Universe

17 pages, 523 KiB

Open AccessArticle

Circular Geodesics in a New Generalization of q-Metric

by Shokoufe Faraji

Universe 2022, 8(3), 195; https://doi.org/10.3390/universe8030195 - 21 Mar 2022

Cited by 6 | Viewed by 1530

This paper introduces an alternative generalization of the static solution with quadrupole moment, the

q

-metric, that describes a deformed compact object in the presence of the external fields characterized by multipole moments. In addition, we also examine the impact of the external [...] Read more.

This paper introduces an alternative generalization of the static solution with quadrupole moment, the

q

-metric, that describes a deformed compact object in the presence of the external fields characterized by multipole moments. In addition, we also examine the impact of the external fields up to quadrupole on the circular geodesics and the interplay of these two quadrupoles on the place of the innermost stable circular orbit (ISCO) in the equatorial plane. Full article

(This article belongs to the Section Mathematical Physics)

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21 pages, 1145 KiB

Open AccessArticle

Theory and Phenomenology of a Four-Dimensional String–Corrected Black Hole

by Kimet Jusufi and Dejan Stojkovic

Universe 2022, 8(3), 194; https://doi.org/10.3390/universe8030194 - 21 Mar 2022

Cited by 7 | Viewed by 2058

Abstract

We construct an effective four-dimensional string-corrected black hole (4D SCBH) by rescaling the string coupling parameter in a D-dimensional Callan–Myers–Perry black hole. From the theoretical point of view, the most interesting findings are that the string corrections coincide with the so-called generalized [...] Read more.

We construct an effective four-dimensional string-corrected black hole (4D SCBH) by rescaling the string coupling parameter in a D-dimensional Callan–Myers–Perry black hole. From the theoretical point of view, the most interesting findings are that the string corrections coincide with the so-called generalized uncertainty principle (GUP) corrections to black hole solutions, Bekenstein–Hawking entropy acquires logarithmic corrections, and that there exists a critical value of the coupling parameter for which the black hole temperature vanishes. We also find that, due to the string corrections, the nature of the central singularity may be altered from space-like to time-like singularity. In addition, we study the possibility of testing such a black hole with astrophysical observations. Since the dilaton field does not decouple from the metric, it is not a priori clear that the resulting 4D SCBH offers only small corrections to the Schwarzschild black hole. We used motion of the S2 star around the black hole at the center of our galaxy to constrain the parameters (the string coupling parameter and ADM mass) of the 4D SCBH. To test the weak gravity regime, we calculate the deflection angle in this geometry and apply it to gravitational lensing. To test the strong field regime, we calculate the black hole shadow radius. While we find that the observables change as we change the string coupling parameter, the magnitude of the change is too small to distinguish it from the Schwarzschild black hole. With the current precision, to the leading order terms, the 4D SCBH cannot be distinguished from the Schwarzschild black hole. Full article

(This article belongs to the Special Issue Black Holes in Einstein–Gauss–Bonnet Theories)

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13 pages, 523 KiB

Open AccessArticle

Spin Precession in the Gravity Wave Analogue Black Hole Spacetime

by Chandrachur Chakraborty and Banibrata Mukhopadhyay

Universe 2022, 8(3), 193; https://doi.org/10.3390/universe8030193 - 20 Mar 2022

Viewed by 1819

Abstract

It was predicted that the spin precession frequency of a stationary gyroscope shows various anomalies in the strong gravity regime if its orbit shrinks, and eventually, its precession frequency becomes arbitrarily high very close to the horizon of a rotating black hole. Considering [...] Read more.

It was predicted that the spin precession frequency of a stationary gyroscope shows various anomalies in the strong gravity regime if its orbit shrinks, and eventually, its precession frequency becomes arbitrarily high very close to the horizon of a rotating black hole. Considering the gravity waves of a flowing fluid with a vortex in a shallow basin, which acts as a rotating analogue black hole, one can observe the predicted strong gravity effect on the spin precession in the laboratory. Attaching a thread with the buoyant particles and anchoring it to the bottom of the fluid container with a short-length miniature chain, one can construct a simple local test gyroscope to measure the spin precession frequency in the vicinity of the gravity wave analogue black hole. The thread acts as the axis of the gyroscope. By regulating the orbital frequency of the test gyroscope, one can also measure the strong gravity Lense–Thirring effect and geodetic/de-Sitter effect with this experimental set-up as the special cases. For example, to measure the Lense–Thirring effect, the length of the miniature chain can be set to zero, so that the gyroscope becomes static. One can also measure the geodetic precession with this system by orbiting the test gyroscope in the so-called Keplerian frequency around the non-rotating analogue black hole that can be constructed by making the rotation of the fluid/vortex negligible compared to its radial velocity. Full article

(This article belongs to the Special Issue Analogue Gravity)

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9 pages, 304 KiB

Open AccessArticle

A Subtle Aspect of Minimal Lengths in the Generalized Uncertainty Principle

by Michael Bishop, Joey Contreras and Douglas Singleton

Universe 2022, 8(3), 192; https://doi.org/10.3390/universe8030192 - 20 Mar 2022

Cited by 11 | Viewed by 1782

Abstract

In this work, we point out an overlooked and subtle feature of the generalized uncertainty principle (GUP) approach to quantizing gravity: namely that different pairs of modified operators with the same modified commutator, [...] Read more.

