Universe

10 pages, 334 KiB

Open AccessArticle

A Generic Analysis of Nucleon Decay Branching Fractions in Flipped SU(5) Grand Unification

by Koichi Hamaguchi, Shihwen Hor, Natsumi Nagata and Hiroki Takahashi

Universe 2024, 10(9), 379; https://doi.org/10.3390/universe10090379 - 23 Sep 2024

Viewed by 358

In flipped SU(5) grand unified theories, the partial decay lifetimes of certain nucleon decay channels depend generically on an unknown unitary matrix, which arises when left-handed lepton fields are embedded into anti-fundamental representations of SU(5). This dependency is particularly relevant when the neutrino [...] Read more.

In flipped SU(5) grand unified theories, the partial decay lifetimes of certain nucleon decay channels depend generically on an unknown unitary matrix, which arises when left-handed lepton fields are embedded into anti-fundamental representations of SU(5). This dependency is particularly relevant when the neutrino mass matrix has a generic structure, introducing uncertainty in the prediction of nucleon decay branching fractions within flipped SU(5). In this paper, we demonstrate that this uncertainty can be parametrized using two parameters, which can be determined by measuring the partial lifetimes of

p \to π^{0} e^{+}

,

p \to π^{0} μ^{+}

, and

n \to π^{0} \bar{ν}

. In addition, we establish upper limits on the ratios of the decay widths of these channels, offering a potential method to test flipped SU(5) in future nucleon decay experiments. Full article

(This article belongs to the Special Issue Particle Physics and Cosmology: A Themed Issue in Honor of Professor Dimitri Nanopoulos)

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10 pages, 1492 KiB

Open AccessCommunication

Sgr A* Shadow Study with KTN Space Time and Investigation of NUT Charge Existence

by Masoumeh Ghasemi-Nodehi

Universe 2024, 10(9), 378; https://doi.org/10.3390/universe10090378 - 23 Sep 2024

Viewed by 325

Abstract

In this paper, I investigate the existence of the NUT charge through the KTN spacetime using shadow observations of Sgr A*. I report that the range of my constraint for the NUT charge is between −0.5 and 0.5 for Schwarzschild-like and very slowly [...] Read more.

In this paper, I investigate the existence of the NUT charge through the KTN spacetime using shadow observations of Sgr A*. I report that the range of my constraint for the NUT charge is between −0.5 and 0.5 for Schwarzschild-like and very slowly rotating KTN black holes. This range extends to 1.5 for spins up to −2 and −1.5 for spins up to 2 based on Keck observations for both 40° and 10° viewing angles. For VLTI observations, Schwarzschild-like and very slowly rotating KTN black holes are excluded for a 40° viewing angle, and the NUT charge is constrained to a very narrow range for a 10° viewing angle. I report that the possibility of having KTN naked singularities in Sgr A* is small, considering the uncertainties in the shadow size. Full article

(This article belongs to the Section Gravitation)

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10 pages, 284 KiB

Open AccessArticle

Topological Susceptibility of the Gluon Plasma in the Stochastic-Vacuum Approach

by Dmitry Antonov

Universe 2024, 10(9), 377; https://doi.org/10.3390/universe10090377 - 23 Sep 2024

Viewed by 349

Abstract

Topological susceptibility of the SU(3) gluon plasma is calculated by accounting for both factorized and non-factorized contributions to the two-point correlation function of topological-charge densities. It turns out that, while the factorized contribution keeps this correlation function non-positive away from the origin, the [...] Read more.

Topological susceptibility of the SU(3) gluon plasma is calculated by accounting for both factorized and non-factorized contributions to the two-point correlation function of topological-charge densities. It turns out that, while the factorized contribution keeps this correlation function non-positive away from the origin, the non-factorized contribution makes it positive at the origin, in accordance with the reflection positivity condition. Matching the obtained result for topological susceptibility to its lattice value at the deconfinement critical temperature, we fix the parameters of the quartic cumulant of gluonic field strengths, and calculate the contribution of that cumulant to the string tension. This contribution reduces the otherwise too large value of the string tension, which stems from the quadratic cumulant, making it much closer to the standard phenomenological value. Full article

(This article belongs to the Special Issue Quantum Field Theory, 2nd Edition)

13 pages, 431 KiB

Open AccessArticle

Tensor Amplitudes for Partial Wave Analysis of

ψ \to Δ \bar{Δ}

within Helicity Frame

by Xiang Dong, Kexin Su, Hao Cai and Kai Zhu

Universe 2024, 10(9), 376; https://doi.org/10.3390/universe10090376 - 23 Sep 2024

Viewed by 298

Abstract

We have derived the tensor amplitudes for partial wave analysis of

ψ \to Δ \bar{Δ}

,

Δ \to p π

within the helicity frame, as well as the amplitudes for the other decay sequences with same final states. These formulae are practical [...] Read more.

