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Universe, Volume 9, Issue 9 (September 2023) – 50 articles

Cover Story (view full-size image): The Hubble tension is one of the most important open problems in cosmology, yet we are still very far from a solution. It is often stated that the tension can be solved by introducing “early-time” new physics operating before recombination, the epoch when neutral Hydrogen formed 380,000 years after the Big Bang. This paper presents seven hints suggesting that even the most successful early-time models will fall short of fully solving the tension and that other ingredients, either new physics at late times or on local scales, will always be needed. Additional new physics may also help in solving another conundrum, the S8 tension. These results may pave the way towards resolving one of the most exciting open problems in cosmology, and building a new concordance model beyond ΛCDM. View this paper
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18 pages, 330 KiB  
Article
Model for Origin and Modification of Mass and Coupling Constant
by Yu-Jie Chen, Shi-Lin Li, Yuan-Yuan Liu, Xin Gu, Wen-Du Li and Wu-Sheng Dai
Universe 2023, 9(9), 426; https://doi.org/10.3390/universe9090426 - 21 Sep 2023
Viewed by 871
Abstract
We build a model of the origin of physical constants, including masses and coupling constants. We consider the quantum correction of masses and coupling constants. Some exactly solved leading quantum corrections are given. In the model, the physical constant originates from a coupling [...] Read more.
We build a model of the origin of physical constants, including masses and coupling constants. We consider the quantum correction of masses and coupling constants. Some exactly solved leading quantum corrections are given. In the model, the physical constant originates from a coupling between the matter field and a background field. We show that if such a background field as it should be in the real physical world is a quantum field, then the physical constant will have a space- and time-dependent quantum correction and will no longer be a constant. We build a scalar field model and a mechanics model. In these two models, we discuss the quantum correction of masses and coupling constants in the field framework and in the mechanics framework. Full article
(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)
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17 pages, 3354 KiB  
Article
DPQP: A Detection Pipeline for Quasar Pair Candidates Based on QSO Photometric Images and Spectra
by Yuanbo Liu, Bo Qiu, A-li Luo, Xia Jiang, Lin Yao, Kun Wang and Guiyu Zhao
Universe 2023, 9(9), 425; https://doi.org/10.3390/universe9090425 - 21 Sep 2023
Viewed by 1193
Abstract
Quasars have an important role in the studies of galaxy evolution and star formation. The rare close projection of two quasars in the sky allows us to study the environment and matter exchange around the foreground quasar (QSOfg [...] Read more.
Quasars have an important role in the studies of galaxy evolution and star formation. The rare close projection of two quasars in the sky allows us to study the environment and matter exchange around the foreground quasar (QSOfg) and the background quasar (QSObg). This paper proposes a pipeline DPQP for quasar pair (QP) candidates’ detection based on photometric images and the corresponding spectra. The pipeline consists of three main parts: a target source detector, a regressor, and a discriminator. In the first part, the target source detection network–YOLOv4 (TSD-YOLOv4) and the target source classification network (TSCNet) are used in sequence to detect quasars in SDSS photometric images. In the second part, a depth feature extraction network of quasar images (DE-QNet) is constructed to estimate the redshifts of quasars from photometric images. In the third part, a quasar pair score (Q-Score) metric is proposed based on the spectral analysis. The larger the Q-Score, the greater the possibility of two pairs being a quasar pair. The experimental results show that between redshift 1.0 and 4.0, the MAE of DE-QNet is 0.316, which is 16.1% lower than the existing method. Samples with |Δz| < 0.15 account for 77.1% of the test dataset. A new table with 1025 QP candidates is provided by traversing 50,000 SDSS photometric images. Full article
(This article belongs to the Section Astroinformatics and Astrostatistics)
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11 pages, 721 KiB  
Article
Rotational Curves of the Milky Way Galaxy and Andromeda Galaxy in Light of Vacuum Polarization around Eicheon
by Sergey L. Cherkas and Vladimir L. Kalashnikov
Universe 2023, 9(9), 424; https://doi.org/10.3390/universe9090424 - 21 Sep 2023
Viewed by 920
Abstract
Eicheon properties are discussed. It is shown that the eicheon surface allows setting a boundary condition for the vacuum polarization and obtaining a solution describing the dark matter tail in the Milky Way Galaxy. That is, the dark matter in the Milky Way [...] Read more.
Eicheon properties are discussed. It is shown that the eicheon surface allows setting a boundary condition for the vacuum polarization and obtaining a solution describing the dark matter tail in the Milky Way Galaxy. That is, the dark matter in the Milky Way Galaxy is explained as the F-type of vacuum polarization, which could be treated as dark radiation. The model presented is spherically symmetric, but a surface density of a baryonic galaxy disk is taken into account approximately by smearing the disk over a sphere. This allows the reproduction of the large distance shape of the Milky Way Galaxy rotational curve. Andromeda Galaxy’s rotational curve is also discussed. Full article
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10 pages, 255 KiB  
Article
The Quantum Yang–Mills Theory
by Dimitrios Metaxas
Universe 2023, 9(9), 423; https://doi.org/10.3390/universe9090423 - 20 Sep 2023
Viewed by 861
Abstract
In axiomatic quantum field theory, the postulate of the uniqueness of the vacuum (a pure vacuum state) is independent from the other axioms and equivalent to the cluster decomposition property. The latter, however, implies a Coulomb or Yukawa attenuation of the interactions at [...] Read more.
