Universe

14 pages, 302 KiB

Open AccessArticle

The Quantum Electromagnetic Field in the Weyl–Wigner Representation

by Emilio Santos

Universe 2024, 10(12), 452; https://doi.org/10.3390/universe10120452 - 9 Dec 2024

The quantum electromagnetic (EM) field is formulated in the Weyl–Wigner representation (WW), which is equivalent to the standard Hilbert space one (HS). In principle, it is possible to interpret within WW all experiments involving the EM field interacting with macroscopic bodies, the latter [...] Read more.

The quantum electromagnetic (EM) field is formulated in the Weyl–Wigner representation (WW), which is equivalent to the standard Hilbert space one (HS). In principle, it is possible to interpret within WW all experiments involving the EM field interacting with macroscopic bodies, the latter treated classically. In the WW formalism, the essential difference between classical electrodynamics and the quantum theory of the EM field is just the assumption that there is a random EM field-filling space, i.e., the existence of a zero-point field with a Gaussian distribution for the field amplitudes. I analyze a typical optical test of a Bell inequality. The model admits an interpretation compatible with local realism, modulo a number of assumptions assumed plausible. Full article

(This article belongs to the Special Issue Quantum Field Theory, 2nd Edition)

40 pages, 2476 KiB

Open AccessTutorial

A Tutorial on the Strong Gravity Effects in Black Hole X-Ray Spectra

by Cosimo Bambi

Universe 2024, 10(12), 451; https://doi.org/10.3390/universe10120451 - 8 Dec 2024

Abstract

This is a tutorial on the strong gravity effects (motion of massive and massless particles in a curved spacetime, evaluation of redshift factors, estimate of physical quantities in different reference frames, etc.) necessary to calculate the electromagnetic spectra of geometrically thin and optically [...] Read more.

This is a tutorial on the strong gravity effects (motion of massive and massless particles in a curved spacetime, evaluation of redshift factors, estimate of physical quantities in different reference frames, etc.) necessary to calculate the electromagnetic spectra of geometrically thin and optically thick accretion disks around black holes. The presentation is intentionally pedagogical, and most calculations are reported step by step. In the disk–corona model, the spectrum of a source has three components: a thermal component from the disk, a Comptonized component from the corona, and a reflection component from the disk. This tutorial reviews only the strong gravity effects, which can be decoupled from the physical processes involving the interaction between matter and radiation. The formulas presented here are valid for stationary, axisymmetric, asymptotically flat, circular spacetimes, so they can be potentially used for a large class of black hole solutions. Full article

(This article belongs to the Special Issue Recent Advances in Gravitational Lensing and Galactic Dynamics)

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16 pages, 16722 KiB

Open AccessArticle

Modal Frequency and Damping Identification of the FAST Cabin-Cable System

by Mingzhe Li, Caihong Sun, Qingwei Li and Rui Yao

Universe 2024, 10(12), 450; https://doi.org/10.3390/universe10120450 - 7 Dec 2024

Abstract

The Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) faces challenges in establishing high-precision rigid connections between the receiver and the reflective surface due to its vast spatial span. Innovatively, FAST suspends the feed cabin in mid-air using six supporting cables. The precise positioning of [...] Read more.

The Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) faces challenges in establishing high-precision rigid connections between the receiver and the reflective surface due to its vast spatial span. Innovatively, FAST suspends the feed cabin in mid-air using six supporting cables. The precise positioning of the feed focal point is achieved through the coordinated control of cable extension and retraction, along with the A-B axis and the Stewart platform within the cabin. The cables and the feed cabin form a large parallel mechanism. Since the cables are flexible, and the feed cabin remains at a high altitude during observations, it is inevitably subject to internal and external disturbances. To quickly dissipate these disturbances, the system requires a certain level of damping, which directly affects the pointing and tracking accuracy of FAST. During the 2022–2023 operational period, there were multiple instances where the pulleys of the curtain mechanism on the supporting cables became stuck and were carried to the top of the towers by the cables. This also led to the phenomenon where the pulleys, after being stuck, would rapidly slide down the cables due to accumulation. At such moments, the cabin-cable system would experience instantaneous excitation, causing vibrations. This study uses the intrinsic time-scale decomposition (ITD) method to analyze the inertial navigation data installed in the cabin during these events, identifying modal frequencies and damping ratios. The analysis results show that the lowest primary vibration frequency of the FAST cabin-cable suspension system ranges from approximately 0.12 to 0.2 Hz, with a damping ratio of no less than 0.004. These data indicate that the current structure of FAST has a strong energy dissipation capability, providing important reference points for improving the control accuracy of FAST and for the upgrade of the feed support system. Full article

(This article belongs to the Special Issue Planetary Radar Astronomy)

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12 pages, 535 KiB

Open AccessArticle

Reanalysis of the MACHO Constraints on PBH in the Light of Gaia DR3 Data

by Juan García-Bellido and Michael Hawkins

Universe 2024, 10(12), 449; https://doi.org/10.3390/universe10120449 - 6 Dec 2024

Abstract

The recent astrometric data of hundreds of millions of stars from Gaia DR3 has allowed for a precise determination of the Milky Way rotation curve up to 28 kpc. The data suggest a rapid decline in the density of dark matter beyond 19 [...] Read more.

The recent astrometric data of hundreds of millions of stars from Gaia DR3 has allowed for a precise determination of the Milky Way rotation curve up to 28 kpc. The data suggest a rapid decline in the density of dark matter beyond 19 kpc. We fit the whole rotation curve with four components (gas, disk, bulge, and halo), and compute the microlensing optical depth to the Large Magellanic Cloud. With this model of the galaxy we reanalyse the microlensing events of the MACHO and EROS-2 Collaborations. Using the published MACHO efficiency function for the duration of their survey, together with the rate of expected events according to the new density profile, we find that the Dark Matter halo could be composed of up to 20% of massive compact halo objects for any mass between

0.001

to

1 M_{⊙}

. For the EROS-2 survey, using a modified efficiency curve for consistency with the MACHO analysis, we also find compatibility with a MACHO halo, but with a tighter constraint around

0.005 M_{⊙}

where the halo fraction cannot be larger than ∼10%. This result assumes that all the lenses have the same mass. If these were distributed in an extended mass function like that of the Thermal History Model, the constraints are weakened, allowing 100% of all DM in the form of Primordial Black Holes. Full article

(This article belongs to the Section Cosmology)

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21 pages, 397 KiB

Open AccessReview

Enlargement of Symmetry Groups in Physics: A Practitioner’s Guide

by Lehel Csillag, Julio Marny Hoff da Silva and Tudor Pătuleanu

Universe 2024, 10(12), 448; https://doi.org/10.3390/universe10120448 - 6 Dec 2024

Abstract

Wigner’s classification has led to the insight that projective unitary representations play a prominent role in quantum mechanics. The physics literature often states that the theory of projective unitary representations can be reduced to the theory of ordinary unitary representations by enlarging the [...] Read more.

Wigner’s classification has led to the insight that projective unitary representations play a prominent role in quantum mechanics. The physics literature often states that the theory of projective unitary representations can be reduced to the theory of ordinary unitary representations by enlarging the group of physical symmetries. Nevertheless, the enlargement process is not always described explicitly: it is unclear in which cases the enlargement has to be conducted on the universal cover, a central extension, or a central extension of the universal cover. On the other hand, in the mathematical literature, projective unitary representations have been extensively studied, and famous theorems such as the theorems of Bargmann and Cassinelli have been achieved. The present article bridges the two: we provide a precise, step-by-step guide on describing projective unitary representations as unitary representations of the enlarged group. Particular focus is paid to the difference between algebraic and topological obstructions. To build the bridge mentioned above, we present a detailed review of the difference between group cohomology and Lie group cohomology. This culminates in classifying Lie group central extensions by smooth cocycles around the identity. Finally, the take-away message is a hands-on algorithm that takes the symmetry group of a given quantum theory as input and provides the enlarged group as output. This algorithm is applied to several cases of physical interest. We also briefly outline a generalization of Bargmann’s theory to time-dependent phases using Hilbert bundles. Full article

