Universe

76 pages, 1379 KiB

Open AccessArticle

Effective Scalar Potential in Asymptotically Safe Quantum Gravity

by Christof Wetterich

Universe 2021, 7(2), 45; https://doi.org/10.3390/universe7020045 - 17 Feb 2021

Cited by 20 | Viewed by 2510

We compute the effective potential for scalar fields in asymptotically safe quantum gravity. A scaling potential and other scaling functions generalize the fixed point values of renormalizable couplings. The scaling potential takes a non-polynomial form, approaching typically a constant for large values of [...] Read more.

We compute the effective potential for scalar fields in asymptotically safe quantum gravity. A scaling potential and other scaling functions generalize the fixed point values of renormalizable couplings. The scaling potential takes a non-polynomial form, approaching typically a constant for large values of scalar fields. Spontaneous symmetry breaking may be induced by non-vanishing gauge couplings. We strengthen the arguments for a prediction of the ratio between the masses of the top quark and the Higgs boson. Higgs inflation in the standard model is unlikely to be compatible with asymptotic safety. Scaling solutions with vanishing relevant parameters can be sufficient for a realistic description of particle physics and cosmology, leading to an asymptotically vanishing “cosmological constant” or dynamical dark energy. Full article

(This article belongs to the Special Issue Asymptotic Safety in Quantum Gravity)

► Show Figures

Figure 1

10 pages, 284 KiB

Open AccessCommunication

Determining the Topology and Deflection Angle of Ringholes via Gauss-Bonnet Theorem

by Kimet Jusufi

Universe 2021, 7(2), 44; https://doi.org/10.3390/universe7020044 - 16 Feb 2021

Cited by 2 | Viewed by 1898

Abstract

In this letter, we use a recent wormhole metric known as a ringhole [Gonzalez-Diaz, Phys. Rev. D 54, 6122, 1996] to determine the surface topology and the deflection angle of light in the weak limit approximation using the Gauss-Bonnet theorem (GBT). We [...] Read more.

In this letter, we use a recent wormhole metric known as a ringhole [Gonzalez-Diaz, Phys. Rev. D 54, 6122, 1996] to determine the surface topology and the deflection angle of light in the weak limit approximation using the Gauss-Bonnet theorem (GBT). We apply the GBT and show that the surface topology at the wormhole throat is indeed a torus by computing the Euler characteristic number. As a special case of the ringhole solution, one can find the Ellis wormhole which has the surface topology of a 2-sphere at the wormhole throat. The most interesting results of this paper concerns the problem of gravitational deflection of light in the spacetime of a ringhole geometry by applying the GBT to the optical ringhole geometry. It is shown that, the deflection angle of light depends entirely on the geometric structure of the ringhole geometry encoded by the parameters

b_{0}

and a, being the ringhole throat radius and the radius of the circumference generated by the circular axis of the torus, respectively. As special cases of our general result, the deflection angle by Ellis wormhole is obtained. Finally, we work out the problem of deflection of relativistic massive particles and show that the deflection angle remains unaltered by the speed of the particles. Full article

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

15 pages, 3097 KiB

Open AccessArticle

A Novel Method to Identify the Physical Mechanism and Source Region of ELF/VLF Waves Generated by Beat-Wave Modulation Using Preheating Technique

by Zhe Guo, Hanxian Fang and Farideh Honary

Universe 2021, 7(2), 43; https://doi.org/10.3390/universe7020043 - 15 Feb 2021

Cited by 2 | Viewed by 2068

Abstract

One of the most important effects of ionospheric heating by HF (high-frequency) waves is the generation of ELF/VLF (extremely low-frequency/very low-frequency) waves by modulated heating. An important limitation of amplitude modulation (AM) is its dependence on ionospheric electrojet, which means to achieve better [...] Read more.

One of the most important effects of ionospheric heating by HF (high-frequency) waves is the generation of ELF/VLF (extremely low-frequency/very low-frequency) waves by modulated heating. An important limitation of amplitude modulation (AM) is its dependence on ionospheric electrojet, which means to achieve better modulation effect, some strict spatio-temporal conditions must be met. To solve this problem, some possible methods have been proposed including beat-wave (BW) modulation. However, due to the controversy of its mechanism and the source region of the stimulated ELF/VLF waves, it is not clear whether it is an electrojet-independent method or not, which has become one of the hot topics in recent years. In this paper, we found that the effect of preheating on modulation efficiency of BW based on different theories is the opposite. We suppose the opposite character of the influence and effect on the efficiency of BW in D region and F region as a base for a novel method to identify the physical mechanism and source region of BW. This method can be feasible to solve the controversy of BW. The feasibility of this method is verified by simulation results in the paper. Full article

(This article belongs to the Section Space Science)

► Show Figures

Figure 1

9 pages, 415 KiB

Open AccessCommunication

Positronia’ Clouds in Universe

by Igor M. Dremin

Universe 2021, 7(2), 42; https://doi.org/10.3390/universe7020042 - 12 Feb 2021

Cited by 2 | Viewed by 1829

Abstract

The intense emission of 511 keV photons from the Galactic center and within terrestrial thunderstorms is attributed to the formation of parapositronia clouds. Unbound electron–positron pairs and positronia can be created by strong electromagnetic fields produced in interactions of electrically charged objects, in [...] Read more.

