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Universe, Volume 8, Issue 2 (February 2022) – 82 articles

Cover Story (view full-size image): Regular rotating black holes and spinning solitons with de Sitter/phantom interiors are specifified by text. All of them have internal equatorial de Sitter vacuum disks. Some of them can have additional de Sitter vacuum surface incorporating de Sitter disk and phantom substance in cavities between them. In this case, the innermost circular orbits around spinning solitons exist within ergoregions related to phantom regions, and processes of energy extraction can involve phantom energy. Around a regular rotating black hole, there exists one co-rotating light ring, while around a spinning soliton, four light rings can exist. The innermost light rings around solitons and a certain class of black holes are stable. View this paper
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17 pages, 938 KiB  
Article
Excitation Function of Kinetic Freeze-Out Parameters at 6.3, 17.3, 31, 900 and 7000 GeV
by Muhammad Waqas, Abd Al Karim Haj Ismail, Muhammad Ajaz and Atef AbdelKader
Universe 2022, 8(2), 138; https://doi.org/10.3390/universe8020138 - 21 Feb 2022
Cited by 8 | Viewed by 1920
Abstract
The transverse momentum spectra of π+ (π)(π++π) at 6.3, 17.3, 31, 900 and 7000 GeV are analyzed by the blast-wave model with Tsallis statistics (TBW) in proton-proton collisions. We took the value of [...] Read more.
The transverse momentum spectra of π+ (π)(π++π) at 6.3, 17.3, 31, 900 and 7000 GeV are analyzed by the blast-wave model with Tsallis statistics (TBW) in proton-proton collisions. We took the value of flow profile n0 = 1 and 2 in order to see the difference in the results of the extracted parameters in the two cases. Different rapidity slices at 31 GeV are also analyzed, and the values of the related parameters, such as kinetic freeze-out temperature, transverse flow velocity and kinetic freeze-out volume, are obtained. The above parameters rise with the increase of collision energy, while at 31 GeV, they decrease with increasing rapidity, except for the kinetic freeze-out volume, which increases. We also extracted the parameter q, which is an entropy-based parameter, and its rising trend is noticed with increasing collision energy, while at 31 GeV, no specific dependence of q is observed on rapidity. In addition, the multiplicity parameter N0 and mean transverse momentum are extracted, which increase with increasing collision energy and decrease with increasing rapidity. We notice that the kinetic freeze-out temperature and mean transverse momentum are slightly larger with n0 = 2, while the transverse flow velocity is larger in the case of n0 = 1, but the difference is very small and hence insignificant. Full article
(This article belongs to the Special Issue Hydrodynamics and Thermodynamics in High Energy Physics)
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7 pages, 365 KiB  
Communication
Constraining MOdified Gravity with the S2 Star
by Riccardo Della Monica, Ivan de Martino and Mariafelicia de Laurentis
Universe 2022, 8(2), 137; https://doi.org/10.3390/universe8020137 - 21 Feb 2022
Cited by 10 | Viewed by 1480
Abstract
We have used publicly available kinematic data for the S2 star to constrain the parameter space of MOdified Gravity. Integrating geodesics and using a Markov Chain Monte Carlo algorithm, we have provided the first constraint on the scales of the Galactic Centre for [...] Read more.
We have used publicly available kinematic data for the S2 star to constrain the parameter space of MOdified Gravity. Integrating geodesics and using a Markov Chain Monte Carlo algorithm, we have provided the first constraint on the scales of the Galactic Centre for the parameter α of the theory, which represents the fractional increment of the gravitational constant G with respect to its Newtonian value. Namely, α0.662 at 99.7% confidence level (where α=0 reduces the theory to General Relativity). Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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15 pages, 324 KiB  
Article
Black Hole Surface Gravity in Doubly Special Relativity Geometries
by José Javier Relancio and Stefano Liberati
Universe 2022, 8(2), 136; https://doi.org/10.3390/universe8020136 - 21 Feb 2022
Cited by 7 | Viewed by 1479
Abstract
In a quantum gravity theory, spacetime at mesoscopic scales can acquire a novel structure very different from the classical concept of general relativity. A way to effectively characterize the quantum nature of spacetime is through a momentum dependent space-time metric. There is a [...] Read more.
In a quantum gravity theory, spacetime at mesoscopic scales can acquire a novel structure very different from the classical concept of general relativity. A way to effectively characterize the quantum nature of spacetime is through a momentum dependent space-time metric. There is a vast literature showing that this geometry is related to relativistic deformed kinematics, which is precisely a way to capture residual effects of a quantum gravity theory. In this work, we study the notion of surface gravity in a momentum dependent Schwarzschild black hole geometry. We show that using the two main notions of surface gravity in general relativity we obtain a momentum independent result. However, there are several definitions of surface gravity, all of them equivalent in general relativity when there is a Killing horizon. We show that in our scheme, despite the persistence of a Killing horizon, these alternative notions only agree in a very particular momentum basis, obtained in a previous work, so further supporting its physical relevance. Full article
(This article belongs to the Special Issue Beyond Riemannian Geometry in Classical and Quantum Gravity)
13 pages, 785 KiB  
Review
Recent Achievements of the ERNA Collaboration
by Raffaele Buompane, Antonino Di Leva, Lucio Gialanella, Gianluca Imbriani, Lizeth Morales-Gallegos and Mauro Romoli
Universe 2022, 8(2), 135; https://doi.org/10.3390/universe8020135 - 21 Feb 2022
Cited by 2 | Viewed by 1536 | Correction
Abstract
For more than two decades, the ERNA collaboration has investigated nuclear processes of astrophysical interest through the direct measurement of cross sections or the identification of the nucleosynthesis effects. Measurements of cross-section, reported in this publication, of radiative capture reactions have been mainly [...] Read more.
