Pub Date : 2024-08-16DOI: 10.3390/universe10080330
Michael H. Siegel, Caryl Gronwall
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 uvm2 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.
{"title":"Hot Stars, Young Stellar Populations and Dust with Swift/UVOT","authors":"Michael H. Siegel, Caryl Gronwall","doi":"10.3390/universe10080330","DOIUrl":"https://doi.org/10.3390/universe10080330","url":null,"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 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 uvm2 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.","PeriodicalId":48646,"journal":{"name":"Universe","volume":"44 1","pages":""},"PeriodicalIF":2.9,"publicationDate":"2024-08-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210918","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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 r0 and the equation of state parameter ω across four scenarios, and it has been seen that the larger throat radii r0 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 r0 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 rc.
{"title":"Weak Deflection Angle by the Einstein–Cartan Traversable Wormhole Using Gauss–Bonnet Theorem with Time Delay","authors":"Susmita Sarkar, Nayan Sarkar, Abhisek Dutta, Farook Rahaman","doi":"10.3390/universe10080331","DOIUrl":"https://doi.org/10.3390/universe10080331","url":null,"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 r0 and the equation of state parameter ω across four scenarios, and it has been seen that the larger throat radii r0 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 r0 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 rc.","PeriodicalId":48646,"journal":{"name":"Universe","volume":"67 1","pages":""},"PeriodicalIF":2.9,"publicationDate":"2024-08-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210917","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-15DOI: 10.3390/universe10080327
Tudor Alexandru Calafeteanu, Paula Gina Isar, Emil Ioan Sluşanschi
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 (Xmax). 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 F1 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.
超高能宇宙射线（UHECRs）是宇宙中普通物质中极为罕见的高能粒子，在到达地球大气层之前会飞行天文数字的距离。当原生宇宙射线与大气核相互作用时，就会产生次级基本粒子的级联大范围空气阵雨（EAS）。无线电探测器已被证明是重建 EAS 特性的可靠方法，如气雨的轴线、能量和最大值（Xmax）。这有助于理解基本的天体物理现象，如活动星系核和伽玛射线暴。与此同时，数据科学已成为 UHECR 研究中不可或缺的部分。通过将统计、计算和深度学习方法应用于真实世界和模拟无线电数据，研究人员可以提取见解并做出预测。我们介绍了一种卷积神经网络（CNN）架构，旨在将模拟的气雨事件分类为由质子或铁核产生。该分类的测试误差稳定在 10%，准确度和 F1 分数分别为 0.9 和 0.8。这些指标表明，基于皮埃尔-奥格天文台（包括无线电天线、水切伦科夫探测器和荧光望远镜）这一世界上最大的地球宇宙射线实验的探测器所能收集到的数据，对 UHECR 的核组成具有很强的预测能力。
{"title":"Convolutional Neural Network Processing of Radio Emission for Nuclear Composition Classification of Ultra-High-Energy Cosmic Rays","authors":"Tudor Alexandru Calafeteanu, Paula Gina Isar, Emil Ioan Sluşanschi","doi":"10.3390/universe10080327","DOIUrl":"https://doi.org/10.3390/universe10080327","url":null,"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 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 (Xmax). 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 F1 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.","PeriodicalId":48646,"journal":{"name":"Universe","volume":"6 1","pages":""},"PeriodicalIF":2.9,"publicationDate":"2024-08-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142227850","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-15DOI: 10.3390/universe10080328
Changjun Gao, Jianhui Qiu
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.