In this work, we point out an overlooked and subtle feature of the generalized uncertainty principle (GUP) approach to quantizing gravity: namely that different pairs of modified operators with the same modified commutator,

[\hat{X}, \hat{P}] = i ħ (1 + β p^{2})

, may have different physical consequences such as having no minimal length at all. These differences depend on how the position and/or momentum operators are modified rather than only on the resulting modified commutator. This provides guidance when constructing GUP models since it distinguishes those GUPs that have a minimal length scale, as suggested by some broad arguments about quantum gravity, versus GUPs without a minimal length scale. Full article

(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)

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38 pages, 2772 KiB

Open AccessReview

RR Lyrae Stars and Anomalous Cepheids as Population Tracers in Local Group Galaxies

by Matteo Monelli and Giuliana Fiorentino

Universe 2022, 8(3), 191; https://doi.org/10.3390/universe8030191 - 19 Mar 2022

Cited by 11 | Viewed by 2307

Abstract

We discuss the use and importance of pulsating variable stars as population tracers in Local Group galaxies. Among bright variable crossing the classical instability strip, we mostly focus on RR Lyrae stars and Anomalous Cepheids. We discuss their pulsational properties and how it [...] Read more.

We discuss the use and importance of pulsating variable stars as population tracers in Local Group galaxies. Among bright variable crossing the classical instability strip, we mostly focus on RR Lyrae stars and Anomalous Cepheids. We discuss their pulsational properties and how it is possible to use them to constrain the evolution and star formation history of the host galaxy. We discuss RR Lyrae stars as tracers of the old population, and how they can be used to trace the accretion history of large galaxies such as the Milky Way and M31, and also the early chemical evolution. Moreover, we show that the frequency of Anomalous Cepheids follows different relations, and therefore trace the intermediate-age star formation. Finally, we discuss the different methods to derive distances and the impact of the Gaia mission. Full article

(This article belongs to the Special Issue Recent Advances in Pulsating Stars)

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13 pages, 5837 KiB

Open AccessArticle

Can Asteroid Belts Exist in the Luyten’s System?

by Mattia Galiazzo, Elizabeth A. Silber and Rudolf Dvorak

Universe 2022, 8(3), 190; https://doi.org/10.3390/universe8030190 - 19 Mar 2022

Viewed by 1809

Abstract

The extra-solar planetary system Luyten is relatively close (12.3 light years) to our Sun. The Luyten’s red dwarf star is orbited by four planets, two of them Earth-like (in mass) and in 4:1 resonance. Extra-solar systems might contain asteroid belts such as ours. [...] Read more.

The extra-solar planetary system Luyten is relatively close (12.3 light years) to our Sun. The Luyten’s red dwarf star is orbited by four planets, two of them Earth-like (in mass) and in 4:1 resonance. Extra-solar systems might contain asteroid belts such as ours. Therefore, it is important to investigate whether it is possible to have a stable population of minor bodies and compare them to those in our system. The study of extra-solar systems is crucial for understanding the evolution of planetary systems in general. Here, we investigate the stability of two possible asteroid populations in the Luyten’s system: the main asteroid belt between the two inner and two outer planets, and an outer asteroid belt, situated beyond the planets. We also explore the likelihood of observing an asteroid or a dwarf planet in this system. Our study suggests that the existence of asteroid belts is possible, notably the main belt at 0.09–0.53 au from the star and an outer belt (with the inner boundary at 0.85 au and the outer boundary at ∼66,000 au). The average Yarkovsky drift for the Luyten’s main asteroid belt is ∼

0.5 \times 10^{- 4}

au/Myr for km-size objects. The Luyten’s system might host extra-solar minor bodies, some of which could be capable of entering our own system. Presently, no asteroids can be detected in the Luyten’s system, not even a Ceres-sized body, because the detection signal using the radial velocity method is at least two orders of magnitude less than that required for discerning such objects. The detection probability of an asteroid in the Luyten belt similar to Ceres is about 1.3%, which is less than the probability of finding Luyten B (∼3%). Full article

(This article belongs to the Special Issue Small Bodies in the Solar System)

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25 pages, 1683 KiB

Open AccessArticle

The “ER = EPR” Conjecture and Generic Gravitational Properties: A Universal Topological Linking Model of the Correspondence between Tripartite Entanglement and Planck-Scale Wormholes

by Elias Zafiris and Albrecht von Müller

Universe 2022, 8(3), 189; https://doi.org/10.3390/universe8030189 - 18 Mar 2022

Cited by 1 | Viewed by 3257

Abstract

The “ER = EPR” conjecture, conceived by Maldacena and Susskind, is grounded on the notion that a gravitational theory in the bulk is dual to the corresponding quantum field theory on the boundary in accordance to the AdS/CFT correspondence. The conjecture pertains to [...] Read more.

The “ER = EPR” conjecture, conceived by Maldacena and Susskind, is grounded on the notion that a gravitational theory in the bulk is dual to the corresponding quantum field theory on the boundary in accordance to the AdS/CFT correspondence. The conjecture pertains to the idea that Einstein-Rosen (ER) spacetime bridges and Einstein-Podolsky-Rosen (EPR) quantum entanglement may be considered as dually equivalent. Since ER bridges refer to the connectivity between black holes, the “ER = EPR” conjecture implies that black holes connected by ER bridges are entangled, and conversely, that entangled black holes are connected by ER bridges. However, the instance of the maximally entangled tripartite (GHZ) quantum state points to the necessity of devising a model of non-classical Planck scale ER bridges going beyond the standard description of these bridges in spacetime. Based on the topological structure of the maximally entangled GHZ state, we propose that a universal topological link, called the Borromean rings, furnishes a particular linking structure that is able to unravel the equavalence between entanglement and wormholes, and thus, to address the validity of the “ER = EPR” conjecture beyond the initial context of the AdS/CFT correspondence. As a consequence, we propose the explicit construction of distinguishable extensions of the smooth classical spacetime manifold taking place in the transition to the quantum gravity regime according to a naturally induced physical criterion of gravitational generic properties following from this intrinsic topological qualification of the “ER = EPR” conjecture. Full article

(This article belongs to the Special Issue Recent Advances in Wormhole Physics)

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18 pages, 592 KiB

Open AccessFeature PaperReview

Lattice Computations for Beyond Standard Model Physics

by Kari Rummukainen and Kimmo Tuominen

Universe 2022, 8(3), 188; https://doi.org/10.3390/universe8030188 - 17 Mar 2022

Cited by 6 | Viewed by 1552

Abstract

Understanding the dynamics of strongly coupled non-Abelian gauge theories constitutes one of the ongoing grand challenges in theoretical physics. This has been motivated by the need to understand long-distance behavior of quantum chromodynamics, and by the possible phenomenological applications in dynamical electroweak symmetry [...] Read more.