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

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13 pages, 632 KiB

Open AccessArticle

Using the Difference of the Inclinations of a Pair of Counter-Orbiting Satellites to Measure the Lense–Thirring Effect

by Lorenzo Iorio

Universe 2024, 10(12), 447; https://doi.org/10.3390/universe10120447 - 5 Dec 2024

Abstract

Let two test particles A and B, revolving about a spinning primary along ideally identical orbits in opposite directions, be considered. From the general expressions of the precessions of the orbital inclination induced by the post-Newtonian gravitomagnetic and Newtonian quadrupolar fields of the [...] Read more.

Let two test particles A and B, revolving about a spinning primary along ideally identical orbits in opposite directions, be considered. From the general expressions of the precessions of the orbital inclination induced by the post-Newtonian gravitomagnetic and Newtonian quadrupolar fields of the central object, it turns out that the Lense–Thirring inclination rates of A and B are equal and opposite, while the Newtonian ones oblateness are identical, due to the primary’s oblateness. Thus, the differences in the inclination shifts of the two orbiters would allow, in principle, for the classical effects to be cancelled out by enhancing the general relativistic ones. The conditions affecting the orbital configurations that must be satisfied for this to occur and possible observable consequences regarding the Earth are investigated. In particular, a scenario involving two spacecraft in polar orbits, branded POLAr RElativity Satellites (POLARES) and reminiscent of an earlier proposal by Van Patten and Everitt in the mid-1970s, is considered. A comparison with the ongoing experiment with the LAser GEOdynamics Satellite (LAGEOS) and LAser RElativity Satellite (LARES) 2 is made. Full article

(This article belongs to the Section Gravitation)

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21 pages, 6592 KiB

Open AccessReview

High-Energy Spectra of Black Hole and Neutron Star Low-Mass X-Ray Binaries

by Gábor Pszota

Universe 2024, 10(12), 446; https://doi.org/10.3390/universe10120446 - 3 Dec 2024

Abstract

In the case of low-mass X-ray binaries, the companion star is often too faint for detection; therefore, there is no chance for dynamical studies to independently determine the mass of the compact object. In the absence of a mass estimate, one cannot make [...] Read more.

In the case of low-mass X-ray binaries, the companion star is often too faint for detection; therefore, there is no chance for dynamical studies to independently determine the mass of the compact object. In the absence of a mass estimate, one cannot make a distinction as to whether the binary hosts a black hole or neutron star. Therefore, the question arises whether this distinction can be made based on the X-ray data alone, even when there are no bursts or pulsations. These would automatically imply a neutron star, but they are not always present. Black hole systems are known to emit radiation with an unbroken power–law shape up to several hundred keV energies in their high/soft states. If the non-thermal Comptonization processes that are responsible for this are somehow related to the lack of a solid surface, and to the fact that more gravitational potential energy can be released for a black hole, then there would be a definite method to reliably distinguish between the two sources. This work intends to review the available observations and studies to compare how these two populations behave during their different spectral states. A conclusion can be made that high/soft-state spectra are really different for black holes and neutron stars, for the low/hard state; however, the same conclusion cannot be safely made. Full article

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24 pages, 7225 KiB

Open AccessReview

CESE Schemes for Solar Wind Plasma MHD Dynamics

by Yun Yang and Huichao Li

Universe 2024, 10(12), 445; https://doi.org/10.3390/universe10120445 - 30 Nov 2024

Abstract

Magnetohydrodynamic (MHD) numerical simulation has emerged as a pivotal tool in space physics research, witnessing significant advancements. This methodology offers invaluable insights into diverse space physical phenomena based on solving the fundamental MHD equations. Various numerical methods are utilized to approximate the MHD [...] Read more.

