Universe

14 pages, 404 KiB

Open AccessArticle

A Study of the Accretion–Jet Coupling of Black Hole Objects at Different Scales

by Zhou Yang, Qing-Chen Long, Wei-Jia Yang and Ai-Jun Dong

Universe 2024, 10(8), 335; https://doi.org/10.3390/universe10080335 - 22 Aug 2024

Viewed by 669

The fundamental plane of black hole activity is a very important tool to study accretion and jets. However, we found that the SEDs of AGNs and XRBs are different in the 2–10 keV energy band, and it seems inappropriate to use 2–10 keV [...] Read more.

The fundamental plane of black hole activity is a very important tool to study accretion and jets. However, we found that the SEDs of AGNs and XRBs are different in the 2–10 keV energy band, and it seems inappropriate to use 2–10 keV X-ray luminosities to study the fundamental plane. In this work, we use the luminosity near the peak of the blackbody radiation of the active galactic nuclei and black hole binaries to replace the 2–10 keV luminosity. We re-explore the fundamental plane of black hole activity by using the 2500

\overset{˚}{A}

luminosity as the peak luminosity of the blackbody radiation of AGNs and 1 keV luminosity as the peak luminosity of the blackbody radiation of XRBs. We compile samples of black hole binaries and active galactic nuclei with luminosity near the peak luminosity of blackbody radiation and study the fundamental plane between radio luminosity (

L_{R}

), the peak luminosity of blackbody radiation (

L_{peak}

), and black hole mass (

M_{BH}

). We find that the radio–peak luminosity correlations are

L_{5 GHz} / L_{Edd} \propto {(L_{2500 \overset{˚}{A}} / L_{Edd})}^{1.55}

and

L_{5 GHz} / L_{Edd} \propto {(L_{1 keV} / L_{Edd})}^{1.53}

for AGN and XRB, respectively, in the radiatively efficient sample, and

L_{5 GHz} / L_{Edd} \propto {(L_{2500 \overset{˚}{A}} / L_{Edd})}^{0.48}

and

L_{5 GHz} / L_{Edd} \propto {(L_{1 keV} / L_{Edd})}^{0.53}

in the radiatively inefficient sample, respectively. Based on the similarities in radio–peak correlations, we further propose a fundamental plane in radio luminosity, the peak luminosity of blackbody radiation, and black hole mass, which is radiatively efficient:

log L_{5 GHz} = 1 . 57_{- 0.01}^{+ 0.01} log L_{peak} - 0 . 32_{- 0.16}^{+ 0.16} log M_{BH} - 27 . 73_{- 0.34}^{+ 0.34}

with a scatter of

σ_{R}

= 0.48 dex, and radiatively inefficient:

log L_{5 GHz} = 0 . 45_{- 0.01}^{+ 0.01} log L_{peak} + 0 . 91_{- 0.10}^{+ 0.12} log M_{BH} + 12 . 58_{- 0.38}^{+ 0.38}

with a scatter of

σ_{R}

= 0.63 dex. Our results are similar to those of previous studies on the fundamental plane for radiatively efficient and radiatively inefficient black hole activity. However, our results exhibit a smaller scatter, so when using the same part of blackbody radiation (i.e., the peak luminosity of the blackbody radiation), the fundamental plane becomes a little bit tighter. Full article

► Show Figures

Figure 1

17 pages, 1724 KiB

Open AccessArticle

Slow Body MHD Waves in Inhomogeneous Photospheric Waveguides

by Istvan Ballai, Fisal Asiri, Viktor Fedun, Gary Verth, Emese Forgács-Dajka and Abdulrahman B. Albidah

Universe 2024, 10(8), 334; https://doi.org/10.3390/universe10080334 - 21 Aug 2024

Viewed by 635

Abstract

The present study deals with the investigation of the oscillatory morphology of guided slow body MHD modes in inhomogeneous magnetic waveguides that appear in the solar photospheric plasmas in the forms of pores or sunspots. The eigenvalues and eigenfunctions related to these waves [...] Read more.

The present study deals with the investigation of the oscillatory morphology of guided slow body MHD modes in inhomogeneous magnetic waveguides that appear in the solar photospheric plasmas in the forms of pores or sunspots. The eigenvalues and eigenfunctions related to these waves in an isothermal plasma are obtained numerically by solving a Sturm-Liouville problem with Dirichlet boundary conditions set at the boundary of the waveguide. Our results show that the inhomogeneities in density (pressure) and magnetic field have a strong influence on the morphology of waves, and higher-order more are sensitive to the presence of inhomogeneity. Our results suggest that he identification of modes just by a simple visual inspection can lead to a misinterpretation of the nature of modes. Full article

(This article belongs to the Special Issue Solar and Stellar Activity: Exploring the Cosmic Nexus)

► Show Figures

Figure 1

38 pages, 465 KiB

Open AccessArticle

Quantum Effects on Cosmic Scales as an Alternative to Dark Matter and Dark Energy

by Da-Ming Chen and Lin Wang

Universe 2024, 10(8), 333; https://doi.org/10.3390/universe10080333 - 19 Aug 2024

Viewed by 799

Abstract

The spin-torsion theory is a gauge theory approach to gravity that expands upon Einstein’s general relativity (GR) by incorporating the spin of microparticles. In this study, we further develop the spin-torsion theory to examine spherically symmetric and static gravitational systems that involve free-falling [...] Read more.

The spin-torsion theory is a gauge theory approach to gravity that expands upon Einstein’s general relativity (GR) by incorporating the spin of microparticles. In this study, we further develop the spin-torsion theory to examine spherically symmetric and static gravitational systems that involve free-falling macroscopic particles. We posit that the quantum spin of macroscopic matter becomes noteworthy at cosmic scales. We further assume that the Dirac spinor and Dirac equation adequately capture all essential physical characteristics of the particles and their associated processes. A crucial aspect of our approach involves substituting the constant mass in the Dirac equation with a scale function, allowing us to establish a connection between quantum effects and the scale of gravitational systems. This mechanism ensures that the quantum effect of macroscopic matter is scale-dependent and diminishes locally, a phenomenon not observed in microparticles. For any given matter density distribution, our theory predicts an additional quantum term, the quantum potential energy (QPE), within the mass expression. The QPE induces time dilation and distance contraction, and thus mimics a gravitational well. When applied to cosmology, our theory yields a static cosmological model. The QPE serves as a counterpart to the cosmological constant introduced by Einstein to balance gravity in his static cosmological model. The QPE also offers a plausible explanation for the origin of Hubble redshift (traditionally attributed to the universe’s expansion). The predicted luminosity distance–redshift relation aligns remarkably well with SNe Ia data from the cosmological sample of SNe Ia. In the context of galaxies, the QPE functions as the equivalent of dark matter. The predicted circular velocities align well with rotation curve data from the SPARC (Spitzer Photometry and Accurate Rotation Curves database) sample. Importantly, our conclusions in this paper are reached through a conventional approach, with the sole assumption of the quantum effects of macroscopic matter at large scales, without the need for additional modifications or assumptions. Full article

► Show Figures

Figure 1

20 pages, 8778 KiB

Open AccessReview

Fluctuations and Correlations of Conserved Charges Serving as Signals for QGP Production: An Overview from Polyakov Loop Enhanced Nambu–Jona-Lasinio Model

by Sudipa Upadhaya

Universe 2024, 10(8), 332; https://doi.org/10.3390/universe10080332 - 19 Aug 2024

Viewed by 694

Abstract

Quark–Gluon plasma driven by the strong force is subject to the conservativeness of the baryon number, net electric charge, strangeness, etc. However, the fluctuations around their mean values at specific temperatures and chemical potentials can provide viable signals for the production of Quark–Gluon [...] Read more.