We have derived the tensor amplitudes for partial wave analysis of

ψ \to Δ \bar{Δ}

,

Δ \to p π

within the helicity frame, as well as the amplitudes for the other decay sequences with same final states. These formulae are practical for the experiments measuring

ψ

decaying into

p \bar{p} π^{+} π^{-}

final states, such as BESIII with its recently collected huge

J / ψ

and

ψ (2 S)

data samples. Full article

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

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11 pages, 296 KiB

Open AccessArticle

On the Euler–Type Gravitomagnetic Orbital Effects in the Field of a Precessing Body

by Lorenzo Iorio

Universe 2024, 10(9), 375; https://doi.org/10.3390/universe10090375 - 21 Sep 2024

Viewed by 337

Abstract

To the first post–Newtonian order, the gravitational action of mass–energy currents is encoded by the off–diagonal gravitomagnetic components of the spacetime metric tensor. If they are time–dependent, a further acceleration enters the equations of motion of a moving test particle. Let the source [...] Read more.

To the first post–Newtonian order, the gravitational action of mass–energy currents is encoded by the off–diagonal gravitomagnetic components of the spacetime metric tensor. If they are time–dependent, a further acceleration enters the equations of motion of a moving test particle. Let the source of the gravitational field be an isolated, massive body rigidly rotating whose spin angular momentum experiences a slow precessional motion. The impact of the aforementioned acceleration on the orbital motion of a test particle is analytically worked out in full generality. The resulting averaged rates of change are valid for any orbital configuration of the satellite; furthermore, they hold for an arbitrary orientation of the precessional velocity vector of the spin of the central object. In general, all the orbital elements, with the exception of the mean anomaly at epoch, undergo nonvanishing long–term variations which, in the case of the Juno spacecraft currently orbiting Jupiter and the double pulsar PSR J0737–3039 A/B turn out to be quite small. Such effects might become much more relevant in a star–supermassive black hole scenario; as an example, the relative change of the semimajor axis of a putative test particle orbiting a Kerr black hole as massive as the one at the Galactic Centre at, say, 100 Schwarzschild radii may amount up to about

7 %

per year if the hole’s spin precessional frequency is

10 %

of the particle’s orbital one. Full article

(This article belongs to the Special Issue Testing General Relativity from the Solar System, Black Holes to Cosmology)

23 pages, 12934 KiB

Open AccessArticle

Dynamics of Two Planets near a 2:1 Resonance: Case Studies of Known and Synthetic Exosystems on a Grid of Initial Configurations

by Valeri Makarov, Alexey Goldin and Dimitri Veras

Universe 2024, 10(9), 374; https://doi.org/10.3390/universe10090374 - 19 Sep 2024

Viewed by 421

Abstract

The distribution of period ratios for 580 known two-planet systems is apparently nonuniform, with several sharp peaks and troughs. In particular, the vicinity of the 2:1 commensurability seems to have a deficit of systems. Using Monte Carlo simulations and an empirically inferred population [...] Read more.

The distribution of period ratios for 580 known two-planet systems is apparently nonuniform, with several sharp peaks and troughs. In particular, the vicinity of the 2:1 commensurability seems to have a deficit of systems. Using Monte Carlo simulations and an empirically inferred population distribution of period ratios, we prove that this apparent dearth of near-resonant systems is not statistically significant. The excess of systems with period ratios in the wider vicinity of the 2:1 resonance is significant, however. Long-term WHFast integrations of a synthetic two-planet system on a grid period ratios from 1.87 through 2.12 reveal that the eccentricity and inclination exchange mechanism between non-resonant planets represents the orbital evolution very well in all cases, except at the exact 2:1 mean motion resonance. This resonance destroys the orderly exchange of eccentricity, while the exchange of inclination still takes place. Additional simulations of the Kepler-113 system on a grid of initial inclinations show that the secular periods of eccentricity and inclination variations are well fitted by a simple hyperbolic cosine function of the initial mutual inclination. We further investigate the six known two-planet systems with period ratios within 2% of the exact 2:1 resonance (TOI-216, KIC 5437945, Kepler-384, HD 82943, HD 73526, HD 155358) on a grid of initial inclinations and for two different initial periastron longitudes corresponding to the aligned and anti-aligned states. All these systems are found to be long-term stable except HD 73526, which is likely a false positive. The periodic orbital momentum exchange is still at work in some of these systems, albeit with much shorter cycling periods of a few years. Full article

(This article belongs to the Special Issue Formation and Evolution of Exoplanets)

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17 pages, 426 KiB

Open AccessArticle

Time-Series Feature Selection for Solar Flare Forecasting

by Yagnashree Velanki, Pouya Hosseinzadeh, Soukaina Filali Boubrahimi and Shah Muhammad Hamdi

Universe 2024, 10(9), 373; https://doi.org/10.3390/universe10090373 - 19 Sep 2024

Viewed by 416

Abstract

Solar flares are significant occurrences in solar physics, impacting space weather and terrestrial technologies. Accurate classification of solar flares is essential for predicting space weather and minimizing potential disruptions to communication, navigation, and power systems. This study addresses the challenge of selecting the [...] Read more.