杰尼斯-纽曼-维尼库尔时空对应于爱因斯坦方程的静态球对称解，其能量动量张量为无质量五元场。据了解，该时空描述了一个裸奇点。据我们所知，这两个参数的确切物理含义尚不清楚。本文从 Janis-Newman-Winicour 裸奇点解出发，首先通过复变得到虫洞解。然后，让参数 s 接近无穷大，我们得到了著名的指数虫洞解。之后，我们将杰尼斯-纽曼-维尼柯裸奇点及其虫洞对应方案分别嵌入到具有大质量五元场和大质量幻象场的能量动量张量的德西特宇宙或反德西特宇宙背景中。令我们惊讶的是，所得到的五元势实际上就是我们中的一个人发现的稀拉顿势。这表明，通过调节带电稀拉顿黑洞解的参数，我们可以得到杰尼斯-纽曼-维尼库尔解。此外，在德西特或反德西特宇宙背景下，通过对带电稀拉顿黑洞解进行复杂变换，可以得到带电虫洞解。我们最终发现，s 实际上与稀拉顿场与麦克斯韦场的耦合常数有关，而 b 则与稀拉顿黑洞的负质量有关。负的黑洞质量在物理上是被禁止的。因此，我们得出结论：物理上不允许出现杰尼斯-纽曼-维尼库尔裸奇点解。
{"title":"From the Janis–Newman–Winicour Naked Singularities to the Einstein–Maxwell Phantom Wormholes","authors":"Changjun Gao, Jianhui Qiu","doi":"10.3390/universe10080328","DOIUrl":"https://doi.org/10.3390/universe10080328","url":null,"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 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.","PeriodicalId":48646,"journal":{"name":"Universe","volume":"58 1","pages":""},"PeriodicalIF":2.9,"publicationDate":"2024-08-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210919","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-13DOI: 10.3390/universe10080326
Ankur Sharma
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.
{"title":"Multi-Messenger Connection in High-Energy Neutrino Astronomy","authors":"Ankur Sharma","doi":"10.3390/universe10080326","DOIUrl":"https://doi.org/10.3390/universe10080326","url":null,"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 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.","PeriodicalId":48646,"journal":{"name":"Universe","volume":"11 1","pages":""},"PeriodicalIF":2.9,"publicationDate":"2024-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210921","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-11DOI: 10.3390/universe10080325
Vladimir N. Yershov
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
{"title":"Constraint on the Cosmic Curvature in a Model with the Schwarzschild–de Sitter Metric from Supernovae and Gamma-Ray Burst Observational Data","authors":"Vladimir N. Yershov","doi":"10.3390/universe10080325","DOIUrl":"https://doi.org/10.3390/universe10080325","url":null,"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 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: 2.4×1015 Mpc. This means that the observational data indicate a negligible curvature of space.","PeriodicalId":48646,"journal":{"name":"Universe","volume":"5 1","pages":""},"PeriodicalIF":2.9,"publicationDate":"2024-08-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940634","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-10DOI: 10.3390/universe10080324
Paulo M. Sá
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.
{"title":"Coupled Quintessence Inspired by Warm Inflation","authors":"Paulo M. Sá","doi":"10.3390/universe10080324","DOIUrl":"https://doi.org/10.3390/universe10080324","url":null,"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 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.","PeriodicalId":48646,"journal":{"name":"Universe","volume":"42 1","pages":""},"PeriodicalIF":2.9,"publicationDate":"2024-08-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940746","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-09DOI: 10.3390/universe10080322
Francesco Di Clemente, Alessandro Drago, Giuseppe Pagliara
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.
{"title":"Strange Dwarfs: A Review on the (in)Stability","authors":"Francesco Di Clemente, Alessandro Drago, Giuseppe Pagliara","doi":"10.3390/universe10080322","DOIUrl":"https://doi.org/10.3390/universe10080322","url":null,"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 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.","PeriodicalId":48646,"journal":{"name":"Universe","volume":"30 1","pages":""},"PeriodicalIF":2.9,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940747","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-08DOI: 10.3390/universe10080320
K. Sravan Kumar, João Marto
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 (PT) 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.
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Pub Date : 2024-08-05DOI: 10.3390/universe10080318
Judit Muraközy
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.
对活动区（AR）寿命的研究大多局限于短寿命的 AR。本文的目的是将反复出现的 AR 囊括进来，并对其进行明确识别。第一步是通过算法列出可能返回的太阳黑子；然后，利用德布勒森太阳黑子数据库的独特 HTML 功能对候选太阳黑子进行目视检查。最后一步是应用以前文章中介绍的非对称高斯函数来处理短寿命的太阳黑子。该函数与正确识别出的重复出现的太阳黑子群数据的拟合效果出奇地好，通过几次旋转，可以重建太阳远侧的发展情况。面积-寿命关系的 Gnevyshev-Waldmeier 规则不适用于周期性太阳黑子；但是，作为一种新方法，线性回归分析扩展到了长寿命，从而可以识别出两种大小的群体，并得到两种不同的面积-寿命关系。寿命与太阳纬度和太阳周期相位有微弱的关系。如果无法对数据进行非对称高斯拟合，那么它们可能属于一个活动巢穴的连续成员。
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