Understanding the dynamics of strongly coupled non-Abelian gauge theories constitutes one of the ongoing grand challenges in theoretical physics. This has been motivated by the need to understand long-distance behavior of quantum chromodynamics, and by the possible phenomenological applications in dynamical electroweak symmetry breaking or strongly coupled and composite dark sectors. In this review, we start by briefly outlining these motivations, and then discuss how first principle lattice methods have been adapted to provide results on vacuum phase diagrams of strongly coupled gauge theories with different gauge groups and various fermion representations. Full article

(This article belongs to the Special Issue Numerical Studies of Strongly Coupled Gauge Theories (SCGTs) in the Search of New Physics)

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18 pages, 667 KiB

Open AccessArticle

Bose–Einstein Condensate Dark Matter That Involves Composites

by Alexandre M. Gavrilik and Andriy V. Nazarenko

Universe 2022, 8(3), 187; https://doi.org/10.3390/universe8030187 - 17 Mar 2022

Cited by 2 | Viewed by 1664

Abstract

Improving the Bose–Einstein condensate model of dark matter through the repulsive three-particle interaction to better reproduce observables such as rotation curves reveals both different thermodynamic phases and few-particle correlations. Using the numerically found solutions of the Gross–Pitaevskii equation for averaging the products of [...] Read more.

Improving the Bose–Einstein condensate model of dark matter through the repulsive three-particle interaction to better reproduce observables such as rotation curves reveals both different thermodynamic phases and few-particle correlations. Using the numerically found solutions of the Gross–Pitaevskii equation for averaging the products of local densities and for calculating thermodynamic functions at zero temperature, it is shown that the few-particle correlations imply a first-order phase transition and are reduced to the product of single-particle averages with a simultaneous increase in pressure, density, and quantum fluctuations. Under given conditions, dark matter exhibits the properties of an ideal gas with an effective temperature determined by quantum fluctuations. Characteristics of oscillations between bound and unbound states of three particles are estimated within a simple random walk approach to qualitatively model the instability of particle complexes. On the other hand, the density-dependent conditions for the formation of composites are analyzed using chemical kinetics without specifying the bonds formed. The obtained results can be extended to the models of multicomponent dark matter consisting of composites formed by particles with a large scattering length. Full article

(This article belongs to the Special Issue Dark Matter as a Bose-Einstein Condensate)

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13 pages, 1103 KiB

Open AccessArticle

Vacuum Radiation from Massive Scalar Field

by Yu-Song Cao

Universe 2022, 8(3), 186; https://doi.org/10.3390/universe8030186 - 16 Mar 2022

Viewed by 1400

Abstract

The vacuum radiation of a massive scalar field is studied by means of a single moving mirror. The field equation with an arbitrary-shaped mirror moving in

(d + 1)

dimensions is given perturbatively in the non-relativistic limit. Explicit results are obtained [...] Read more.

The vacuum radiation of a massive scalar field is studied by means of a single moving mirror. The field equation with an arbitrary-shaped mirror moving in

(d + 1)

dimensions is given perturbatively in the non-relativistic limit. Explicit results are obtained for a flat mirror moving in

(1 + 1)

dimensions and

(3 + 1)

dimensions. The vacuum radiation power and vacuum friction force on the mirror are given in

(1 + 1)

dimensions. The intrinsic mass of the field is found to suppress the vacuum radiation. In

(3 + 1)

dimensions, the modification of the frequency spectra and angular spectra of emitted particles due to the intrinsic mass are obtained. In the limit of

m \to 0

, we recover the results of the massless field. Full article

(This article belongs to the Section Field Theory)

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16 pages, 305 KiB

Open AccessArticle

Weak Gravitation in the 4+1 Formalism

by Martin Land

Universe 2022, 8(3), 185; https://doi.org/10.3390/universe8030185 - 16 Mar 2022

Cited by 4 | Viewed by 1443

Abstract

The 4+1 formalism in general relativity (GR) prescribes field equations for the spacetime metric

γ_{μ ν} (x, τ)

which is local in the spacetime coordinates x and evolves according to an external “worldtime”

τ

. This formalism extends to GR the [...] Read more.