The intense emission of 511 keV photons from the Galactic center and within terrestrial thunderstorms is attributed to the formation of parapositronia clouds. Unbound electron–positron pairs and positronia can be created by strong electromagnetic fields produced in interactions of electrically charged objects, in particular, in collisions of heavy nuclei. Kinematics of this process favors abundant creation of the unbound electron–positron pairs with very small masses and the confined parapositronia states which decay directly to two 511 keV quanta. Therefore, we propose to consider interactions of electromagnetic fields of colliding heavy ions as a source of low-mass pairs which can transform to 511 keV quanta. Intensity of their creation is enlarged by the factor Z

^{4}

(Z is the electric charge of a heavy ion) compared to protons with Z = 1. These processes are especially important at very high energies of nuclear collisions because their cross sections increase proportionally to cube of the logarithm of energy and can even exceed the cross sections of strong interactions which may not increase faster than the squared logarithm of energy. Moreover, production of extremely low-mass

e^{+} e^{-}

-pairs in ultraperipheral nuclear collisions is strongly enhanced due to the Sommerfeld-Gamow-Sakharov (SGS) factor which accounts for mutual Coulomb attraction of non-relativistic electrons to positrons in case of low pair-masses. This attraction may lead to their annihilation and, therefore, to the increased intensity of 511 keV photons. It is proposed to confront the obtained results to forthcoming experimental data at NICA collider. Full article

(This article belongs to the Section High Energy Nuclear and Particle Physics)

► Show Figures

Figure 1

10 pages, 2985 KiB

Open AccessCommunication

Daytime Cloud Detection Method Using the All-Sky Imager over PERMATApintar Observatory

by Mohammad Afiq Dzuan Mohd Azhar, Nurul Shazana Abdul Hamid, Wan Mohd Aimran Wan Mohd Kamil and Nor Sakinah Mohamad

Universe 2021, 7(2), 41; https://doi.org/10.3390/universe7020041 - 9 Feb 2021

Cited by 9 | Viewed by 2194

Abstract

In this study, we explored a new method of cloud detection called the Blue-Green (B-G) Color Difference, which is adapted from the widely used Red-Blue (R-B) Color Difference. The objective of this study was to test the effectiveness of these two methods in [...] Read more.

In this study, we explored a new method of cloud detection called the Blue-Green (B-G) Color Difference, which is adapted from the widely used Red-Blue (R-B) Color Difference. The objective of this study was to test the effectiveness of these two methods in detecting daytime clouds. Three all-sky images were selected from a database system at PERMATApintar Observatory. Each selected all-sky image represented different sky conditions, namely clear, partially cloudy and overcast. Both methods were applied to all three images and compared in terms of cloud coverage detection. Our analysis revealed that both color difference methods were able to detect a thick cloud efficiently. However, the B-G was able to detect thin clouds better compared to the R-B method, resulting in a higher and more accurate cloud coverage detection. Full article

(This article belongs to the Section Space Science)

► Show Figures

Figure 1

18 pages, 1986 KiB

Open AccessArticle

Trapped Gravitational Waves in Jackiw–Teitelboim Gravity

by Jeong-Myeong Bae, Ido Ben-Dayan, Marcelo Schiffer, Gibum Yun and Heeseung Zoe

Universe 2021, 7(2), 40; https://doi.org/10.3390/universe7020040 - 7 Feb 2021

Viewed by 1782

Abstract

We discuss the possibility that gravitational fluctuations (“gravitational-waves”) are trapped in space by gravitational interactions in two dimensional Jackiw–Teitelboim gravity. In the standard geon (gravitational electromagnetic entity) approach, the effective energy is entirely deposited in a thin layer, the active region, that achieves [...] Read more.

We discuss the possibility that gravitational fluctuations (“gravitational-waves”) are trapped in space by gravitational interactions in two dimensional Jackiw–Teitelboim gravity. In the standard geon (gravitational electromagnetic entity) approach, the effective energy is entirely deposited in a thin layer, the active region, that achieves spatial self-confinement and raises doubts about the geon’s stability. In this paper we relinquish the “active region” approach and obtain self-confinement of “gravitational waves” that are trapped by the vacuum geometry and can be stable against the backreaction due to metric fluctuations. Full article

(This article belongs to the Section Gravitation)

