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Universe, Volume 10, Issue 4 (April 2024) – 37 articles

Cover Story (view full-size image): Redshift drift is the phenomenon whereby the observed redshift between an emitter and observer comoving with the Hubble flow in an expanding FLRW universe will slowly evolve—on a timescale comparable to the Hubble time. In a previous article, three of the current authors performed a cosmographic analysis of the redshift drift in an FLRW universe, temporarily putting aside the issue of dynamics (the Friedmann equations). In the current article, we add dynamics while still remaining within the framework of an exact FLRW universe. We developed a suitable generic matter model and applied it to both standard FLRW and various dark energy models. Furthermore, we present an analysis of the utility of alternative cosmographic variables to describe the redshift drift data. View this paper
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11 pages, 637 KiB  
Article
Two-Pion Bose–Einstein Correlations in Au+Au Collisions at sNN = 3 GeV in the STAR Experiment
by Anna Kraeva
Universe 2024, 10(4), 188; https://doi.org/10.3390/universe10040188 - 20 Apr 2024
Viewed by 1182
Abstract
The correlation femtoscopy technique makes it possible to estimate the geometric dimensions and lifetime of the particle emission region after the collision of ions. Measurements of the emission region characteristics not only at midrapidity but also at backward (forward) rapidity can provide new [...] Read more.
The correlation femtoscopy technique makes it possible to estimate the geometric dimensions and lifetime of the particle emission region after the collision of ions. Measurements of the emission region characteristics not only at midrapidity but also at backward (forward) rapidity can provide new information about the source and make it possible to impose constraints on the heavy-ion collision models. This work is devoted to revealing the dependence of the spatial and temporal parameters of the emission region of identical pions in Au+Au collisions at sNN = 3 GeV from the fixed-target program of the STAR experiment. The extracted femtoscopic radii, RoutRsideRlongRoutlong2, and the correlation strength, λ, are presented as a function of collision centrality, pair rapidity, and transverse momentum. Physics implications will be discussed. Full article
(This article belongs to the Special Issue Multiparticle Dynamics)
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30 pages, 31917 KiB  
Review
Future Perspectives for Gamma-ray Burst Detection from Space
by Enrico Bozzo, Lorenzo Amati, Wayne Baumgartner, Tzu-Ching Chang, Bertrand Cordier, Nicolas De Angelis, Akihiro Doi, Marco Feroci, Cynthia Froning, Jessica Gaskin, Adam Goldstein, Diego Götz, Jon E. Grove, Sylvain Guiriec, Margarita Hernanz, C. Michelle Hui, Peter Jenke, Daniel Kocevski, Merlin Kole, Chryssa Kouveliotou, Thomas Maccarone, Mark L. McConnell, Hideo Matsuhara, Paul O’Brien, Nicolas Produit, Paul S. Ray, Peter Roming, Andrea Santangelo, Michael Seiffert, Hui Sun, Alexander van der Horst, Peter Veres, Jianyan Wei, Nicholas White, Colleen Wilson-Hodge, Daisuke Yonetoku, Weimin Yuan and Shuang-Nan Zhangadd Show full author list remove Hide full author list
Universe 2024, 10(4), 187; https://doi.org/10.3390/universe10040187 - 19 Apr 2024
Cited by 3 | Viewed by 1521
Abstract
Since their first discovery in the late 1960s, gamma-ray bursts have attracted an exponentially growing interest from the international community due to their central role in the most highly debated open questions of the modern research of astronomy, astrophysics, cosmology, and fundamental physics. [...] Read more.
Since their first discovery in the late 1960s, gamma-ray bursts have attracted an exponentially growing interest from the international community due to their central role in the most highly debated open questions of the modern research of astronomy, astrophysics, cosmology, and fundamental physics. These range from the intimate nuclear composition of high-density material within the core of ultra-dense neuron stars, to stellar evolution via the collapse of massive stars, the production and propagation of gravitational waves, as well as the exploration of the early universe by unveiling the first stars and galaxies (assessing also their evolution and cosmic re-ionization). GRBs in the past ∼50 years have stimulated the development of cutting-edge technological instruments for observations of high-energy celestial sources from space, leading to the launch and successful operations of many different scientific missions (several of them still in data-taking mode currently). In this review, we provide a brief description of the GRB-dedicated missions from space being designed and developed for the future. The list of these projects, not meant to be exhaustive, shall serve as a reference to interested readers to understand what is likely to come next to lead the further development of GRB research and the associated phenomenology. Full article
(This article belongs to the Special Issue Recent Advances in Gamma Ray Astrophysics and Future Perspectives)
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24 pages, 8562 KiB  
Article
The Changes in Multiscale Solar Wind Fluctuations on the Path from the Sun to Earth
by Igor D. Volodin, Maria O. Riazantseva, Liudmila S. Rakhmanova, Alexander A. Khokhlachev and Yuri I. Yermolaev
Universe 2024, 10(4), 186; https://doi.org/10.3390/universe10040186 - 19 Apr 2024
Viewed by 1058
Abstract
This paper is devoted to the analysis of fluctuations in the solar wind plasma and interplanetary magnetic field parameters observed by Solar Orbiter and WIND spacecraft at different scales ranging from ~103 to 107 km. We consider two long data intervals [...] Read more.
