity and discovery potential. The distinguishing characteristic of IAXO is that it will feature a magnet that is designed to maximise the relevant parameters in sensitivity and which will be equipped with X-ray focusing devices and detectors that have been developed for axion physics. In this paper, we review aspects that motivate IAXO and its prototype, BabyIAXO, in the axion, and ALPs landscape. As part of this Special Issue, some emphasis is given on Spanish participation in the project, of which CAPA (Centro de Astropartículas y Física de Altas Energías of the Universidad de Zaragoza) is a strong promoter. Full article

(This article belongs to the Special Issue Studying the Universe from Spain)

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16 pages, 1357 KiB

Open AccessArticle

Time in Quantum Cosmology

by Claus Kiefer and Patrick Peter

Universe 2022, 8(1), 36; https://doi.org/10.3390/universe8010036 - 8 Jan 2022

Cited by 25 | Viewed by 2379

Abstract

Time in quantum gravity is not a well-defined notion despite its central role in the very definition of dynamics. Using the formalism of quantum geometrodynamics, we briefly review the problem and illustrate it with two proposed solutions. Our main application is quantum cosmology—the [...] Read more.

Time in quantum gravity is not a well-defined notion despite its central role in the very definition of dynamics. Using the formalism of quantum geometrodynamics, we briefly review the problem and illustrate it with two proposed solutions. Our main application is quantum cosmology—the application of quantum gravity to the Universe as a whole. Full article

(This article belongs to the Special Issue Quantum Models for Cosmology)

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18 pages, 1708 KiB

Open AccessArticle

Evaluation of the UMASEP-10 Version 2 Tool for Predicting All >10 MeV SEP Events of Solar Cycles 22, 23 and 24

by Marlon Núñez

Universe 2022, 8(1), 35; https://doi.org/10.3390/universe8010035 - 7 Jan 2022

Cited by 5 | Viewed by 1734

Abstract

The prediction of solar energetic particle (SEP) events may help to improve the mitigation of adverse effects on humans and technology in space. UMASEP (University of Málaga Solar particle Event Predictor) is an empirical model scheme that predicts SEP events. This scheme is [...] Read more.

The prediction of solar energetic particle (SEP) events may help to improve the mitigation of adverse effects on humans and technology in space. UMASEP (University of Málaga Solar particle Event Predictor) is an empirical model scheme that predicts SEP events. This scheme is based on a dual-model approach. The first model predicts well-connected events by using an improved lag-correlation algorithm for analyzing soft X-ray (SXR) and differential proton fluxes to estimate empirically the Sun–Earth magnetic connectivity. The second model predicts poorly connected events by analyzing the evolution of differential proton fluxes. This study presents the evaluation of UMASEP-10 version 2, a tool based on the aforementioned scheme for predicting all >10 MeV SEP events, including those without associated flare. The evaluation of this tool is presented in terms of the probability of detection (POD), false alarm ratio (FAR) and average warning time (AWT). The best performance was achieved for the solar cycle 24 (i.e., 2008–2019), obtaining a POD of 91.1% (41/45), a FAR of 12.8% (6/47) and an AWT of 2 h 46 min. These results show that UMASEP-10 version 2 obtains a high POD and low FAR mainly because it is able to detect true Sun–Earth magnetic connections. Full article

(This article belongs to the Section Solar and Stellar Physics)

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24 pages, 2360 KiB

Open AccessArticle

Radial Oscillations of Quark Stars Admixed with Dark Matter

by José C. Jiménez and Eduardo S. Fraga

Universe 2022, 8(1), 34; https://doi.org/10.3390/universe8010034 - 5 Jan 2022

Cited by 23 | Viewed by 1674

Abstract

We investigated compact stars consisting of cold quark matter and fermionic dark matter treated as two admixed fluids. We computed the stellar structures and fundamental radial oscillation frequencies of different masses of the dark fermion in the cases of weak and strong self-interacting [...] Read more.