Quark–Gluon plasma driven by the strong force is subject to the conservativeness of the baryon number, net electric charge, strangeness, etc. However, the fluctuations around their mean values at specific temperatures and chemical potentials can provide viable signals for the production of Quark–Gluon plasma. These fluctuations can be captured theoretically as moments of different orders in the expansion of pressure or the thermodynamic potential of the system under concern. Here, we look for possible explanations in the methodologies used for capturing them by using the framework of the Polyakov–Nambu–Jona-Lasinio (PNJL) model under the 2 + 1 flavor consideration with mean-field approximation. The various quantities thus explored can act to signify meaningfully near the phase transitions. Justifications are also made for some of the quantities capable of serving necessarily under experimental scenarios. Additionally, variations in certain quantities are also made for the different collision energies explored in the high-energy experiments. Rectification of the quantitative accuracy, especially in the low-temperature hadronic sector, is of prime concern, and it is also addressed. It was found that most of the observables stay in close proximity with the existing lattice QCD results at the continuum limit, with some artifacts still remaining, especially in the strange sector, which needs further attention. Full article

(This article belongs to the Special Issue Exploring Quark Matter under Extreme Scenarios of Temperature and Density)

► Show Figures

Figure 1

22 pages, 739 KiB

Open AccessArticle

Weak Deflection Angle by the Einstein–Cartan Traversable Wormhole Using Gauss–Bonnet Theorem with Time Delay

by Susmita Sarkar, Nayan Sarkar, Abhisek Dutta and Farook Rahaman

Universe 2024, 10(8), 331; https://doi.org/10.3390/universe10080331 - 16 Aug 2024

Viewed by 786

Abstract

In this article, we estimate the gravitational deflection angles of light in the spacetime of Einstein–Cartan wormholes supported by normal matter or phantom energy utilizing the Gauss–Bonnet theorem. The obtained deflection angles are examined in relation to the wormhole throat radius

r_{0}

[...] Read more.

In this article, we estimate the gravitational deflection angles of light in the spacetime of Einstein–Cartan wormholes supported by normal matter or phantom energy utilizing the Gauss–Bonnet theorem. The obtained deflection angles are examined in relation to the wormhole throat radius

r_{0}

and the equation of state parameter

ω

across four scenarios, and it has been seen that the larger throat radii

r_{0}

result in higher deflection angles. Moreover, the wormholes filled with phantom energy exhibit greater deflection angles compared to those filled with normal matter. The reported deflection angles are influenced by dark matter and Maxwell’s fish eye matter: Dark matter, as well as Maxwell’s fish eye matter, increases the deflection angles. The deflection angle is also estimated using the Keeton and Petters method, which is proportional to wormhole throat

r_{0}

and inversely proportional to the impact parameter b. Additionally, a comparative study is performed on the deflection angles obtained from four different scenarios. Finally, analytical results for time delay due to Einstein–Cartan wormholes are estimated for the four

ω

cases which are decreasing for increasing values of

r_{c}

. Full article

(This article belongs to the Section Field Theory)

► Show Figures

Figure 1

24 pages, 18810 KiB

Open AccessReview

Hot Stars, Young Stellar Populations and Dust with Swift/UVOT

by Michael H. Siegel and Caryl Gronwall

Universe 2024, 10(8), 330; https://doi.org/10.3390/universe10080330 - 16 Aug 2024

Viewed by 611

Abstract

In this review, we highlight the contributions made by the Swift/UVOT instrument to the understanding of the ultraviolet (UV) attenuation and extinction properties of interstellar dust and provide insight into hot stars and young stellar populations. The study of these two fields is [...] Read more.

In this review, we highlight the contributions made by the Swift/UVOT instrument to the understanding of the ultraviolet (UV) attenuation and extinction properties of interstellar dust and provide insight into hot stars and young stellar populations. The study of these two fields is interconnected: UV-bright objects can only be understood if the effects of foreground dust are accounted for, but foreground dust can only be accounted for by studying the properties of UV-bright objects. Decades worth of work have established that the effects of dust on background starlight vary in the ultraviolet, with proposed extinction laws having a wide variety of slopes and a strong “bump” spectroscopic feature at 2175 Å. We show that UVOT is uniquely suited to probe variations in the UV extinction law, specifically because of the

u v m 2

filter that is centered on the bump and the telescope’s ability to resolve nearby stellar populations. When used in combination with optical and infrared imaging, UVOT can provide strong constraints on variations in the extinction law, both from galaxy to galaxy and within individual galaxies, as well as the properties of young stellar populations. Surveys of UVOT have included the Milky Way, the galaxies of the Local Group, the Local Volume Legacy Survey (LVLS) and two deep fields. All of these are being utilized to provide the most detailed information yet about the UV dust attenuation law and the connection of its variation to underlying physical processes as well as the UV properties of hot stars and young stellar populations. Full article

(This article belongs to the Special Issue 18 Years of Science with the Neil Gehrels Swift Observatory's Ultra-Violet/Optical Telescope)

► Show Figures

Figure 1

12 pages, 273 KiB

Open AccessEditorial

Centenary of Alexander Friedmann’s Prediction of Universe Expansion and the Prospects of Modern Cosmology

by Galina L. Klimchitskaya, Vladimir M. Mostepanenko and Sergey V. Sushkov

Universe 2024, 10(8), 329; https://doi.org/10.3390/universe10080329 - 16 Aug 2024

Viewed by 707

Abstract

In this Editorial to the Special Issue “The Friedmann Cosmology: A Century Later”, we consider an outstanding character of Friedmann’s prediction of Universe expansion, which laid the foundation of modern cosmology. The list of the main discoveries made in cosmology during the last [...] Read more.