In axiomatic quantum field theory, the postulate of the uniqueness of the vacuum (a pure vacuum state) is independent from the other axioms and equivalent to the cluster decomposition property. The latter, however, implies a Coulomb or Yukawa attenuation of the interactions at growing distances and hence cannot accommodate the confining properties of the strong interaction. Thesolution of the Yang–Mills quantum theory given previously uses an auxiliary field to incorporate Gauss’s law and demonstrates the existence of two separate vacua, the perturbative and the confining vacuum, therefore resulting in a mixed vacuum state, deriving confinement, as well as the related, expected properties of the strong interaction. The existence of multiple vacua is, in fact, expected by the axiomatic, algebraic quantum field theory, via the decomposition of the vacuum state to eigenspaces of the auxiliary field. The general vacuum state is a mixed quantum state, and the cluster decomposition property does not hold. Because of the energy density difference between the two vacua, the physics of the strong interactions does not admit a Lagrangian description. I clarify the above remarks related to the previous solution of the Yang–Mills interaction and conclude with some discussion a criticism of a related mathematical problem and some tentative comments regarding the spin-2 case. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Field Theory)
26 pages, 1599 KiB  
Review
The Standard Model Theory of Neutron Beta Decay
by Mikhail Gorchtein and Chien-Yeah Seng
Universe 2023, 9(9), 422; https://doi.org/10.3390/universe9090422 - 19 Sep 2023
Cited by 5 | Viewed by 1100
Abstract
We review the status of the Standard Model theory of neutron beta decay. Particular emphasis is put on the recent developments in the electroweak radiative corrections. Given that some existing approaches give slightly different results, we thoroughly review the origin of discrepancies, and [...] Read more.
We review the status of the Standard Model theory of neutron beta decay. Particular emphasis is put on the recent developments in the electroweak radiative corrections. Given that some existing approaches give slightly different results, we thoroughly review the origin of discrepancies, and provide our recommended value for the radiative correction to the neutron and nuclear decay rates. The use of dispersion relation, lattice Quantum Chromodynamics, and an effective field theory framework allows for high-precision theory calculations at the level of 104, turning neutron beta decay into a powerful tool to search for new physics, complementary to high-energy collider experiments. We offer an outlook to the future improvements. Full article
(This article belongs to the Special Issue Neutron Lifetime)
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22 pages, 595 KiB  
Review
The Statistics of Primordial Black Holes in a Radiation-Dominated Universe: Recent and New Results
by Cristiano Germani and Ravi K. Sheth
Universe 2023, 9(9), 421; https://doi.org/10.3390/universe9090421 - 16 Sep 2023
Cited by 8 | Viewed by 977
Abstract
We review the nonlinear statistics of Primordial Black Holes that form from the collapse of over-densities in a radiation-dominated Universe. We focus on the scenario in which large over-densities are generated by rare and Gaussian curvature perturbations during inflation. As new results, we [...] Read more.
We review the nonlinear statistics of Primordial Black Holes that form from the collapse of over-densities in a radiation-dominated Universe. We focus on the scenario in which large over-densities are generated by rare and Gaussian curvature perturbations during inflation. As new results, we show that the mass spectrum follows a power law determined by the critical exponent of the self-similar collapse up to a power spectrum dependent cutoff, and that the abundance related to very narrow power spectra is exponentially suppressed. Related to this, we discuss and explicitly show that both the Press–Schechter approximation and the statistics of mean profiles lead to wrong conclusions for the abundance and mass spectrum. Finally, we clarify that the transfer function in the statistics of initial conditions for Primordial Black Holes formation (the abundance) does not play a significant role. Full article
(This article belongs to the Special Issue Primordial Black Holes from Inflation)
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7 pages, 266 KiB  
Opinion
Considerations Concerning the Little Group
by Jens Erler
Universe 2023, 9(9), 420; https://doi.org/10.3390/universe9090420 - 15 Sep 2023
Cited by 1 | Viewed by 768
Abstract
I very briefly review both the historical and constructive approaches to relativistic quantum mechanics and relativistic quantum field theory, including remarks on the possibility of a non-vanishing photon mass, as well as a foolhardy speculation regarding dark matter. Full article
(This article belongs to the Section High Energy Nuclear and Particle Physics)
12 pages, 276 KiB  
Article
Spherically Symmetric C3 Matching in General Relativity
by Hernando Quevedo
Universe 2023, 9(9), 419; https://doi.org/10.3390/universe9090419 - 14 Sep 2023
Viewed by 791
Abstract
We study the problem of matching interior and exterior solutions to Einstein’s equations along a particular hypersurface. We present the main aspects of the C3 matching approach that involve third-order derivatives of the corresponding metric tensors in contrast to the standard [...] Read more.
We study the problem of matching interior and exterior solutions to Einstein’s equations along a particular hypersurface. We present the main aspects of the C3 matching approach that involve third-order derivatives of the corresponding metric tensors in contrast to the standard C2 matching procedures known in general relativity, which impose conditions on the second-order derivatives only. The C3 alternative approach does not depend on coordinates and allows us to determine the matching surface by using the invariant properties of the eigenvalues of the Riemann curvature tensor. As a particular example, we apply the C3 procedure to match the exterior Schwarzschild metric with a general spherically symmetric interior spacetime with a perfect fluid source and obtain that on the matching hypersurface, the density and pressure should vanish, which is in accordance with the intuitive physical expectation. Full article
(This article belongs to the Special Issue Remo Ruffini Festschrift)
10 pages, 674 KiB  
Article
Unconventional Mechanisms of Heavy Quark Fragmentation
by Boris Kopeliovich, Jan Nemchik, Irina Potashnikova and Ivan Schmidt
Universe 2023, 9(9), 418; https://doi.org/10.3390/universe9090418 - 13 Sep 2023
Viewed by 974
Abstract
Heavy and light quarks produced in high-pT partonic collisions radiate differently. Heavy quarks regenerate their color field, stripped-off in the hard reaction, much faster than the light ones and radiate a significantly smaller fraction of the initial quark energy. This peculiar [...] Read more.