We investigated compact stars consisting of cold quark matter and fermionic dark matter treated as two admixed fluids. We computed the stellar structures and fundamental radial oscillation frequencies of different masses of the dark fermion in the cases of weak and strong self-interacting dark matter. We found that the fundamental frequency can be dramatically modified and, in some cases, stable dark strange planets and dark strangelets with very low masses and radii can be formed. Full article

(This article belongs to the Special Issue Properties and Dynamics of Neutron Stars and Proto-Neutron Stars)

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27 pages, 519 KiB

Open AccessReview

E₆ GUT and Baryon Asymmetry Generation in the E₆CHM

by Roman Nevzorov

Universe 2022, 8(1), 33; https://doi.org/10.3390/universe8010033 - 5 Jan 2022

Cited by 3 | Viewed by 1770

Abstract

Grand unified theories (GUTs) may result in the

E_{6}

-inspired composite Higgs model (E

_{6}

CHM) at low energies, almost stabilizing the electroweak scale. We consider an orbifold GUT in 6 dimensions in which the

E_{6}

-gauge group is broken to [...] Read more.

Grand unified theories (GUTs) may result in the

E_{6}

-inspired composite Higgs model (E

_{6}

CHM) at low energies, almost stabilizing the electroweak scale. We consider an orbifold GUT in 6 dimensions in which the

E_{6}

-gauge group is broken to the gauge symmetry of the standard model (SM) while different multiplets of the SM fermions come from different 27-plets. The strongly coupled sector of the E

_{6}

CHM is confined on the brane where

E_{6}

is broken down to its

S U (6)

subgroup. Near the scale of

f ≳ 5 TeV

, this approximate

S U (6)

symmetry is expected to be further broken down to its

S U (5)

subgroup, which contains the SM-gauge group. Such a breakdown leads to a set of pseudo-Nambu–Goldstone bosons (pNGBs) that includes an SM-like Higgs doublet. The approximate gauge coupling unification in the E

_{6}

CHM takes place at high energies when the right-handed top quark is a composite fermion. To ensure anomaly cancellation, the weakly coupled sector of this model contains extra exotic matter beyond the SM. We discuss the mechanism of the generation of matter–antimatter asymmetry within the variant of the E

_{6}

CHM in which the baryon number and CP invariance are violated. Full article

(This article belongs to the Special Issue Investigating the Matter-Antimatter Asymmetry)

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24 pages, 1694 KiB

Open AccessArticle

Particle Acceleration in Mildly Relativistic Outflows of Fast Energetic Transient Sources

by Andrei Bykov, Vadim Romansky and Sergei Osipov

Universe 2022, 8(1), 32; https://doi.org/10.3390/universe8010032 - 5 Jan 2022

Cited by 6 | Viewed by 2965

Abstract

Recent discovery of fast blue optical transients (FBOTs)—a new class of energetic transient sources—can shed light on the long-standing problem of supernova—long gamma-ray burst connections. A distinctive feature of such objects is the presence of modestly relativistic outflows which place them in between [...] Read more.

Recent discovery of fast blue optical transients (FBOTs)—a new class of energetic transient sources—can shed light on the long-standing problem of supernova—long gamma-ray burst connections. A distinctive feature of such objects is the presence of modestly relativistic outflows which place them in between the non-relativistic and relativistic supernovae-related events. Here we present the results of kinetic particle-in-cell and Monte Carlo simulations of particle acceleration and magnetic field amplification by shocks with the velocities in the interval between 0.1 and 0.7 c. These simulations are needed for the interpretation of the observed broad band radiation of FBOTs. Their fast, mildly to moderately relativistic outflows may efficiently accelerate relativistic particles. With particle-in-cell simulations we demonstrate that synchrotron radiation of accelerated relativistic electrons in the shock downstream may fit the observed radio fluxes. At longer timescales, well beyond those reachable within a particle-in-cell approach, our nonlinear Monte Carlo model predicts that protons and nuclei can be accelerated to petaelectronvolt (PeV) energies. Therefore, such fast and energetic transient sources can contribute to galactic populations of high energy cosmic rays. 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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15 pages, 511 KiB