In this Editorial to the Special Issue “The Friedmann Cosmology: A Century Later”, we consider an outstanding character of Friedmann’s prediction of Universe expansion, which laid the foundation of modern cosmology. The list of the main discoveries made in cosmology during the last one hundred years is followed by a formulation of the standard cosmological model. The articles contributing to the Special Issue are considered in relation to this model, and to several alternative theoretical approaches. Special attention is paid to unresolved problems, such as the nature of dark matter and dark energy, Hubble tension and the pre-inflationary stage of the Universe evolution. The conclusion is made that astrophysics and cosmology are on the threshold of new fundamental discoveries. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

25 pages, 353 KiB

Open AccessArticle

From the Janis–Newman–Winicour Naked Singularities to the Einstein–Maxwell Phantom Wormholes

by Changjun Gao and Jianhui Qiu

Universe 2024, 10(8), 328; https://doi.org/10.3390/universe10080328 - 15 Aug 2024

Cited by 2 | Viewed by 581

Abstract

The Janis–Newman–Winicour spacetime corresponds to a static spherically symmetric solution of Einstein equations with the energy momentum tensor of a massless quintessence field. It is understood that the spacetime describes a naked singularity. The solution has two parameters, b and s. To [...] Read more.

The Janis–Newman–Winicour spacetime corresponds to a static spherically symmetric solution of Einstein equations with the energy momentum tensor of a massless quintessence field. It is understood that the spacetime describes a naked singularity. The solution has two parameters, b and s. To our knowledge, the exact physical meaning of the two parameters is still unclear. In this paper, starting from the Janis–Newman–Winicour naked singularity solution, we first obtain a wormhole solution by a complex transformation. Then, letting the parameter s approach infinity, we obtain the well-known exponential wormhole solution. After that, we embed both the Janis–Newman–Winicour naked singularity and its wormhole counterpart in the background of a de Sitter or anti-de Sitter universe with the energy momentum tensor of massive quintessence and massive phantom fields, respectively. To our surprise, the resulting quintessence potential is actually the dilaton potential found by one of us. It indicates that, by modulating the parameters in the charged dilaton black hole solutions, we can obtain the Janis–Newman–Winicour solution. Furthermore, a charged wormhole solution is obtained by performing a complex transformation on the charged dilaton black hole solutions in the background of a de Sitter or anti-de Sitter universe. We eventually find that s is actually related to the coupling constant of the dilaton field to the Maxwell field and b is related to a negative mass for the dilaton black holes. A negative black hole mass is physically forbidden. Therefore, we conclude that the Janis–Newman–Winicour naked singularity solution is not physically allowed. Full article

(This article belongs to the Collection Open Questions in Black Hole Physics)

► Show Figures

Figure 1

16 pages, 1918 KiB

Open AccessArticle

Convolutional Neural Network Processing of Radio Emission for Nuclear Composition Classification of Ultra-High-Energy Cosmic Rays

by Tudor Alexandru Calafeteanu, Paula Gina Isar and Emil Ioan Sluşanschi

Universe 2024, 10(8), 327; https://doi.org/10.3390/universe10080327 - 15 Aug 2024

Viewed by 959

Abstract

Ultra-high-energy cosmic rays (UHECRs) are extremely rare energetic particles of ordinary matter in the Universe, traveling astronomical distances before reaching the Earth’s atmosphere. When primary cosmic rays interact with atmospheric nuclei, cascading extensive air showers (EASs) of secondary elementary particles are developed. Radio [...] Read more.

Ultra-high-energy cosmic rays (UHECRs) are extremely rare energetic particles of ordinary matter in the Universe, traveling astronomical distances before reaching the Earth’s atmosphere. When primary cosmic rays interact with atmospheric nuclei, cascading extensive air showers (EASs) of secondary elementary particles are developed. Radio detectors have proven to be a reliable method for reconstructing the properties of EASs, such as the shower’s axis, its energy, and its maximum (

X_{\max}

). This aids in understanding fundamental astrophysical phenomena, like active galactic nuclei and gamma-ray bursts. Concurrently, data science has become indispensable in UHECR research. By applying statistical, computational, and deep learning methods to both real-world and simulated radio data, researchers can extract insights and make predictions. We introduce a convolutional neural network (CNN) architecture designed to classify simulated air shower events as either being generated by protons or by iron nuclei. The classification achieved a stable test error of 10%, with Accuracy and

F_{1}

scores of 0.9 and an MCC of 0.8. These metrics indicate strong prediction capability for UHECR’s nuclear composition, based on data that can be gathered by detectors at the world’s largest cosmic rays experiment on Earth, the Pierre Auger Observatory, which includes radio antennas, water Cherenkov detectors, and fluorescence telescopes. Full article

(This article belongs to the Special Issue Advanced Studies in Ultra-High-Energy Cosmic Rays)

► Show Figures

Figure 1

19 pages, 1544 KiB

Open AccessReview

Multi-Messenger Connection in High-Energy Neutrino Astronomy

by Ankur Sharma

Universe 2024, 10(8), 326; https://doi.org/10.3390/universe10080326 - 13 Aug 2024

Viewed by 1014

Abstract

Low fluxes of astrophysical neutrinos at TeV energies, and the overwhelming background of atmospheric neutrinos below that, render the current paradigm of neutrino astronomy a severely statistics-limited one. While many hints have emerged, all the evidence gathered by IceCube and ANTARES, over the [...] Read more.

Low fluxes of astrophysical neutrinos at TeV energies, and the overwhelming background of atmospheric neutrinos below that, render the current paradigm of neutrino astronomy a severely statistics-limited one. While many hints have emerged, all the evidence gathered by IceCube and ANTARES, over the course of almost a decade and a half of operation, has fallen short of providing any conclusive answer to the puzzle of the origin of high-energy cosmic rays and neutrinos. The advancement of the field is thus closely associated with not only the neutrino observatories coming online in the next few years, but also on the coordinated efforts of the EM, GW and cosmic ray communities to develop dedicated channels and infrastructure that allow for the swift and comprehensive multi-messenger follow-up of relevant events detected in any of these sectors. This paper highlights the strides that have been already taken in that direction and the fruits that they have borne, as well as the challenges that lie ahead. Full article

(This article belongs to the Special Issue Neutrinos across Different Energy Scales)

► Show Figures

Figure 1

14 pages, 563 KiB

Open AccessArticle

Constraint on the Cosmic Curvature in a Model with the Schwarzschild–de Sitter Metric from Supernovae and Gamma-Ray Burst Observational Data

by Vladimir N. Yershov

Universe 2024, 10(8), 325; https://doi.org/10.3390/universe10080325 - 11 Aug 2024

Viewed by 1127

Abstract

In developing his cosmological model of 1917, de Sitter theoretically predicted the phenomenon of cosmological redshift (the de Sitter effect), which he did long before the discovery of this phenomenon in observations. The de Sitter effect is gravitational by its nature, as it [...] Read more.