► Show Figures

Figure 1

1 pages, 178 KiB

Open AccessCorrection

Correction: Mashhoon, B. Critical Tidal Currents in General Relativity. Universe 2020, 6, 104

by Bahram Mashhoon

Universe 2021, 7(2), 39; https://doi.org/10.3390/universe7020039 - 5 Feb 2021

Viewed by 1380

Abstract

In writing out the tidal equation for X in my recent paper [...] Full article

23 pages, 422 KiB

Open AccessEditor’s ChoiceReview

To Conserve, or Not to Conserve: A Review of Nonconservative Theories of Gravity

by Hermano Velten and Thiago R. P. Caramês

Universe 2021, 7(2), 38; https://doi.org/10.3390/universe7020038 - 4 Feb 2021

Cited by 37 | Viewed by 2817

Abstract

Apart from the familiar structure firmly-rooted in the general relativistic field equations where the energy–momentum tensor has a null divergence i.e., it conserves, there exists a considerable number of extended theories of gravity allowing departures from the usual conservative framework. Many of these [...] Read more.

Apart from the familiar structure firmly-rooted in the general relativistic field equations where the energy–momentum tensor has a null divergence i.e., it conserves, there exists a considerable number of extended theories of gravity allowing departures from the usual conservative framework. Many of these theories became popular in the last few years, aiming to describe the phenomenology behind dark matter and dark energy. However, within these scenarios, it is common to see attempts to preserve the conservative property of the energy–momentum tensor. Most of the time, it is done by means of some additional constraint that ensures the validity of the standard conservation law, as long as this option is available in the theory. However, if no such extra constraint is available, the theory will inevitably carry a non-trivial conservation law as part of its structure. In this work, we review some of such proposals discussing the theoretical construction leading to the non-conservation of the energy–momentum tensor. Full article

(This article belongs to the Special Issue Universe: 5th Anniversary)

12 pages, 355 KiB

Open AccessArticle

On the 2PN Pericentre Precession in the General Theory of Relativity and the Recently Discovered Fast-Orbiting S-Stars in Sgr A*

by Lorenzo Iorio

Universe 2021, 7(2), 37; https://doi.org/10.3390/universe7020037 - 4 Feb 2021

Cited by 11 | Viewed by 2297

Abstract

Recently, the secular pericentre precession was analytically computed to the second post-Newtonian (2PN) order by the present author with the Gauss equations in terms of the osculating Keplerian orbital elements in order to obtain closer contact with the observations in astronomical and astrophysical [...] Read more.

Recently, the secular pericentre precession was analytically computed to the second post-Newtonian (2PN) order by the present author with the Gauss equations in terms of the osculating Keplerian orbital elements in order to obtain closer contact with the observations in astronomical and astrophysical scenarios of potential interest. A discrepancy in previous results from other authors was found. Moreover, some of such findings by the same authors were deemed as mutually inconsistent. In this paper, it is demonstrated that, in fact, some calculation errors plagued the most recent calculations by the present author. They are explicitly disclosed and corrected. As a result, all of the examined approaches mutually agree, yielding the same analytical expression for the total 2PN pericentre precession once the appropriate conversions from the adopted parameterisations are made. It is also shown that, in the future, it may become measurable, at least in principle, for some of the recently discovered short-period S-stars in Sgr A*, such as S62 and S4714. Full article

(This article belongs to the Special Issue Current and Future Tests of General Relativity)

► Show Figures

Figure 1

11 pages, 2288 KiB

Open AccessCommunication

Holocene Millennial-Scale Solar Variability and the Climatic Responses on Earth

by Xinhua Zhao, Willie Soon and Victor M. Velasco Herrera

Universe 2021, 7(2), 36; https://doi.org/10.3390/universe7020036 - 4 Feb 2021

Cited by 3 | Viewed by 4643

Abstract

The solar impact on Earth’s climate is both a rich and open-ended topic with intense debates. In this study, we use the reconstructed data available to investigate periodicities of solar variability (i.e., variations of sunspot numbers) and temperature changes (10 sites spread all [...] Read more.

The solar impact on Earth’s climate is both a rich and open-ended topic with intense debates. In this study, we use the reconstructed data available to investigate periodicities of solar variability (i.e., variations of sunspot numbers) and temperature changes (10 sites spread all over the Earth) as well as the statistical inter-relations between them on the millennial scale during the past 8640 years (BC 6755–AD 1885) before the modern industrial era. We find that the variations of the Earth’s temperatures show evidence for the Eddy cycle component, i.e., the 1000-year cyclicity, which was discovered in variations of sunspot numbers and believed to be an intrinsic periodicity of solar variability. Further wavelet time-frequency analysis demonstrates that the co-variation between the millennium cycle components of solar variability and the temperature change held stable and statistically strong for five out of these 10 sites during our study interval. In addition, the Earth’s climatic response to solar forcing could be different region-by-region, and the temperatures in the southern hemisphere seemed to have an opposite changing trend compared to those in the northern hemisphere on this millennial scale. These findings reveal not only a pronounced but also a complex relationship between solar variability and climatic change on Earth on the millennial timescale. More data are needed to further verify these preliminary results in the future. Full article