Solar flares are significant occurrences in solar physics, impacting space weather and terrestrial technologies. Accurate classification of solar flares is essential for predicting space weather and minimizing potential disruptions to communication, navigation, and power systems. This study addresses the challenge of selecting the most relevant features from multivariate time-series data, specifically focusing on solar flares. We employ methods such as Mutual Information (MI), Minimum Redundancy Maximum Relevance (mRMR), and Euclidean Distance to identify key features for classification. Recognizing the performance variability of different feature selection techniques, we introduce an ensemble approach to compute feature weights. By combining outputs from multiple methods, our ensemble method provides a more comprehensive understanding of the importance of features. Our results show that the ensemble approach significantly improves classification performance, achieving values 0.15 higher in True Skill Statistic (TSS) values compared to individual feature selection methods. Additionally, our method offers valuable insights into the underlying physical processes of solar flares, leading to more effective space weather forecasting and enhanced mitigation strategies for communication, navigation, and power system disruptions. Full article

(This article belongs to the Section Solar System)

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113 pages, 1053 KiB

Open AccessArticle

Quantum Field Theory of Black Hole Perturbations with Backreaction: I General Framework

by Thomas Thiemann

Universe 2024, 10(9), 372; https://doi.org/10.3390/universe10090372 - 18 Sep 2024

Viewed by 399

Abstract

In a seminal work, Hawking showed that natural states for free quantum matter fields on classical spacetimes that solve the spherically symmetric vacuum Einstein equations are KMS states of non-vanishing temperature. Although Hawking’s calculation does not include the backreaction of matter on geometry, [...] Read more.

In a seminal work, Hawking showed that natural states for free quantum matter fields on classical spacetimes that solve the spherically symmetric vacuum Einstein equations are KMS states of non-vanishing temperature. Although Hawking’s calculation does not include the backreaction of matter on geometry, it is more than plausible that the corresponding Hawking radiation leads to black hole evaporation which is, in principle, observable. Obviously, an improvement of Hawking’s calculation including backreaction is a problem of quantum gravity. Since no commonly accepted quantum field theory of general relativity is available yet, it has been difficult to reliably derive the backreaction effect. An obvious approach is to use the black hole perturbation theory of a Schwarzschild black hole of fixed mass and to quantize those perturbations. However, it is not clear how to reconcile perturbation theory with gauge invariance beyond linear perturbations. In recent work, we proposed a new approach to this problem that applies when the physical situation has an approximate symmetry, such as homogeneity (cosmology), spherical symmetry (Schwarzschild), or axial symmetry (Kerr). The idea, which is surprisingly feasible, is to first construct the non-perturbative physical (reduced) Hamiltonian of the reduced phase space of fully gauge invariant observables and only then apply perturbation theory directly in terms of observables. The task to construct observables is then disentangled from perturbation theory, thus allowing to unambiguously develop perturbation theory to arbitrary orders. In this first paper of the series we outline and showcase this approach for spherical symmetry and second order in the perturbations for Einstein–Klein–Gordon–Maxwell theory. Details and generalizations to other matter and symmetry and higher orders will appear in subsequent companion papers. Full article

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15 pages, 353 KiB

Open AccessArticle

Constraints on Metastable Dark Energy Decaying into Dark Matter

by Jônathas S. T. de Souza, Gustavo S. Vicente and Leila L. Graef

Universe 2024, 10(9), 371; https://doi.org/10.3390/universe10090371 - 18 Sep 2024

Viewed by 427

Abstract

We revisit the proposal that an energy transfer from dark energy into dark matter can be described in field theory by a first order phase transition. We analyze a metastable dark energy model proposed in the literature, using updated constraints on the decay [...] Read more.

We revisit the proposal that an energy transfer from dark energy into dark matter can be described in field theory by a first order phase transition. We analyze a metastable dark energy model proposed in the literature, using updated constraints on the decay time of a metastable dark energy from recent data. The results of our analysis show no prospects for potentially observable signals that could distinguish this scenario from the

Λ CDM

. We analyze, for the first time, the process of bubble nucleation in this model, showing that such model would not drive a complete transition to a dark matter dominated phase even in a distant future. Nevertheless, the model is not excluded by the latest data and we confirm that the mass of the dark matter particle that would result from such a process corresponds to the mass of an axion-like particle, which is currently one of the best motivated dark matter candidates. We argue that extensions to this model, possibly with additional couplings, still deserve further attention as it could provide an interesting and viable description for an interacting dark sector scenario based in a single scalar field. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

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14 pages, 647 KiB

Open AccessArticle

Bayesian Knowledge Infusion for Studying Historical Sunspot Numbers

by Wenxin Jiang and Haisheng Ji

Universe 2024, 10(9), 370; https://doi.org/10.3390/universe10090370 - 14 Sep 2024

Viewed by 349

Abstract

A scientific method that proposes a value Y to estimate a target value

ρ

is often subject to some level of uncertainty. In the Bayesian framework, the level of uncertainty can be measured by the width of the

68 %

interval, which is [...] Read more.