Open AccessArticle

Analyzing Transverse Momentum Spectra by a New Method in High-Energy Collisions

by Li-Li Li, Fu-Hu Liu, Muhammad Waqas and Muhammad Ajaz

Universe 2022, 8(1), 31; https://doi.org/10.3390/universe8010031 - 5 Jan 2022

Cited by 14 | Viewed by 1765

Abstract

We analyzed the transverse momentum spectra of positively and negatively charged pions (

π^{+}

and

π^{-}

), positively and negatively charged kaons (

K^{+}

and

K^{-}

), protons and antiprotons (p and

\bar{p}

), as well as [...] Read more.

We analyzed the transverse momentum spectra of positively and negatively charged pions (

π^{+}

and

π^{-}

), positively and negatively charged kaons (

K^{+}

and

K^{-}

), protons and antiprotons (p and

\bar{p}

), as well as

ϕ

produced in mid-(pseudo)rapidity region in central nucleus–nucleus (AA) collisions over a center-of-mass energy range from 2.16 to 2760 GeV per nucleon pair. The transverse momentum of the considered particle is regarded as the joint contribution of two participant partons which obey the modified Tsallis-like transverse momentum distribution and have random azimuths in superposition. The calculation of transverse momentum distribution of particles is performed by the Monte Carlo method and compared with the experimental data measured by international collaborations. The excitation functions of effective temperature and other parameters are obtained in the considered energy range. With the increase of collision energy, the effective temperature parameter increases quickly and then slowly. The boundary appears at around 5 GeV, which means the change of reaction mechanism and/or generated matter. Full article

(This article belongs to the Special Issue Hydrodynamics and Thermodynamics in High Energy Physics)

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14 pages, 4975 KiB

Open AccessArticle

Predicting the Daily 10.7-cm Solar Radio Flux Using the Long Short-Term Memory Method

by Wanting Zhang, Xinhua Zhao, Xueshang Feng, Cheng’ao Liu, Nanbin Xiang, Zheng Li and Wei Lu

Universe 2022, 8(1), 30; https://doi.org/10.3390/universe8010030 - 5 Jan 2022

Cited by 17 | Viewed by 2809

Abstract

As an important index of solar activity, the 10.7-cm solar radio flux (F_10.7) can indicate changes in the solar EUV radiation, which plays an important role in the relationship between the Sun and the Earth. Therefore, it is valuable to [...] Read more.

As an important index of solar activity, the 10.7-cm solar radio flux (F_10.7) can indicate changes in the solar EUV radiation, which plays an important role in the relationship between the Sun and the Earth. Therefore, it is valuable to study and forecast F_10.7. In this study, the long short-term memory (LSTM) method in machine learning is used to predict the daily value of F_10.7. The F_10.7 series from 1947 to 2019 are used. Among them, the data during 1947–1995 are adopted as the training dataset, and the data during 1996–2019 (solar cycles 23 and 24) are adopted as the test dataset. The fourfold cross validation method is used to group the training set for multiple validations. We find that the root mean square error (RMSE) of the prediction results is only 6.20~6.35 sfu, and the correlation coefficient (R) is as high as 0.9883~0.9889. The overall prediction accuracy of the LSTM method is equivalent to those of the widely used autoregressive (AR) and backpropagation neural network (BP) models. Especially for 2-day and 3-day forecasts, the LSTM model is slightly better. All this demonstrates the potentiality of the LSTM method in the real-time forecasting of F_10.7 in future. Full article

(This article belongs to the Section Space Science)

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2 pages, 171 KiB

Open AccessEditorial

Editorial for the Special Issue “Relativistic Astrophysics”

by Francesco De Paolis, Achille A. Nucita and Alexander F. Zakharov

Universe 2022, 8(1), 29; https://doi.org/10.3390/universe8010029 - 4 Jan 2022

Viewed by 1057

Abstract

Relativistic Astrophysics is the branch of astrophysics that studies astronomical phenomena and celestial bodies, for which classical mechanics and Newton’s law of gravitation are inapplicable to creation of suitable models and we have to generalize these approaches following general relativistic prescriptions [...] Full article