For more than two decades, the ERNA collaboration has investigated nuclear processes of astrophysical interest through the direct measurement of cross sections or the identification of the nucleosynthesis effects. Measurements of cross-section, reported in this publication, of radiative capture reactions have been mainly conducted using the ERNA Recoil Mass Separator, and more recently with an array of charged particle detector telescopes designed for nuclear astrophysics measurements. Some results achieved with ERNA will be reviewed, with a focus on the results most relevant for nucleosynthesis in AGB and advanced burning phases. Full article
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11 pages, 4444 KiB  
Article
Experimental Study of the Positive Ion Feedback from Gas to Liquid in a Dual-Phase Argon Chamber and Measurement of the Ion Mobility in Argon Gas
by Luciano Romero, Roberto Santorelli, Edgar Sánchez García, Thorsten Lux, Michael Leyton, Silvestro di Luise, Pablo García Abia, Rodrigo López Manzano, José Manuel Cela-Ruiz, Sebastián Quizhpi and Vicente Pesudo
Universe 2022, 8(2), 134; https://doi.org/10.3390/universe8020134 - 21 Feb 2022
Cited by 1 | Viewed by 1342
Abstract
The dynamics of the positive ions created by particle interactions inside argon time projection chambers plays an important role in characterizing the next generation of massive detectors planned for the direct search for dark matter and the study of neutrino properties. We have [...] Read more.
The dynamics of the positive ions created by particle interactions inside argon time projection chambers plays an important role in characterizing the next generation of massive detectors planned for the direct search for dark matter and the study of neutrino properties. We have constructed a 1 L liquid argon chamber (ARION: ARgon ION experiment) with a high voltage pulse generator capable of injecting, in a controlled manner, a sizeable ion current into the drift region. This chamber is capable of reproducing a volume charge similar to that found in large detectors, allowing its effects to be studied systematically. New experimental results regarding ion dynamics in the liquid and direct demonstration of ion feedback from the gas to the liquid are discussed in this paper. In addition, a novel technique to measure the drift velocity of argon ions is introduced along with preliminary results obtained in gas. Full article
(This article belongs to the Special Issue Studying the Universe from Spain)
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16 pages, 1054 KiB  
Article
The Equation of State of Neutron-Rich Matter at Fourth Order of Chiral Effective Field Theory and the Radius of a Medium-Mass Neutron Star
by Francesca Sammarruca and Randy Millerson
Universe 2022, 8(2), 133; https://doi.org/10.3390/universe8020133 - 19 Feb 2022
Cited by 9 | Viewed by 1947
Abstract
We report neutron star predictions based on our most recent equations of state. These are derived from chiral effective field theory, which allows for a systematic development of nuclear forces, order by order. We utilize high-quality two-nucleon interactions and include all three-nucleon forces [...] Read more.
We report neutron star predictions based on our most recent equations of state. These are derived from chiral effective field theory, which allows for a systematic development of nuclear forces, order by order. We utilize high-quality two-nucleon interactions and include all three-nucleon forces up to fourth order in the chiral expansion. Our ab initio predictions are restricted to the domain of applicability of chiral effective field theory. However, stellar matter in the interior of neutron stars can be up to several times denser than normal nuclear matter at saturation, and its composition is essentially unknown. Following established practices, we extend our microscopic predictions to higher densities matching piecewise polytropes. The radius of the average-size neutron star, about 1.4 solar masses, is sensitive to the pressure at normal densities, and thus it is suitable to constrain ab initio theories of the equation of state. For this reason, we focus on the radius of medium-mass stars. We compare our results with other theoretical predictions and recent constraints. Full article
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19 pages, 350 KiB  
Article
Gravitational Waves: The Theorist’s Swiss Knife
by Mairi Sakellariadou
Universe 2022, 8(2), 132; https://doi.org/10.3390/universe8020132 - 19 Feb 2022
Cited by 4 | Viewed by 1441
Abstract
Gravitational waves provide a novel and powerful way to test astrophysical models of compact objects, early universe processes, beyond the Standard Model particle physics, dark matter candidates, Einstein’s theory of General Relativity and extended gravity models, and even quantum gravity candidate theories. A [...] Read more.
Gravitational waves provide a novel and powerful way to test astrophysical models of compact objects, early universe processes, beyond the Standard Model particle physics, dark matter candidates, Einstein’s theory of General Relativity and extended gravity models, and even quantum gravity candidate theories. A short introduction to the gravitational-wave background and the method we are using to detect it will be presented. Constraints on various astrophysical/cosmological models from the non-detectability of the gravitational-wave background will be discussed. Gravitational waves from transients will be highlighted and their physical implications will be summarised. Full article
(This article belongs to the Special Issue The Quantum & The Gravity)
17 pages, 375 KiB  
Article
Spatially Hyperbolic Gravitating Sources in Λ-Dominated Era
by Z. Yousaf
Universe 2022, 8(2), 131; https://doi.org/10.3390/universe8020131 - 18 Feb 2022
Cited by 30 | Viewed by 1372
Abstract
This study focuses on the impact of the cosmological constant on hyperbolically symmetric matter configurations in a static background. I extend the work of Herrera et al. 2021. and describe the influences of such a repulsive character on a few realistic features of [...] Read more.