Magnetohydrodynamic (MHD) numerical simulation has emerged as a pivotal tool in space physics research, witnessing significant advancements. This methodology offers invaluable insights into diverse space physical phenomena based on solving the fundamental MHD equations. Various numerical methods are utilized to approximate the MHD equations. Among these, the space–time conservation element and solution element (CESE) method stands out as an effective computational approach. Unlike traditional numerical schemes, the CESE method significantly enhances accuracy, even at the same base point. The concurrent discretization of space and time for conserved variables inherently achieves higher-order accuracy in both dimensions, without the need for intricate higher-order time discretization processes, which are often challenging in other methods. Additionally, this scheme can be readily extended to multidimensional cases, without relying on operator splitting or direction alternation. This paper primarily delves into the remarkable progress of CESE MHD models and their applications in studying solar wind, solar eruption activities, and the Earth’s magnetosphere. We aim to illuminate potential avenues for future solar–interplanetary CESE MHD models and their applications. Furthermore, we hope that the discussions presented in this review will spark new research endeavors in this dynamic field. Full article

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26 pages, 996 KiB

Open AccessReview

Primordial Black Holes: Formation, Spin and Type II

by Tomohiro Harada

Universe 2024, 10(12), 444; https://doi.org/10.3390/universe10120444 - 30 Nov 2024

Abstract

Primordial black holes (PBHs) may have formed through the gravitational collapse of cosmological perturbations that were generated and stretched during the inflationary era, later entering the cosmological horizon during the decelerating phase, if their amplitudes were sufficiently large. In this review paper, we [...] Read more.

Primordial black holes (PBHs) may have formed through the gravitational collapse of cosmological perturbations that were generated and stretched during the inflationary era, later entering the cosmological horizon during the decelerating phase, if their amplitudes were sufficiently large. In this review paper, we will briefly introduce the basic concept of PBHs and review the formation dynamics through this mechanism, the estimation of the initial spins of PBHs and the time evolution of type II fluctuations, with a focus on the radiation-dominated and (early) matter-dominated phases. Full article

(This article belongs to the Special Issue Primordial Black Holes from Inflation)

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21 pages, 899 KiB

Open AccessArticle

The Shape of the Chameleon Fifth-Force on the Mass Components of Galaxy Clusters

by Lorenzo Pizzuti, Valentina Amatori, Alexandre M. Pombo and Sandeep Haridasu

Universe 2024, 10(12), 443; https://doi.org/10.3390/universe10120443 - 30 Nov 2024

Abstract

In the context of chameleon gravity, we present a semi-analytical solution of the chameleon field profile in an accurately modelled galaxy cluster’s mass components, namely the stellar mass of the Brightest Cluster Galaxy (BCG), the baryonic mass in galaxies other than the BCG, [...] Read more.

In the context of chameleon gravity, we present a semi-analytical solution of the chameleon field profile in an accurately modelled galaxy cluster’s mass components, namely the stellar mass of the Brightest Cluster Galaxy (BCG), the baryonic mass in galaxies other than the BCG, the mass of the Intra-Cluster Medium (ICM) and the diffuse cold dark matter (CDM). The obtained semi-analytic profile is validated against the numerical solution of the chameleon field equation and implemented in the MG-MAMPOSSt code for kinematic analyses of galaxy clusters in modified gravity scenarios. By means of mock halos, simulated both in GR and in modified gravity, we show that the combination of the velocities and positions of cluster member galaxies, along with the data of the stellar velocity dispersion profile of the BCG, can impose constraints on the parameter space of the chameleon model; for a cluster generated in GR, these constraints are at the same level as a joint lensing+kinematics analysis of a cluster modelled with a single mass profile, without the BCG data. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')