Heavy and light quarks produced in high-pT partonic collisions radiate differently. Heavy quarks regenerate their color field, stripped-off in the hard reaction, much faster than the light ones and radiate a significantly smaller fraction of the initial quark energy. This peculiar feature of heavy-quark jets leads to a specific shape of the fragmentation functions observed in e+e annihilation. Differently from light flavors, the heavy quark fragmentation function strongly peaks at large fractional momentum z, i.e., the produced heavy–light mesons, B or D, carry the main fraction of the jet momentum. This is a clear evidence of the dead-cone effect, and of a short production time of a heavy–light mesons. Contrary to propagation of a small qq¯ dipole, which survives in the medium due to color transparency, a heavy–light Qq¯ dipole promptly expands to a large size. Such a big dipole has no chance to remain intact in a dense medium produced in relativistic heavy ion collisions. On the other hand, a breakup of such a dipole does not affect much the production rate of Qq¯ mesons, differently from the case of light qq¯ meson production. Full article
(This article belongs to the Special Issue Jet and Heavy Flavor Production)
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14 pages, 2230 KiB  
Article
Starspot Activity without Pulsation in the Binary System KIC 5444392 Revisited
by Mengqi Jin, Jianning Fu, Jiaxing Wang and Weikai Zong
Universe 2023, 9(9), 417; https://doi.org/10.3390/universe9090417 - 12 Sep 2023
Viewed by 920
Abstract
We revisited the short-period (∼1.5 days) binary system KIC 5444392, which shows quasi-period modulated light variations. Previous studies indicated that these variations might be caused by stellar pulsations. In our work, we used the PHOEBE program, which revealed that this binary is an [...] Read more.
We revisited the short-period (∼1.5 days) binary system KIC 5444392, which shows quasi-period modulated light variations. Previous studies indicated that these variations might be caused by stellar pulsations. In our work, we used the PHOEBE program, which revealed that this binary is an almost circular (e 0.007) detached system with two G-type stars. The masses and radii of the primary and secondary stars were obtained as M1=1.21±0.06M, R1=1.69±0.09R and M2=1.27±0.06M, R2=1.69±0.09R, respectively. Based on these parameters, the isochrone fitting showed that this system consists of a subgiant and a main-sequence star, whose ages are 3.890.34+0.37 Gyr. Neither the primary nor the secondary star is in the mass range of Cepheid and Gamma Dor. Fourier analysis showed that the fitting residuals varied stochastically in a frequency around the orbital frequency, which means that the quasi-periodic signals resulted from starspots rather than stellar pulsation. Similar stellar parameters of both components of KIC 5444392 and the frequency analysis lead us to believe that starspots are in both stars. The autocorrelation analysis on the residuals indicates that the decay timescale of the starspots is about 53 days, and the rotational periods of both stars are very close to the orbital period of the binary. This result adheres to the trend that the decay timescale increases following the rotational frequency. Thus, studying this binary could increase our understanding of the light variations in the binary system. Full article
(This article belongs to the Section Stellar Astronomy)
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19 pages, 12312 KiB  
Article
ESNet: Estimating Stellar Parameters from LAMOST Low-Resolution Stellar Spectra
by Kun Wang, Bo Qiu, A-li Luo, Fuji Ren and Xia Jiang
Universe 2023, 9(9), 416; https://doi.org/10.3390/universe9090416 - 11 Sep 2023
Viewed by 956
Abstract
Stellar parameters are estimated through spectra and are crucial in studying both stellar evolution and the history of the galaxy. To extract features from the spectra efficiently, we present ESNet (encoder selection network for spectra), a novel architecture that incorporates three essential modules: [...] Read more.
Stellar parameters are estimated through spectra and are crucial in studying both stellar evolution and the history of the galaxy. To extract features from the spectra efficiently, we present ESNet (encoder selection network for spectra), a novel architecture that incorporates three essential modules: a feature encoder (FE), feature selection (FS), and feature mapping (FM). FE is responsible for extracting advanced spectral features through encoding. The role of FS, on the other hand, is to acquire compressed features by reducing the spectral dimension and eliminating redundant information. FM comes into play by fusing the advanced and compressed features, establishing a nonlinear mapping between spectra and stellar parameters. The stellar spectra used for training and testing are obtained through crossing LAMOST and SDSS. The experimental results demonstrate that for low signal-to-noise spectra (0–10), ESNet achieves excellent performance on the test set, with mean absolute error (MAE) values of 82 K for Teff (effective temperature), 0.20 dex for logg (logarithm of the gravity), and 0.10 dex for [Fe/H] (metallicity). The results indeed indicate that ESNet has an excellent ability to extract spectral features. Furthermore, this paper validates the consistency between ESNet predictions and the SDSS catalog. The experimental results prove that the model can be employed for the evaluation of stellar parameters. Full article
(This article belongs to the Section Astroinformatics and Astrostatistics)
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9 pages, 2591 KiB  
Article
Exploring the Spectral Line Broadening of the Bulk Motions in the High Mass Star Forming Region with Radiative Transfer Simulations
by Shixian Mo and Keping Qiu
Universe 2023, 9(9), 415; https://doi.org/10.3390/universe9090415 - 10 Sep 2023
Viewed by 888
Abstract
The Davis–Chandrasekhar–Fermi (DCF) method is widely used to indirectly estimate the strength of magnetic fields in star-forming regions. However, recent developments in this method have primarily focused on improving the measurement of angular dispersion of the field, neglecting other physical quantities, especially turbulence [...] Read more.