This study focuses on the impact of the cosmological constant on hyperbolically symmetric matter configurations in a static background. I extend the work of Herrera et al. 2021. and describe the influences of such a repulsive character on a few realistic features of hyperbolical anisotropic fluids. After describing the Einstein-Λ equations of motion, I elaborate the corresponding mass function along with its conservation laws. In our study, besides observing negative energy density, I notice the formation of a Minkowskian core as matter content is compelled not to follow inward motion near the axis of symmetry. Three families of solutions are found in the Λ-dominated epoch. The first is calculated by keeping the Weyl scalar to a zero value, while the second solution maintains zero complexity in the subsequent changes of the hyperbolical compact object. However, the last model encompasses stiff fluid within the self-gravitating system. Such a type of theoretical setup suggests its direct link to study a few particular quantum scenarios where negative behavior of energy density is noticed at the Λ-dominated regime. Full article
(This article belongs to the Section Gravitation)
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16 pages, 2066 KiB  
Article
The RADIOSTAR Project
by Maria Lugaro, Benoit Côté, Marco Pignatari, Andrés Yagüe López, Hannah Brinkman, Borbála Cseh, Jacqueline Den Hartogh, Carolyn Louise Doherty, Amanda Irene Karakas, Chiaki Kobayashi, Thomas Lawson, Mária Pető, Benjámin Soós, Thomas Trueman and Blanka Világos
Universe 2022, 8(2), 130; https://doi.org/10.3390/universe8020130 - 17 Feb 2022
Cited by 2 | Viewed by 1659
Abstract
Radioactive nuclei are the key to understanding the circumstances of the birth of our Sun because meteoritic analysis has proven that many of them were present at that time. Their origin, however, has been so far elusive. The ERC-CoG-2016 RADIOSTAR project is dedicated [...] Read more.
Radioactive nuclei are the key to understanding the circumstances of the birth of our Sun because meteoritic analysis has proven that many of them were present at that time. Their origin, however, has been so far elusive. The ERC-CoG-2016 RADIOSTAR project is dedicated to investigating the production of radioactive nuclei by nuclear reactions inside stars, their evolution in the Milky Way Galaxy, and their presence in molecular clouds. So far, we have discovered that: (i) radioactive nuclei produced by slow (107Pd and 182Hf) and rapid (129I and 247Cm) neutron captures originated from stellar sources —asymptotic giant branch (AGB) stars and compact binary mergers, respectively—within the galactic environment that predated the formation of the molecular cloud where the Sun was born; (ii) the time that elapsed from the birth of the cloud to the birth of the Sun was of the order of 107 years, and (iii) the abundances of the very short-lived nuclei 26Al, 36Cl, and 41Ca can be explained by massive star winds in single or binary systems, if these winds directly polluted the early Solar System. Our current and future work, as required to finalise the picture of the origin of radioactive nuclei in the Solar System, involves studying the possible origin of radioactive nuclei in the early Solar System from core-collapse supernovae, investigating the production of 107Pd in massive star winds, modelling the transport and mixing of radioactive nuclei in the galactic and molecular cloud medium, and calculating the galactic chemical evolution of 53Mn and 60Fe and of the p-process isotopes 92Nb and 146Sm. Full article
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40 pages, 565 KiB  
Article
Observables in Quantum Mechanics and the Importance of Self-Adjointness
by Tajron Jurić
Universe 2022, 8(2), 129; https://doi.org/10.3390/universe8020129 - 17 Feb 2022
Cited by 11 | Viewed by 2126
Abstract
We are focused on the idea that observables in quantum physics are a bit more then just hermitian operators and that this is, in general, a “tricky business”. The origin of this idea comes from the fact that there is a subtle difference [...] Read more.
We are focused on the idea that observables in quantum physics are a bit more then just hermitian operators and that this is, in general, a “tricky business”. The origin of this idea comes from the fact that there is a subtle difference between symmetric, hermitian, and self-adjoint operators which are of immense importance in formulating Quantum Mechanics. The theory of self-adjoint extensions is presented through several physical examples and some emphasis is given on the physical implications and applications. Full article
(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)
25 pages, 882 KiB  
Review
Trojan Horse Investigation for AGB Stellar Nucleosynthesis
by Maria Letizia Sergi, Giuseppe D’Agata, Giovanni Luca Guardo, Giuseppe Gabriele Rapisarda, Vaclav Burjan, Silvio Cherubini, Marisa Gulino, Iolanda Indelicato, Marco La Cognata, Livio Lamia, Dario Lattuada, Jaromir Mrázek, Alessandro Alberto Oliva, Rosario Gianluca Pizzone, Stefano Romano, Roberta Spartá, Oscar Trippella and Aurora Tumino
Universe 2022, 8(2), 128; https://doi.org/10.3390/universe8020128 - 16 Feb 2022
Cited by 3 | Viewed by 2090
Abstract
Asymptotic Giant Branch (AGB) stars are among the most important astrophysical sites influencing the nucleosynthesis and the chemical abundances in the Universe. From a pure nuclear point of view, several processes take part during this peculiar stage of stellar evolution thus requiring detailed [...] Read more.
Asymptotic Giant Branch (AGB) stars are among the most important astrophysical sites influencing the nucleosynthesis and the chemical abundances in the Universe. From a pure nuclear point of view, several processes take part during this peculiar stage of stellar evolution thus requiring detailed experimental cross section measurements. Here, we report on the most recent results achieved via the application of the Trojan Horse Method (THM) and Asymptotic Normalization Coefficient (ANC) indirect techniques, discussing the details of the experimental procedure and the deduced reaction rates. In addition, we report also on the on going studies of interest for AGB nucleosynthesis. Full article
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22 pages, 586 KiB  
Article
Renormalizing Open Quantum Field Theories
by Sandor Nagy and Janos Polonyi
Universe 2022, 8(2), 127; https://doi.org/10.3390/universe8020127 - 16 Feb 2022
Cited by 4 | Viewed by 1829
Abstract
The functional renormalization group flow of a scalar field theory with quartic couplings and a sharp spatial momentum cutoff is presented in four-dimensional Minkowski space-time for the bare action by retaining the entanglement of the IR and the UV particle modes. It is [...] Read more.