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13 pages, 690 KiB

Open AccessArticle

The Impact of Electron Phase Shifts on ββ-Decay Kinematics

by Ovidiu Niţescu, Stefan Ghinescu and Fedor Šimkovic

Universe 2024, 10(12), 442; https://doi.org/10.3390/universe10120442 - 30 Nov 2024

Abstract

We reexamine the angular correlation between the emitted electrons in the double beta decay (DBD) of ¹⁰⁰Mo, with particular attention to the impact of electronic wave function phase shifts. In the two-neutrino mode, the angular correlation factor increases modestly compared to calculations [...] Read more.

We reexamine the angular correlation between the emitted electrons in the double beta decay (DBD) of ¹⁰⁰Mo, with particular attention to the impact of electronic wave function phase shifts. In the two-neutrino mode, the angular correlation factor increases modestly compared to calculations without phase shifts. However, a more detailed analysis of the angular correlation energy distributions uncovered a striking feature: electrons are most likely emitted in the same direction when one of them is below a certain energy threshold. We show that this feature is absent in previous Standard Model (SM) predictions and that phase shifts could also influence the angular correlations predicted by new physics models in two-neutrino DBD. For the neutrinoless mode, the direction flip is also present when phase shifts are included in the calculation. However, the angular correlation factor does not change much when phase shifts are taken into account, though our analysis is limited to the light neutrino exchange as the dominant mechanism. These findings highlight the subtle yet significant role that phase shifts can play in shaping electron emission patterns, influencing both SM and new physics predictions in DBD. Full article

(This article belongs to the Special Issue Exploring Double Beta Decay: Probing Fundamental Properties of Neutrinos and Beyond)

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77 pages, 5464 KiB

Open AccessReview

Premerger Phenomena in Neutron Star Binary Coalescences

by Arthur G. Suvorov, Hao-Jui Kuan and Kostas D. Kokkotas

Universe 2024, 10(12), 441; https://doi.org/10.3390/universe10120441 - 29 Nov 2024

Abstract

A variety of high-energy events can take place in the seconds leading up to a binary neutron star merger. Mechanisms involving tidal resonances, electrodynamic interactions, or shocks in mass-loaded wakes have been proposed as instigators of these precursors. With a view of gravitational-wave [...] Read more.

A variety of high-energy events can take place in the seconds leading up to a binary neutron star merger. Mechanisms involving tidal resonances, electrodynamic interactions, or shocks in mass-loaded wakes have been proposed as instigators of these precursors. With a view of gravitational-wave and multimessenger astrophysics, more broadly, premerger observations and theory are reviewed, emphasising how gamma-ray precursors and dynamical tides can constrain the neutron-star equation of state, thermodynamic microphysics, and evolutionary pathways. Connections to post-merger phenomena, notably gamma-ray bursts, are discussed together with how magnetic fields, spin and misalignment, crustal elasticity, and stratification gradients impact observables. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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12 pages, 7215 KiB

Open AccessArticle

Radio Spectrum Observations and Studies of the Solar Broadband Radio Dynamic Spectrometer (SBRS)

by Jing Huang and Baolin Tan

Universe 2024, 10(12), 440; https://doi.org/10.3390/universe10120440 - 29 Nov 2024

Abstract

Solar radio spectral observation is one of the essential approaches for solar physics research, which helps us study the plasma dynamics in the solar atmosphere. The Solar Broadband Radio Dynamic Spectrometer (SBRS) started observing the Sun at Huairou Solar Observing Station in Beijing, [...] Read more.