This paper is devoted to the analysis of fluctuations in the solar wind plasma and interplanetary magnetic field parameters observed by Solar Orbiter and WIND spacecraft at different scales ranging from ~103 to 107 km. We consider two long data intervals where the distances between the spacecraft are 0.1 and 0.5 AU, respectively, and they are located close to the Sun–Earth line. Transformation of the fluctuation’s properties on the way from the Sun to Earth is analyzed for different types of solar wind associated with quasi-stationary and transient solar phenomena. The time series of bulk speed are shown to undergo a slight modification, even for large spacecraft separation, while the time series of the interplanetary magnetic field magnitude and components as well as proton density may be transformed even at a relatively short distance. Though the large-scale solar wind structures propagate the distance up to 0.5 AU without significant change, local structures at smaller scales may be modified. The statistical properties of the fluctuations such as relative standard deviation or probability distribution function and its moments remain nearly unchanged at different distances between the two spacecraft and are likely to depend mostly on the type of the solar wind. Full article
(This article belongs to the Section Solar and Stellar Physics)
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20 pages, 3333 KiB  
Article
The High Mass Accretion in the Innermost Regions of a Viscously Evolved Protoplanetary Disk
by Chunjian Liu, Zhen Yao and Yue Quan
Universe 2024, 10(4), 185; https://doi.org/10.3390/universe10040185 - 18 Apr 2024
Viewed by 941
Abstract
In this paper, we investigate the mass accretion properties in the innermost regions of a viscously evolved protoplanetary disk and try to find some clues to the outburst events. In our newly developed one-dimensional time-dependent disk model based on the diffusion equation for [...] Read more.
In this paper, we investigate the mass accretion properties in the innermost regions of a viscously evolved protoplanetary disk and try to find some clues to the outburst events. In our newly developed one-dimensional time-dependent disk model based on the diffusion equation for surface density, we take into account the following physical effects: the gravitational collapse of the parent molecular cloud core, the irradiation from the central star to the disk, the effect of the photoevaporation mechanism, the viscosity due to the magnetorotational instability (MRI) and the gravitational instability (GI), and the thermal ionization mechanism in the inner regions. We find that the mass accretion rate M·disk in the innermost regions is statistically high enough to generate outbursts, although there are regions where the accretion rate is low. Additionally, we find that there is a weak correlation between the high mass accretion rate M·disk and the molecular cloud core’s properties (angular velocity ω and mass Mcd), as well as a strong correlation with the minimum viscosity parameter αmin. In general, there are two regions of outburst, the inner Region I and outer Region II. The outburst of Region I is caused by the MRI mechanism and thermal instability, while neither the MRI, the GI, nor the thermal instability causes the outburst of Region II. Our analysis suggests that the outer Region II is dominated by, or largely related to, the Rosseland mean opacity κR and the αmin parameter. Full article
(This article belongs to the Section Planetary Sciences)
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6 pages, 210 KiB  
Editorial
Special Issue on Modified Gravity Approaches to the Tensions of ΛCDM: Goals and Highlights
by Eleonora Di Valentino, Leandros Perivolaropoulos and Jackson Levi Said
Universe 2024, 10(4), 184; https://doi.org/10.3390/universe10040184 - 18 Apr 2024
Cited by 2 | Viewed by 1209
Abstract
The standard cosmological model, known as ΛCDM, has been remarkably successful in providing a coherent and predictive framework for understanding the Universe’s evolution, its large-scale structure, and cosmic microwave background (CMB) radiation [...] Full article
(This article belongs to the Special Issue Modified Gravity Approaches to the Tensions of ΛCDM)
18 pages, 2427 KiB  
Article
Linear Stability Analysis of Relativistic Magnetized Jets: The Minimalist Approach
by Nektarios Vlahakis
Universe 2024, 10(4), 183; https://doi.org/10.3390/universe10040183 - 17 Apr 2024
Cited by 1 | Viewed by 1342
Abstract
A minimalist approach to the linear stability problem in fluid dynamics is developed that ensures efficiency by utilizing only the essential elements required to find the eigenvalues for given boundary conditions. It is shown that the problem is equivalent to a single first-order [...] Read more.
A minimalist approach to the linear stability problem in fluid dynamics is developed that ensures efficiency by utilizing only the essential elements required to find the eigenvalues for given boundary conditions. It is shown that the problem is equivalent to a single first-order ordinary differential equation, and that studying the argument of the unknown complex function in the eigenvalue space is sufficient to find the dispersion relation. The method is applied to a model for relativistic magnetized astrophysical jets. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)
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17 pages, 1296 KiB  
Article
The Statistical Analysis of Exoplanet and Host Stars Based on Multi-Satellite Data Observations
by Yanke Tang, Xiaolu Li, Kai Xiao, Ning Gai, Shijie Li, Futong Dong, Yifan Wang and Yang Gao
Universe 2024, 10(4), 182; https://doi.org/10.3390/universe10040182 - 16 Apr 2024
Cited by 1 | Viewed by 1275
Abstract
In recent years, the rapid development of exoplanet research has provided us with an opportunity to better understand planetary systems in the universe and to search for signs of life. In order to further investigate the prevalence of habitable exoplanets and to validate [...] Read more.
In recent years, the rapid development of exoplanet research has provided us with an opportunity to better understand planetary systems in the universe and to search for signs of life. In order to further investigate the prevalence of habitable exoplanets and to validate planetary formation theories, as well as to comprehend planetary evolution, we have utilized confirmed exoplanet data obtained from the NASA Exoplanet Archive database, including data released by telescopes such as Kepler and TESS. By analyzing these data, we have selected a sample of planets around F, G, K, and M-type stars within a radius range of 1 to 20 R and with orbital periods ranging from 0.4 days to 400 days. Using the IDEM method based on these data, we calculated the overall formation rate, which is estimated to be 2.02%. Then, we use these data to analyze the relationship among planet formation rates, stellar metallicity, and stellar gravitational acceleration (logg). We firstly find that the formation rate of giant planets is higher around metal-rich stellars, but it inhibits the formation of gas giants when logg > 4.5, yet the stellar metallicity seems to have no effect on the formation rate of smaller planets. Secondly, the host stellar gravitational acceleration affects the relationship between planet formation rate and orbital period. Thirdly, there is a robust power-law relationship between the orbital period of smaller planets and their formation rate. Finally, we find that, for a given orbital period, there is a positive correlation between the planet formation rate and the logg. Full article
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27 pages, 662 KiB  
Article
Causal Structure in Spin Foams
by Eugenio Bianchi and Pierre Martin-Dussaud
Universe 2024, 10(4), 181; https://doi.org/10.3390/universe10040181 - 14 Apr 2024
Cited by 1 | Viewed by 1397
Abstract
The metric field of general relativity is almost fully determined by its causal structure. Yet, in spin foam models of quantum gravity, the role played by the causal structure is still largely unexplored. The goal of this paper is to clarify how causality [...] Read more.