(This article belongs to the Special Issue Space Weather)

► Show Figures

Figure 1

30 pages, 7763 KiB

Open AccessArticle

Limits on Magnetized Quark-Nugget Dark Matter from Episodic Natural Events

by J. Pace VanDevender, Aaron P. VanDevender, Peter Wilson, Benjamin F. Hammel and Niall McGinley

Universe 2021, 7(2), 35; https://doi.org/10.3390/universe7020035 - 4 Feb 2021

Cited by 7 | Viewed by 3141

Abstract

A quark nugget is a hypothetical dark-matter candidate composed of approximately equal numbers of up, down, and strange quarks. Most models of quark nuggets do not include effects of their intrinsic magnetic field. However, Tatsumi used a mathematically tractable approximation of the Standard [...] Read more.

A quark nugget is a hypothetical dark-matter candidate composed of approximately equal numbers of up, down, and strange quarks. Most models of quark nuggets do not include effects of their intrinsic magnetic field. However, Tatsumi used a mathematically tractable approximation of the Standard Model of Particle Physics and found that the cores of magnetar pulsars may be quark nuggets in a ferromagnetic liquid state with surface magnetic field B_o = 10^12±1 T. We have applied that result to quark-nugget dark matter. Previous work addressed the formation and aggregation of magnetized quark nuggets (MQNs) into a broad and magnetically stabilized mass distribution before they could decay and addressed their interaction with normal matter through their magnetopause, losing translational velocity while gaining rotational velocity and radiating electromagnetic energy. The two orders of magnitude uncertainty in Tatsumi’s estimate for B_o precludes the practical design of systematic experiments to detect MQNs through their predicted interaction with matter. In this paper, we examine episodic events consistent with a unique signature of MQNs. If they are indeed caused by MQNs, they constrain the most likely values of B_o to 1.65 × 10¹² T +/− 21% and support the design of definitive tests of the MQN dark-matter hypothesis. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy)

► Show Figures

Figure 1

13 pages, 415 KiB

Open AccessEditor’s ChoiceArticle

Benefit of New High-Precision LLR Data for the Determination of Relativistic Parameters

by Liliane Biskupek, Jürgen Müller and Jean-Marie Torre

Universe 2021, 7(2), 34; https://doi.org/10.3390/universe7020034 - 3 Feb 2021

Cited by 18 | Viewed by 3691

Abstract

Since 1969, Lunar Laser Ranging (LLR) data have been collected by various observatories and analysed by different analysis groups. In the recent years, observations with bigger telescopes (APOLLO) and at infra-red wavelength (OCA) are carried out, resulting in a better distribution of precise [...] Read more.

Since 1969, Lunar Laser Ranging (LLR) data have been collected by various observatories and analysed by different analysis groups. In the recent years, observations with bigger telescopes (APOLLO) and at infra-red wavelength (OCA) are carried out, resulting in a better distribution of precise LLR data over the lunar orbit and the observed retro-reflectors on the Moon. This is a great advantage for various investigations in the LLR analysis. The aim of this study is to evaluate the benefit of the new LLR data for the determination of relativistic parameters. Here, we show current results for relativistic parameters like a possible temporal variation of the gravitational constant

\dot{G} / G_{0} = (- 5.0 \pm 9.6) \times 10^{- 15} {yr}^{- 1}

, the equivalence principle with

Δ {(m_{g} / m_{i})}_{EM} = (- 2.1 \pm 2.4) \times 10^{- 14}

, and the PPN parameters

β - 1 = (6.2 \pm 7.2) \times 10^{- 5}

and

γ - 1 = (1.7 \pm 1.6) \times 10^{- 4}

. The results show a significant improvement in the accuracy of the various parameters, mainly due to better coverage of the lunar orbit, better distribution of measurements over the lunar retro-reflectors, and last but not least, higher accuracy of the data. Within the estimated accuracies, no violation of Einstein’s theory is found and the results set improved limits for the different effects. Full article

(This article belongs to the Special Issue Current and Future Tests of General Relativity)

► Show Figures

Figure 1

16 pages, 2851 KiB

Open AccessArticle

Possibilities of Detecting Light Dark Matter Produced via Drell-Yan Channel in a Fixed Target Experiment

by Eduard Ursov, Anna Anokhina, Emil Khalikov, Ivan Vidulin and Tatiana Roganova

Universe 2021, 7(2), 33; https://doi.org/10.3390/universe7020033 - 1 Feb 2021

Viewed by 2577

Abstract

This work presents the complete modeling scheme of production and detection of two types of light dark matter (LDM)—Dirac fermionic and scalar particles—in a fixed target experiment using SHiP experiment as an example. The Drell-Yan process was chosen as a channel of LDM [...] Read more.