A scientific method that proposes a value Y to estimate a target value

ρ

is often subject to some level of uncertainty. In the Bayesian framework, the level of uncertainty can be measured by the width of the

68 %

interval, which is the range of the middle

68 %

of the ranked

ρ

values sampled from the posterior distribution

p (ρ | Y)

. This paper considers Bayesian knowledge infusion (BKI) to reduce the uncertainty of the posterior distribution

p (ρ | Y)

based on additional knowledge that an event A happens. BKI is achieved by using a conditional prior distribution

p (ρ | A)

in the Bayes theorem, assuming that given the true

ρ

, its error-contaminated value Y is independent of event A. We use two examples to illustrate how to study whether or not it is possible to reduce uncertainty from ¹⁴C reconstruction (Y) of the annual sunspot number (SSN) (

ρ

) by infusing additional information (A) using BKI. Information (A) that SSN is from a year that has a Far Eastern record of naked eye sunspots is found to be not so effective in reducing the uncertainty. In contrast, information that SSN is from a year at a cycle minimum is found to be very effective, producing much narrower 68% intervals. The resulting Bayesian point estimates of SSN (the posterior medians of

ρ

) are cross-validated and tested on a subset of telescopically observed SSNs that were unused in the process of Bayes computation. Full article

(This article belongs to the Section Astroinformatics and Astrostatistics)

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12 pages, 835 KiB

Open AccessArticle

Primordial Axion Stars and Galaxy Halo Formation

by Alexander I. Nesterov

Universe 2024, 10(9), 369; https://doi.org/10.3390/universe10090369 - 12 Sep 2024

Viewed by 352

Abstract

Primordial axion stars, hypothetical stars formed from axions, could play an essential role in forming galaxy halos. These stars could have originated in the early universe shortly after the Big Bang. We show that the ultralight axions forming primordial stars can act as [...] Read more.

Primordial axion stars, hypothetical stars formed from axions, could play an essential role in forming galaxy halos. These stars could have originated in the early universe shortly after the Big Bang. We show that the ultralight axions forming primordial stars can act as the initial seeds for galaxy halos. Full article

(This article belongs to the Special Issue Dark Energy and Dark Matter)

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50 pages, 3558 KiB

Open AccessArticle

Dark Atoms of Nuclear Interacting Dark Matter

by Vitaly A. Beylin, Timur E. Bikbaev, Maxim Yu. Khlopov, Andrey G. Mayorov and Danila O. Sopin

Universe 2024, 10(9), 368; https://doi.org/10.3390/universe10090368 - 11 Sep 2024

Viewed by 636

Abstract

The lack of positive evidence for Weakly Interacting Massive Particles (WIMPs) as well as the lack of discovery of supersymmetric (SUSY) particles at the LHC may appeal to a non-supersymmetric solution for the Standard Model problem of the Higgs boson mass divergence, the [...] Read more.

The lack of positive evidence for Weakly Interacting Massive Particles (WIMPs) as well as the lack of discovery of supersymmetric (SUSY) particles at the LHC may appeal to a non-supersymmetric solution for the Standard Model problem of the Higgs boson mass divergence, the origin of the electroweak energy scale and the physical nature of the cosmological dark matter in the approach of composite Higgs boson. If the Higgs boson consists of charged constituents, their binding can lead to stable particles with electroweak charges. Such particles can take part in sphaleron transitions in the early Universe, which balance their excess with baryon asymmetry. Constraints on exotic charged species leave only stable particles with charge

- 2 n

possible, which can bind with n nuclei of primordial helium in neutral dark atoms. The predicted ratio of densities of dark atoms and baryonic matter determines the condition for dark atoms to dominate in the cosmological dark matter. To satisfy this condition of the dark-atom nature of the observed dark matter, the mass of new stable

- 2 n

charged particles should be within reach of the LHC for their searches. We discuss the possibilities of dark-atom binding in multi-atom systems and present state-of-the-art quantum mechanical descriptions of dark-atom interactions with nuclei. Annual modulations in such interactions with nuclei of underground detectors can explain the positive results of DAMA/NaI and DAMA/LIBRA experiments and the negative results of the underground WIMP searches. Full article

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

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19 pages, 712 KiB

Open AccessArticle

The Operational Meaning of Total Energy of Isolated Systems in General Relativity

by Abhay Ashtekar and Simone Speziale

Universe 2024, 10(9), 367; https://doi.org/10.3390/universe10090367 - 11 Sep 2024

Viewed by 435

Abstract

We present thought experiments to measure the Arnowitt–Deser–Misner

E_{ADM}

and Bondi–Sachs energy

E_{BS}

of isolated systems in general relativity. The expression of

E_{BS}

used in the protocol is likely to have other applications. In particular, it is well-suited to be [...] Read more.

We present thought experiments to measure the Arnowitt–Deser–Misner

E_{ADM}

and Bondi–Sachs energy

E_{BS}

of isolated systems in general relativity. The expression of

E_{BS}

used in the protocol is likely to have other applications. In particular, it is well-suited to be promoted to an operator in non-perturbative loop quantum gravity. Full article

(This article belongs to the Special Issue Recent Advances in Gravity: A Themed Issue in Honor of Prof. Jorge Pullin on His 60th Anniversary)

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12 pages, 266 KiB

Open AccessArticle

Initial State in Quantum Cosmology and the Proper Mass of the Universe

by Natalia Gorobey, Alexander Lukyanenko and Alexander V. Goltsev

Universe 2024, 10(9), 366; https://doi.org/10.3390/universe10090366 - 11 Sep 2024

Viewed by 333

Abstract

In the Euclidean form of the theory of gravity, where there is no dedicated time parameter, a generalized canonical form of the principle of least action is proposed. On its basis, the quantum principle of least action is formulated, in which the “dynamics” [...] Read more.