The metric field of general relativity is almost fully determined by its causal structure. Yet, in spin foam models of quantum gravity, the role played by the causal structure is still largely unexplored. The goal of this paper is to clarify how causality is encoded in such models. The quest unveils the physical meaning of the orientation of the two-complex and its role as a dynamical variable. We propose a causal version of the EPRL spin foam model and discuss the role of the causal structure in the reconstruction of a semiclassical space–time geometry. Full article
(This article belongs to the Special Issue Loop Quantum Gravity: A Themed Issue in Honor of Prof. Abhay Ashtekar)
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28 pages, 577 KiB  
Article
Small-Scale Cosmology Independent of the Standard Model
by Georgy I. Burde
Universe 2024, 10(4), 180; https://doi.org/10.3390/universe10040180 - 13 Apr 2024
Viewed by 1180
Abstract
‘Small-scale cosmology’ is a theory designed to incorporate the linear redshift versus distance relation, which is inferred from observations, into the theoretical framework independent of the global Robertson–Walker–Friedman (RWF)-type models. The motivation behind this is that the RWF cosmological models, based on the [...] Read more.
‘Small-scale cosmology’ is a theory designed to incorporate the linear redshift versus distance relation, which is inferred from observations, into the theoretical framework independent of the global Robertson–Walker–Friedman (RWF)-type models. The motivation behind this is that the RWF cosmological models, based on the assumptions of homogeneity and a constant matter density, as well as the concept of expanding space inherent to them are not applicable on the scales of observations from which the linear Hubble law is inferred. Therefore, explaining the Hubble law as the small redshift limit of the RWF model or as an effect of expanding space is inconsistent. Thus, the Hubble linear relation between the redshift of an extragalactic object and its distance should be considered an independent law of nature valid in the range of the distances where the RWF cosmology is not valid. In general, the theory, based on that concept, can be developed in different ways. In the present paper, ‘small-scale cosmology’ is formulated as a theory operating in the (redshift–object coordinates) space, which allows developing a conceptual and computational basis of the theory along the lines of that of special relativity. In such a theory, the condition of invariance of the Hubble law with respect to a change in the observer acceleration plays a central role. In pursuing this approach, the effectiveness of group theoretical methods is exploited. Applying the Lie group method yields transformations of the variables (the redshift and space coordinates of a cosmological object) between the reference frames of the accelerated observers. In this paper, the transformations are applied to studying the effects of the solar system observer acceleration on the observed shape, distribution and rotation curves of galaxy clusters. Full article
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22 pages, 1626 KiB  
Article
Minkowskian Approach to the Pseudorange Navigation Equations
by Ramón Serrano Montesinos and Juan Antonio Morales-Lladosa
Universe 2024, 10(4), 179; https://doi.org/10.3390/universe10040179 - 12 Apr 2024
Cited by 1 | Viewed by 978
Abstract
Our starting point is the covariant coordinate transformation equation of a relativistic positioning system in Minkowski space–time that maps the receiver’s emission coordinates (proper times broadcast by the emitters) to its coordinates in some inertial reference frame. Bancroft’s analytical (closed-form) solution to the [...] Read more.
Our starting point is the covariant coordinate transformation equation of a relativistic positioning system in Minkowski space–time that maps the receiver’s emission coordinates (proper times broadcast by the emitters) to its coordinates in some inertial reference frame. Bancroft’s analytical (closed-form) solution to the basic pseudorange navigation equations with four emitters is recovered, and the subjacent elements are geometrically interpreted. The case of four static beacons is analysed as a clarifying situation. Full article
(This article belongs to the Section Gravitation)
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15 pages, 5835 KiB  
Article
A New Solution of the Pulsar Equation
by Ioannis Contopoulos, Ioannis Dimitropoulos, Dimitris Ntotsikas and Konstantinos N. Gourgouliatos
Universe 2024, 10(4), 178; https://doi.org/10.3390/universe10040178 - 12 Apr 2024
Cited by 1 | Viewed by 1167
Abstract
We present the first new type of solution of the pulsar equation since 1999. In it, the whole magnetosphere is confined inside the light cylinder and an electrically charged layer wraps around it and holds it together. The reason this new solution has [...] Read more.
We present the first new type of solution of the pulsar equation since 1999. In it, the whole magnetosphere is confined inside the light cylinder and an electrically charged layer wraps around it and holds it together. The reason this new solution has never been obtained before is that all current time-dependent simulations are initialized with a vacuum dipole configuration that extends to infinity; thus, their final steady-state solution also extends to infinity. Under special conditions, such a confined configuration may be attained when the neutron star first forms in the interior of a collapsing star during a supernova explosion, or when it accretes from an external wind or disk from a donor star. It is shown that this new maximally closed non-decelerating solution is the limit of a continuous sequence of standard magnetospheres with open and closed field lines when the amount of open field lines gradually drops to zero. The minimum energy solution in this sequence is a standard magnetosphere in which the closed field line region extends up to about 80% of the light cylinder. We estimate that the released energy when the new solution transitions to the minimum energy one is enough to power a fast radio burst. Full article
(This article belongs to the Special Issue A New Horizon of Pulsar and Neutron Star: The 55-Year Anniversary)
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5 pages, 215 KiB  
Communication
Upper Bound of Barrow Entropy Index from Black Hole Fragmentation
by Jiayi Xia and Yen Chin Ong
Universe 2024, 10(4), 177; https://doi.org/10.3390/universe10040177 - 11 Apr 2024
Cited by 1 | Viewed by 1394
Abstract
Both classical and quantum arguments suggest that if Barrow entropy is correct, its index δ must be energy-dependent, which would affect the very early universe. Based on thermodynamic stability that sufficiently large black holes should not fragment, we argue that Barrow entropy correction [...] Read more.