In developing his cosmological model of 1917, de Sitter theoretically predicted the phenomenon of cosmological redshift (the de Sitter effect), which he did long before the discovery of this phenomenon in observations. The de Sitter effect is gravitational by its nature, as it is due to differences between the coordinate systems of the observer and the distant source. However, the relationship between the redshift and distance derived from the de Sitter metric is at odds with observations, since this relationship is nonlinear (quadratic) for small redshifts, while the observed relationship between the same quantities is strictly linear. This paper discusses the possibility that cosmological redshift is gravitational by its nature, as in de Sitter’s 1917 model. At the same time, here, as in de Sitter’s model, an elliptical space is used, the main characteristic of which is the identification of its antipodal points. But, unlike de Sitter’s model, here, in order to ensure strict linear dependence of the redshift on distance, the origin of the reference system is transferred to the observer’s antipodal point. The Schwarzschild–de Sitter metric used in this model allows you to estimate the curvature of space from observational data. To achieve this, a theoretical Hubble diagram is built within the framework of the model with the Schwarzschild–de Sitter metric, which is compared with observations from the Pantheon+ catalogue of type Ia supernovae and the Amati catalogue of gamma-ray bursts in the redshift range of

0 < z < 8

. As a result of this comparison, we found that the lower estimate of the radius of curvature of space was quite large:

2.4 \times 10^{15}

Mpc. This means that the observational data indicate a negligible curvature of space. Full article

(This article belongs to the Special Issue Cosmological Models of the Universe)

► Show Figures

Figure 1

13 pages, 485 KiB

Open AccessArticle

Coupled Quintessence Inspired by Warm Inflation

by Paulo M. Sá

Universe 2024, 10(8), 324; https://doi.org/10.3390/universe10080324 - 10 Aug 2024

Cited by 3 | Viewed by 576

Abstract

We investigate a coupled quintessence cosmological model in which a dark-energy scalar field with an exponential potential interacts directly with a dark-matter fluid through a dissipative term inspired by warm inflation. The evolution equations of this model give rise to a three-dimensional dynamical [...] Read more.

We investigate a coupled quintessence cosmological model in which a dark-energy scalar field with an exponential potential interacts directly with a dark-matter fluid through a dissipative term inspired by warm inflation. The evolution equations of this model give rise to a three-dimensional dynamical system for which a thorough qualitative analysis is performed for all values of the relevant parameters. We find that the model is able to replicate the observed sequence of late-time cosmological eras, namely, a long enough matter-dominated era followed by a present era of accelerated expansion. In situations where there is a significant transfer of energy from dark energy to dark matter, temporary scaling-type solutions may arise, but, asymptotically, all solutions are dominated by dark energy. Full article

(This article belongs to the Special Issue Cosmological Models of the Universe)

► Show Figures

Figure 1

26 pages, 11389 KiB

Open AccessArticle

UHECR Clustering: Lightest Nuclei from Local Sheet Galaxies

by Daniele Fargion, Pier Giorgio De Sanctis Lucentini and Maxim Yu. Khlopov

Universe 2024, 10(8), 323; https://doi.org/10.3390/universe10080323 - 9 Aug 2024

Cited by 1 | Viewed by 717

Abstract

The ultra-high-energy cosmic ray (UHECR) puzzle is reviewed under the hints of a few basic results: clustering, anisotropy, asymmetry, bending, and composition changes with energies. We show how the lightest UHECR nuclei from the nearest AGN or Star-Burst sources, located inside a few [...] Read more.

The ultra-high-energy cosmic ray (UHECR) puzzle is reviewed under the hints of a few basic results: clustering, anisotropy, asymmetry, bending, and composition changes with energies. We show how the lightest UHECR nuclei from the nearest AGN or Star-Burst sources, located inside a few Mpc Local Sheets, may explain, at best, the observed clustering of Hot Spots at tens EeV energy. Among the possible local extragalactic candidate sources, we derived the main contribution of very few galactic sources. These are located in the Local Sheet plane within a distance of a few Mpc, ejecting UHECR at a few tens of EeV energy. UHECR also shine at lower energies of several EeV, partially feeding the Auger dipole by LMC and possibly a few nearer galactic sources. For the very recent highest energy UHECR event, if a nucleon, it may be explained by a model based on the scattering of UHE ZeV neutrinos on low-mass relic neutrinos. Such scatterings are capable of correlating, via Z boson resonance, the most distant cosmic sources above the GZK bound with such an enigmatic UHECR event. Otherwise, these extreme events, if made by the heaviest composition, could originate from the largest bending trajectory of heaviest nuclei or from nearby sources, even galactic ones. In summary, the present lightest to heavy nuclei model UHECR from the Local Sheet could successfully correlate UHECR clustering with the nearest galaxies and AGN. Heavy UHECR may shine by being widely deflected from the Local Sheet or from past galactic, GRB, or SGR explosive ejection. Full article

(This article belongs to the Special Issue High Energy Emission from Clusters, Groups, and Filaments: Current Observations and Future Prospects)

► Show Figures

Figure 1

17 pages, 998 KiB

Open AccessReview

Strange Dwarfs: A Review on the (in)Stability

by Francesco Di Clemente, Alessandro Drago and Giuseppe Pagliara

Universe 2024, 10(8), 322; https://doi.org/10.3390/universe10080322 - 9 Aug 2024

Cited by 1 | Viewed by 815

Abstract

White dwarfs are the remnants of stars not massive enough to become supernovae. This review explores the concept of strange dwarfs, a unique class of white dwarfs that contain cores of strange quark matter. Strange dwarfs have different sizes, masses, and evolutionary paths [...] Read more.

White dwarfs are the remnants of stars not massive enough to become supernovae. This review explores the concept of strange dwarfs, a unique class of white dwarfs that contain cores of strange quark matter. Strange dwarfs have different sizes, masses, and evolutionary paths with respect to white dwarfs. They might form through the accumulation of normal matter on strange quark stars or by the capture of strangelets. The stability of strange dwarfs has been debated, with initial studies suggesting stability, while later analyses indicated potential instability. This review revisits these discussions, focusing on the critical role of boundary conditions between nuclear and quark matter in determining stability. It also offers insights into their formation, structure, and possible detection in the universe. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

► Show Figures

Figure 1

22 pages, 3326 KiB

Open AccessArticle

CME Forecasting System: Event Selection Algorithm, Dimming Data Application Limitations, and Analysis of the Results for Events of the Solar Cycle 24

by Ksenia Kaportseva, Yulia Shugay, Anna Vakhrusheva, Vladimir Kalegaev, Anton Shiryaev and Valeriy Eremeev

Universe 2024, 10(8), 321; https://doi.org/10.3390/universe10080321 - 9 Aug 2024

Viewed by 959

Abstract

The modeling of coronal mass ejections (CMEs) arrival to Earth was carried out using a one-dimensional drag-based model (DBM) over the period from 2010 to 2018. The CME propagation model includes a simulation of the interaction of the CME with background solar wind [...] Read more.