Solar radio spectral observation is one of the essential approaches for solar physics research, which helps us study the plasma dynamics in the solar atmosphere. The Solar Broadband Radio Dynamic Spectrometer (SBRS) started observing the Sun at Huairou Solar Observing Station in Beijing, China, in 1999. It has obtained a large amount of high-quality observation data of solar radio dynamic spectra in the centimeter–decimeter wavelengths (1.10–7.60 GHz). In particular, the observations with high-temporal resolution of millisecond and high-frequency resolution of MHz display plenty of superfine structures in the dynamic spectrum, which provide crucial information on the radiation process of various radio bursts. We review the past history of solar radio spectral observation and scientific results of SBRS. It is meaningful and will undoubtedly help us inspire new ideas for future research. The understanding of the basic plasma processes in solar plasma could also promote the development of solar physics, astrophysics, and space weather. To broaden the observation frequency range, we propose a new spectrometer at millimeter wavelengths (20–100 GHz) with ultra-wideband and high time–frequency resolution to study the physical processes in the solar transition region. This will open a new window for solar physics research and will provide crucial observational evidence for exploring a series of major issues in solar physics, including coronal heating, solar eruptions, and the origin of solar winds. Full article

(This article belongs to the Special Issue Measurements, Observations and Theoretical Studies on the Solar Magnetic Field—Celebrating the 40th Anniversary of the Huairou Solar Observing Station)

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11 pages, 904 KiB

Open AccessArticle

An Analysis of Variance of the Pantheon+ Dataset: Systematics in the Covariance Matrix?

by Ryan E. Keeley, Arman Shafieloo and Benjamin L’Huillier

Universe 2024, 10(12), 439; https://doi.org/10.3390/universe10120439 - 28 Nov 2024

Abstract

We investigate the statistics of the available Pantheon+ dataset. Noticing that the

χ^{2}

value for the best-fit

Λ

CDM model to the real data is small, we quantify how significant its smallness is by calculating the distribution of

χ^{2}

values for [...] Read more.

We investigate the statistics of the available Pantheon+ dataset. Noticing that the

χ^{2}

value for the best-fit

Λ

CDM model to the real data is small, we quantify how significant its smallness is by calculating the distribution of

χ^{2}

values for the best-fit

Λ

CDM model fit to mock Pantheon+-like datasets, using the provided covariance matrix. We further investigate the distribution of the residuals of the Pantheon+ dataset with respect to the best-fit

Λ

CDM model, and notice that they scatter less than would be expected from the covariance matrix but find no significant kurtosis. These results point to the conclusion that the Pantheon+ covariance matrix is over-estimated. One simple interpretation of these results is a ∼7% overestimation of errors on SN distance moduli in Pantheon+ data. When the covariance matrix is reduced by subtracting an intrinsic scatter term from the diagonal terms of the covariance matrix, the best-fit

χ^{2}

for the

Λ

CDM model achieves a normal value of 1580 and no deviation from

Λ

CDM is detected. We further quantify how consistent the

Λ

CDM model is with respect to the modified data with the subtracted covariance matrix using model-independent reconstruction techniques such as the iterative smoothing method. We find that the standard model is consistent with the data. There are a number of potential explanations for this smallness of the

χ^{2}

, such as a Malmquist bias at high redshift, or accounting for systematic uncertainties by adding them to the covariance matrix, thus approximating systematic uncertainties as statistical ones. Full article

(This article belongs to the Section Cosmology)

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18 pages, 1635 KiB

Open AccessReview

Jet Precession in Gamma-Ray Bursts

by Bao-Quan Huang and Tong Liu

Universe 2024, 10(12), 438; https://doi.org/10.3390/universe10120438 - 27 Nov 2024

Abstract

Jet precession is thought to be a ubiquitous phenomenon in astronomical events of various scales, including gamma-ray bursts (GRBs). If GRB jets undergo precession, periodic features might be introduced into their light curves. Detecting these periodic signals is therefore crucial for confirming the [...] Read more.