The functional renormalization group flow of a scalar field theory with quartic couplings and a sharp spatial momentum cutoff is presented in four-dimensional Minkowski space-time for the bare action by retaining the entanglement of the IR and the UV particle modes. It is argued that the open interaction channels have to be taken into account in quantum field theory defined by the help of a cutoff, and a non-perturbative UV-IR entanglement is found in closed or almost closed models. Full article
(This article belongs to the Special Issue Quantum Field Theory of Open Systems)
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13 pages, 951 KiB  
Article
Palatini f(R) Gravity and Variants of k-/Constant Roll/Warm Inflation within Variation of Strong Coupling Scenario
by Mahmoud AlHallak, Amer AlRakik, Nidal Chamoun and Moustafa Sayem El-Daher
Universe 2022, 8(2), 126; https://doi.org/10.3390/universe8020126 - 15 Feb 2022
Cited by 13 | Viewed by 1639
Abstract
We show that upon applying Palatini f(R), characterized by an αR2 term, within a scenario motivated by a temporal variation of strong coupling constant, then one obtains a quadratic kinetic energy. We do not drop this term, [...] Read more.
We show that upon applying Palatini f(R), characterized by an αR2 term, within a scenario motivated by a temporal variation of strong coupling constant, then one obtains a quadratic kinetic energy. We do not drop this term, but rather study two extreme cases: α<<1 and α>>1. In both cases, one can generate a kinematically-induced inflationary paradigm. In order to fit the Planck 2018 data, the α>>1 case, called k-inflation, requires a fine tuning adjustment with nonvanishing nonminimal coupling to gravity parameter ξ, whereas the α<<1 case, studied in the constant-roll regime, can fit the data for vanishing ξ. The varying strong coupling inflation scenario remains viable when implemented through a warm inflation scenario with or without f(R) gravity. Full article
(This article belongs to the Special Issue Modified Theories of Gravity and Cosmological Applications)
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14 pages, 5371 KiB  
Article
Performance of NeQuick-2 and IRI-Plas 2017 Models during Solar Maximum Years in 2013–2014 over Equatorial and Low Latitude Regions
by Kenneth Iluore, Jianyong Lu, Francisca Okeke and Kesyton Oyamenda Ozegin
Universe 2022, 8(2), 125; https://doi.org/10.3390/universe8020125 - 13 Feb 2022
Cited by 2 | Viewed by 2200
Abstract
This paper carries out a comparative investigation of the Total Electron Content (TEC) values calculated by using the NeQuick-2 and IRI-Plas 2017 models. The investigation was carried out for the solar maximum year of 2013–2014 with data from eight GPS stations within the [...] Read more.
This paper carries out a comparative investigation of the Total Electron Content (TEC) values calculated by using the NeQuick-2 and IRI-Plas 2017 models. The investigation was carried out for the solar maximum year of 2013–2014 with data from eight GPS stations within the equatorial and low latitude regions. The results show that both models agree quite well with the observed TEC values obtained from GPS measurements in all the stations, although with some overestimations and underestimations observed during the daytime and nighttime hours. The NeQuick-2 model, in general, performed better in months, seasons, and in most of the stations when the IRI-Plas overestimates the GPS-TEC. However, it is interesting to know that with an increase in solar activity in some seasons, the quality of forecasting IRI-Plas can improve, whereas for the NeQuick-2 model, it decreases, but this is not true for all the seasons and all the stations. Factors causing the discrepancies in the IRI-Plas data model might be caused by the plasmaspheric part included in the IRI, and it is found to be maximum at the MBAR (34%) station, whereas that of the NeQuick-2 data model is found to be maximum at the ADIS (47.7%) station. There is a latitudinal dependence for both models in which the prediction error decreases with the increasing latitudes. Full article
(This article belongs to the Special Issue Space Weather in the Sun–Earth System)
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9 pages, 380 KiB  
Article
Statistical Thermal Model for Particle Production in pp Collisions at RHIC and LHC Energies
by Jiayu Chen, Baochun Li, Xinjian Wen and Hongwei Dong
Universe 2022, 8(2), 124; https://doi.org/10.3390/universe8020124 - 13 Feb 2022
Cited by 1 | Viewed by 1660
Abstract
In this paper, an improved multi-source thermal model is used to analyze the transverse momentum spectra in pp collisions at high energies, ranging from sNN = 62.4 GeV to 7 TeV. Via a detailed comparison between the model results and experimental [...] Read more.
In this paper, an improved multi-source thermal model is used to analyze the transverse momentum spectra in pp collisions at high energies, ranging from sNN = 62.4 GeV to 7 TeV. Via a detailed comparison between the model results and experimental data at RHIC and LHC energies, the thermodynamic properties of particle production are decided. It is shown that the excitation factors of emission sources depend linearly on lnsNN in the framework. Based on the variation regularity of the source excitation factors, transverse momentum spectra are predicted in pp collisions at higher energies; potential future pp colliders operating at sNN = 33 and 100 TeV. Full article
(This article belongs to the Special Issue Hydrodynamics and Thermodynamics in High Energy Physics)
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13 pages, 18868 KiB  
Review
Low Energy Antimatter Physics
by Marco Giammarchi and Giuseppe Vinelli
Universe 2022, 8(2), 123; https://doi.org/10.3390/universe8020123 - 13 Feb 2022
Viewed by 1830
Abstract
We will review the motivations and the general features of experiments devoted to testing fundamental laws with antimatter at low energies, namely the study of CPT invariance and the Weak Equivalence Principle. A summary of the recent experimental results will be presented. Full article
(This article belongs to the Section High Energy Nuclear and Particle Physics)
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26 pages, 7708 KiB  
Review
RR Lyrae and Type II Cepheid Variables in Globular Clusters: Optical and Infrared Properties
by Anupam Bhardwaj
Universe 2022, 8(2), 122; https://doi.org/10.3390/universe8020122 - 13 Feb 2022
Cited by 15 | Viewed by 2764
Abstract
Globular clusters are both primary fossils of galactic evolution and formation and are ideal laboratories for constraining the evolution of low-mass and metal-poor stars. RR Lyrae and type II Cepheid variables are low-mass, radially pulsating stars that trace old-age stellar populations. These stellar [...] Read more.