The 4+1 formalism in general relativity (GR) prescribes field equations for the spacetime metric

γ_{μ ν} (x, τ)

which is local in the spacetime coordinates x and evolves according to an external “worldtime”

τ

. This formalism extends to GR the Stueckelberg Horwitz Piron (SHP) framework, developed to address the various issues known as the problem of time as they appear in electrodynamics. SHP field theories exhibit a formal 5D symmetry on

(x, τ)

that is strategically broken to 4+1 representations of the Lorentz group, resulting in a manifestly covariant canonical formalism describing the

τ

-evolution of spacetime structures as an initial value problem. Einstein equations for

γ_{μ ν} (x, τ)

are found by constructing a 5D pseudo-manifold (combining 4D geometry and

τ

-dynamics) and performing the natural foliation under broken 5D symmetry. This paper discusses weak gravitation in the 4+1 formalism, demonstrating the natural decomposition of the field equations into first-order evolution equations for the unconstrained 4D metric, and the propagation of constraints associated with the Bianchi identity. Full article

(This article belongs to the Special Issue Numerical Relativity)

10 pages, 266 KiB

Open AccessArticle

Abelianized Structures in Spherically Symmetric Hypersurface Deformations

by Martin Bojowald

Universe 2022, 8(3), 184; https://doi.org/10.3390/universe8030184 - 15 Mar 2022

Cited by 5 | Viewed by 1394

Abstract

In canonical gravity, general covariance is implemented by hypersurface-deformation symmetries on thephase space. The different versions of hypersurface deformations required for full covariance have complicated interplays with one another, governed by non-Abelian brackets with structure functions. For spherically symmetric space-times, it is possible [...] Read more.

In canonical gravity, general covariance is implemented by hypersurface-deformation symmetries on thephase space. The different versions of hypersurface deformations required for full covariance have complicated interplays with one another, governed by non-Abelian brackets with structure functions. For spherically symmetric space-times, it is possible to identify a certain Abelian substructure within general hypersurface deformations, which suggests a simplified realization as a Lie algebra. The generators of this substructure can be quantized more easily than full hypersurface deformations, but the symmetries they generate do not directly correspond to hypersurface deformations. The availability of consistent quantizations therefore does not guarantee general covariance or a meaningful quantum notion thereof. In addition to placing the Abelian substructure within the full context of spherically symmetric hypersurface deformation, this paper points out several subtleties relevant for attempted applications in quantized space-time structures. In particular, it follows that recent constructions by Gambini, Olmedo, and Pullin in an Abelianized setting fail to address the covariance crisis of loop quantum gravity. Full article

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

10 pages, 454 KiB

Open AccessArticle

Take the A-Metric: Interpretations of Some Known Solutions of Einstein’s Vacuum Field Equations

by Charles W. Robson and Marco Ornigotti

Universe 2022, 8(3), 183; https://doi.org/10.3390/universe8030183 - 14 Mar 2022

Cited by 1 | Viewed by 2493

Abstract

In this work, we present a new interpretation of the only static vacuum solution of Einstein’s field equations with planar symmetry, the Taub solution. This solution is a member of the

A I I I

class of metrics, along with the type D [...] Read more.

In this work, we present a new interpretation of the only static vacuum solution of Einstein’s field equations with planar symmetry, the Taub solution. This solution is a member of the

A I I I

class of metrics, along with the type D Kasner solution. Various interpretations of these solutions have been put forward previously in the literature, however, some of these interpretations have suspect features and are not generally considered physical. Using a simple mathematical analysis, we show that a novel interpretation of the Taub solution is possible and that it naturally emerges from the radial, near-singularity limit of negative-mass Schwarzschild spacetime. A new, more transparent derivation is also given, showing that the type D Kasner metric can be interpreted as a region of spacetime deep within a positive-mass Schwarzschild black hole. The dual nature of this class of A-metrics is thereby demonstrated. Full article

(This article belongs to the Section Field Theory)

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18 pages, 869 KiB

Open AccessArticle

Optical Features of AdS Black Holes in the Novel 4D Einstein-Gauss-Bonnet Gravity Coupled to Nonlinear Electrodynamics

by Khadije Jafarzade, Mahdi Kord Zangeneh and Francisco S. N. Lobo

Universe 2022, 8(3), 182; https://doi.org/10.3390/universe8030182 - 14 Mar 2022

Cited by 14 | Viewed by 1561

Abstract

An alternative theory of gravity that has attracted much attention recently is the novel four-dimensional Einstein-Gauss-Bonnet (4D EGB) gravity. The theory is rescaled by the Gauss-Bonnet (GB) coupling constant

α \to α / (D - 4)

in D dimensions and redefined [...] Read more.

An alternative theory of gravity that has attracted much attention recently is the novel four-dimensional Einstein-Gauss-Bonnet (4D EGB) gravity. The theory is rescaled by the Gauss-Bonnet (GB) coupling constant

α \to α / (D - 4)

in D dimensions and redefined as four-dimensional gravity in the limit

D \to 4

. Thus, in this manner, the GB term yields a non-trivial contribution to the gravitational dynamics. In fact, regularized black hole solutions and applications in the novel 4D EGB gravity have also been extensively explored. In this work, motivated by recent astrophysical observations, we present an in-depth study of the optical features of AdS black holes in the novel 4D EGB gravity coupled to exponential nonlinear electrodynamics (NED), such as the shadow geometrical shape, the energy emission rate, the deflection angle and quasinormal modes. Taking into account these dynamic quantities, we investigate the effects on the black hole solution by varying the parameters of the models. More specifically, we show that the variation of the GB and NED parameters, and of the cosmological constant, imprints specific signatures on the optical features of AdS black holes in the novel 4D EGB gravity coupled to nonlinear electrodynamics, thus leading to the possibility of directly testing these black hole models by using astrophysical observations. Full article

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

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14 pages, 907 KiB

Open AccessArticle

Prospects for the Detection of the Diffuse Supernova Neutrino Background with the Experiments SK-Gd and JUNO

by Yu-Feng Li, Mark Vagins and Michael Wurm

Universe 2022, 8(3), 181; https://doi.org/10.3390/universe8030181 - 14 Mar 2022

Cited by 12 | Viewed by 1980

Abstract

The advent of gadolinium-loaded Super-Kamiokande (SK-Gd) and of the soon-to-start JUNO liquid scintillator detector marks a substantial improvement in global sensitivity for the Diffuse Supernova Neutrino Background (DSNB). The present article reviews the detector properties most relevant for the DSNB searches in both [...] Read more.