Both classical and quantum arguments suggest that if Barrow entropy is correct, its index δ must be energy-dependent, which would affect the very early universe. Based on thermodynamic stability that sufficiently large black holes should not fragment, we argue that Barrow entropy correction must be small, except possibly at the Planckian regime. Furthermore, the fact that a solar mass black hole does not fragment implies an upper bound δO(103), which surprisingly lies in the same range as the bound obtained from some cosmological considerations assuming fixed δ. This indicates that allowing δ to run does not raise its allowed value. We briefly comment on the case of Kaniadakis entropy. Full article
(This article belongs to the Special Issue Recent Advances in Quantum Cosmology)
13 pages, 767 KiB  
Article
Revisiting a Realistic Intersecting D6-Brane with Modified Soft SUSY Terms
by Imtiaz Khan, Waqas Ahmed, Tianjun Li and Shabbar Raza
Universe 2024, 10(4), 176; https://doi.org/10.3390/universe10040176 - 11 Apr 2024
Viewed by 971
Abstract
Because there are a few typos in the supersymmetry-breaking sfermion masses and trilinear soft term, regarding the current Large Hadron Collider (LHC) and dark matter searches, we revisit a three-family Pati–Salam model based on intersecting D6-branes in Type IIA string theory on a [...] Read more.
Because there are a few typos in the supersymmetry-breaking sfermion masses and trilinear soft term, regarding the current Large Hadron Collider (LHC) and dark matter searches, we revisit a three-family Pati–Salam model based on intersecting D6-branes in Type IIA string theory on a T6/(Z2×Z2) orientifold with a realistic phenomenology. We study the viable parameter space and discuss the spectrum consistent with the current LHC Supersymmetry searches and the dark matter relic density bounds from the Planck 2018 data. For the gluinos and first two generations of sfermions, we observe that the gluino mass is in the range [2, 14] TeV, the squarks mass range is [2, 13] TeV and the sleptons mass is in the range [1, 5] TeV. We achieve the cold dark matter relic density consistent with 5σ Planck 2018 bounds via A-funnel and coannihilation channels such as stop–neutralino, stau–neutralino, and chargino–neutralino. Except for a few chargino–neutralino coannihilation solutions, these solutions satisfy current nucleon-neutralino spin-independent and spin-dependent scattering cross-sections and may be probed by future dark matter searches. Full article
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12 pages, 1503 KiB  
Article
Statistical Study on the Q Parameter Based on Parkes Data
by Xu Zhu, Hui Liu, Xinji Wu, Rushuang Zhao, Qijun Zhi, Shijun Dang, Lunhua Shang, Shuo Xiao, Hongwei Xu, Weilan Li, Ruwen Tian, Shidong Wang and Zefeng Tu
Universe 2024, 10(4), 175; https://doi.org/10.3390/universe10040175 - 11 Apr 2024
Viewed by 1113
Abstract
Using the rotating vector model (RVM) and aiming to constrain the value of the magnetic inclination angle (α), we perform a least-squares fit on the linearly polarized position angles of 125 pulsars from Parkes 64 m archive data at 1400 MHz. [...] Read more.
Using the rotating vector model (RVM) and aiming to constrain the value of the magnetic inclination angle (α), we perform a least-squares fit on the linearly polarized position angles of 125 pulsars from Parkes 64 m archive data at 1400 MHz. Subsequently, a statistical analysis of the normalized Q parameters is carried out. Furthermore, based on the Q-parameter, we provide a further understanding of the geometry of the radio emission region of the pulsar. In this statistical sample, about 1/5 of the sample is clustered at 0, suggesting that this part of the pulsar is viewed from the center of the radiation cone. For the rest of the pulsars, the Q parameters follow a uniform distribution, supporting the conclusion that the interface of the radiation cone is non-elliptical. Full article
(This article belongs to the Section Compact Objects)
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12 pages, 377 KiB  
Article
New Timing Results of MSPs from NICER Observations
by Shijie Zheng, Dawei Han, Heng Xu, Kejia Lee, Jianping Yuan, Haoxi Wang, Mingyu Ge, Liang Zhang, Yongye Li, Yitao Yin, Xiang Ma, Yong Chen and Shuangnan Zhang
Universe 2024, 10(4), 174; https://doi.org/10.3390/universe10040174 - 7 Apr 2024
Viewed by 1278
Abstract
Millisecond pulsars (MSPs) are known for their long-term stability. Using six years of observations from the Neutron Star Interior Composition Explorer (NICER), we have conducted an in-depth analysis of the X-ray timing results for six MSPs: PSRs B1937+21, B1821-24, J0437-4715, J0030+0451, J0218+4232, and [...] Read more.
Millisecond pulsars (MSPs) are known for their long-term stability. Using six years of observations from the Neutron Star Interior Composition Explorer (NICER), we have conducted an in-depth analysis of the X-ray timing results for six MSPs: PSRs B1937+21, B1821-24, J0437-4715, J0030+0451, J0218+4232, and J2124-3358. The timing stability parameter σz has been calculated, revealing remarkable timing precision on the order of 1014 for PSRs B1937+21 and J0437-4715, and 1013 for PSRs B1821-24, J0218+4232, and J0030+0451 over a timescale of 1000 days. These findings underscore the feasibility of autonomous in-orbit timekeeping using X-ray observations of MSPs. In addition, the consistency of long-term spin-down noise in the X-ray and radio bands has been investigated by comparison with IPTA radio data. Full article
(This article belongs to the Special Issue Pulsar Astronomy)
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33 pages, 1038 KiB  
Article
QED Meson Description of the Anomalous Particles at ∼17 and ∼38 MeV
by Cheuk-Yin Wong
Universe 2024, 10(4), 173; https://doi.org/10.3390/universe10040173 - 7 Apr 2024
Cited by 1 | Viewed by 1515
Abstract
The Schwinger confinement mechanism stipulates that a massless fermion and a massless antifermion are confined as a massive boson when they interact in the Abelian QED interaction in (1+1)D.If we approximate light quarks as massless and apply the Schwinger confinement mechanism to quarks, [...] Read more.