The Davis–Chandrasekhar–Fermi (DCF) method is widely used to indirectly estimate the strength of magnetic fields in star-forming regions. However, recent developments in this method have primarily focused on improving the measurement of angular dispersion of the field, neglecting other physical quantities, especially turbulence velocity. Most DCF studies tend to overlook or fail to acknowledge the influence of bulk motions on the linewidth, and directly obtain the turbulence velocity based on the non-thermal linewidth. Therefore, to explore the contributions of bulk motions to the linewidth, we conducted radiative transfer simulations using a rotating and infalling envelope–disk model to a high-mass star formation region, IRAS18360-0537. The main conclusion from our work is that the bulk motions contribute significantly to the linewidth and cannot be fully eliminated by simply deducing velocity gradients. Hence, fully attributing the observed non-thermal velocity dispersion derived from fitting a spectral line profile to the turbulence can result in significantly overestimated magnetic field strength and may yield unscientific results of star-forming regions. Full article
(This article belongs to the Special Issue Advances in Star Formation in the Milky Way)
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12 pages, 1127 KiB  
Article
Collisional Broadening within a Hadronic Transport Approach
by Branislav Balinovic, Renan Hirayama and Hannah Elfner
Universe 2023, 9(9), 414; https://doi.org/10.3390/universe9090414 - 09 Sep 2023
Viewed by 825
Abstract
We explore the emergence of the collisional broadening of hadrons under the influence of different media using the hadronic transport approach SMASH (Simulating Many Accelerated Strongly interacting Hadrons), which employs vacuum properties and contains no a priori information about in-medium effects. In this [...] Read more.
We explore the emergence of the collisional broadening of hadrons under the influence of different media using the hadronic transport approach SMASH (Simulating Many Accelerated Strongly interacting Hadrons), which employs vacuum properties and contains no a priori information about in-medium effects. In this context, we define collisional broadening as a decrease in the lifetime of hadrons, and it arises from an interplay between the cross-sections for inelastic processes and the available phase space. We quantify this effect for various hadron species, in both a thermal gas in equilibrium and in nuclear collisions. Furthermore, we distinguish the individual contribution of each process and verify the medium response to different vacuum assumptions; we see that the decay width that depends on the resonance mass leads to a larger broadening than a mass-independent scenario. Full article
(This article belongs to the Special Issue Zimányi School – Heavy Ion Physics)
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17 pages, 777 KiB  
Article
Suppression of the Multiplicity Fluctuations in Particle Correlations
by Chong Ye, Hong-Hao Ma, Dan Wen, Philipe Mota, Wei-Liang Qian and Rui-Hong Yue
Universe 2023, 9(9), 413; https://doi.org/10.3390/universe9090413 - 09 Sep 2023
Viewed by 731
Abstract
Multiplicity fluctuations play a crucial role in relativistic heavy-ion collisions. In this work, we explore how the multiplicity fluctuations can be effectively suppressed in the measurement of particle correlations. In particular, through proper normalization, particle correlations can be evaluated in a manner irrelevant [...] Read more.
Multiplicity fluctuations play a crucial role in relativistic heavy-ion collisions. In this work, we explore how the multiplicity fluctuations can be effectively suppressed in the measurement of particle correlations. In particular, through proper normalization, particle correlations can be evaluated in a manner irrelevant to multiplicity. When the multiplicity fluctuations are adequately extracted, Monte Carlo simulations show that the remaining correlations possess distinct features buried in the otherwise overwhelming fluctuations. Moreover, we argue that such a normalization scheme naturally agrees with the multi-particle correlator, which can be evaluated using the Q-vectors. The implications of the present study in the data analysis are also addressed. Full article
(This article belongs to the Section High Energy Nuclear and Particle Physics)
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14 pages, 3992 KiB  
Article
Automatic Identification of Auroral Substorms Based on Ultraviolet Spectrographic Imager Aboard Defense Meteorological Satellite Program (DMSP) Satellite
by Ze-Jun Hu, Hui-Fang Lian, Bai-Ru Zhao, Bing Han and Yi-Sheng Zhang
Universe 2023, 9(9), 412; https://doi.org/10.3390/universe9090412 - 08 Sep 2023
Viewed by 899
Abstract
An auroral substorm is an important physical process of energy accumulation and explosive release in the Earth’s magnetosphere, and is an important research object of space environment monitoring and space weather warnings. A westward traveling surge (WTS) is a typical auroral physical process [...] Read more.