The advent of gadolinium-loaded Super-Kamiokande (SK-Gd) and of the soon-to-start JUNO liquid scintillator detector marks a substantial improvement in global sensitivity for the Diffuse Supernova Neutrino Background (DSNB). The present article reviews the detector properties most relevant for the DSNB searches in both experiments and estimates the expected signal and background levels. Based on these inputs, we evaluate the sensitivity of both experiments individually and combined. Using a simplified statistical approach, we find that both SK-Gd and JUNO have the potential to reach >3

σ

evidence of the DSNB signal within 10 years of measurement. Combination of their results is likely to enable a

5 σ

discovery of the DSNB signal within the next decade. Full article

(This article belongs to the Special Issue Neutrinos from Astrophysical Sources)

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17 pages, 1369 KiB

Open AccessFeature PaperArticle

The Contribution of Large Recurrent Sunspot Groups to Solar Activity: Empirical Evidence

by Alexander Shapoval

Universe 2022, 8(3), 180; https://doi.org/10.3390/universe8030180 - 13 Mar 2022

Cited by 1 | Viewed by 1657

Abstract

We identify large sunspot nestlets (SN) mostly containing recurrent sunspot groups and investigate the indices of solar activity defined as the 11- or 22-year moving average of the daily areas of the SN. These nestlets, 667 in total, are constructed from the daily [...] Read more.

We identify large sunspot nestlets (SN) mostly containing recurrent sunspot groups and investigate the indices of solar activity defined as the 11- or 22-year moving average of the daily areas of the SN. These nestlets, 667 in total, are constructed from the daily 1874–2020 RGO/SOON catalogue, which contains 41,394 groups according to their IDs, with a machine-learning technique. Within solar cycles 15–19, the index contributed disproportionately strongly to the overall solar activity: the index is normalized to a quasi-constant shape by a power function of the activity, where the exponent is approximately

1.35

. Large SN contribute to solar activity even more in cycle 22, underlying the second largest peak of solar activity within the last Gleissberg cycle in ∼1985. Introducing another composite, moderate SN normalized by the overall activity, we observe its quasi-constant shape in cycles 15–19 and a general anti-correlation with the first normalized composite. The constructed sunspot nestlets constitute a modified catalogue of solar activity. We define the average lifetime per day in 22-year windows for the modified catalogue, in line with Henwood et al. (SoPhys 262, 299, 2010), and reproduce the dynamics of this quantity they revealed for 1900–1965. The average lifetime derived from the moderate SN is found to form a wave with minima at the beginning of the 20th and 21st centuries, resembling the Gleissberg cycle with long minima. The average lifetime characterizing large SN exhibited a deeper minimum at the beginning of the 20th century than 100 years later. Full article

(This article belongs to the Special Issue Solar Activity Cycle)

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10 pages, 342 KiB

Open AccessArticle

Post-Newtonian Jeans Equation for Stationary and Spherically Symmetrical Self-Gravitating Systems

by Gilberto Medeiros Kremer

Universe 2022, 8(3), 179; https://doi.org/10.3390/universe8030179 - 13 Mar 2022

Cited by 2 | Viewed by 1659

Abstract

The post-Newtonian Jeans equation for stationary self-gravitating systems is derived from the post-Newtonian Boltzmann equation in spherical coordinates. The Jeans equation is coupled with the three Poisson equations from the post-Newtonian theory. The Poisson equations are functions of the energy-momentum tensor components which [...] Read more.

The post-Newtonian Jeans equation for stationary self-gravitating systems is derived from the post-Newtonian Boltzmann equation in spherical coordinates. The Jeans equation is coupled with the three Poisson equations from the post-Newtonian theory. The Poisson equations are functions of the energy-momentum tensor components which are determined from the post-Newtonian Maxwell–Jüttner distribution function. As an application, the effect of a central massive black hole on the velocity dispersion profile of the host galaxy is investigated and the influence of the post-Newtonian corrections are determined. Full article

(This article belongs to the Special Issue Kinetic Processes in Relativistic Domain)

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17 pages, 883 KiB

Open AccessEditor’s ChoiceReview

Low Energy Supersymmetry Confronted with Current Experiments: An Overview

by Fei Wang, Wenyu Wang, Jinmin Yang, Yang Zhang and Bin Zhu

Universe 2022, 8(3), 178; https://doi.org/10.3390/universe8030178 - 12 Mar 2022

Cited by 18 | Viewed by 2511

Abstract

This study provides a brief overview of low energy supersymmetry (SUSY) in light of current experimental constraints, such as collider searches, dark matter searches, and muon

g - 2

measurements. In addition, we survey a variety of low energy supersymmetric models: the phenomenological [...] Read more.

This study provides a brief overview of low energy supersymmetry (SUSY) in light of current experimental constraints, such as collider searches, dark matter searches, and muon

g - 2

measurements. In addition, we survey a variety of low energy supersymmetric models: the phenomenological minimal supersymmetric model (MSSM); the supersymmetric models with cut-off-scale boundary conditions, i.e., the minimal supergravity (mSUGRA) or the constrained MSSM (CMSSM), the gauge mediation of SUSY breaking (GMSB), and the anomaly mediation of SUSY breaking (AMSB), as well as their extensions. The conclusion is that the low energy SUSY can survive all current experimental constraints and remains compelling, albeit suffering from a slight fine-tuning problem. The advanced models such as mSUGRA, GMSB, and AMSB need to be extended if the muon

g - 2

anomaly comes from new physics. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

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18 pages, 362 KiB

Open AccessEditor’s ChoiceArticle

Minisuperspace Quantum Cosmology in Metric and Affine Theories of Gravity

by Salvatore Capozziello and Francesco Bajardi

Universe 2022, 8(3), 177; https://doi.org/10.3390/universe8030177 - 10 Mar 2022

Cited by 11 | Viewed by 2187

Abstract

Minisuperspace Quantum Cosmology is an approach by which it is possible to infer initial conditions for dynamical systems which can suitably represent observable and non-observable universes. Here we discuss theories of gravity which, from various points of view, extend Einstein’s General Relativity. Specifically, [...] Read more.