In the Euclidean form of the theory of gravity, where there is no dedicated time parameter, a generalized canonical form of the principle of least action is proposed. On its basis, the quantum principle of least action is formulated, in which the “dynamics” of the universe in the Origin is described by the eigenvector of the action operator—the wave functional on the space of 4D Riemannian geometries and configurations of matter fields in some compact region of 4D space. The corresponding eigenvalue of the action operator determines the initial state for the world history of the universe outside this region, where the metric signature is Lorentzian and, thus, the time parameter exists. The boundary of the Origin region is determined by the condition that the rate of change of the determinant of the 3D metric tensor is zero on it. The size of the Origin is interpreted as a reciprocal temperature of the universe in the initial state. It has been suggested that in the initial state, the universe contains a certain distribution of its own mass, which is not directly related to the fields of matter. Full article

(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)

28 pages, 1121 KiB

Open AccessArticle

Comparing Analytic and Numerical Studies of Tensor Perturbations in Loop Quantum Cosmology

by Guillermo A. Mena Marugán, Antonio Vicente-Becerril and Jesús Yébana Carrilero

Universe 2024, 10(9), 365; https://doi.org/10.3390/universe10090365 - 11 Sep 2024

Viewed by 418

Abstract

We investigate the implications of different quantization approaches in Loop Quantum Cosmology for the primordial power spectrum of tensor modes. Specifically, we consider the hybrid and dressed metric approaches to derive the effective mass that governs the evolution of the tensor modes. Our [...] Read more.

We investigate the implications of different quantization approaches in Loop Quantum Cosmology for the primordial power spectrum of tensor modes. Specifically, we consider the hybrid and dressed metric approaches to derive the effective mass that governs the evolution of the tensor modes. Our study comprehensively examines the two resulting effective masses and how to estimate them in order to obtain approximated analytic solutions to the tensor perturbation equations. Since Loop Quantum Cosmology incorporates preinflationary effects in the dynamics of the perturbations, we do not have at our disposal a standard choice of privileged vacuum, like the Bunch–Davies state in quasi-de Sitter inflation. We then select the vacuum state by a recently proposed criterion which removes unwanted oscillations in the power spectrum and guarantees an asymptotic diagonalization of the Hamiltonian in the ultraviolet. This vacuum is usually called the NO-AHD (from the initials of Non-Oscillating with Asymptotic Hamiltonian Diagonalization) vacuum. Consequently, we compute the power spectrum by using our analytic approximations and by introducing a suitable numerical procedure, adopting in both cases an NO-AHD vacuum. With this information, we compare the different spectra obtained from the hybrid and the dressed metric approaches, as well as from the analytic and numerical procedures. In particular, this proves the remarkable accuracy of our approximations. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology, Second Edition)

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11 pages, 535 KiB

Open AccessArticle

Predicting Solar Cycles with a Parametric Time Series Model

by Kristof Petrovay

Universe 2024, 10(9), 364; https://doi.org/10.3390/universe10090364 - 11 Sep 2024

Viewed by 408

Abstract

The objective of this paper is to reproduce and predict the series of solar cycle amplitudes using a simple time-series model that takes into account the variable time scale of the Gleissberg oscillation and the absence of clear evidence for odd–even alternation prior [...] Read more.

The objective of this paper is to reproduce and predict the series of solar cycle amplitudes using a simple time-series model that takes into account the variable time scale of the Gleissberg oscillation and the absence of clear evidence for odd–even alternation prior to Solar Cycle 9 (SC9). It is demonstrated that the Gleissberg oscillation can be quite satisfactorily modelled as a sinusoidal variation of constant amplitude with a period increasing linearly with time. Subtracting this model from the actual cycle amplitudes, a clear even–odd alternating pattern is discerned in the time series of the residuals since SC9. For this period of time, the mean value of the residuals for odd-numbered cycles is shown to exceed the value for even-numbered cycles by more than

4 σ

, providing the clearest evidence yet for a persistent odd–even–odd alternation in cycle amplitudes. Random deviations from these means are less than half the standard deviation of the raw cycle amplitude time series for the same period, which allows the use of these regularities for solar cycle prediction with substantially better confidence than the simple climatological average. Predicted cycle amplitudes are found to be robust against the addition or omission of some data points from the input set, and the method correctly hindcasts SC23 and SC24. The potential physical background of the regularities is also discussed. Our predictions for the amplitudes of SC25, SC26, and SC27 are

155.8 \pm 20.7

,

96.9 \pm 25.1

and

140.8 \pm 20.7

, respectively. This suggests that the amplitude of SC26 will be even lower than that of SC24, making it the weakest cycle since the Dalton Minimum. Full article

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

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12 pages, 272 KiB

Open AccessArticle

Ellipsoidal Universe and Cosmic Shear

by Luigi Tedesco

Universe 2024, 10(9), 363; https://doi.org/10.3390/universe10090363 - 10 Sep 2024

Viewed by 456

Abstract

We consider a Bianchi I geometry of the universe. We obtain a cosmic shear expression related to the eccentricity of the universe. In particular, we study the connections among cosmic shear, eccentricity, and CMB. The equations are self-contained, with only two parameters. Full article

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

19 pages, 452 KiB

Open AccessArticle

From de Sitter to de Sitter: A Thermal Approach to Running Vacuum Cosmology and the Non-Canonical Scalar Field Description

by Pedro Eleuterio Mendonça Almeida, Rose Clivia Santos and Jose Ademir Sales Lima

Universe 2024, 10(9), 362; https://doi.org/10.3390/universe10090362 - 9 Sep 2024

Viewed by 462

Abstract

The entire classical cosmological history between two extreme de Sitter vacuum solutions is discussed based on Einstein’s equations and non-equilibrium thermodynamics. The initial non-singular de Sitter state is characterised by a very high energy scale, which is equal or smaller than the reduced [...] Read more.