Jet precession is thought to be a ubiquitous phenomenon in astronomical events of various scales, including gamma-ray bursts (GRBs). If GRB jets undergo precession, periodic features might be introduced into their light curves. Detecting these periodic signals is therefore crucial for confirming the properties of GRBs’ central engines. However, periodic signals are always missing from observed GRB light curves. Against this backdrop, the broader effects of jet precession on GRBs have been widely studied. In this review, we summarize recent research progress on jet precession in GRBs. The main content focuses on four aspects of the effects of jet precession on GRBs: light curves, jet structures, polarization, and gravitational waves. Full article

(This article belongs to the Special Issue New Insights into High-Energy Astrophysics, Galaxies, and Cosmology—Celebrating the 10th Anniversary of the Re-establishment of the Department of Astronomy at Xiamen University (2012–2022))

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17 pages, 5124 KiB

Open AccessArticle

Pulsation in Hot Main-Sequence Stars: Comparison of Observations with Models

by Luis A. Balona

Universe 2024, 10(12), 437; https://doi.org/10.3390/universe10120437 - 25 Nov 2024

Abstract

The locations of hot pulsating variables in the H–R diagram are found using the effective temperatures derived from spectroscopic analysis and luminosities from Gaia parallaxes. Frequency peaks extracted from TESS photometry were used to compare with model predictions. A large number of stars [...] Read more.

The locations of hot pulsating variables in the H–R diagram are found using the effective temperatures derived from spectroscopic analysis and luminosities from Gaia parallaxes. Frequency peaks extracted from TESS photometry were used to compare with model predictions. A large number of stars with pulsation frequencies similar to

δ

Scuti variables were found between the predicted

δ

Scuti and

β

Cephei instability regions, contrary to the models. These Maia variables cannot be explained by rapid rotation. There is a serious mismatch between the observed and predicted frequencies for stars within the known

δ

Scuti instability strip. In

δ

Scuti and Maia stars, the frequency at the maximum amplitude as a function of the effective temperature was found to have a surprisingly well-defined upper envelope. The majority of

γ

Doradus stars were found within the

δ

Scuti instability strip. This is difficult to understand unless pulsational driving is non-linear. Non-linearity may also explain the huge variety in frequency patterns and the presence of low frequencies in hot

δ

Scuti stars.

γ

Doradus stars were found all along the main sequence and into the B-star region, where they merged with SPB variables. There seemed to be no distinct instability regions in the H–R diagram. It was concluded that current models do not offer a satisfactory description of observations. Full article

(This article belongs to the Special Issue Variable Stars in the 21st Century: From Microvariability to Megavariability)

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9 pages, 312 KiB

Open AccessArticle

On the Hypothesis of Exact Conservation of Charged Weak Hadronic Vector Current in the Standard Model

by Derar Altarawneh, Roman Höllwieser and Markus Wellenzohn

Universe 2024, 10(12), 436; https://doi.org/10.3390/universe10120436 - 22 Nov 2024

Abstract

We investigate the reliability of the conservation of the vector current (CVC) hypothesis in the neutron beta decay (n

β^{-}

decay). We calculate the contribution of the phenomenological term, responsible for the CVC in the hadronic current of the n [...] Read more.

We investigate the reliability of the conservation of the vector current (CVC) hypothesis in the neutron beta decay (n

β^{-}

decay). We calculate the contribution of the phenomenological term, responsible for the CVC in the hadronic current of the n

β^{-}

decay (or the CVC effect), to the neutron lifetime. We show that the CVC effect increases the neutron lifetime with a relative contribution of

8.684 \times 10^{- 2}

. This leads to the increase of the neutron lifetime by 76.4 s with respect to the world averaged value

τ_{n} = 880.2 (1.0) s

from the Particle Data Group. We show that since in the Standard Model there are no interactions that are able to cancel such a huge increase in the neutron lifetime, we have to turn to the interactions beyond the Standard Model, the contribution of which to the neutron lifetime reduces to the Fierz interference term

b_{F}

only. Cancelling the CVC effect at the level of the experimental accuracy, we obtain

b_{F} = 0.1219 (12)

. If this value cannot be accepted for the Fierz interference term, the CVC effect induces irresistible problems for description and understanding of the n