Minisuperspace Quantum Cosmology is an approach by which it is possible to infer initial conditions for dynamical systems which can suitably represent observable and non-observable universes. Here we discuss theories of gravity which, from various points of view, extend Einstein’s General Relativity. Specifically, the Hamiltonian formalism for

f (R)

,

f (T)

, and

f (G)

gravity, with R, T, and

G

being the curvature, torsion and Gauss–Bonnet scalars, respectively, is developed starting from the Arnowitt–Deser–Misner approach. The Minisuperspace Quantum Cosmology is derived for all these models and cosmological solutions are obtained thanks to the existence of Noether symmetries. The Hartle criterion allows the interpretation of solutions in view of observable universes. Full article

(This article belongs to the Special Issue Quantum Cosmology)

14 pages, 315 KiB

Open AccessArticle

Gauge Gravity Vacuum in Constraintless Clairaut-Type Formalism

by Michael L. Walker and Steven Duplij

Universe 2022, 8(3), 176; https://doi.org/10.3390/universe8030176 - 10 Mar 2022

Cited by 2 | Viewed by 1502

Abstract

The gauged Lorentz theory with torsion has been argued to have an effective theory whose non-trivial background is responsible for background gravitational curvature if torsion is treated as a quantum-mechanical variable against a background of constant curvature. We use the CDG decomposition to [...] Read more.

The gauged Lorentz theory with torsion has been argued to have an effective theory whose non-trivial background is responsible for background gravitational curvature if torsion is treated as a quantum-mechanical variable against a background of constant curvature. We use the CDG decomposition to argue that such a background can be found without including torsion. Adapting our previously published Clairaut-based treatment of QCD, we go on to study the implications for second quantisation. Full article

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

16 pages, 1755 KiB

Open AccessReview

Two Sides of the Same Coin: Sterile Neutrinos and Dark Radiation, Status and Perspectives

by Maria Archidiacono and Stefano Gariazzo

Universe 2022, 8(3), 175; https://doi.org/10.3390/universe8030175 - 10 Mar 2022

Cited by 13 | Viewed by 2237

Abstract

The presence of light sterile neutrinos is one of the unanswered questions of particle physics. The cosmological counterpart is represented by dark radiation, i.e., any form of radiation present in the early Universe besides photons and standard (active) neutrinos. This short review provides [...] Read more.

The presence of light sterile neutrinos is one of the unanswered questions of particle physics. The cosmological counterpart is represented by dark radiation, i.e., any form of radiation present in the early Universe besides photons and standard (active) neutrinos. This short review provides a comprehensive overview of the two problems and of their connection. We review the status of neutrino oscillation anomalies, commenting on the most recent oscillation data and their mutual tensions, and we discuss the constraints from other terrestrial probes. We show the shortcomings of translating light sterile neutrinos in cosmology as additional thermalised relativistic species, produced by neutrino oscillations, and we detail alternative solutions, specifically focusing on neutrino nonstandard interactions, and on their link to the Hubble constant problem. The impact of a new force leading to dark radiation–dark matter interactions is also discussed in the realm of new physics in the dark sector. Full article

(This article belongs to the Special Issue Dark Matter and Dark Energy: Particle Physics, Cosmology, and Experimental Searches)

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19 pages, 1648 KiB

Open AccessArticle

Multiplicity Dependencies of Midrapidity Transverse Momentum Distributions of Identified Charged Particles in Proton-Proton Collisions at (s)^1/2 = 7 TeV at the LHC

by Khusniddin K. Olimov, Fu-Hu Liu, Kobil A. Musaev and Maratbek Z. Shodmonov

Universe 2022, 8(3), 174; https://doi.org/10.3390/universe8030174 - 10 Mar 2022

Cited by 9 | Viewed by 1762

Abstract

Dependencies of midrapidity p_t distributions of the charged pions and kaons, protons and antiprotons on charged-particle multiplicity density (<dN_ch/dη>) in inelastic proton-proton collisions at (s)^1/2 = 7 TeV at the LHC, measured by ALICE [...] Read more.

Dependencies of midrapidity p_t distributions of the charged pions and kaons, protons and antiprotons on charged-particle multiplicity density (<dN_ch/dη>) in inelastic proton-proton collisions at (s)^1/2 = 7 TeV at the LHC, measured by ALICE Collaboration, are investigated. The simultaneous minimum χ² fits with the Tsallis function with thermodynamical consistence and the Hagedorn function with included transverse flow have well-described the p_t spectra of the particle species in the ten studied groups of charged-particle multiplicity density. The effective temperatures, T, of the Tsallis function with thermodynamical consistence have shown a steady rise with increasing the charged-particle multiplicity in proton-proton collisions at (s)^1/2 = 7 TeV, in agreement with the similar result obtained recently in proton-proton collisions at (s)^1/2 = 13 TeV at the LHC. The respective T versus <dN_ch/dη> dependence in proton-proton collisions at (s)^1/2 = 7 TeV is reproduced quite well by the simple power function with the same value (≈ 1/3) of the exponent parameter as that extracted in proton-proton collisions at (s)^1/2 = 13 TeV. The identical power dependence