This work presents the complete modeling scheme of production and detection of two types of light dark matter (LDM)—Dirac fermionic and scalar particles—in a fixed target experiment using SHiP experiment as an example. The Drell-Yan process was chosen as a channel of LDM production; the deep inelastic scattering on lead nuclei was simulated and analyzed in the detector; the production of secondary particles was modeled with the aid of PYTHIA6 toolkit. Obtained observable parameters of secondary particles produced in events associated with LDM were compared with the background neutrino events that were simulated using GENIE toolkit. The yield of LDM events was calculated with various model parameter values. Using machine learning methods, a classifier that is able to distinguish LDM events from neutrino background events based on the observed parameters with high precision has been developed. Full article

(This article belongs to the Special Issue Enhancing Sensitivity to Physics beyond the Standard Model and Detector Performance Monitoring in HEP Experiments with Machine Learning)

► Show Figures

Figure 1

40 pages, 1715 KiB

Open AccessArticle

Quaternion Algebra on 4D Superfluid Quantum Space-Time: Can Dark Matter Be a Manifestation of the Superfluid Ether?

by Valeriy Sbitnev

Universe 2021, 7(2), 32; https://doi.org/10.3390/universe7020032 - 31 Jan 2021

Cited by 4 | Viewed by 3769

Abstract

Quaternions are a natural framework of 4D space-time, where the unit element relates to time, and three others relate to 3D space. We define a quaternion set of differential torsion operators (shifts with rotations) that act to the energy-momentum tensor written on the [...] Read more.

Quaternions are a natural framework of 4D space-time, where the unit element relates to time, and three others relate to 3D space. We define a quaternion set of differential torsion operators (shifts with rotations) that act to the energy-momentum tensor written on the same quaternion basis. It results in the equations of gravity-torsion (gravitomagnetic) fields that are similar to Maxwell’s equations. These equations are parent equations, generating the following equations: (a) equations of the transverse gravity-torsion waves; (b) the vorticity equation describing vortices orbital speed of which grows monotonically in the vortex core but far from it, it goes to a permanent level; (c) the modified Navier–Stokes equation leading to the Schrödinger equation in the nonrelativistic limit and to the Dirac equation in the relativistic limit. The Ginsburg–Landau theory of superfluidity resulting from the Schrödinger equation shows the emergence of coupled proton-antiproton pairs forming the Bose–Einstein condensate. In the final part of the article, we describe Samokhvalov’s experiment with rotating nonelectric, nonferromagnetic massive disks in a vacuum. It demonstrates an unknown force transferring the rotational moment from the driving disk to a driven one. It can be a manifestation of the dark matter. For studying this phenomenon, we propose a neutron interference experiment that is like the Aharonov–Bohm one. Full article

(This article belongs to the Special Issue Foundations of Quantum Mechanics)

► Show Figures

Figure 1

20 pages, 3832 KiB

Open AccessArticle

Dark Matter Constraints and the Neutralino Sector of the scNMSSM

by Elham Aldufeery and Maien Binjonaid

Universe 2021, 7(2), 31; https://doi.org/10.3390/universe7020031 - 31 Jan 2021

Cited by 5 | Viewed by 2632

Abstract

The neutralino sector of the semi-constrained next-to-minimal supersymmetric standard model is explored under recent experimental constraints, with special attention to dark matter (DM) limits. The effects of the upper and lower bounds of dark matter relic density and recent direct detection constraints on [...] Read more.

The neutralino sector of the semi-constrained next-to-minimal supersymmetric standard model is explored under recent experimental constraints, with special attention to dark matter (DM) limits. The effects of the upper and lower bounds of dark matter relic density and recent direct detection constraints on spin-independent and -dependent cross-sections are thoroughly analyzed. Particularly, we show which regions of the parameter space are ruled out due to the different dark matter constraints and the corresponding model-specific parameters:

λ, κ, A_{λ}

, and

A_{κ}

. We analyze all annihilation and co-annihilation processes (with heavier neutralinos and charginos) that contribute to the dark matter relic density. The mass components of the dark matter candidate, the lightest neutralino

{\tilde{χ}}_{1}^{0}

, are studied, and the decays of heavy neutralinos and charginos, especially

{\tilde{χ}}_{2}^{0}

and

{\tilde{χ}}_{1}^{+}

, into the lightest neutralino are examined. We impose semi-universal boundary conditions at the Grand Unified Theory scale, and require a moderate range of

tan β ≲ 10

. We find that the allowed parameter space is associated with a heavy mass spectrum in general and that the lightest neutralino is mostly Higgsino with a mass range that resides mostly between 1000 and 1500 GeV. However, smaller mass values can be achieved if the DM candidate is bino-like or singlino-like. Full article

(This article belongs to the Section High Energy Nuclear and Particle Physics)

► Show Figures

Figure 1

9 pages, 668 KiB

Open AccessCommunication

Quantization of Gravitationally Bound Systems

by Michael Fil’chenkov and Yuri Laptev

Universe 2021, 7(2), 30; https://doi.org/10.3390/universe7020030 - 30 Jan 2021

Cited by 1 | Viewed by 1817

Abstract

Some of the approaches to quantization in gravity theory concerning gravitationally bound systems are considered. Grades of quantization applicable to these systems have been classified in terms of quantum mechanics, quantum field theory, and quantum geometrodynamics. Energy levels for the graviatom, Lemaître’s atom, [...] Read more.