The modeling of coronal mass ejections (CMEs) arrival to Earth was carried out using a one-dimensional drag-based model (DBM) over the period from 2010 to 2018. The CME propagation model includes a simulation of the interaction of the CME with background solar wind via the quasi-stationary solar wind (QSW) model. An analysis of the results of forecasting CME speed and time of arrival to Earth was performed. Input data were obtained from the CACTus database. To ensure real-time operation, a new algorithm was established to select events that can reach Earth more likely. Coronal dimming data were used to obtain coordinates of the CME source location. Forecasting results have been compared with interplanetary CME (ICME) catalogs. The system has predicted 189 of 280 events (68%), with a tolerance of 48 h for the period of maximum solar activity (from 2010 to 2015). The average absolute error of predicted CME arrival speed is about 90 km/s. Our system has predicted 80% of ICMEs associated with extreme geomagnetic storms (Dst_min ≤ −100 nT) within a tolerance of 24 h. Full article

(This article belongs to the Special Issue Solar and Stellar Activity: Exploring the Cosmic Nexus)

► Show Figures

Figure 1

23 pages, 849 KiB

Open AccessArticle

Revisiting Quantum Field Theory in Rindler Spacetime with Superselection Rules

by K. Sravan Kumar and João Marto

Universe 2024, 10(8), 320; https://doi.org/10.3390/universe10080320 - 8 Aug 2024

Cited by 4 | Viewed by 841

Abstract

Quantum field theory (QFT) in Rindler spacetime is a gateway to understanding unitarity and information loss paradoxes in curved spacetime. Rindler coordinates map Minkowski spacetime onto regions with horizons, effectively dividing accelerated observers into causally disconnected sectors. Employing standard quantum field theory techniques [...] Read more.

Quantum field theory (QFT) in Rindler spacetime is a gateway to understanding unitarity and information loss paradoxes in curved spacetime. Rindler coordinates map Minkowski spacetime onto regions with horizons, effectively dividing accelerated observers into causally disconnected sectors. Employing standard quantum field theory techniques and Bogoliubov transformations between Minkowski and Rindler coordinates yields entanglement between states across these causally separated regions of spacetime. This results in a breakdown of unitarity, implying that information regarding the entangled partner may be irretrievably lost beyond the Rindler horizon. As a consequence, one has a situation of pure states evolving into mixed states. In this paper, we introduce a novel framework for comprehending this phenomenon using a recently proposed formulation of direct-sum quantum field theory (DQFT), which is grounded in superselection rules formulated by the parity and time reversal (

P T

) symmetry of Minkowski spacetime. In the context of DQFT applied to Rindler spacetime, we demonstrate that each Rindler observer can, in principle, access pure states within the horizon, thereby restoring unitarity. However, our analysis also reveals the emergence of a thermal spectrum of Unruh radiation. This prompts a reevaluation of entanglement in Rindler spacetime, where we propose a novel perspective on how Rindler observers may reconstruct complementary information beyond the horizon. Furthermore, we revisit the implications of the Reeh-Schlieder theorem within the framework of DQFT. Lastly, we underscore how our findings contribute to ongoing efforts aimed at elucidating the role of unitarity in quantum field theory within the context of de Sitter and black hole spacetimes. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

► Show Figures

Figure 1

11 pages, 4044 KiB

Open AccessArticle

Characteristics of Powerful Radio Galaxies

by Chandra B. Singh, Michael Williams, David Garofalo, Luis Rojas Castillo, Landon Taylor and Eddie Harmon

Universe 2024, 10(8), 319; https://doi.org/10.3390/universe10080319 - 8 Aug 2024

Viewed by 1096

Abstract

Mature radio galaxies such as M87 belong to a specific subclass of active galaxies (AGN) whose evolution in time endows them with five distinguishing characteristics, including (1) low excitation emission, (2) low star formation rates, (3) high bulge stellar-velocity dispersion, (4) bright stellar [...] Read more.

Mature radio galaxies such as M87 belong to a specific subclass of active galaxies (AGN) whose evolution in time endows them with five distinguishing characteristics, including (1) low excitation emission, (2) low star formation rates, (3) high bulge stellar-velocity dispersion, (4) bright stellar nuclei, and (5) weak or nonexistent merger signatures. We show how to understand these seemingly disparate characteristics as originating from the time evolution of powerful radio quasars and describe a new model prediction that tilted accretion disks in AGN are expected to occur in bright quasars but not in other subclasses of AGN. The picture we present should be understood as the most compelling evidence for counter-rotation as a key element in feedback from accreting black holes. Full article

(This article belongs to the Section Compact Objects)

► Show Figures

Figure 1

24 pages, 801 KiB

Open AccessArticle

Lifetime of Long-Lived Sunspot Groups

by Judit Muraközy

Universe 2024, 10(8), 318; https://doi.org/10.3390/universe10080318 - 5 Aug 2024

Viewed by 717

Abstract

Studies of active region (AR) lifetimes are mostly restricted to short-lived ARs. The aim of this paper is to include recurrent ARs, which should be identified unambiguously. The first step is the algorithmic listing of possible returns; then, the candidates are visually checked [...] Read more.

Studies of active region (AR) lifetimes are mostly restricted to short-lived ARs. The aim of this paper is to include recurrent ARs, which should be identified unambiguously. The first step is the algorithmic listing of possible returns; then, the candidates are visually checked using the unique HTML-feature of the Debrecen sunspot database. The final step is application of an asymmetric Gaussian function, introduced in previous articles, for short-lived ARs. This function has a surprisingly good fit to the data on correctly identified recurrent sunspot groups over several rotations enabling the reconstruction of the development on the far side of the sun. The Gnevyshev–Waldmeier rule for the area–lifetime relationship is not applicable for recurrent ARs; however, as a novel approach, a linear regression analysis extended to long lifetimes made it possible to recognize two populations of sizes for which two different area–lifetime relationships can be obtained. The lifetimes exhibit weak dependencies on the heliographic latitude and solar cycle phase. If an asymmetric Gaussian cannot be fit to the data, then they presumably belong to consecutive members of an active nest. Full article

(This article belongs to the Special Issue Solar and Stellar Activity: Exploring the Cosmic Nexus)

► Show Figures

Figure 1

40 pages, 795 KiB

Open AccessReview

Measuring a Mass: The Puzzling History of an Elusive Particle

by Elisabetta Di Grezia, Salvatore Esposito and Adele Naddeo

Universe 2024, 10(8), 317; https://doi.org/10.3390/universe10080317 - 3 Aug 2024

Viewed by 776

Abstract

Since Pauli’s hypothesis of their existence in 1930, neutrinos never ceased to bring into play novel ideas and to add new pieces of physics in the whole picture of fundamental interactions. They are only weakly interacting and, at odds with Standard Model’s predictions, [...] Read more.