The Schwinger confinement mechanism stipulates that a massless fermion and a massless antifermion are confined as a massive boson when they interact in the Abelian QED interaction in (1+1)D.If we approximate light quarks as massless and apply the Schwinger confinement mechanism to quarks, we can infer that a light quark and a light antiquark interacting in the Abelian QED interaction are confined as a QED meson in (1+1)D. Similarly, a light quark and a light antiquark interacting in the QCD interaction in the quasi-Abelian approximation will be confined as a QCD meson in (1+1)D. The QED and QCD mesons in (1+1)D can represent physical mesons in (3+1)D when the flux tube radius is properly taken into account. Such a theory leads to a reasonable description of the masses of π0,η, and η, and its extrapolation to the unknown QED sector yields an isoscalar QED meson at about 17 MeV and an isovector QED meson at about 38 MeV. The observations of the anomalous soft photons, the hypothetical X17 particle, and the hypothetical E38 particle bear promising evidence for the possible existence of the QED mesons. Pending further confirmation, they hold important implications on the properties on the quarks and their interactions. Full article
(This article belongs to the Special Issue Multiparticle Dynamics)
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18 pages, 436 KiB  
Article
Landau Levels versus Hydrogen Atom
by Tekin Dereli, Philippe Nounahon and Todor Popov
Universe 2024, 10(4), 172; https://doi.org/10.3390/universe10040172 - 7 Apr 2024
Cited by 1 | Viewed by 1976
Abstract
The Landau problem and harmonic oscillator in the plane share a Hilbert space that carries the structure of Dirac’s remarkable so(2,3) representation. We show that the orthosymplectic algebra osp(1|4) is [...] Read more.
The Landau problem and harmonic oscillator in the plane share a Hilbert space that carries the structure of Dirac’s remarkable so(2,3) representation. We show that the orthosymplectic algebra osp(1|4) is the spectrum generating algebra for the Landau problem and, hence, for the 2D isotropic harmonic oscillator. The 2D harmonic oscillator is in duality with the 2D quantum Coulomb–Kepler systems, with the osp(1|4) symmetry broken down to the conformal symmetry so(2,3). The even so(2,3) submodule (coined Rac) generated from the ground state of zero angular momentum is identified with the Hilbert space of a 2D hydrogen atom. An odd element of the superalgebra osp(1|4) creates a pseudo-vacuum with intrinsic angular momentum 1/2 from the vacuum. The odd so(2,3)-submodule (coined Di) built upon the pseudo-vacuum is the Hilbert space of a magnetized 2D hydrogen atom: a quantum system of a dyon and an electron. Thus, the Hilbert space of the Landau problem is a direct sum of two massless unitary so(2,3) representations, namely, the Di and Rac singletons introduced by Flato and Fronsdal. Full article
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20 pages, 851 KiB  
Article
Search for Extreme Mass Ratio Inspirals Using Particle Swarm Optimization and Reduced Dimensionality Likelihoods
by Xiao-Bo Zou, Soumya D. Mohanty, Hong-Gang Luo and Yu-Xiao Liu
Universe 2024, 10(4), 171; https://doi.org/10.3390/universe10040171 - 6 Apr 2024
Viewed by 1250
Abstract
Extreme-mass-ratio inspirals (EMRIs) are significant observational targets for spaceborne gravitational wave detectors, namely, LISA, Taiji, and Tianqin, which involve the inspiral of stellar-mass compact objects into massive black holes (MBHs) with a mass range of approximately 104107M [...] Read more.
Extreme-mass-ratio inspirals (EMRIs) are significant observational targets for spaceborne gravitational wave detectors, namely, LISA, Taiji, and Tianqin, which involve the inspiral of stellar-mass compact objects into massive black holes (MBHs) with a mass range of approximately 104107M. EMRIs are estimated to produce long-lived gravitational wave signals with more than 105 cycles before plunge, making them an ideal laboratory for exploring the strong-gravity properties of the spacetimes around the MBHs, stellar dynamics in galactic nuclei, and properties of the MBHs itself. However, the complexity of the waveform model, which involves the superposition of multiple harmonics, as well as the high-dimensional and large-volume parameter space, make the fully coherent search challenging. In our previous work, we proposed a 10-dimensional search using Particle Swarm Optimization (PSO) with local maximization over the three initial angles. In this study, we extend the search to an 8-dimensional PSO with local maximization over both the three initial angles and the angles of spin direction of the MBH, where the latter contribute a time-independent amplitude to the waveforms. Additionally, we propose a 7-dimensional PSO search by using a fiducial value for the initial orbital frequency and shifting the corresponding 8-dimensional Time Delay Interferometry responses until a certain lag returns the corresponding 8-dimensional log-likelihood ratio’s maximum. The reduced dimensionality likelihoods enable us to successfully search for EMRI signals with a duration of 0.5 years and signal-to-noise ratio of 50 within a wider search range than our previous study. However, the ranges used by both the LISA Data Challenge (LDC) and Mock LISA Data Challenge (MLDC) to generate their simulated signals are still wider than the those we currently employ in our direct searches. Consequently, we discuss further developments, such as using a hierarchical search to narrow down the search ranges of certain parameters and applying Graphics Processing Units to speed up the code. These advances aim to improve the efficiency, accuracy, and generality of the EMRI search algorithm. Full article
(This article belongs to the Special Issue Newest Results in Gravitational Waves and Machine Learning)
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20 pages, 1110 KiB  
Article
Quantum Field Theory of Neutrino Mixing in Spacetimes with Torsion
by Antonio Capolupo, Giuseppe De Maria, Simone Monda, Aniello Quaranta and Raoul Serao
Universe 2024, 10(4), 170; https://doi.org/10.3390/universe10040170 - 3 Apr 2024
Cited by 4 | Viewed by 1431
Abstract
In the framework of quantum field theory, we analyze the neutrino oscillations in the presence of a torsion background. We consider the Einstein–Cartan theory and we study the cases of constant torsion and of linearly time-dependent torsion. We derive new neutrino oscillation formulae [...] Read more.