Some of the approaches to quantization in gravity theory concerning gravitationally bound systems are considered. Grades of quantization applicable to these systems have been classified in terms of quantum mechanics, quantum field theory, and quantum geometrodynamics. Energy levels for the graviatom, Lemaître’s atom, quantum gravitational collapse have been calculated, and relationships for the masses of bound system components, as well as Universe’s birth probabilities, are presented to exemplify the properties of gravitationally bound systems. Objects and processes in them have been analyzed to construct quantum models of compact astrophysical objects and the early Universe. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

► Show Figures

Figure 1

18 pages, 3369 KiB

Open AccessReview

The Generation of ULF/ELF/VLF Waves in the Ionosphere by Modulated Heating

by Zhe Guo, Hanxian Fang and Farideh Honary

Universe 2021, 7(2), 29; https://doi.org/10.3390/universe7020029 - 29 Jan 2021

Cited by 9 | Viewed by 5094

Abstract

One of the most important effects of ionospheric modification by high power, high frequency (HF) waves is the generation of ultra low frequency/extremely low frequency/very low frequency (ULF/ELF/VLF) waves by modulated heating. This paper reviews the scientific achievements of the past five decades [...] Read more.

One of the most important effects of ionospheric modification by high power, high frequency (HF) waves is the generation of ultra low frequency/extremely low frequency/very low frequency (ULF/ELF/VLF) waves by modulated heating. This paper reviews the scientific achievements of the past five decades regarding the main mechanisms of excitation of ULF/ELF/VLF waves and discusses their characteristics, such as their electrojet dependency, the location of the source region, continuous and discontinuous waves, the number of HF arrays, and the suitable range of the modulation frequency for different proposed mechanisms. Finally, the outlook for future research in this area is presented. Full article

(This article belongs to the Section Space Science)

► Show Figures

Figure 1

35 pages, 534 KiB

Open AccessArticle

Superfluid Dynamics in Neutron Star Crusts: The Iordanskii Force and Chemical Gauge Covariance

by Lorenzo Gavassino, Marco Antonelli and Brynmor Haskell

Universe 2021, 7(2), 28; https://doi.org/10.3390/universe7020028 - 29 Jan 2021

Cited by 11 | Viewed by 2610

Abstract

We present a geometrical derivation of the relativistic dynamics of the superfluid inner crust of a neutron star. The resulting model is analogous to the Hall-Vinen-Bekarevich-Khalatnikov hydrodynamics for a single-component superfluid at finite temperature, but particular attention should be paid to the fact [...] Read more.

We present a geometrical derivation of the relativistic dynamics of the superfluid inner crust of a neutron star. The resulting model is analogous to the Hall-Vinen-Bekarevich-Khalatnikov hydrodynamics for a single-component superfluid at finite temperature, but particular attention should be paid to the fact that some fraction of the neutrons is locked to the motion of the protons in nuclei. This gives rise to an ambiguity in the definition of the two currents (the normal and the superfluid one) on which the model is built, a problem that manifests itself as a chemical gauge freedom of the theory. To ensure chemical gauge covariance of the hydrodynamic model, the phenomenological equation of motion for a quantized vortex should contain an extra transverse force, that is the relativistic version of the Iordanskii force discussed in the context of superfluid Helium. Hence, we extend the mutual friction model of Langlois et al. (1998) to account for the possible presence of this Iordanskii-like force. Furthermore, we propose that a better understanding of the (still not completely settled) controversy around the presence of the Iordanskii force in superfluid Helium, as well as in neutron stars, may be achieved by considering that the different incompatible results present in the literature pertain to two, opposite, dynamical regimes of the fluid system. Full article

(This article belongs to the Special Issue Superfluidity and Superconductivity in Neutron Stars)

► Show Figures

Figure 1

18 pages, 312 KiB

Open AccessArticle

Deconstructing Frame-Dragging

by Luis Herrera

Universe 2021, 7(2), 27; https://doi.org/10.3390/universe7020027 - 27 Jan 2021

Cited by 2 | Viewed by 2412

Abstract

The vorticity of world-lines of observers associated with the rotation of a massive body was reported by Lense and Thirring more than a century ago. In their example, the frame-dragging effect induced by the vorticity is directly (explicitly) related to the rotation of [...] Read more.