The entire classical cosmological history between two extreme de Sitter vacuum solutions is discussed based on Einstein’s equations and non-equilibrium thermodynamics. The initial non-singular de Sitter state is characterised by a very high energy scale, which is equal or smaller than the reduced Planck mass. It is structurally unstable, and all of the continuous created matter, energy, and entropy of the material component comes from the irreversible flow powered by the primeval vacuum energy density. The analytical expression describing the running vacuum is obtained from the thermal approach. It opens a new perspective to solve the old puzzles and current observational challenges plaguing the cosmic concordance model driven by a rigid vacuum. Such a scenario is also modelled through a non-canonical scalar field. It is demonstrated that the resulting scalar field model is shown to be a step-by-step a faithful analytical representation of the thermal running vacuum cosmology. Full article

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

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11 pages, 270 KiB

Open AccessArticle

On the Propagation of Gravitational Waves in the Weyl Invariant Theory of Gravity

by Mauro Duarte, Fabio Dahia and Carlos Romero

Universe 2024, 10(9), 361; https://doi.org/10.3390/universe10090361 - 9 Sep 2024

Viewed by 437

Abstract

We revisit Weyl’s unified field theory, which arose in 1918, shortly after general relativity was discovered. As is well known, in order to extend the program of the geometrization of physics started by Einstein to include the electromagnetic field, H. Weyl developed a [...] Read more.

We revisit Weyl’s unified field theory, which arose in 1918, shortly after general relativity was discovered. As is well known, in order to extend the program of the geometrization of physics started by Einstein to include the electromagnetic field, H. Weyl developed a new geometry which constitutes a kind of generalization of Riemannian geometry. In this paper, our aim is to discuss Weyl’s proposal anew and examine its consistency and completeness as a physical theory. We propose new directions and possible conceptual changes in the original work. Among these, we investigate with some detail the propagation of gravitational waves, and the new features arising in this recent modified gravity theory, in which the presence of a massive vector field appears somewhat unexpectedly. We also speculate whether the results could be examined in the context of primordial gravitational waves. Full article

(This article belongs to the Section Cosmology)

19 pages, 12053 KiB

Open AccessArticle

A Comprehensive Study on the Mid-Infrared Variability of Blazars

by Xuemei Zhang, Zhipeng Hu, Weitian Huang and Lisheng Mao

Universe 2024, 10(9), 360; https://doi.org/10.3390/universe10090360 - 7 Sep 2024

Viewed by 587

Abstract

We present a comprehensive investigation of mid-infrared (MIR) flux variability at 3.4 μm (W1 band) for a large sample of 3816 blazars, using Wide-field Infrared Survey Explorer (WISE) data through December 2022. The sample consists of 1740 flat-spectrum radio quasars (FSRQs), 1281 BL [...] Read more.

We present a comprehensive investigation of mid-infrared (MIR) flux variability at 3.4 μm (W1 band) for a large sample of 3816 blazars, using Wide-field Infrared Survey Explorer (WISE) data through December 2022. The sample consists of 1740 flat-spectrum radio quasars (FSRQs), 1281 BL Lac objects (BL Lacs), and 795 blazars of uncertain type (BCUs). Considering Fermi Large Area Telescope detection, we classify 2331 as Fermi blazars and 1485 as non-Fermi blazars. Additionally, based on synchrotron peak frequency, the sample includes 2264 low-synchrotron peaked (LSP), 512 intermediate-synchrotron peaked (ISP), and 655 high-synchrotron peaked (HSP) sources. We conduct a comparative analysis of short- and long-term intrinsic variability amplitude (

σ_{m}

), duty cycle (DC), and ensemble structure function (ESF) across blazar subclasses. The median short-term

σ_{m}

values were

0 . 181_{- 0.106}^{+ 0.153}

,

0 . 104_{- 0.054}^{+ 0.101}

,

0 . 135_{- 0.076}^{+ 0.154}

,

0 . 173_{- 0.097}^{+ 0.158}

,

0 . 177_{- 0.100}^{+ 0.156}

,

0 . 096_{- 0.050}^{+ 0.109}

, and

0 . 106_{- 0.058}^{+ 0.100}

mag for FSRQs, BL Lacs, Fermi blazars, non-Fermi blazars, LSPs, ISPs, and HSPs, respectively. The median DC values were

71 . 03_{- 22.48}^{+ 14.17}

,

64 . 02_{- 22.86}^{+ 16.97}

,

68 . 96_{- 25.52}^{+ 15.66}

,

69 . 40_{- 22.17}^{+ 14.42}

,

71 . 24_{- 21.36}^{+ 14.25}

,

63 . 03_{- 33.19}^{+ 16.93}

, and

64 . 63_{- 24.26}^{+ 15.88}

percent for the same subclasses. The median long-term

σ_{m}

values were

0 . 137_{- 0.105}^{+ 0.408}

,

0 . 171_{- 0.132}^{+ 0.206}

,

0 . 282_{- 0.184}^{+ 0.332}

,

0 . 071_{- 0.062}^{+ 0.143}

,

0 . 218_{- 0.174}^{+ 0.386}

,

0 . 173_{- 0.132}^{+ 0.208}

, and

0 . 101_{- 0.077}^{+ 0.161}

mag for the same subclasses, respectively. Our results reveal significant differences in 3.4