Globular clusters are both primary fossils of galactic evolution and formation and are ideal laboratories for constraining the evolution of low-mass and metal-poor stars. RR Lyrae and type II Cepheid variables are low-mass, radially pulsating stars that trace old-age stellar populations. These stellar standard candles in globular clusters are crucial for measuring their precise distances and, in turn, absolute ages, and for the calibration of the extragalactic distance scale. Herein, the evolutionary stages of RR Lyrae and type II Cepheids are discussed, and their pulsation properties, including the light curves, color–magnitude and period–amplitude diagrams, and period–luminosity relations in globular clusters at optical and infrared wavelengths are presented. The RR Lyrae visual magnitude–metallicity relation and the multiband period–luminosity–metallicity relations in globular clusters covering a wide metallicity range are also discussed in detail for their application to the RR Lyrae-based distance scale. Full article
(This article belongs to the Special Issue Recent Advances in Pulsating Stars)
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16 pages, 349 KiB  
Article
Strongly Nonlinear Diffusion in Turbulent Environment: A Problem with Infinitely Many Couplings
by Nikolay V. Antonov, Andrew A. Babakin and Polina I. Kakin
Universe 2022, 8(2), 121; https://doi.org/10.3390/universe8020121 - 13 Feb 2022
Cited by 3 | Viewed by 1498
Abstract
The field theoretic renormalization group is applied to the strongly nonlinear stochastic advection-diffusion equation. The turbulent advection is modelled by the Kazantsev–Kraichnan “rapid-change” ensemble. As a requirement of the renormalizability, the model necessarily involves infinite number of coupling constants (“charges”). The one-loop counterterm [...] Read more.
The field theoretic renormalization group is applied to the strongly nonlinear stochastic advection-diffusion equation. The turbulent advection is modelled by the Kazantsev–Kraichnan “rapid-change” ensemble. As a requirement of the renormalizability, the model necessarily involves infinite number of coupling constants (“charges”). The one-loop counterterm is calculated explicitly. The corresponding renormalization group equation demonstrates existence of a pair of two-dimensional surfaces of fixed points in the infinite-dimensional parameter space. If the surfaces contain infrared attractive regions, the problem allows for the large-scale, long-time scaling behaviour. For the first surface (advection is irrelevant), the critical dimensions of the scalar field Δθ, the response field Δθ and the frequency Δω are nonuniversal (through the dependence on the effective couplings) but satisfy certain exact identities. For the second surface (advection is relevant), the dimensions are universal and they are found exactly. Full article
(This article belongs to the Section Mathematical Physics)
23 pages, 914 KiB  
Review
Observational Cosmology with Artificial Neural Networks
by Juan de Dios Rojas Olvera, Isidro Gómez-Vargas and Jose Alberto Vázquez
Universe 2022, 8(2), 120; https://doi.org/10.3390/universe8020120 - 12 Feb 2022
Cited by 8 | Viewed by 3586
Abstract
In cosmology, the analysis of observational evidence is very important when testing theoretical models of the Universe. Artificial neural networks are powerful and versatile computational tools for data modelling and have recently been considered in the analysis of cosmological data. The main goal [...] Read more.
In cosmology, the analysis of observational evidence is very important when testing theoretical models of the Universe. Artificial neural networks are powerful and versatile computational tools for data modelling and have recently been considered in the analysis of cosmological data. The main goal of this paper is to provide an introduction to artificial neural networks and to describe some of their applications to cosmology. We present an overview on the fundamentals of neural networks and their technical details. Through three examples, we show their capabilities in the modelling of cosmological data, numerical tasks (saving computational time), and the classification of stellar objects. Artificial neural networks offer interesting qualities that make them viable alternatives for data analysis in cosmological research. Full article
(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)
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17 pages, 333 KiB  
Article
Two Approaches to Hamiltonian Bigravity
by Vladimir Olegovich Soloviev
Universe 2022, 8(2), 119; https://doi.org/10.3390/universe8020119 - 12 Feb 2022
Cited by 1 | Viewed by 1538
Abstract
Bigravity is one of the most natural modifications of General Relativity (GR), as it is based on the equivalence principle. However, its canonical structure appears rather complicated because of the unusual form of the interaction between two metrics. As a consequence, there are [...] Read more.