An auroral substorm is an important physical process of energy accumulation and explosive release in the Earth’s magnetosphere, and is an important research object of space environment monitoring and space weather warnings. A westward traveling surge (WTS) is a typical auroral physical process of an auroral substorm. Its static characteristic is auroral folding at the polar boundary of an auroral oval and its dynamic characteristic is the westward motion of auroral folding. According to the static characteristic of a WTS, we defined a set of feature parameters based on the morphology and designed a set of automatic detection and discrimination methods; that is, the WTS was identified by using the extracted features and pattern recognition approaches. This approach was tested by using the aurora data of the ultraviolet auroral spectral imager of the Defense Meteorological Satellite Program (DMSP) satellite. The results showed that the accuracy rate of automatic recognition was 61.39%~63.61% and the precision rate was 55.52%~57.92%. The experimental results showed that the approach was effective at detecting the typical characteristics of an auroral substorm (WTS). Full article
(This article belongs to the Section Space Science)
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25 pages, 7759 KiB  
Article
Phase Transitions in the Interacting Relativistic Boson Systems
by Dmitry Anchishkin, Volodymyr Gnatovskyy, Denys Zhuravel, Vladyslav Karpenko, Igor Mishustin and Horst Stoecker
Universe 2023, 9(9), 411; https://doi.org/10.3390/universe9090411 - 07 Sep 2023
Cited by 2 | Viewed by 935
Abstract
The thermodynamic properties of the interacting particle–antiparticle boson system at high temperatures and densities were investigated within the framework of scalar and thermodynamic mean-field models. We assume isospin (charge) density conservation in the system. The equations of state and thermodynamic functions are determined [...] Read more.
The thermodynamic properties of the interacting particle–antiparticle boson system at high temperatures and densities were investigated within the framework of scalar and thermodynamic mean-field models. We assume isospin (charge) density conservation in the system. The equations of state and thermodynamic functions are determined after solving the self-consistent equations. We study the relationship between attractive and repulsive forces in the system and the influence of these interactions on the thermodynamic properties of the bosonic system, especially on the development of the Bose–Einstein condensate. It is shown that under “weak” attraction, the boson system has a phase transition of the second order, which occurs every time the dependence of the particle density crosses the critical curve or even touches it. It was found that with a “strong” attractive interaction, the system forms a Bose condensate during a phase transition of the first order, and, despite the finite value of the isospin density, these condensate states are characterized by a zero chemical potential. That is, such condensate states cannot be described by the grand canonical ensemble since the chemical potential is involved in the conditions of condensate formation, so it cannot be a free variable when the system is in the condensate phase. Full article
(This article belongs to the Special Issue Zimányi School – Heavy Ion Physics)
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7 pages, 924 KiB  
Communication
Binary Neutron-Star Mergers with a Crossover Transition to Quark Matter
by Grant J. Mathews, Atul Kedia, Hee Il Kim and In-Saeng Suh
Universe 2023, 9(9), 410; https://doi.org/10.3390/universe9090410 - 07 Sep 2023
Viewed by 893
Abstract
This paper summarizes recent work on the possible gravitational-wave signal from binary neutron-star mergers in which there is a crossover transition to quark matter. Although this is a small piece of a much more complicated problem, we discuss how the power spectral density [...] Read more.
This paper summarizes recent work on the possible gravitational-wave signal from binary neutron-star mergers in which there is a crossover transition to quark matter. Although this is a small piece of a much more complicated problem, we discuss how the power spectral density function may reveal the presence of a crossover transition to quark matter. Full article
(This article belongs to the Special Issue Remo Ruffini Festschrift)
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10 pages, 280 KiB  
Article
Universality in the Exact Renormalization Group: Comparison to Perturbation Theory
by José Gaite
Universe 2023, 9(9), 409; https://doi.org/10.3390/universe9090409 - 07 Sep 2023
Cited by 1 | Viewed by 834
Abstract
Various formulations of the exact renormalization group can be compared in the perturbative domain, in which we have reliable expressions for regularization-independent (universal) quantities. We consider the renormalization of the λϕ4 theory in three dimensions and make a comparison between the [...] Read more.
Various formulations of the exact renormalization group can be compared in the perturbative domain, in which we have reliable expressions for regularization-independent (universal) quantities. We consider the renormalization of the λϕ4 theory in three dimensions and make a comparison between the sharp-cutoff regularization method and other more recent methods. They all give good results, which only differ by small non-universal terms. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Field Theory)
16 pages, 814 KiB  
Article
Plane Symmetric Cosmological Model with Strange Quark Matter in f(R,T) Gravity
by Vijay Singh, Siwaphiwe Jokweni and Aroonkumar Beesham
Universe 2023, 9(9), 408; https://doi.org/10.3390/universe9090408 - 06 Sep 2023
Viewed by 922
Abstract
A plane symmetric Bianchi-I model filled with strange quark matter (SQM) was explored in f(R,T)=R+2λT gravity, where R is the Ricci scalar, T is the trace of the energy-momentum tensor, and λ [...] Read more.
A plane symmetric Bianchi-I model filled with strange quark matter (SQM) was explored in f(R,T)=R+2λT gravity, where R is the Ricci scalar, T is the trace of the energy-momentum tensor, and λ is an arbitrary constant. Three different types of solutions were obtained. In each model, comparisons of the outcomes in f(R,T) gravity and bag constant were made to comprehend their roles. The first power-law solution was obtained by assuming that the expansion scalar is proportional to the shear scalar. This solution was compared with a similar one obtained earlier. The second solution was derived by assuming a constant deceleration parameter q. This led to two solutions: one power-law and the other exponential. Just as in the case of general relativity, we can obtain solutions for each of the different eras of the universe, but we cannot obtain a model which shows transitional behavior from deceleration to acceleration. However, the third solution is a hybrid solution, which shows the required transition. The models start off with anisotropy, but are shear free at late times. In general relativity, the effect of SQM is to accelerate the universe, so we expect the same in f(R,T) gravity. Full article
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11 pages, 651 KiB  
Article
Parameter Inference for Coalescing Massive Black Hole Binaries Using Deep Learning
by Wenhong Ruan, He Wang, Chang Liu and Zongkuan Guo
Universe 2023, 9(9), 407; https://doi.org/10.3390/universe9090407 - 06 Sep 2023
Cited by 3 | Viewed by 1021
Abstract
In the 2030s, a new era of gravitational wave (GW) observations will dawn as multiple space-based GW detectors, such as the Laser Interferometer Space Antenna, Taiji, and TianQin, will open the millihertz window for GW astronomy. These detectors are poised to detect a [...] Read more.