Since Pauli’s hypothesis of their existence in 1930, neutrinos never ceased to bring into play novel ideas and to add new pieces of physics in the whole picture of fundamental interactions. They are only weakly interacting and, at odds with Standard Model’s predictions, have a mass less than one millionth of the electron mass, which makes the investigation of their properties very challenging. The issue of the measurement of neutrino’s rest mass gained a wider and wider consensus since its discovery through neutrino oscillations in 1998. Various neutrino sources are available for experiments, ranging from nuclear collisions of cosmic rays in the Earth atmosphere and supernova explosions to neutrino beams produced by accelerators and power reactors. These suggest different approaches to the experimental detection and measurement of the absolute value of the neutrino mass. In this paper, we retrace the intriguing story of this endeavor, focusing mainly on direct mass determination methods. The puzzling issue of the nature of massive neutrinos is addressed as well with explicit reference to the phenomenon of double beta-decay as a viable experimental tool to discriminate between Dirac’s and Majorana’s nature. Full article

(This article belongs to the Special Issue Neutrinos across Different Energy Scales)

► Show Figures

Figure 1

31 pages, 2024 KiB

Open AccessArticle

Prospects for Time-Domain and Multi-Messenger Science with AXIS

by Riccardo Arcodia, Franz E. Bauer, S. Bradley Cenko, Kristen C. Dage, Daryl Haggard, Wynn C. G. Ho, Erin Kara, Michael Koss, Tingting Liu, Labani Mallick, Michela Negro, Pragati Pradhan, J. Quirola-Vásquez, Mark T. Reynolds, Claudio Ricci, Richard E. Rothschild, Navin Sridhar, Eleonora Troja and Yuhan Yao

Universe 2024, 10(8), 316; https://doi.org/10.3390/universe10080316 - 2 Aug 2024

Cited by 5 | Viewed by 2142

Abstract

The Advanced X-ray Imaging Satellite (AXIS) promises revolutionary science in the X-ray and multi-messenger time domain. AXIS will leverage excellent spatial resolution (<1.5 arcsec), sensitivity (

80 \times

that of Swift), and a large collecting area (5–10× that of Chandra) across a 24-arcmin [...] Read more.

The Advanced X-ray Imaging Satellite (AXIS) promises revolutionary science in the X-ray and multi-messenger time domain. AXIS will leverage excellent spatial resolution (<1.5 arcsec), sensitivity (

80 \times

that of Swift), and a large collecting area (5–10× that of Chandra) across a 24-arcmin diameter field of view at soft X-ray energies (0.3–10.0 keV) to discover and characterize a wide range of X-ray transients from supernova-shock breakouts to tidal disruption events to highly variable supermassive black holes. The observatory’s ability to localize and monitor faint X-ray sources opens up new opportunities to hunt for counterparts to distant binary neutron star mergers, fast radio bursts, and exotic phenomena like fast X-ray transients. AXIS will offer a response time of <2 h to community alerts, enabling studies of gravitational wave sources, high-energy neutrino emitters, X-ray binaries, magnetars, and other targets of opportunity. This white paper highlights some of the discovery science that will be driven by AXIS in this burgeoning field of time domain and multi-messenger astrophysics. This White Paper is part of a series commissioned for the AXIS Probe Concept Mission; additional AXIS White Papers can be found at the AXIS website. Full article

(This article belongs to the Section Galaxies and Clusters)

► Show Figures

Figure 1

15 pages, 879 KiB

Open AccessArticle

Solar Energetic Particles Propagation under 3D Corotating Interaction Regions with Different Characteristic Parameters

by Yuji Zhu and Fang Shen

Universe 2024, 10(8), 315; https://doi.org/10.3390/universe10080315 - 2 Aug 2024

Cited by 1 | Viewed by 886

Abstract

Solar energetic particles (SEPs) are bursts of high-energy particles that originate from the Sun and can last for hours or even days. The aim of this study is to understand how the characteristics of energetic particles ware affected by the characteristic parameters of [...] Read more.

Solar energetic particles (SEPs) are bursts of high-energy particles that originate from the Sun and can last for hours or even days. The aim of this study is to understand how the characteristics of energetic particles ware affected by the characteristic parameters of corotating interaction regions (CIRs). In particular, the particle intensity distribution with time and space in CIRs with different characteristics were studied. The propagation and acceleration of particles were described by the focused transport equation (FTE). We used a three-dimensional magnetohydrodynamic (MHD) model to simulate the background solar wind with CIRs. By changing the inner boundary conditions, we constructed CIRs with different solar wind speeds, angles between the polar axis and rotation axis, and the azimuthal widths of the fast streams. Particles were impulsively injected at the inner boundary of the MHD model. We then studied the particle propagation and compression acceleration in different background solar wind. The results showed that the CIR widths are related to the solar wind speed, tilt angles, and the azimuthal widths of the fast stream. The acceleration of particles in the reverse and forward compression regions are mainly influenced by the solar wind speed difference and the slow solar wind speed, respectively. Particles with lower energy (sub-MeV) are more sensitive to the solar wind speed difference and the tilt angle. The particle intensity variation with time and the radial distance is mainly influenced by the solar wind speed. The longitudinal distribution of particle intensity is affected by the solar wind speed, tilt angles, and the azimuthal widths of the fast stream. Full article

(This article belongs to the Section Space Science)

► Show Figures

Figure 1

20 pages, 11521 KiB

Open AccessArticle

Calibration of Polarization Data for Vector Magnetographs at the Huairou Solar Observing Station over the Past Four Decades

by Jiangtao Su, Haiqing Xu, Suo Liu, Jiaben Lin, Hui Wang, Yongliang Song, Xianyong Bai, Shangbin Yang, Jie Chen, Xiaofan Wang, Yingzi Sun, Xiao Yang and Yuanyong Deng

Universe 2024, 10(8), 314; https://doi.org/10.3390/universe10080314 - 31 Jul 2024

Viewed by 868

Abstract

The Huairou Solar Observing Station (HSOS) has conducted solar vector magnetic field observations for 40 years and developed multiple vector magnetographs (including one space magnetic field observation instrument). Using these accumulated magnetic field observation data, HSOS has achieved significant progress in solar physics [...] Read more.

The Huairou Solar Observing Station (HSOS) has conducted solar vector magnetic field observations for 40 years and developed multiple vector magnetographs (including one space magnetic field observation instrument). Using these accumulated magnetic field observation data, HSOS has achieved significant progress in solar physics research, including important advancements in the helicity sign rule of solar active regions, the helicity characteristics of strong and weak magnetic fields in active regions, the chromospheric magnetic field characteristics of the Sun, the evolution of magnetic fields in active regions, and the extraction of magnetic field characteristics for flare precursors. However, due to historical reasons, the calibration of vector magnetic field data in HSOS are not standardized. Therefore, this paper summarizes past historical experiences and introduces the standardized calibration procedure for vector magnetic field processing in detail. These calibration procedures are the basic steps of the calibration process for the space vector magnetograph (Full-Disk Vector MagnetoGraph, abbreviated as FMG) observation data, and are also applicable to the calibration of other instrument observation data at HSOS. They mainly include basic processing of polarization data and in-depth processing of vector magnetic fields. We believe that such calibration processing of the historical data collected by HSOS over the past 40 years will help us to accurately measure and analyze the solar magnetic field, further revealing the laws of solar activity and its impact on the Earth’s environment. 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)

► Show Figures

Figure 1

30 pages, 2416 KiB

Open AccessReview

Stellar Flares, Superflares, and Coronal Mass Ejections—Entering the Big Data Era

by Krisztián Vida, Zsolt Kővári, Martin Leitzinger, Petra Odert, Katalin Oláh, Bálint Seli, Levente Kriskovics, Robert Greimel and Anna Mária Görgei

Universe 2024, 10(8), 313; https://doi.org/10.3390/universe10080313 - 31 Jul 2024

Cited by 2 | Viewed by 998

Abstract

Flares, sometimes accompanied by coronal mass ejections (CMEs), are the result of sudden changes in the magnetic field of stars with high energy release through magnetic reconnection, which can be observed across a wide range of the electromagnetic spectrum from radio waves to [...] Read more.