(This article belongs to the Special Issue Relativistic Astrophysics)

8 pages, 633 KiB

Open AccessCommunication

On the Counter-Rotation of Closed Time-like Curves

by Yuanyuan Duan, Fangxun Liu, Yu Wang and Yen Chin Ong

Universe 2022, 8(1), 28; https://doi.org/10.3390/universe8010028 - 4 Jan 2022

Cited by 3 | Viewed by 1630

Abstract

While it is tempting to think of closed time-like curves (CTCs) around rotating bodies, such as a black hole, as being “caused” by the rotation of the source, Andréka et al. pointed out that the underlying physics are not as straightforward as this, [...] Read more.

While it is tempting to think of closed time-like curves (CTCs) around rotating bodies, such as a black hole, as being “caused” by the rotation of the source, Andréka et al. pointed out that the underlying physics are not as straightforward as this, since such CTCs are “counter-rotating”, i.e., the time orientation (the opening of the local light cones) of the CTCs is opposite to the direction in which the singularity or the ergosphere rotates. It was also suggested that this is a generic phenomenon that calls for a deeper intuitive physical understanding. In this short note, we point out—with Kerr–Taub–NUT as an example—that CTCs are counter-rotating with respect to the local angular velocity of the spacetime, not the global angular momentum, nor the angular velocity of the black hole horizon, which makes the physical interpretation of CTCs being “caused” by a rotating source even more problematic. Full article

(This article belongs to the Special Issue The Physics of Time Travel)

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39 pages, 2287 KiB

Open AccessArticle

No Intrinsic Decoherence of Inflationary Cosmological Perturbations

by Jen-Tsung Hsiang and Bei-Lok Hu

Universe 2022, 8(1), 27; https://doi.org/10.3390/universe8010027 - 3 Jan 2022

Cited by 16 | Viewed by 1645

Abstract

After a brief summary of the four main veins in the treatment of decoherence and quantum to classical transition in cosmology since the 1980s, we focus on one of these veins in the study of quantum decoherence of cosmological perturbations in inflationary universe, [...] Read more.

After a brief summary of the four main veins in the treatment of decoherence and quantum to classical transition in cosmology since the 1980s, we focus on one of these veins in the study of quantum decoherence of cosmological perturbations in inflationary universe, the case when it does not rely on any environment. This is what ‘intrinsic’ in the title refers to—a closed quantum system, consisting of a quantum field which drives inflation. The question is whether its quantum perturbations, which interact with the density contrast giving rise to structures in the universe, decohere with an inflationary expansion of the universe. A dominant view which had propagated for a quarter of a century asserts yes, based on the belief that the large squeezing of a quantum state after a duration of inflation renders the system effectively classical. This paper debunks this view by identifying the technical fault-lines in its derivations and revealing the pitfalls in its arguments which drew earlier authors to this wrong conclusion. We use a few simple quantum mechanical models to expound where the fallacy originated: The highly squeezed ellipse quadrature in phase space cannot be simplified to a line, and the Wigner function cannot be replaced by a delta function. These measures amount to taking only the leading order in the relevant parameters in seeking the semiclassical limit and ignoring the subdominant contributions where quantum features reside. Doing so violates the bounds of the Wigner function, and its wave functions possess negative eigenvalues. Moreover, the Robertson-Schrödinger uncertainty relation for a pure state is violated. For inflationary cosmological perturbations, in addition to these features, entanglement exists between the created pairs. This uniquely quantum feature cannot be easily argued away. Indeed, it could be our best hope to retroduce the quantum nature of cosmological perturbations and the trace of an inflation field. All this points to the invariant fact that a closed quantum system, even when highly squeezed, evolves unitarily without loss of coherence; quantum cosmological perturbations do not decohere by themselves. Full article

(This article belongs to the Special Issue Quantum Aspects of the Universe)

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