In the 2030s, a new era of gravitational wave (GW) observations will dawn as multiple space-based GW detectors, such as the Laser Interferometer Space Antenna, Taiji, and TianQin, will open the millihertz window for GW astronomy. These detectors are poised to detect a multitude of GW signals emitted by different sources. It is a challenging task for GW data analysis to recover the parameters of these sources at a low computational cost. Generally, the matched filtering approach entails exploring an extensive parameter space for all resolvable sources, incurring a substantial cost owing to the generation of GW waveform templates. To alleviate the challenge, we make an attempt to perform parameter inference for coalescing massive black hole binaries (MBHBs) using deep learning. The model trained in this work has the capability to produce 50,000 posterior samples for the redshifted total mass, mass ratio, coalescence time, and luminosity distance of an MBHB in about twenty seconds. Our model can serve as an effective data pre-processing tool, reducing the volume of parameter space by more than four orders of magnitude for MBHB signals with a signal-to-noise ratio larger than 100. Moreover, the model exhibits robustness when handling input data that contain multiple MBHB signals. Full article
(This article belongs to the Special Issue Newest Results in Gravitational Waves and Machine Learning)
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14 pages, 6874 KiB  
Article
Revise the Phase-Space Analysis of the Dynamical Spacetime Unified Dark Energy Cosmology
by Andronikos Paliathanasis
Universe 2023, 9(9), 406; https://doi.org/10.3390/universe9090406 - 05 Sep 2023
Viewed by 796
Abstract
We analyze the phase-space of an alternate scalar field cosmology that aims to combine the concepts of dark energy and the dark sector. The investigation focuses on stationary points within this phase-space, considering different functional forms of the two potential functions. Our findings [...] Read more.
We analyze the phase-space of an alternate scalar field cosmology that aims to combine the concepts of dark energy and the dark sector. The investigation focuses on stationary points within this phase-space, considering different functional forms of the two potential functions. Our findings indicate that a de Sitter universe is achievable solely when at the asymptotic limit the potential function is constant. For constant potential function, the de Sitter universe is recovered in the finite regime; however, for the exponential potential, the de Sitter universe exists at the infinity regime. The cosmological viability of the present theory is discussed. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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16 pages, 347 KiB  
Review
Quantum Imprints on CMBR
by Shreya Banerjee
Universe 2023, 9(9), 405; https://doi.org/10.3390/universe9090405 - 04 Sep 2023
Viewed by 889
Abstract
Quantum cosmology aims to develop a quantum theory of the universe, attempting to answer open questions of physical cosmology, mainly related to the early epochs of the universe. Such a theory aims to unite relativity theory and quantum theory. Here, the whole universe [...] Read more.
Quantum cosmology aims to develop a quantum theory of the universe, attempting to answer open questions of physical cosmology, mainly related to the early epochs of the universe. Such a theory aims to unite relativity theory and quantum theory. Here, the whole universe is treated as a quantum mechanical system and is described by a wave function rather than by a classical spacetime. In this review, I shall describe the mathematical structure and primary formulations that form the backbone of quantum cosmology. We know that over a period of time, several approaches were developed to form a quantum theory of gravity. However, in order to decide which approach is the best, we need testable predictions, effects that can be observed in cosmic microwave background radiation (CMBR). I shall discuss the methodologies for generating quantum gravitational corrections to inflationary background leading to testable predictions. Another aspect of finding quantum imprints on CMBR results through the application of resolution of the ‘quantum measurement problem’ to early universe physics. In this article, I shall also discuss two such promising models explaining the classicalization of inflationary perturbation and are capable of leaving distinct observational imprints on the observables. Full article
15 pages, 4222 KiB  
Review
X-ray Spectroscopic Study of Low-Mass X-ray Binaries: A Review of Recent Progress via the Example of GX 339-4
by Gábor Pszota and Endre Kovács
Universe 2023, 9(9), 404; https://doi.org/10.3390/universe9090404 - 04 Sep 2023
Viewed by 862
Abstract
Low-mass X-ray binaries (LMXB) serve as natural laboratories, where the predictions of general relativity can be tested in the strong field regime. The primary object of such sources can be a neutron star (NS) or a black hole (BH), and this object captures [...] Read more.