Flares, sometimes accompanied by coronal mass ejections (CMEs), are the result of sudden changes in the magnetic field of stars with high energy release through magnetic reconnection, which can be observed across a wide range of the electromagnetic spectrum from radio waves to the optical range to X-rays. In our observational review, we attempt to collect some fundamental new results, which can largely be linked to the Big Data era that has arrived due to the expansion of space photometric observations over the last two decades. We list the different types of stars showing flare activity and their observation strategies and discuss how their main stellar properties relate to the characteristics of the flares (or even CMEs) they emit. Our goal is to focus, without claiming to be complete, on those results that may, in one way or another, challenge the “standard” flare model based on the solar paradigm. Full article

(This article belongs to the Special Issue Solar and Stellar Activity: Exploring the Cosmic Nexus)

► Show Figures

Figure 1

33 pages, 2278 KiB

Open AccessReview

Axion-like Particle Effects on Photon Polarization in High-Energy Astrophysics

by Giorgio Galanti

Universe 2024, 10(8), 312; https://doi.org/10.3390/universe10080312 - 30 Jul 2024

Viewed by 752

Abstract

In this review, we present a self-contained introduction to axion-like particles (ALPs) with a particular focus on their effects on photon polarization: both theoretical and phenomenological aspects are discussed. We derive the photon survival probability in the presence of photon–ALP interaction, the corresponding [...] Read more.

In this review, we present a self-contained introduction to axion-like particles (ALPs) with a particular focus on their effects on photon polarization: both theoretical and phenomenological aspects are discussed. We derive the photon survival probability in the presence of photon–ALP interaction, the corresponding final photon degree of linear polarization, and the polarization angle in a wide energy interval. The presented results can be tested by current and planned missions such as IXPE (already operative), eXTP, XL-Calibur, NGXP, XPP in the X-ray band and like COSI (approved to launch), e-ASTROGAM, and AMEGO in the high-energy range. Specifically, we describe ALP-induced polarization effects on several astrophysical sources, such as galaxy clusters, blazars, and gamma-ray bursts, and we discuss their real detectability. In particular, galaxy clusters appear as very good observational targets in this respect. Moreover, in the very-high-energy (VHE) band, we discuss a peculiar ALP signature in photon polarization, in principle capable of proving the ALP existence. Unfortunately, present technologies cannot detect photon polarization up to such high energies, but the observational capability of the latter ALP signature in the VHE band could represent an interesting challenge for the future. As a matter of fact, the aim of this review is to show new ways to make progress in the physics of ALPs, thanks to their effects on photon polarization, a topic that has aroused less interest in the past, but which is now timely with the advent of many new polarimetric missions. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—High Energy Nuclear and Particle Physics)

► Show Figures

Figure 1

11 pages, 2922 KiB

Open AccessArticle

The Algorithm of the Two Neutron Monitors for the Analysis of the Rigidity Spectrum Variations of Galactic Cosmic Ray Intensity Flux in Solar Cycle 24

by Krzysztof Iskra, Marek Siluszyk and Witold Wozniak

Universe 2024, 10(8), 311; https://doi.org/10.3390/universe10080311 - 30 Jul 2024

Viewed by 650

Abstract

The method of the two neutron monitors was used to analyze the parameters of the rigidity spectrum variations (RSV) of galactic cosmic ray intensity (GCR) flux in solar cycle 24 based on the data from the global network of neutron monitors. This method [...] Read more.

The method of the two neutron monitors was used to analyze the parameters of the rigidity spectrum variations (RSV) of galactic cosmic ray intensity (GCR) flux in solar cycle 24 based on the data from the global network of neutron monitors. This method is an alternative to the least squares method when there are few monitors working stably in a given period, and their use in the least squares method is impossible. Analyses of the changes in exponent γ in the RSV of GCR flux from 2009 to 2019 were studied. The soft RSV (γ = 1.2–1.3) of the GCR flux around the maximum epoch and the hard RSV (γ = 0.6–0.9) around the minimum epoch of solar activity (SA) is the general feature of GCR modulation in the GeV energy scale (5, 50), to which neutron monitors were found to correspond. Therefore, various values of the RSV γ in the considered period show that during the decrease and increase period of SA, the essential changes in the large-scale structure of the heliospheric magnetic field (HMF) fluctuations/turbulence take place. The exponent γ of the RSV of the GCR flux can be considered a significant parameter to investigate the long-period changes in the GCR flux. Full article

► Show Figures

Figure 1

13 pages, 417 KiB

Open AccessArticle

Low-Energy Cosmic Rays and Associated MeV Gamma-Ray Emissions in the Protoplanetary System

by Xulei Sun, Shuying Zheng, Zhaodong Shi, Bing Liu and Ruizhi Yang

Universe 2024, 10(8), 310; https://doi.org/10.3390/universe10080310 - 27 Jul 2024

Viewed by 976

Abstract

Low-energy cosmic rays (LECRs) play a crucial role in the formation of planetary systems, and detecting and reconstructing the properties of early LECRs is essential for understanding the mechanisms of planetary system formation. Given that LECRs interact with the surrounding medium to produce [...] Read more.

Low-energy cosmic rays (LECRs) play a crucial role in the formation of planetary systems, and detecting and reconstructing the properties of early LECRs is essential for understanding the mechanisms of planetary system formation. Given that LECRs interact with the surrounding medium to produce nuclear de-excitation line emissions, which are gamma-ray emissions with energy mainly within 0.1–10 MeV and are unaffected by stellar wind modulation, these emissions can accurately reflect the properties of LECRs. This study introduces an innovative method for using gamma-ray emissions to infer LECR properties. We employed the Parker transport equation to simulate the propagation and spectral evolution of LECRs in a protoplanetary disk and calculated the characteristic gamma-ray emissions resulting from interactions between LECRs and disk material. These gamma-ray emissions encapsulate the spectral information of LECRs, providing a powerful tool to reconstruct the cosmic ray environment at that time. This method, supported by further theoretical developments and observations, will fundamentally enhance our understanding of the impact of CRs on the origin and evolution of planetary systems and address significant scientific questions regarding the cosmic ray environment at the origin of life. Full article

(This article belongs to the Special Issue Studying Astrophysics with High-Energy Cosmic Particles)

► Show Figures

Figure 1

13 pages, 557 KiB

Open AccessArticle

Construction of Ground-State Solutions of the Gross–Pitaevskii–Poisson System Using Genetic Algorithms

by Carlos Tena-Contreras, Iván Alvarez-Ríos and Francisco S. Guzmán

Universe 2024, 10(8), 309; https://doi.org/10.3390/universe10080309 - 26 Jul 2024

Viewed by 653

Abstract

We present the construction of the ground state of the Gross–Pitaevskii–Poisson equations using genetic algorithms. By employing numerical solutions, we develop an empirical formula for the density that works within the considered parameter space. Through the analysis of both numerical and empirical solutions, [...] Read more.