Bigravity is one of the most natural modifications of General Relativity (GR), as it is based on the equivalence principle. However, its canonical structure appears rather complicated because of the unusual form of the interaction between two metrics. As a consequence, there are different approaches that are difficult to compare in detail. This work is a first attempt to obtain a synthetic picture of the Hamiltonian formalism for bigravity. Here, we are trying to combine two rather different approaches to gain a binocular view of the theory. The first publications on the subject were based on metric formalism. It was proved that both massive gravity and bigravity with de Rham–Gabadadze–Tolley (dRGT) potential were free of Boulware–Deser (BD) ghosts. This proof was based on the transformation of variables involving a 3×3-matrix which could be treated as the root of a quadratic equation involving two spatial metrics and a new 3-vector introduced instead of the standard shift variable. Therefore, this matrix occurred as an implicit function of the abovementioned variables. After a substantial amount of time, it became possible to calculate the algebra of constraints in full using this method. However, in another approach also based on metric variables and implicit functions, similar calculations were completed earlier. It is not a new matrix, but the potential itself has been taken as an implicit function of two spatial metrics and four functions constructed of two pairs of lapses and shifts. Finally, a straightforward route to canonical bigravity is to apply tetrad (or vierbein) variables. The matrix square root involved in the dRGT potential can be explicitly extracted if tetrads fulfill the symmetry condition. A full treatment has been developed in first-order formalism by treating tetrads and connections as independent variables. In that case, the theory contains many more variables and constraints than in metric formalism. An essential simplification occurs in second-order vierbein formalism. The potential is given explicitly as a polynomial of bilinear combinations of the two tetrads. The 3×3-matrix introduced in the pioneer papers can be expressed explicitly through canonical coordinates, and the celebrated transformation of variables arises in the Dirac constraint analysis. Full article
(This article belongs to the Special Issue Advances in Cosmology and Subatomic Particle Physics)
7 pages, 526 KiB  
Article
Impact of Warm Dark Matter on the Cosmic Neutrino Background Anisotropies
by Christopher G. Tully and Gemma Zhang
Universe 2022, 8(2), 118; https://doi.org/10.3390/universe8020118 - 12 Feb 2022
Cited by 3 | Viewed by 1870
Abstract
The Cosmic Neutrino Background (CνB) anisotropies for massive neutrinos are a unique probe of large-scale structure formation. The redshift-distance measure is completely different for massive neutrinos as compared to electromagnetic radiation. The CνB anisotropies in massive neutrinos grow in [...] Read more.
The Cosmic Neutrino Background (CνB) anisotropies for massive neutrinos are a unique probe of large-scale structure formation. The redshift-distance measure is completely different for massive neutrinos as compared to electromagnetic radiation. The CνB anisotropies in massive neutrinos grow in response to non-relativistic motion in gravitational potentials seeded by relatively high k-modes. Differences in the early phases of large-scale structure formation in warm dark matter (WDM) versus cold dark matter (CDM) cosmologies have an impact on the magnitude of the CνB anisotropies for contributions to the angular power spectrum that peak at high k-modes. We take the examples of WDM consisting of 2, 3, or 7 keV sterile neutrinos and show that the CνB anisotropies for 0.05 eV neutrinos drop off at high-l multipole moment in the angular power spectrum relative to CDM. At the same angular scales that one can observe baryonic acoustical oscillations in the CMB, the CνB anisotropies begin to become sensitive to differences in WDM and CDM cosmologies. The precision measurement of high-l multipoles in the CνB neutrino sky map is a potential possibility for the PTOLEMY experiment with thin film targets of spin-polarized atomic tritium superfluid that exhibit significant quantum liquid amplification for non-relativistic relic neutrino capture. Full article
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9 pages, 280 KiB  
Article
On Emergent Particles and Stable Neutral Plasma Balls in SU(2) Yang-Mills Thermodynamics
by Ralf Hofmann and Thierry Grandou
Universe 2022, 8(2), 117; https://doi.org/10.3390/universe8020117 - 12 Feb 2022
Cited by 5 | Viewed by 4283
Abstract
For a pure SU(2) Yang–Mills theory in 4D, we revisit the spatial (3D), ball-like region of radius r0 in its bulk subject to the pressureless, deconfining phase at T0=1.32Tc, where Tc denotes the critical temperature [...] Read more.
For a pure SU(2) Yang–Mills theory in 4D, we revisit the spatial (3D), ball-like region of radius r0 in its bulk subject to the pressureless, deconfining phase at T0=1.32Tc, where Tc denotes the critical temperature for the onset of the deconfining–preconfining phase transition. Such a region possesses finite energy density and represents the self-intersection of a figure-eight shaped center-vortex loop if a BPS monopole of core radius ∼r052.4, isolated from its antimonopole by repulsion externally invoked through a transient shift of (anti)caloron holonomy (pair creation), is trapped therein. The entire soliton (vortex line plus region of self-intersection of mass m0 containing the monopole) can be considered an excitation of the pressureless and energyless ground state of the confining phase. Correcting an earlier estimate of r0, we show that the vortex-loop self-intersection region associates to the central part of a(n) (anti)caloron and that this region carries one unit of electric U(1) charge via the (electric-magnetic dually interpreted) charge of the monopole. The monopole core quantum vibrates at a thermodynamically determined frequency ω0 and is unresolved. For a deconfining-phase plasma oscillation about the zero-pressure background at T=T0, we compute the lowest frequency Ω0 within a neutral and homogeneous spatial ball (no trapped monopole) in dependence of its radius R0. For R0=r0 a comparison of Ω0 with ω0 reveals that the neutral plasma oscillates much slower than the same plasma driven by the oscillation of a monopole core. Full article
(This article belongs to the Special Issue Quantum Field Theory)
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8 pages, 271 KiB  
Article
The Laplace Transform of Quantum Gravity
by Jorge Gamboa, Fernando Méndez and Natalia Tapia-Arellano
Universe 2022, 8(2), 116; https://doi.org/10.3390/universe8020116 - 12 Feb 2022
Viewed by 1904
Abstract
Following analogies with relativistic point particles and Schild strings, we show that the Einstein gravity and its strong coupling regime (or the Planck mass going to zero) are related to each other through a Laplace transform. The Feynman propagator of gravity in the [...] Read more.