Low-mass X-ray binaries (LMXB) serve as natural laboratories, where the predictions of general relativity can be tested in the strong field regime. The primary object of such sources can be a neutron star (NS) or a black hole (BH), and this object captures material from the secondary object through the inner Lagrange point via a process called Roche lobe overflow. Because of the angular momentum of the infalling matter, an accretion disk is formed, in which viscous effects transport the angular momentum radially outward. In the high/soft state of these sources, the accretion disk can extend all the way to the innermost stable circular orbit (ISCO); therefore, when the primary object is a BH, its X-ray spectrum contains information about the region very close to the event horizon. This paper aims to review the theoretical and observational works related to the X-ray spectroscopy of such sources via the example of GX 339-4, which is one of the most well-known and well-studied LMXBs. Full article
(This article belongs to the Section Compact Objects)
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15 pages, 1821 KiB  
Article
Influence of the Surface Temperature Evolution over Organic and Inorganic Compounds on Iapetus
by Katherine Villavicencio-Valero, Emilio Ramírez-Juidias, Antonio Madueño-Luna, José Miguel Madueño-Luna and Miguel Calixto López-Gordillo
Universe 2023, 9(9), 403; https://doi.org/10.3390/universe9090403 - 04 Sep 2023
Viewed by 1047
Abstract
In this manuscript, there were performed simulations of the evolution of the surface temperature for each of the two hemispheres of Iapetus. This icy moon of Saturn shows the most differentiated albedo dichotomy of the Solar System. The dark leading side has a [...] Read more.
In this manuscript, there were performed simulations of the evolution of the surface temperature for each of the two hemispheres of Iapetus. This icy moon of Saturn shows the most differentiated albedo dichotomy of the Solar System. The dark leading side has a lower albedo than the bright trailing side. Spectral data on the visible light reveal the existence of two types of materials on the surface. The darkening in the leading side is thought to be due to the presence of organic material and carbonaceous compounds on the surface, while the trailing side is covered by water ice due to migration processes from the dark side. On airless bodies like Iapetus, the surface escape speed is greater than the speed of water molecules, resulting in the retention of a H2O atmosphere that allows some species to diffuse through it. Results showed a slow yet steady increment of temperatures for both sides, with a steeper slope for the dark hemisphere. It was also simulated how much energy can be accumulated on both sides and the consequences of that. Finally, we calculated the diffusion coefficients for ammonia, methane, and water ice. The results allowed us to infer how these compounds could evolve over time. Full article
(This article belongs to the Section Planetary Sciences)
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15 pages, 6166 KiB  
Review
Evolved Pulsar Wind Nebulae
by Barbara Olmi
Universe 2023, 9(9), 402; https://doi.org/10.3390/universe9090402 - 01 Sep 2023
Cited by 3 | Viewed by 839
Abstract
Based on the expected population of core collapse supernova remnants and the huge number of detected pulsars in the Galaxy, still representing only a fraction of the real population, pulsar wind nebulae are likely to constitute one of the largest classes of extended [...] Read more.
Based on the expected population of core collapse supernova remnants and the huge number of detected pulsars in the Galaxy, still representing only a fraction of the real population, pulsar wind nebulae are likely to constitute one of the largest classes of extended Galactic sources in many energy bands. For simple evolutionary reasons, the majority of the population is made of evolved systems, whose detection and identification are complicated by their reduced luminosity, the possible lack of X-ray emission (that fades progressively away with the age of the pulsar), and by their modified morphology with respect to young systems. Nevertheless they have gained renewed attention in recent years, following the detection of misaligned X-ray tails protruding from an increasing number of nebulae created by fast moving pulsars, and of extended TeV halos surrounding aged systems. Both these features are clear signs of an efficient escape of particles, with energy close to the maximum acceleration limit of the pulsar. Here we discuss the properties of those evolved systems and what we have understood about the process of particle escape, and the formation of observed features. Full article
(This article belongs to the Special Issue Pulsar Wind Nebulae)
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13 pages, 2525 KiB  
Article
Primordial Planets with an Admixture of Dark Matter Particles and Baryonic Matter
by O. V. Kiren, Arun Kenath and Chandra Sivaram
Universe 2023, 9(9), 401; https://doi.org/10.3390/universe9090401 - 31 Aug 2023
Viewed by 787
Abstract
It has been suggested that primordial planets could have formed in the early universe and the missing baryons in the universe could be explained by primordial free-floating planets of solid hydrogen. Many such planets were recently discovered around the old and metal-poor stars, [...] Read more.
It has been suggested that primordial planets could have formed in the early universe and the missing baryons in the universe could be explained by primordial free-floating planets of solid hydrogen. Many such planets were recently discovered around the old and metal-poor stars, and such planets could have formed in early epochs. Another possibility for missing baryons in the universe could be that these baryons are admixed with DM particles inside the primordial planets. Here, we discuss the possibility of the admixture of baryons in the DM primordial planets discussed earlier. We consider gravitationally bound DM objects with the DM particles constituting them varying in mass from 20 to100 GeV. Different fractions of DM particles mixed with baryonic matter in forming the primordial planets are discussed. For the different mass range of DM particles forming DM planets, we have estimated the radius and density of these planets with different fractions of DM and baryonic particles. It is found that for heavier-mass DM particles with the admixture of certain fractions of baryonic particles, the mass of the planet increases and can reach or even substantially exceed Jupiter mass. The energy released during the process of merger of such primordial planets is discussed. The energy required for the tidal breakup of such an object in the vicinity of a black hole is also discussed. Full article
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14 pages, 574 KiB  
Article
Non-Local Interactions Are Essential Elements for Dark Matter Halo Stability: A Cross-Model Study
by Ahmad Borzou
Universe 2023, 9(9), 400; https://doi.org/10.3390/universe9090400 - 31 Aug 2023
Viewed by 734
Abstract
This paper introduces a comprehensive methodology for examining the stability of dark matter (DM) halos, emphasizing the necessity for non-local inter-particle interactions, whether they are fundamental or effective in nature, to maintain halo stability. We highlight the inadequacy of vanilla cold, collisionless DM [...] Read more.