In the framework of quantum field theory, we analyze the neutrino oscillations in the presence of a torsion background. We consider the Einstein–Cartan theory and we study the cases of constant torsion and of linearly time-dependent torsion. We derive new neutrino oscillation formulae which depend on the spin orientation. Indeed, the energy splitting induced by the torsion influences oscillation amplitudes and frequencies. This effect is maximal for values of torsion of the same order of the neutrino masses and for very low momenta, and disappears for large values of torsion. Moreover, neutrino oscillation is inhibited for intensities of torsion term much larger than neutrino masses and momentum. The modifications induced by torsion on the CP-asymmetry are also presented. Future experiments, such as PTOLEMY, which have as a goal the analysis of the cosmological background of neutrino (which have very low momenta), can provide insights into the effect shown here. Full article
(This article belongs to the Special Issue Quantum Field Theory, 2nd Edition)
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19 pages, 2294 KiB  
Review
Neutrino at Different Epochs of the Friedmann Universe
by Alexandre V. Ivanchik, Oleg A. Kurichin and Vlad Yu. Yurchenko
Universe 2024, 10(4), 169; https://doi.org/10.3390/universe10040169 - 2 Apr 2024
Cited by 1 | Viewed by 1573
Abstract
At least two relics of the Big Bang have survived: the cosmological microwave background (CMB) and the cosmological neutrino background (CνB). Being the second most abundant particle in the universe, the neutrino has a significant impact on its evolution from the [...] Read more.
At least two relics of the Big Bang have survived: the cosmological microwave background (CMB) and the cosmological neutrino background (CνB). Being the second most abundant particle in the universe, the neutrino has a significant impact on its evolution from the Big Bang to the present day. Neutrinos affect the following cosmological processes: the expansion rate of the universe, its chemical and isotopic composition, the CMB anisotropy and the formation of the large-scale structure of the universe. Another relic neutrino background is theoretically predicted, it consists of non-equilibrium antineutrinos of Primordial Nucleosynthesis arising as a result of the decay of neutrons and tritium nuclei. Such antineutrinos are an indicator of the baryon asymmetry of the universe. In addition to experimentally detectable active neutrinos, the existence of sterile neutrinos is theoretically predicted to generate neutrino masses and explain their oscillations. Sterile neutrinos can also solve such cosmological problems as the baryonic asymmetry of the universe and the nature of dark matter. The recent results of several independent experiments point to the possibility of the existence of a light sterile neutrino. However, the existence of such a neutrino is inconsistent with the predictions of the Standard Cosmological Model. The inclusion of a non-zero lepton asymmetry of the universe and/or increasing the energy density of active neutrinos can eliminate these contradictions and reconcile the possible existence of sterile neutrinos with Primordial Nucleosynthesis, the CMB anisotropy, and also reduce the H0-tension. In this brief review, we discuss the influence of the physical properties of active and sterile neutrinos on the evolution of the universe from the Big Bang to the present day. Full article
(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)
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11 pages, 826 KiB  
Article
Checking the 8Be Anomaly with a Two-Arm Electron Positron Pair Spectrometer
by Tran The Anh, Tran Dinh Trong, Attila J. Krasznahorkay, Attila Krasznahorkay, József Molnár, Zoltán Pintye, Nguyen Ai Viet, Nguyen The Nghia, Do Thi Khanh Linh, Bui Thi Hoa, Le Xuan Chung and Nguyen Tuan Anh
Universe 2024, 10(4), 168; https://doi.org/10.3390/universe10040168 - 1 Apr 2024
Cited by 6 | Viewed by 1619
Abstract
We have repeated the experiment performed recently by ATOMKI Laboratory (Debrecen, Hungary), which may indicate a new particle called X17 in the literature. In order to obtain a reliable and independent result, we used a different structure of the electron–positron pair spectrometer at [...] Read more.
We have repeated the experiment performed recently by ATOMKI Laboratory (Debrecen, Hungary), which may indicate a new particle called X17 in the literature. In order to obtain a reliable and independent result, we used a different structure of the electron–positron pair spectrometer at the VNU University of Science. The spectrometer has two arms and simpler acceptance and efficiency as a function of the correlation angle, but the other conditions of the experiment were very similar to the published ones. We could confirm the presence of the anomaly measured at Ep = 1225 keV, which is above the Ep = 1040 keV resonance. Full article
(This article belongs to the Special Issue Multiparticle Dynamics)
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11 pages, 495 KiB  
Article
The 1932 Majorana Equation: A Forgotten but Surprisingly Modern Particle Theory
by Luca Nanni
Universe 2024, 10(4), 167; https://doi.org/10.3390/universe10040167 - 1 Apr 2024
Viewed by 1318
Abstract
The Standard Model is an up-to-date theory that best summarizes current knowledge in particle physics. Although some problems still remain open, it represents the leading model which all physicists refer to. One of the pillars which underpin the Standard Model is represented by [...] Read more.
The Standard Model is an up-to-date theory that best summarizes current knowledge in particle physics. Although some problems still remain open, it represents the leading model which all physicists refer to. One of the pillars which underpin the Standard Model is represented by the Lorentz invariance of the equations that form its backbone. These equations made it possible to predict the existence of particles and phenomena that experimental physics had not yet been able to detect. The first hint of formulating a fundamental theory of particles can be found in the 1932 Majorana equation, formulated when electrons and protons were the only known particles. Today we know that parts of the hypotheses set by Majorana were not correct, but his equation hid concepts that are found in the Standard Model. In this study, the Majorana equation is revisited and solved for free particles. The time-like, light-like and space-like solutions, represented by infinite-component wave functions, are discussed. Full article
18 pages, 349 KiB  
Article
Primordial Black Holes from Spatially Varying Cosmological Constant Induced by Field Fluctuations in Extra Dimensions
by Arkady A. Popov, Sergey G. Rubin and Alexander S. Sakharov
Universe 2024, 10(4), 166; https://doi.org/10.3390/universe10040166 - 31 Mar 2024
Cited by 1 | Viewed by 1518
Abstract
The origin and evolution of supermassive black holes (SMBHs) in our universe have sparked controversy. In this study, we explore the hypothesis that some of these black holes may have seeded from the direct collapse of dark energy domains with density significantly higher [...] Read more.