The vorticity of world-lines of observers associated with the rotation of a massive body was reported by Lense and Thirring more than a century ago. In their example, the frame-dragging effect induced by the vorticity is directly (explicitly) related to the rotation of the source. However, in many other cases, it is not so, and the origin of vorticity remains obscure and difficult to identify. Accordingly, in order to unravel this issue, and looking for the ultimate origin of vorticity associated to frame-dragging, we analyze in this manuscript very different scenarios where the frame-dragging effect is present. Specifically, we consider general vacuum stationary spacetimes, general electro-vacuum spacetimes, radiating electro-vacuum spacetimes, and Bondi–Sachs radiating spacetimes. We identify the physical quantities present in all these cases, which determine the vorticity and may legitimately be considered as responsible for the frame-dragging. Doing so, we provide a comprehensive, physical picture of frame-dragging. Some observational consequences of our results are discussed. Full article

(This article belongs to the Special Issue Frame-Dragging and Gravitomagnetism)

11 pages, 272 KiB

Open AccessArticle

Anti-BRST in the Causal Approach

by Dan-Radu Grigore

Universe 2021, 7(2), 26; https://doi.org/10.3390/universe7020026 - 26 Jan 2021

Viewed by 1789

Abstract

It is known that the elimination of anomalies in all orders of perturbation theory is an open problem. The constraints given by usual invariance properties and the Wess–Zumino identities are not enough to eliminate the anomalies in the general case of a Yang–Mills [...] Read more.

It is known that the elimination of anomalies in all orders of perturbation theory is an open problem. The constraints given by usual invariance properties and the Wess–Zumino identities are not enough to eliminate the anomalies in the general case of a Yang–Mills theory. So, any new symmetry of the model could restrict further the anomalies and be a solution of the problem. We consider the anti-BRST transform of Ojima in the causal approach and investigate if such new restrictions are obtained. Unfortunately, the result is negative: if we have BRST invariance up to the second order of perturbation theory, we also have anti-BRST invariance up to the same order. Probably, this result is true in all orders of perturbation theory. So, anti-BRST transform gives nothing new, and we have to find other ideas to restrict and eventually eliminate the anomalies for a general Yang–Mills theory. Full article

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

16 pages, 1087 KiB

Open AccessArticle

Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

by Umberto Battino, Claudia Lederer-Woods, Borbála Cseh, Pavel Denissenkov and Falk Herwig

Universe 2021, 7(2), 25; https://doi.org/10.3390/universe7020025 - 26 Jan 2021

Cited by 6 | Viewed by 3271

Abstract

The slow neutron-capture process (s-process) efficiency in low-mass AGB stars (1.5 < M/M

_{⊙}

< 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution [...] Read more.

The slow neutron-capture process (s-process) efficiency in low-mass AGB stars (1.5 < M/M

_{⊙}

< 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution and nucleosynthesis of low-mass AGB stars at low metallicities using the MESA stellar evolution code. The combined data set includes models with initial masses M

_{i n i}

/M

_{⊙} = 2

and 3 for initial metallicities

Z = 0.001

and

0.002

. The nucleosynthesis was calculated for all relevant isotopes by post-processing with the NuGrid mppnp code. Using these models, we show the impact of the uncertainties affecting the main mixing processes on heavy element nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on low-metallicity stars. We find that mixing at the interface between the He-intershell and the CO-core has a critical impact on the s-process at low metallicities, and its importance is comparable to convective boundary mixing processes under the convective envelope, which determine the formation and size of the ¹³C-pocket. Additionally, our results indicate that models with very low to no mixing below the He-intershell during thermal pulses, and with a ¹³C-pocket size of at least ∼3 × 10

^{- 4}

M

_{⊙}

, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided. Full article

(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)

► Show Figures

Figure 1

23 pages, 545 KiB

Open AccessFeature PaperArticle

Superfluid Neutron Matter with a Twist

by Georgios Palkanoglou and Alexandros Gezerlis

Universe 2021, 7(2), 24; https://doi.org/10.3390/universe7020024 - 26 Jan 2021

Cited by 4 | Viewed by 3122

Abstract

Superfluid neutron matter is a key ingredient in the composition of neutron stars. The physics of the inner crust are largely dependent on those of its S-wave neutron superfluid, which has made its presence known through pulsar glitches and modifications in neutron [...] Read more.

Superfluid neutron matter is a key ingredient in the composition of neutron stars. The physics of the inner crust are largely dependent on those of its S-wave neutron superfluid, which has made its presence known through pulsar glitches and modifications in neutron star cooling. Moreover, with recent gravitational-wave observations of neutron star mergers, the need for an equation of state for the matter of these compact stars is further accentuated and a model-independent treatment of neutron superfluidity is important. Ab initio techniques developed for finite systems can be guided to perform extrapolations to the thermodynamic limit and attain this model-independent extraction of various quantities of infinite superfluid neutron matter. To inform such an extrapolation scheme, we performed calculations of the neutron