T ~ ε^{1 / 3}

between the initial energy density and effective temperature of the system has been observed in proton-proton collisions at (s)^1/2 = 7 and 13 TeV. We have observed that the transverse radial flow emerges at <dN_ch/dη> ≈ 6 and then increases, becoming substantial at larger multiplicity events in proton-proton collisions at (s)^1/2 = 7 TeV. We have estimated, analyzing T₀ and

⟨ β_{t} ⟩

versus <dN_ch/dη> dependencies, that the possible onset of deconfinement phase transition in proton-proton collisions at (s)^1/2 = 7 TeV occurs at <dN_ch/dη> ≈ 6.1 ± 0.3, which is close to the corresponding recent estimate (<dN_ch/dη> ≈ 7.1 ± 0.2) in proton-proton collisions at (s)^1/2 = 13 TeV. The corresponding critical energy densities for probable onset of deconfinement phase transition in proton-proton collisions at (s)^1/2 = 7 and 13 TeV at the LHC have been estimated to be 0.67 ± 0.03 and 0.76 ± 0.02 GeV/fm³, respectively. Full article

(This article belongs to the Special Issue Universe: Feature Papers–High Energy Nuclear and Particle Physics)

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13 pages, 2270 KiB

Open AccessReview

Impact of AGB Stars on the Chemical Evolution of Neutron-Capture Elements

by Gabriele Cescutti and Francesca Matteucci

Universe 2022, 8(3), 173; https://doi.org/10.3390/universe8030173 - 9 Mar 2022

Cited by 8 | Viewed by 2147

Abstract

In this review, we discuss the impact of s-process nucleosynthesis in asymptotic giant branch stars on the enrichment of heavy elements. We review the main steps made on this subject in the last 40 years and discuss the importance of modelling the evolution [...] Read more.

In this review, we discuss the impact of s-process nucleosynthesis in asymptotic giant branch stars on the enrichment of heavy elements. We review the main steps made on this subject in the last 40 years and discuss the importance of modelling the evolution of the abundances of such elements in our Milky Way. From the comparison between model results and observations, we can impose strong constraints on stellar nucleosynthesis, as well as on the evolution of the Milky Way. 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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14 pages, 1769 KiB

Open AccessArticle

Numerical Equilibrium Configurations and Quadrupole Moments of Post-Merger Differentially Rotating Relativistic Stars

by Kevin Franceschetti, Luca Del Zanna, Jacopo Soldateschi and Niccolò Bucciantini

Universe 2022, 8(3), 172; https://doi.org/10.3390/universe8030172 - 9 Mar 2022

Cited by 3 | Viewed by 1761

Abstract

Numerical simulations of binary neutron star mergers invariably show that, when a long-lived remnant forms, its rotation profile is never a simple decaying function of the radius but rather exhibits a maximum rotation rate shifted away from the center. This is in contrast [...] Read more.

Numerical simulations of binary neutron star mergers invariably show that, when a long-lived remnant forms, its rotation profile is never a simple decaying function of the radius but rather exhibits a maximum rotation rate shifted away from the center. This is in contrast to the usual differential rotation profile employed for the numerical modeling of axisymmetric equilibria of relativistic stars. Two families of rotation rate functions that mimic post-merger profiles were proposed by Uryū et al. (2017). In this work we implement Uryū’s profiles into the XNS code by Bucciantini and Del Zanna (2011) and we present novel equilibrium sequences of differentially rotating neutron stars. These are constructed by using three different equations of state, in order to study the dependence of mass, radius, angular momentum, and other important physical quantities, especially the quadrupole deformation and metric quadrupole moment, from the rotation properties. Full article

(This article belongs to the Section Compact Objects)

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26 pages, 366 KiB

Open AccessArticle

Singular Lagrangians, Constrained Hamiltonian Systems and Gauge Invariance: An Example of the Dirac–Bergmann Algorithm

by J. David Brown

Universe 2022, 8(3), 171; https://doi.org/10.3390/universe8030171 - 9 Mar 2022

Cited by 7 | Viewed by 2136

Abstract

The Dirac–Bergmann algorithm is a recipe for converting a theory with a singular Lagrangian into a constrained Hamiltonian system. Constrained Hamiltonian systems include gauge theories—general relativity, electromagnetism, Yang–Mills, string theory, etc. The Dirac–Bergmann algorithm is elegant but at the same time rather complicated. [...] Read more.

The Dirac–Bergmann algorithm is a recipe for converting a theory with a singular Lagrangian into a constrained Hamiltonian system. Constrained Hamiltonian systems include gauge theories—general relativity, electromagnetism, Yang–Mills, string theory, etc. The Dirac–Bergmann algorithm is elegant but at the same time rather complicated. It consists of a large number of logical steps linked together by a subtle chain of reasoning. Examples of the Dirac–Bergmann algorithm found in the literature are designed to isolate and illustrate just one or two of those logical steps. In this paper, I analyze a finite-dimensional system that exhibits all of the major steps in the algorithm. The system includes primary and secondary constraints, first and second class constraints, restrictions on Lagrange multipliers, and both physical and gauge degrees of freedom. This relatively simple system provides a platform for discussing the Dirac conjecture, constructing Dirac brackets, and applying gauge conditions. Full article

(This article belongs to the Special Issue Numerical Relativity)

10 pages, 558 KiB

Open AccessArticle

The NuGrid AGB Evolution and Nucleosynthesis Data Set

by Umberto Battino, Marco Pignatari, Ashley Tattersall, Pavel Denissenkov and Falk Herwig

Universe 2022, 8(3), 170; https://doi.org/10.3390/universe8030170 - 9 Mar 2022

Cited by 2 | Viewed by 2399

Abstract

Asymptotic Giant Branch (AGB) stars play a key role in the chemical evolution of galaxies. These stars are the fundamental stellar site for the production of light elements such as C, N and F, and half of the elements heavier than Fe via [...] Read more.