The origin and evolution of supermassive black holes (SMBHs) in our universe have sparked controversy. In this study, we explore the hypothesis that some of these black holes may have seeded from the direct collapse of dark energy domains with density significantly higher than the surrounding regions. The mechanism of the origin of such domains relies on the inflationary evolution of a scalar field acting in D dimensions, which is associated with the cosmological constant in our four-dimensional spacetime manifold. Inner space quantum fluctuations of the field during inflation are responsible for the spatial variations of the dark energy density in our space. This finding holds particular significance, especially considering recent evidence from pulsar timing array observations, which supports the existence of a stochastic gravitational wave background consisting of SMBH mergers. Full article
(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)
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18 pages, 1219 KiB  
Article
Elliptical Space with the McVittie Metrics
by Vladimir N. Yershov
Universe 2024, 10(4), 165; https://doi.org/10.3390/universe10040165 - 31 Mar 2024
Viewed by 1384
Abstract
The main feature of elliptical space—the topological identification of its antipodal points—could be fundamental for understanding the nature of the cosmological redshift. The physical interpretation of the mathematical (topological) structure of elliptical space is made by using physical connections in the form of [...] Read more.
The main feature of elliptical space—the topological identification of its antipodal points—could be fundamental for understanding the nature of the cosmological redshift. The physical interpretation of the mathematical (topological) structure of elliptical space is made by using physical connections in the form of Einstein-Rosen bridges (also called “wormholes”). The Schwarzschild metric of these structures embedded into a dynamic (expanding) spacetime corresponds to McVittie’s solution of Einstein’s field equations. The cosmological redshift of spectral lines of remote sources in this metric is a combination of gravitational redshift and the time-dependent scale factor of the Friedmann-Lemaitre-Robertson-Walker metric. I compare calculated distance moduli of type-Ia supernovae, which are commonly regarded as “standard candles” in cosmology, with the observational data published in the catalogue “Pantheon+”. The constraint based on these accurate data gives a much smaller expansion rate of the Universe than is currently assumed by modern cosmology, the major part of the cosmological redshift being gravitational by its nature. The estimated age of the Universe within the discussed model is 1.48·1012 yr, which is more than two orders of magnitude larger than the age assumed by using the standard cosmological model parameters. Full article
(This article belongs to the Section Cosmology)
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20 pages, 514 KiB  
Review
Standard and Non-Standard Aspects of Neutrino Physics
by Alessandro Granelli
Universe 2024, 10(4), 164; https://doi.org/10.3390/universe10040164 - 30 Mar 2024
Viewed by 1471
Abstract
This review provides a succinct overview of the basic aspects of neutrino physics. The topics covered include neutrinos in the standard model and the three-neutrino mixing scheme; the current status of neutrino oscillation measurements and what remains to be determined; the seesaw mechanisms [...] Read more.
This review provides a succinct overview of the basic aspects of neutrino physics. The topics covered include neutrinos in the standard model and the three-neutrino mixing scheme; the current status of neutrino oscillation measurements and what remains to be determined; the seesaw mechanisms for neutrino mass generation and the associated phenomenology, including the leptogenesis mechanism to explain the observed matter–antimatter asymmetry of the Universe; and models for the origin of the pattern of neutrino mixing and lepton masses based on discrete flavour symmetries and modular invariance. Full article
(This article belongs to the Special Issue Neutrinos across Different Energy Scales)
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33 pages, 7875 KiB  
Review
A Very-High-Energy Gamma-Ray View of the Transient Sky
by Alessandro Carosi and Alicia López-Oramas
Universe 2024, 10(4), 163; https://doi.org/10.3390/universe10040163 - 29 Mar 2024
Viewed by 1557
Abstract
The development of the latest generation of Imaging Atmospheric Cherenkov Telescopes (IACTs) over recent decades has led to the discovery of new extreme astrophysical phenomena in the very-high-energy (VHE, E > 100 GeV) gamma-ray regime. Time-domain and multi-messenger astronomy are inevitably connected to [...] Read more.
The development of the latest generation of Imaging Atmospheric Cherenkov Telescopes (IACTs) over recent decades has led to the discovery of new extreme astrophysical phenomena in the very-high-energy (VHE, E > 100 GeV) gamma-ray regime. Time-domain and multi-messenger astronomy are inevitably connected to the physics of transient VHE emitters, which show unexpected (and mostly unpredictable) flaring or exploding episodes at different timescales. These transients often share the physical processes responsible for the production of the gamma-ray emission, through cosmic-ray acceleration, magnetic reconnection, jet production and/or outflows, and shocks interactions. In this review, we present an up-to-date overview of the VHE transients field, spanning from novae to supernovae, neutrino counterparts or fast radio bursts, among others, and we outline the expectations for future facilities. Full article
(This article belongs to the Special Issue Recent Advances in Gamma Ray Astrophysics and Future Perspectives)
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23 pages, 1383 KiB  
Article
Dynamical Analysis of the Redshift Drift in FLRW Universes
by Francisco S. N. Lobo, José Pedro Mimoso, Jessica Santiago and Matt Visser
Universe 2024, 10(4), 162; https://doi.org/10.3390/universe10040162 - 29 Mar 2024
Cited by 2 | Viewed by 1171
Abstract
Redshift drift is the phenomenon whereby the observed redshift between an emitter and observer comoving with the Hubble flow in an expanding FLRW universe will slowly evolve—on a timescale comparable to the Hubble time. In a previous article, three of the current authors [...] Read more.