μ

m flux variability among these subclasses. FSRQs (LSPs) exhibit larger

σ_{m}

and DC values compared to BL Lacs (ISPs and HSPs). Fermi blazars display higher long-term

σ_{m}

but lower short-term

σ_{m}

relative to non-Fermi blazars, while DC distributions between the two groups are similar. ESF analysis further confirms the greater variability of FSRQs, LSPs, and Fermi blazars across a wide range of time scales compared to BL Lacs, ISPs/HSPs, and non-Fermi blazars. These findings highlight a close correlation between MIR variability and blazar properties, providing valuable insights into the underlying physical mechanisms responsible for their emission. Full article

(This article belongs to the Section Galaxies and Clusters)

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28 pages, 2303 KiB

Open AccessArticle

Probing the Nonlinear Density Wave Theory of Spiral Galaxies by Baryonic Tully–Fisher Relation

by Miroslava Vukcevic, Djordje Savic and Predrag Jovanović

Universe 2024, 10(9), 359; https://doi.org/10.3390/universe10090359 - 6 Sep 2024

Viewed by 397

Abstract

The baryonic mass–velocity relation provides an important test of different galaxy dynamics models such as Lambda–cold dark matter (

Λ

CDM) and alternatives like Modified Newtonian Dynamics (MOND). Novel nonlinear density wave theory with a soliton solution gives an opportunity to test whether [...] Read more.

The baryonic mass–velocity relation provides an important test of different galaxy dynamics models such as Lambda–cold dark matter (

Λ

CDM) and alternatives like Modified Newtonian Dynamics (MOND). Novel nonlinear density wave theory with a soliton solution gives an opportunity to test whether the derived rotational velocity expression is able to support the well known Tully–Fisher empirical relation between mass and rotation velocity in disk galaxies. Initial assumptions do not involve any larger dark matter halo that supports the stability of the very thin galactic disk nor any modified gravitational acceleration acting on galactic scales. It rather follows an important gravitational interaction between constituents of disk mass in the outer part of the disk via gravitational potential. Data are obtained by a fitting procedure applied on the sample of 81 rotational curves of late type spirals using expressions for the rotational velocity derived as an exact, a self-consistent solution of the nonlinear Schrodinger (NLS) equation for galactic surface mass density. The location of these selected objects in the baryonic mass–rotation velocity plane follows the relation

log M_{b} = (3.7 \pm 0.2) log V_{f l a t} + (2.7 \pm 0.4)

in marginal agreement with the findings in the literature. Full article

(This article belongs to the Special Issue Recent Advances in Gravitational Lensing and Galactic Dynamics)

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119 pages, 3293 KiB

Open AccessReview

Three-Dimensional Quantum Black Holes: A Primer

by Emanuele Panella, Juan F. Pedraza and Andrew Svesko

Universe 2024, 10(9), 358; https://doi.org/10.3390/universe10090358 - 6 Sep 2024

Viewed by 485

Abstract

We review constructions of three-dimensional ‘quantum’ black holes. Such spacetimes arise via holographic braneworlds and are exact solutions to an induced higher-derivative theory of gravity consistently coupled to a large-c quantum field theory with an ultraviolet cutoff, accounting for all orders of [...] Read more.

We review constructions of three-dimensional ‘quantum’ black holes. Such spacetimes arise via holographic braneworlds and are exact solutions to an induced higher-derivative theory of gravity consistently coupled to a large-c quantum field theory with an ultraviolet cutoff, accounting for all orders of semi-classical backreaction. Notably, such quantum-corrected black holes are much larger than the Planck length. We describe the geometry and horizon thermodynamics of a host of asymptotically (anti-) de Sitter and flat quantum black holes. A summary of higher-dimensional extensions is given. We survey multiple applications of quantum black holes and braneworld holography. Full article

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

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24 pages, 2750 KiB

Open AccessArticle

Exploring New Physics with Deep Underground Neutrino Experiment High-Energy Flux: The Case of Lorentz Invariance Violation, Large Extra Dimensions and Long-Range Forces

by Alessio Giarnetti, Simone Marciano and Davide Meloni

Universe 2024, 10(9), 357; https://doi.org/10.3390/universe10090357 - 5 Sep 2024

Viewed by 509

Abstract

DUNE is a next-generation long-baseline neutrino oscillation experiment. It is expected to measure, with unprecedented precision, the atmospheric oscillation parameters, including the CP-violating phase

δ_{C P}

. Moreover, several studies have suggested that its unique features should allow DUNE to probe several [...] Read more.