^{1} S_{0}

pairing gap using model-independent odd–even staggering in the context of the particle-conserving, projected Bardeen–Cooper–Schrieffer (BCS) theory under twisted boundary conditions. While the practice of twisted boundary conditions is standard in solid-state physics and has been used repeatedly in the past to reduce finite-size effects, this is the first time that it has been employed in the context of pairing. We find that a twist-averaging approach results in a substantial reduction of the finite-size effects, bringing systems with

N ⪆ 50

within a

2 %

error margin from the infinite system. This can significantly reduce extrapolation-related errors in the extraction of superfluid neutron matter quantities. Full article

(This article belongs to the Special Issue Superfluidity and Superconductivity in Neutron Stars)

► Show Figures

Graphical abstract

13 pages, 2903 KiB

Open AccessArticle

Comparison of EEJ Longitudinal Variation from Satellite and Ground Measurements over Different Solar Activity Levels

by Wan Nur Izzaty Ismail, Nurul Shazana Abdul Hamid, Mardina Abdullah, Akimasa Yoshikawa, Teiji Uozumi and Zahira Mohd Radzi

Universe 2021, 7(2), 23; https://doi.org/10.3390/universe7020023 - 23 Jan 2021

Cited by 8 | Viewed by 2950

Abstract

The longitudinal variability and local time of equatorial electrojet (EEJ) current using simultaneous data recorded by ground and satellite magnetometers at different levels of solar activity were investigated. In this study, we used data from the CHAMP and Swarm satellites to obtain EEJ [...] Read more.

The longitudinal variability and local time of equatorial electrojet (EEJ) current using simultaneous data recorded by ground and satellite magnetometers at different levels of solar activity were investigated. In this study, we used data from the CHAMP and Swarm satellites to obtain EEJ current measurements around the globe. The ground data were provided by the MAGDAS, INTERMAGNET, and IIG networks. The ground observation was carried out by analyzing magnetometer data in four different sectors: the South American, Indian, African, and Southeast Asian sectors. These ground data were normalized to the dip equator to overcome the latitudinal variation of each station. The analysis for both measurements was performed using quiet day data. Both the ground and satellite data were categorized according to solar activity level; low, moderate, and high. The results revealed that, during the low solar activity, there was a good agreement between the longitudinal profiles of the EEJ measured using the satellite and the ground data. In general, strong correlations were obtained in most of the sectors where ground data were available between 11 and 13 local time (LT). Besides that, our analysis revealed that the different times of maximum EEJ appearances were seasonally dependent only at certain longitude sectors. Full article

(This article belongs to the Special Issue Space Weather)

► Show Figures

Figure 1

6 pages, 164 KiB

Open AccessEditorial

Acknowledgment to Reviewers of Universe in 2020

by Universe Editorial Office

Universe 2021, 7(2), 22; https://doi.org/10.3390/universe7020022 - 22 Jan 2021

Viewed by 1602

Abstract

Peer review is the driving force of journal development, and reviewers are gatekeepers who ensure that Universe maintains its standards for the high quality of its published papers [...] Full article

33 pages, 7518 KiB

Open AccessArticle

Curvature Invariants for the Alcubierre and Natário Warp Drives

by Brandon Mattingly, Abinash Kar, Matthew Gorban, William Julius, Cooper K. Watson, MD Ali, Andrew Baas, Caleb Elmore, Jeffrey S. Lee, Bahram Shakerin, Eric W. Davis and Gerald B. Cleaver

Universe 2021, 7(2), 21; https://doi.org/10.3390/universe7020021 - 20 Jan 2021

Cited by 11 | Viewed by 4357

Abstract

A process for using curvature invariants is applied to evaluate the metrics for the Alcubierre and the Natário warp drives at a constant velocity. Curvature invariants are independent of coordinate bases, so plotting these invariants will be free of coordinate mapping distortions. As [...] Read more.

A process for using curvature invariants is applied to evaluate the metrics for the Alcubierre and the Natário warp drives at a constant velocity. Curvature invariants are independent of coordinate bases, so plotting these invariants will be free of coordinate mapping distortions. As a consequence, they provide a novel perspective into complex spacetimes, such as warp drives. Warp drives are the theoretical solutions to Einstein’s field equations that allow for the possibility for faster-than-light (FTL) travel. While their mathematics is well established, the visualisation of such spacetimes is unexplored. This paper uses the methods of computing and plotting the warp drive curvature invariants to reveal these spacetimes. The warp drive parameters of velocity, skin depth and radius are varied individually and then plotted to see each parameter’s unique effect on the surrounding curvature. For each warp drive, this research shows a safe harbor and how the shape function forms the warp bubble. The curvature plots for the constant velocity Natário warp drive do not contain a wake or a constant curvature, indicating that these are unique features of the accelerating Natário warp drive. Full article

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

► Show Figures

Figure 1

Journal Menu

Journal Browser

Universe, Volume 7, Issue 2 (February 2021) – 25 articles

Further Information

Guidelines

MDPI Initiatives

Follow MDPI