β^{-}

decay. Full article

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

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28 pages, 10407 KiB

Open AccessArticle

On the Viscous Ringed Disk Evolution in the Kerr Black Hole Spacetime

by Daniela Pugliese, Zdenek Stuchlík and Vladimir Karas

Universe 2024, 10(12), 435; https://doi.org/10.3390/universe10120435 - 22 Nov 2024

Abstract

Supermassive black holes (SMBHs) are observed in active galactic nuclei interacting with their environments, where chaotical, discontinuous accretion episodes may leave matter remnants orbiting the central attractor in the form of sequences of orbiting toroidal structures, with strongly different features as different rotation [...] Read more.

Supermassive black holes (SMBHs) are observed in active galactic nuclei interacting with their environments, where chaotical, discontinuous accretion episodes may leave matter remnants orbiting the central attractor in the form of sequences of orbiting toroidal structures, with strongly different features as different rotation orientations with respect to the central Kerr BH. Such ringed structures can be characterized by peculiar internal dynamics, where co-rotating and counter-rotating accretion stages can be mixed and distinguished by tori interaction, drying–feeding processes, screening effects, and inter-disk jet emission. A ringed accretion disk (RAD) is a full general relativistic model of a cluster of toroidal disks, an aggregate of axi-symmetric co-rotating and counter-rotating disks orbiting in the equatorial plane of a single central Kerr SMBH. In this work, we discuss the time evolution of a ringed disk. Our analysis is a detailed numerical study of the evolving RAD properties formed by relativistic thin disks, using a thin disk model and solving a diffusion-like evolution equation for an RAD in the Kerr spacetime, adopting an initial wavy (ringed) density profile. The RAD reaches a single-disk phase, building accretion to the inner edge regulated by the inner edge boundary conditions. The mass flux, the radial drift, and the disk mass of the ringed disk are evaluated and compared to each of its disk components. During early inter-disk interaction, the ring components spread, destroying the internal ringed structure and quickly forming a single disk with timescales governed by ring viscosity prescriptions. Different viscosities and boundary conditions have been tested. We propose that a system of viscously spreading accretion rings can originate as a product of tidal disruption of a multiple stellar system that comes too close to an SMBH. Full article

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17 pages, 561 KiB

Open AccessArticle

Studying Intra-Night Optical Variability of AGNs Using the TESS Survey Data

by Yujian Yang, Bo Ma and Chen Chen

Universe 2024, 10(12), 434; https://doi.org/10.3390/universe10120434 - 21 Nov 2024

Abstract

Active galactic nuclei (AGNs) exhibit significant luminosity variations across the entire electromagnetic spectrum, with timescales ranging from hours to years. Studying the optical variability of AGNs provides crucial insights into their physical properties. In this study, we explore the intra-night optical variability (INOV) [...] Read more.

Active galactic nuclei (AGNs) exhibit significant luminosity variations across the entire electromagnetic spectrum, with timescales ranging from hours to years. Studying the optical variability of AGNs provides crucial insights into their physical properties. In this study, we explore the intra-night optical variability (INOV) of AGNs using data from the TESS satellite’s all-sky survey. We derive differential light curves for a sample of 56 AGNs from 30-min cadence TESS full-frame images. Our analysis confirms that BL Lac objects typically exhibit stronger INOV compared to quasars, which generally show weaker variability. The duty cycle for INOV with an amplitude greater than 3% is approximately

4 \pm 0.7 %

for quasars and

22 \pm 5 %

for BL Lac objects. For INOV with an amplitude exceeding 10%, the corresponding duty cycle decreases to about

0.5 \pm 0.3 %

and

5 \pm 2 %

, respectively. Furthermore, we identify a potential linear-log relationship between the duty cycle and the INOV amplitude, based on results from this study and previous findings, particularly from the ARIES program. Full article

(This article belongs to the Section Galaxies and Clusters)

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