Following analogies with relativistic point particles and Schild strings, we show that the Einstein gravity and its strong coupling regime (or the Planck mass going to zero) are related to each other through a Laplace transform. The Feynman propagator of gravity in the strong coupling regime satisfies a functional diffusion equation in the three-metric space with the evolution parameter being the volume of spacetime. We conjecture that the relationship between both regimes is consistent with the existence of an evolution operator in which time is replaced by the volume of spacetime. Full article
(This article belongs to the Special Issue Beyond Riemannian Geometry in Classical and Quantum Gravity)
22 pages, 354 KiB  
Article
Geodesics for the Painlevé–Gullstrand Form of Lense–Thirring Spacetime
by Joshua Baines, Thomas Berry, Alex Simpson and Matt Visser
Universe 2022, 8(2), 115; https://doi.org/10.3390/universe8020115 - 10 Feb 2022
Cited by 13 | Viewed by 1512
Abstract
Recently, the current authors have formulated and extensively explored a rather novel Painlevé–Gullstrand variant of the slow-rotation Lense–Thirring spacetime, a variant which has particularly elegant features—including unit lapse, intrinsically flat spatial 3-slices, and a separable Klein–Gordon equation (wave operator). This spacetime also possesses [...] Read more.
Recently, the current authors have formulated and extensively explored a rather novel Painlevé–Gullstrand variant of the slow-rotation Lense–Thirring spacetime, a variant which has particularly elegant features—including unit lapse, intrinsically flat spatial 3-slices, and a separable Klein–Gordon equation (wave operator). This spacetime also possesses a non-trivial Killing tensor, implying separability of the Hamilton–Jacobi equation, the existence of a Carter constant, and complete formal integrability of the geodesic equations. Herein, we investigate the geodesics in some detail, in the general situation demonstrating the occurrence of “ultra-elliptic” integrals. Only in certain special cases can the complete geodesic integrability be explicitly cast in terms of elementary functions. The model is potentially of astrophysical interest both in the asymptotic large-distance limit and as an example of a “black hole mimic”, a controlled deformation of the Kerr spacetime that can be contrasted with ongoing astronomical observations. Full article
(This article belongs to the Section Gravitation)
23 pages, 355 KiB  
Article
Dynamical Symmetry and the Thermofield State at Large N
by Antal Jevicki, Xianlong Liu, Junggi Yoon and Junjie Zheng
Universe 2022, 8(2), 114; https://doi.org/10.3390/universe8020114 - 10 Feb 2022
Cited by 6 | Viewed by 1471
Abstract
We discuss thermofield double QFT at real time, in the large N limit. First, we establish a (dynamical) symmetry, which we argue holds in general for the real-time portion of the Schwinger–Kelydish contour. At large N, this symmetry is seen to generate [...] Read more.
We discuss thermofield double QFT at real time, in the large N limit. First, we establish a (dynamical) symmetry, which we argue holds in general for the real-time portion of the Schwinger–Kelydish contour. At large N, this symmetry is seen to generate a one-parameter degeneracy of stationary collective solutions. The construction is explicitly worked out on an example of the O(N) vector QFT. As a nontrivial application, we describe the construction of the corresponding (large N) thermofield double state in real-time collective formalism. Full article
(This article belongs to the Special Issue Quantum Field Theory of Open Systems)
24 pages, 1670 KiB  
Article
The Effect of Age on the Grouping of Open Clusters: The Primordial Group Hypothesis
by Juan Casado
Universe 2022, 8(2), 113; https://doi.org/10.3390/universe8020113 - 10 Feb 2022
Cited by 6 | Viewed by 1895
Abstract
The Primordial Group hypothesis states that only sufficiently young open clusters (OCs) can be multiple, and old OCs are essentially isolated. We tested this postulate through four different studies using a manual search of Gaia EDR3 and extensive literature. First, we revisited the [...] Read more.
The Primordial Group hypothesis states that only sufficiently young open clusters (OCs) can be multiple, and old OCs are essentially isolated. We tested this postulate through four different studies using a manual search of Gaia EDR3 and extensive literature. First, we revisited the work of de La Fuente Marcos and de La Fuente Marcos (2009), which states that only ca. 40% of OC pairs are of primordial origin. However, no plausible binary system among their proposed OC pairs having at least one member older than 0.1 Gyr was found. Second, we researched the OCs < 0.01 Gyr old in Tarricq et al. (2021) and found that ca. 71% of them remain in their primordial groups. Third, a similar study of the oldest OCs (age > 4 Gyr) showed that they are essentially alone. Forth, the well-known case of the double cluster in Perseus and some other binary systems described in the literature were also shown to accommodate the title hypothesis. A simplified bimodal model allows for retrieval of the overall fraction of related OCs (approximately 12–16%) from our results, assuming that young clusters remain associated at ~0.04 Gyr. The obtained results further support that OCs are born in groups (Casado 2021). Full article
(This article belongs to the Special Issue Star Clusters)
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20 pages, 5368 KiB  
Article
Radon Mitigation Applications at the Laboratorio Subterráneo de Canfranc (LSC)
by Javier Pérez-Pérez, Julio Cesar Amare, Iulian Catalin Bandac, Alberto Bayo, Silvia Borjabad-Sánchez, Jose Maria Calvo-Mozota, Laura Cid-Barrio, Rebecca Hernández-Antolín, Beatriz Hernández-Molinero, Pau Novella, Krzysztof Pelczar, Carlos Peña-Garay, Beatriz Romeo, Alfonso Ortiz de Solórzano, Michel Sorel, Jordi Torrent, Alberto Usón, Anna Wojna-Pelczar and Grzegorz Zuzel
Universe 2022, 8(2), 112; https://doi.org/10.3390/universe8020112 - 9 Feb 2022
Cited by 13 | Viewed by 2562
Abstract
The Laboratorio Subterráneo de Canfranc (LSC) is the Spanish national hub for low radioactivity techniques and the associated scientific and technological applications. The concentration of the airborne radon is a major component of the radioactive budget in the neighborhood of the detectors. The [...] Read more.