We present the construction of the ground state of the Gross–Pitaevskii–Poisson equations using genetic algorithms. By employing numerical solutions, we develop an empirical formula for the density that works within the considered parameter space. Through the analysis of both numerical and empirical solutions, we investigate the stability of these ground-state solutions. Our findings reveal that while the numerical solution outperforms the empirical formula, both solutions lead to similar oscillation modes. We observe that the stability of the solutions depends on specific values of the central density and the nonlinear self-interaction term and establish an empirical criterion delineating the conditions under which the solutions exhibit stability or instability. Full article

(This article belongs to the Section Cosmology)

► Show Figures

Figure 1

13 pages, 5613 KiB

Open AccessArticle

Measurements of Decoherence in Small Sea-Level Extensive Air Showers

by Roger Clay

Universe 2024, 10(8), 308; https://doi.org/10.3390/universe10080308 - 25 Jul 2024

Viewed by 735

Abstract

A study is made of the progressive ‘decoherence’ of cosmic ray extensive air-shower particle-detector signals in small air showers through measurements of coincidence rates for pairs of detectors versus the detector separation. Measurements are made both when only the two separated detectors themselves [...] Read more.

A study is made of the progressive ‘decoherence’ of cosmic ray extensive air-shower particle-detector signals in small air showers through measurements of coincidence rates for pairs of detectors versus the detector separation. Measurements are made both when only the two separated detectors themselves trigger in coincidence, and when that coincidence trigger also requires the detection of a local air shower by a small external air-shower array. The addition of the explicit air-shower trigger ensures that the latter data correspond to showers of a larger particle size, and triggering by very localised shower cores is then unlikely. When including a shower trigger, the decoherence results appear substantially different in form. The coincidence rate between two detectors only can be approximated by a power-law variation with separation distance. When triggering involves an air-shower array, the variation becomes close to an exponential form with characteristic exponent distances varying systematically with increasing detector and air-shower size thresholds. A result is that one can see that small air showers will exhibit clear non-Poissonian density fluctuations near their cores, out to distances of ~5 m, or at shower energies below ~0.05 PeV. These ideas can be helpful in understanding the statistical properties of signals when using large detectors in air-shower arrays. Full article

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

► Show Figures

Figure 1

14 pages, 596 KiB

Open AccessArticle

A Study of Spin 1 Unruh–De Witt Detectors

by F. M. Guedes, M. S. Guimaraes, I. Roditi and S. P. Sorella

Universe 2024, 10(8), 307; https://doi.org/10.3390/universe10080307 - 24 Jul 2024

Cited by 1 | Viewed by 685

Abstract

A study of the interaction of spin 1 Unruh–De Witt detectors with a relativistic scalar quantum field is presented here. After tracing out the field modes, the resulting density matrix for a bipartite qutrit system is employed to investigate the violation of the [...] Read more.

A study of the interaction of spin 1 Unruh–De Witt detectors with a relativistic scalar quantum field is presented here. After tracing out the field modes, the resulting density matrix for a bipartite qutrit system is employed to investigate the violation of the Bell–CHSH inequality. Unlike the case of spin

1 / 2

, for which the effects of the quantum field result in a decrease in the size of violation, in the case of spin 1, both a decrease or an increase in the size of the violation may occur. This effect is ascribed to the fact that Tsirelson’s bound is not saturated in the case of qutrits. Full article

(This article belongs to the Section Field Theory)

► Show Figures

Figure 1

43 pages, 639 KiB

Open AccessTutorial

Graviton Physics: A Concise Tutorial on the Quantum Field Theory of Gravitons, Graviton Noise, and Gravitational Decoherence

by Jen-Tsung Hsiang, Hing-Tong Cho and Bei-Lok Hu

Universe 2024, 10(8), 306; https://doi.org/10.3390/universe10080306 - 24 Jul 2024

Cited by 5 | Viewed by 1571

Abstract

The detection of gravitational waves in 2015 ushered in a new era of gravitational wave (GW) astronomy capable of probing the strong field dynamics of black holes and neutron stars. It has opened up an exciting new window for laboratory and space tests [...] Read more.

The detection of gravitational waves in 2015 ushered in a new era of gravitational wave (GW) astronomy capable of probing the strong field dynamics of black holes and neutron stars. It has opened up an exciting new window for laboratory and space tests of Einstein’s theory of classical general relativity (GR). In recent years, two interesting proposals have aimed to reveal the quantum nature of perturbative gravity: (1) theoretical predictions on how graviton noise from the early universe, after the vacuum of the gravitational field was strongly squeezed by inflationary expansion; (2) experimental proposals using the quantum entanglement between two masses, each in a superposition (gravitational cat, or gravcat) state. The first proposal focuses on the stochastic properties of quantum fields (QFs), and the second invokes a key concept of quantum information (QI). An equally basic and interesting idea is to ask whether (and how) gravity might be responsible for a quantum system becoming classical in appearance, known as gravitational decoherence. Decoherence due to gravity is of special interest because gravity is universal, meaning, gravitational interaction is present for all massive objects. This is an important issue in macroscopic quantum phenomena (MQP), underlining many proposals in alternative quantum theories (AQTs). To fully appreciate or conduct research in these exciting developments requires a working knowledge of classical GR, QF theory, and QI, plus some familiarity with stochastic processes (SPs), namely, noise in quantum fields and decohering environments. Traditionally a new researcher may be conversant in one or two of these four subjects: GR, QFT, QI, and SP, depending on his/her background. This tutorial attempts to provide the necessary connective tissues between them, helping an engaged reader from any one of these four subjects to leapfrog to the frontier of these interdisciplinary research topics. In the present version, we shall address the three topics listed in the title, excluding gravitational entanglement, because, despite the high attention some recent experimental proposals have received, its nature and implications in relation to quantum gravity still contain many controversial elements. Full article

(This article belongs to the Special Issue Quantum Field Theory of Open Systems)

► Show Figures

Figure 1

Journal Menu

Journal Browser

Universe, Volume 10, Issue 8 (August 2024) – 31 articles

Further Information

Guidelines

MDPI Initiatives

Follow MDPI