Asymptotic Giant Branch (AGB) stars play a key role in the chemical evolution of galaxies. These stars are the fundamental stellar site for the production of light elements such as C, N and F, and half of the elements heavier than Fe via the slow neutron capture process (s-process). Hence, detailed computational models of AGB stars’ evolution and nucleosynthesis are essential for galactic chemical evolution. In this work, we discuss the progress in updating the NuGrid data set of AGB stellar models and abundance yields. All stellar models have been computed using the MESA stellar evolution code, coupled with the post-processing mppnp code to calculate the full nucleosynthesis. The final data set will include the initial masses M

_{i n i}

/M

_{⊙}

= 1, 1.65, 2, 3, 4, 5, 6 and 7 for initial metallicities Z = 0.0001, 0.001, 0.006, 0.01, 0.02 and 0.03. Observed s-process abundances on the surfaces of evolved stars as well as the typical light elements in the composition of H-deficient post-AGB stars are reproduced. A key short-term goal is to complete and expand the AGB stars data set for the full metallicity range. Chemical yield tables are provided for the available models. 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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14 pages, 354 KiB

Open AccessReview

Studying ΔL = 2 Lepton Flavor Violation with Muons

by Alexey A. Petrov, Renae Conlin and Cody Grant

Universe 2022, 8(3), 169; https://doi.org/10.3390/universe8030169 - 8 Mar 2022

Cited by 1 | Viewed by 1519

Abstract

Flavor violating processes in the lepton sector have highly suppressed branching ratios in the standard model. Thus, observation of lepton flavor violation (LFV) constitutes a clear indication of physics beyond the standard model (BSM). We review new physics searches in the processes that [...] Read more.

Flavor violating processes in the lepton sector have highly suppressed branching ratios in the standard model. Thus, observation of lepton flavor violation (LFV) constitutes a clear indication of physics beyond the standard model (BSM). We review new physics searches in the processes that violate the conservation of lepton (muon) flavor by two units with muonia and muonium–antimuonium oscillations. Full article

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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9 pages, 291 KiB

Open AccessArticle

Topological Effects of a Spiral Dislocation on Quantum Revivals

by Anderson V. D. M. Maia and Knut Bakke

Universe 2022, 8(3), 168; https://doi.org/10.3390/universe8030168 - 8 Mar 2022

Cited by 11 | Viewed by 1678

Abstract

We analyse the influence of spiral dislocation topology on the revival time for the harmonic oscillator, for a particle confined to one-dimensional quantum ring, and a two-dimensional quantum ring. We first investigate the effects of a cut-off point that stems from the topology [...] Read more.

We analyse the influence of spiral dislocation topology on the revival time for the harmonic oscillator, for a particle confined to one-dimensional quantum ring, and a two-dimensional quantum ring. We first investigate the effects of a cut-off point that stems from the topology of this defect on the harmonic oscillator. Then, we show that the influence of spiral dislocation topology on the harmonic oscillator gives rise to a non-null revival time related to the radial quantum number. In the case of the two-dimensional quantum ring, we show that the revival times related to the radial quantum number and the angular momentum quantum number are influenced by the spiral dislocation topology. Full article

(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)

15 pages, 427 KiB

Open AccessArticle

Metrics on End-Periodic Manifolds as Models for Dark Matter

by Christopher L. Duston

Universe 2022, 8(3), 167; https://doi.org/10.3390/universe8030167 - 8 Mar 2022

Viewed by 1545

Abstract

In this paper we will detail an approach to generate metrics and matter models on end-periodic manifolds, which are used extensively in the study of the exotic smooth structures of

R^{4}

. After an overview of the technique, we will present two [...] Read more.

In this paper we will detail an approach to generate metrics and matter models on end-periodic manifolds, which are used extensively in the study of the exotic smooth structures of

R^{4}

. After an overview of the technique, we will present two specific examples, discuss the associated matter models by solving the Einstein equations, and determine the physical viability by examining the energy conditions. We compare the resulting model directly with existing models of matter distributions in extragalactic systems, to highlight the viability of utilizing exotic smooth structures to understand the existence and distribution of dark matter. Full article

(This article belongs to the Special Issue Geometric and Topological Models of Dark Matter and Dark Energy)

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48 pages, 552 KiB

Open AccessReview

Topics in Cosmology—Clearly Explained by Means of Simple Examples

by Jaume de Haro and Emilio Elizalde

Universe 2022, 8(3), 166; https://doi.org/10.3390/universe8030166 - 7 Mar 2022

Cited by 2 | Viewed by 2268

Abstract

This is a very comprehensible review of some key issues in modern cosmology. Simple mathematical examples and analogies are used, whenever available. The starting point is the well-known Big Bang cosmology (BBC). We deal with the mathematical singularities appearing in this theory and [...] Read more.

This is a very comprehensible review of some key issues in modern cosmology. Simple mathematical examples and analogies are used, whenever available. The starting point is the well-known Big Bang cosmology (BBC). We deal with the mathematical singularities appearing in this theory and discuss some ways to remove them. Next, and before introducing the inflationary paradigm by means of clear examples, we review the horizon and flatness problems of the old BBC model. We then consider the current cosmic acceleration and, as a procedure to deal with both periods of cosmic acceleration in a unified way, we study quintessential inflation. Finally, the reheating stage of the universe via gravitational particle production, which took place after inflation ended, is discussed in clear mathematical terms, by involving the so-called

α

-attractors in the context of quintessential inflation. Full article

(This article belongs to the Section Cosmology)

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Universe, Volume 8, Issue 3 (March 2022) – 57 articles

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