The Laboratorio Subterráneo de Canfranc (LSC) is the Spanish national hub for low radioactivity techniques and the associated scientific and technological applications. The concentration of the airborne radon is a major component of the radioactive budget in the neighborhood of the detectors. The LSC hosts a Radon Abatement System, which delivers a radon suppressed air with 1.1±0.2 mBq/m3 of 222Rn. The radon content in the air is continuously monitored with an Electrostatic Radon Monitor. Measurements with the double beta decay demonstrators NEXT-NEW and CROSS and the gamma HPGe detectors show the important reduction of the radioactive background due to the purified air in the vicinity of the detectors. We also discuss the use of this facility in the LSC current program which includes NEXT-100, low background biology experiments and radiopure copper electroformation equipment placed in the radon-free clean room. Full article
(This article belongs to the Special Issue Studying the Universe from Spain)
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30 pages, 5711 KiB  
Review
Milky Way Star Clusters and Gaia: A Review of the Ongoing Revolution
by Tristan Cantat-Gaudin
Universe 2022, 8(2), 111; https://doi.org/10.3390/universe8020111 - 9 Feb 2022
Cited by 30 | Viewed by 6749
Abstract
The unprecedented quality of the astrometric measurements obtained with the ESA Gaia spacecraft have initiated a revolution in Milky Way astronomy. Studies of star clusters in particular have been transformed by the precise proper motions and parallaxes measured by Gaia over the entire [...] Read more.
The unprecedented quality of the astrometric measurements obtained with the ESA Gaia spacecraft have initiated a revolution in Milky Way astronomy. Studies of star clusters in particular have been transformed by the precise proper motions and parallaxes measured by Gaia over the entire sky as well as Gaia’s deep all-sky photometry. This paper presents an overview of the many topics of cluster science that have been impacted by the Gaia DR1, DR2, and EDR3 catalogues from their release to the end of the year 2021. These topics include the identification of known clusters and the discovery of new objects, the formation of young clusters and associations, and the long-term evolution of clusters and their stellar content. In addition to the abundance of scientific results, Gaia is changing the way astronomers work with high-volume and high-dimensionality datasets and is teaching us precious lessons to deal with its upcoming data releases and with the large-scale astronomical surveys of the future. Full article
(This article belongs to the Special Issue Star Clusters)
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11 pages, 440 KiB  
Article
The Complex Behaviour of s-Process Element Abundances at Young Ages
by Valentina D’Orazi, Martina Baratella, Maria Lugaro, Laura Magrini and Marco Pignatari
Universe 2022, 8(2), 110; https://doi.org/10.3390/universe8020110 - 9 Feb 2022
Cited by 6 | Viewed by 2025
Abstract
Open clusters appear as simple objects in many respects, with a high degree of homogeneity in their (initial) chemical composition, and the typical solar-scaled abundance pattern that they exhibit for the majority of the chemical species. The striking singularity is represented by heavy [...] Read more.
Open clusters appear as simple objects in many respects, with a high degree of homogeneity in their (initial) chemical composition, and the typical solar-scaled abundance pattern that they exhibit for the majority of the chemical species. The striking singularity is represented by heavy elements produced from the slow process of the neutron-capture reactions. In particular, young open clusters (ages less than a few hundred Myr) give rise to the so-called barium puzzle: that is an extreme enhancement in their [Be/Fe] ratios, up to a factor of four of the solar value, which is not followed by other nearby s-process elements (e.g., lanthanum and cerium). The definite explanation for such a peculiar trend is still wanting, as many different solutions have been envisaged. We review the status of this field and present our new results on young open clusters and the pre-main sequence star RZ Piscium. Full article
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40 pages, 5022 KiB  
Review
A Review of the Tension between the T2K and NOνA Appearance Data and Hints to New Physics
by Ushak Rahaman, Soebur Razzaque and Sankagiri Uma Sankar
Universe 2022, 8(2), 109; https://doi.org/10.3390/universe8020109 - 9 Feb 2022
Cited by 11 | Viewed by 2438
Abstract
In this article, we review the status of the tension between the long-baseline accelerator neutrino experiments T2K and NOνA. The tension arises mostly due to the mismatch in the apappearance data of the two experiments. We explain how this tension arises [...] Read more.
In this article, we review the status of the tension between the long-baseline accelerator neutrino experiments T2K and NOνA. The tension arises mostly due to the mismatch in the apappearance data of the two experiments. We explain how this tension arises based on νμνe and ν¯μν¯e oscillation probabilities. We define the reference point of vacuum oscillation, maximal θ23 and δCP and compute the νe/ν¯e appearance events for each experiment. We then study the effects of deviating the unknown parameters from the reference point and the compatibility of any given set of values of unknown parameters with the data from T2K and NOνA. T2K observes a large excess in the νe appearance event sample compared to the expected νe events at the reference point, whereas NOνA observes a moderate excess. The large excess in T2K dictates that δCP be anchored at −90° and that θ23 > π/4 with a preference for normal hierarchy. The moderate excess at NOνA leads to two degenerate solutions: (a) NH, 0 < δCP < 180°, and θ23 > π/4; (b) IH, 180° < δCP < 0, and θ23 > π/4. This is the main cause of tension between the two experiments. We review the status of three beyond standard model (BSM) physics scenarios, (a) non-unitary mixing, (b) Lorentz invariance violation, and (c) non-standard neutrino interactions, to resolve the tension. Full article
(This article belongs to the Special Issue Neutrino Physics of the 21st Century and Future)
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