This paper introduces a comprehensive methodology for examining the stability of dark matter (DM) halos, emphasizing the necessity for non-local inter-particle interactions, whether they are fundamental or effective in nature, to maintain halo stability. We highlight the inadequacy of vanilla cold, collisionless DM models in forecasting a stable halo without considering a “non-local” interaction in the halo’s effective free energy, which could potentially arise from factors like baryonic feedback, self-interactions, or the intrinsic quantum characteristics of dark particles. The stability prerequisite necessitates significant effective interactions between any two points within the halo, regardless of their distance from the center. The methodology proposed herein offers a systematic framework to scrutinize the stability of various DM models and refine their parameter spaces. We deduce that DM halos within a model, where the deviation from the standard cold, collisionless framework is confined to regions near the halo center, are unlikely to exhibit stability in their outer sectors. In our study, we demonstrate that the issue of instability within DM halos cannot be addressed adequately using perturbative quantum effects. This issue is less pronounced for fermionic DM but suffers from a higher degree of severity when considering bosonic DM. We find that halos made of bosons with notable quantum effects have sharp edges, while those made of fermions show more diffuse boundaries extending toward infinity. To present the potentials of the cross-model approach, we explore the broadest form of the effective free energy around a chosen mass profile. Next, as a case study, we employ a model where the deviation from the standard cold, collisionless DM model is represented by a two-body interaction in the effective free energy to show how to use observations to investigate universal classes of DM models. Full article
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13 pages, 6815 KiB  
Article
Influence of Alfvén Ion–Cyclotron Waves on the Anisotropy of Solar Wind Turbulence at Ion Kinetic Scales
by Xin Wang, Linzhi Huang, Yuxin Wang and Haochen Yuan
Universe 2023, 9(9), 399; https://doi.org/10.3390/universe9090399 - 31 Aug 2023
Cited by 1 | Viewed by 796
Abstract
The power spectra of the magnetic field at ion kinetic scales have been found to be significantly influenced by Alfvén ion–cyclotron (AIC) waves. Here, we study whether and how this influence of the AIC wave depends on the θVB angle (the [...] Read more.
The power spectra of the magnetic field at ion kinetic scales have been found to be significantly influenced by Alfvén ion–cyclotron (AIC) waves. Here, we study whether and how this influence of the AIC wave depends on the θVB angle (the angle between the local mean magnetic field and the solar wind velocity direction). The wavelet technique is applied to the high time-resolution (11 vectors per second) magnetic field data from WIND spacecraft measurements in a fast solar wind stream associated with an outward magnetic sector. It is found that around the ion kinetic scales (0.52 Hz–1.21 Hz), the power spectrum in the parallel angular bin 0<θVB<10 has a slope of 4.80±0.15. When we remove the left-handed polarized AIC waves (with normalized reduced magnetic helicity smaller than 0.9) from the fluctuations, the spectral index becomes 4.09±0.11. However, the power spectrum in the perpendicular angular bin 80<θVB<90 changes very little during the wave-removal process, and its slope remains 3.22±0.07. These results indicate that the influence of the AIC waves on the magnetic spectral index at the ion kinetic scales is indeed dependent on θVB, which is due to the anisotropic distribution of the waves. Apparently, when the waves are removed from the original data, the spectral anisotropy weakens. This result may help us to better understand the physical nature of the spectral anisotropy around the ion scales. Full article
(This article belongs to the Special Issue The Multi-Scale Dynamics of Solar Wind)
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11 pages, 334 KiB  
Article
Hartree–Fock Calculations in Semi-Infinite Matter with Gogny Interactions
by Dany Davesne, Alessandro Pastore and Jesus Navarro
Universe 2023, 9(9), 398; https://doi.org/10.3390/universe9090398 - 30 Aug 2023
Viewed by 623
Abstract
Hartree–Fock equations in semi-infinite nuclear matter for finite range Gogny interactions are presented together with a detailed numerical scheme to solve them. The value of the surface energy is then extracted and given for standard Gogny interactions. Full article
(This article belongs to the Special Issue Many Body Theory)
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13 pages, 396 KiB  
Article
Alleviating the H0 Tension in Scalar–Tensor and Bi-Scalar–Tensor Theories
by Maria Petronikolou and Emmanuel N. Saridakis
Universe 2023, 9(9), 397; https://doi.org/10.3390/universe9090397 - 30 Aug 2023
Cited by 3 | Viewed by 985
Abstract
Herein, we investigate scalar–tensor and bi-scalar–tensor modified theories of gravity that can alleviate the H0 tension. In the first class of theories, we show that by choosing particular models with a shift-symmetric friction term we are able to alleviate the tension by [...] Read more.
Herein, we investigate scalar–tensor and bi-scalar–tensor modified theories of gravity that can alleviate the H0 tension. In the first class of theories, we show that by choosing particular models with a shift-symmetric friction term we are able to alleviate the tension by obtaining a smaller effective Newton’s constant at intermediate times, a feature that cannot be easily obtained in modified gravity. In the second class of theories, which involve two extra propagating degrees of freedom, we show that the H0 tension can be alleviated, and the mechanism behind this is the phantom behavior of the effective dark-energy equation-of-state parameter. Hence, scalar–tensor and bi-scalar–tensor theories have the ability to alleviate the H0 tension with both known sufficient late-time mechanisms. Full article
(This article belongs to the Special Issue Modified Gravity Approaches to the Tensions of ΛCDM)
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