Redshift drift is the phenomenon whereby the observed redshift between an emitter and observer comoving with the Hubble flow in an expanding FLRW universe will slowly evolve—on a timescale comparable to the Hubble time. In a previous article, three of the current authors performed a cosmographic analysis of the redshift drift in an FLRW universe, temporarily putting aside the issue of dynamics (the Friedmann equations). In the current article, we add dynamics while still remaining within the framework of an exact FLRW universe. We developed a suitable generic matter model and applied it to both standard FLRW and various dark energy models. Furthermore, we present an analysis of the utility of alternative cosmographic variables to describe the redshift drift data. Full article
(This article belongs to the Section Cosmology)
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6 pages, 407 KiB  
Communication
Femtoscopy for the NAno-Plasmonic Laser Inertial Fusion Experiments (NAPLIFE) Project
by L. P. Csernai, T. Csörgő, I. Papp, K. Tamosiunas, M. Csete, A. Szenes, D. Vass, T. S. Biró and N. Kroó
Universe 2024, 10(4), 161; https://doi.org/10.3390/universe10040161 - 29 Mar 2024
Viewed by 1265
Abstract
Hanbury-Brown and Twiss analysis is used to determine the size and timespan of emitted particles. Here, we propose to adapt this method for laser-induced nanoplasmonic inertial confinement fusion to determine the parameters of emitted Deuterium and Helium4 nuclei. This communication is a [...] Read more.
Hanbury-Brown and Twiss analysis is used to determine the size and timespan of emitted particles. Here, we propose to adapt this method for laser-induced nanoplasmonic inertial confinement fusion to determine the parameters of emitted Deuterium and Helium4 nuclei. This communication is a short article that presents part of a larger study over multiple years. It presents a cutting edge method that is new in the field of Inertial Confinement Fusion. Full article
(This article belongs to the Special Issue Multiparticle Dynamics)
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25 pages, 2208 KiB  
Review
Measuring the Lense–Thirring Orbital Precession and the Neutron Star Moment of Inertia with Pulsars
by Huanchen Hu and Paulo C. C. Freire
Universe 2024, 10(4), 160; https://doi.org/10.3390/universe10040160 - 28 Mar 2024
Cited by 3 | Viewed by 1535
Abstract
Neutron stars (NSs) are compact objects that host the densest forms of matter in the observable universe, providing unique opportunities to study the behaviour of matter at extreme densities. While precision measurements of NS masses through pulsar timing have imposed effective constraints on [...] Read more.
Neutron stars (NSs) are compact objects that host the densest forms of matter in the observable universe, providing unique opportunities to study the behaviour of matter at extreme densities. While precision measurements of NS masses through pulsar timing have imposed effective constraints on the equation of state (EoS) of dense matter, accurately determining the radius or moment of inertia (MoI) of an NS remains a major challenge. This article presents a detailed review on measuring the Lense–Thirring (LT) precession effect in the orbit of binary pulsars, which would give access to the MoI of NSs and offer further constraints on the EoS. We discuss the suitability of certain classes of binary pulsars for measuring the LT precession from the perspective of binary star evolution and highlight five pulsars that exhibit properties promising to realise these goals in the near future. Finally, discoveries of compact binaries with shorter orbital periods hold the potential to greatly enhance measurements of the MoI of NSs. The MoI measurements of binary pulsars are pivotal to advancing our understanding of matter at supranuclear densities, as well as improving the precision of gravity tests, such as the orbital decay due to gravitational wave emission, and of tests of alternative gravity theories. Full article
(This article belongs to the Special Issue Studies in Neutron Stars)
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18 pages, 295 KiB  
Article
Boyer–Lindquist Space-Times and Beyond: Metamaterial Analogues for Arbitrary Space-Times
by Sebastian Schuster and Matt Visser
Universe 2024, 10(4), 159; https://doi.org/10.3390/universe10040159 - 28 Mar 2024
Cited by 10 | Viewed by 1320
Abstract
Analogue space-times (and in particular metamaterial analogue space-times) have a long varied and rather complex history. Much of the previous related work to this field has focused on spherically symmetric models; however, axial symmetry is much more relevant for mimicking astrophysically interesting systems [...] Read more.
Analogue space-times (and in particular metamaterial analogue space-times) have a long varied and rather complex history. Much of the previous related work to this field has focused on spherically symmetric models; however, axial symmetry is much more relevant for mimicking astrophysically interesting systems that are typically subject to rotation. Now it is well known that physically reasonable stationary axisymmetric space-times can, under very mild technical conditions, be put into Boyer–Lindquist form. Unfortunately, a metric presented in Boyer–Lindquist form is not well adapted to the “quasi-Cartesian” metamaterial analysis that we developed in our previous articles on “bespoke analogue space-times”. In the current article, we shall first focus specifically on various space-time metrics presented in Boyer–Lindquist form, and subsequently determine a suitable set of equivalent metamaterial susceptibility tensors in a laboratory setting. We shall then turn to analyzing generic space-times, not even necessarily stationary, again determining a suitable set of equivalent metamaterial susceptibility tensors. Perhaps surprisingly, we find that the well-known ADM formalism proves to be not particularly useful, and that it is instead the dual “threaded” (Kaluza–Klein–inspired) formalism that provides much more tractable results. While the background laboratory metric is (for mathematical simplicity and physical plausibility) always taken to be Riemann flat, we will allow for arbitrary curvilinear coordinate systems on the flat background space-time. Finally, for completeness, we shall reconsider spherically symmetric space-times, but now in general spherical polar coordinates rather than quasi-Cartesian coordinates. In summary, this article provides a set of general-purpose calculational tools that can readily be adapted for mimicking various interesting (curved) space-times by using nontrivial susceptibility tensors in general (background-flat) laboratory settings. Full article
(This article belongs to the Special Issue Analogue Gravity)
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