DUNE is a next-generation long-baseline neutrino oscillation experiment. It is expected to measure, with unprecedented precision, the atmospheric oscillation parameters, including the CP-violating phase

δ_{C P}

. Moreover, several studies have suggested that its unique features should allow DUNE to probe several new physics scenarios. In this work, we explore the performances of the DUNE far detector in constraining new physics if a high-energy neutrino flux is employed (HE-DUNE). We take into account three different scenarios: Lorentz Invariance Violation (LIV), Long-Range Forces (LRFs) and Large Extra Dimensions (LEDs). Our results show that HE-DUNE should be able to set bounds competitive to the current ones and, in particular, it can outperform the standard DUNE capabilities in constraining CPT-even LIV parameters and the compactification radius

R_{E D}

of the LED model. Full article

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

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24 pages, 1325 KiB

Open AccessArticle

Non-Canonical Dark Energy Parameter Evolution in a Canonical Quintessence Cosmology

by Rodger I. Thompson

Universe 2024, 10(9), 356; https://doi.org/10.3390/universe10090356 - 5 Sep 2024

Viewed by 400

Abstract

This study considers the specific case of a flat, minimally coupled to gravity, quintessence cosmology with a dark energy quartic polynomial potential that has the same mathematical form as the Higgs potential. Previous work on this case determined that the scalar field is [...] Read more.

This study considers the specific case of a flat, minimally coupled to gravity, quintessence cosmology with a dark energy quartic polynomial potential that has the same mathematical form as the Higgs potential. Previous work on this case determined that the scalar field is given by a simple expression of the Lambert W function in terms of the easily observable scale factor. This expression provides analytic equations for the evolution of cosmological dark energy parameters as a function of the scale factor for all points on the Lambert W function principal branch. The Lambert W function is zero at a scale factor of zero that marks the big bang. The evolutionary equations beyond the big bang describe a canonical universe that is similar to

Λ

CDM, making it an excellent dynamical template to compare with observational data. The portion of the W function principal before the big bang extends to the infinite pre-bang past. It describes a noncanonical universe with an initially very low mass density that contracts by rolling down the dark energy potential to a singularity, big bang, at the scale factor zero point. This provides a natural origin for the big bang. It also raises the possibility that the universe existed before the big bang and is far older, and that it was once far larger than its current size. The recent increasing interest in the possibility of a dynamical universe instead of

Λ

CDM makes the exploration of the nature of such universes particularly relevant. Full article

(This article belongs to the Special Issue Dark Energy and Dark Matter)

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14 pages, 1789 KiB

Open AccessArticle

4D Embedded Rotating Black Hole as a Particle Accelerator in the Presence of Magnetic Fields

by Abraão J. S. Capistrano, Carlos Henrique Coimbra-Araújo and Rita de Cássia dos Anjos

Universe 2024, 10(9), 355; https://doi.org/10.3390/universe10090355 - 4 Sep 2024

Viewed by 443

Abstract

We analyze a rotating black hole (BH) in a four-dimensional space-time embedded in five-dimensional flat bulk. In Boyer–Lindquist coordinates, we use a generic extension of the Kerr metric by the line element of Gürses–Gürsey metric. We discuss their horizon properties and shadow cast [...] Read more.

We analyze a rotating black hole (BH) in a four-dimensional space-time embedded in five-dimensional flat bulk. In Boyer–Lindquist coordinates, we use a generic extension of the Kerr metric by the line element of Gürses–Gürsey metric. We discuss their horizon properties and shadow cast which is tailored by the influence of the extrinsic curvature. By means of the model based on the Nash–Greene theorem, we analyze the Gürses–Gürsey metric embedded in five dimensions acting as a rotating “charged” BH which may be regarded as a source of ultrahigh-energy cosmic rays (UHECRs). We also show that this type of BH presents a different structure of the accretion disk which is modified by the extrinsic curvature leading to an enlargement of the photons ring and an increase in the BH’s inner shadow. In the presence of a magnetic field, our initial results suggest that such BHs may be efficient free-test particle accelerators orbiting the inner stable circular orbit (ISCO). Full article

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

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12 pages, 621 KiB

Open AccessArticle

Quantum Loop Corrections in the Modified Gravity Model of Starobinsky Inflation with Primordial Black Hole Production

by Sultan Saburov and Sergei V. Ketov

Universe 2024, 10(9), 354; https://doi.org/10.3390/universe10090354 - 4 Sep 2024

Viewed by 410

Abstract

A modified gravity model of Starobinsky inflation and primordial black hole production is proposed in good (within

1 σ

) agreement with current measurements of the cosmic microwave background radiation. The model is an extension of the singularity-free Appleby–Battye–Starobinsky model by the [...] Read more.

A modified gravity model of Starobinsky inflation and primordial black hole production is proposed in good (within

1 σ

) agreement with current measurements of the cosmic microwave background radiation. The model is an extension of the singularity-free Appleby–Battye–Starobinsky model by the

R^{4}

term with different values of the parameters whose fine-tuning leads to the efficient production of primordial black holes on smaller scales with the asteroid-size masses between

10^{16}

g and

10^{20}

g. Those primordial black holes may be part (or the whole) of the current dark matter, while the proposed model can be confirmed or falsified by the detection or absence of the induced gravitational waves with the frequencies in the

10^{- 2}

Hz range. The relative size of quantum (loop) corrections to the power spectrum of scalar perturbations in the model is found to be of the order of

10^{- 3}

or less, so that the model is not ruled out by the quantum corrections. Full article

(This article belongs to the Special Issue Primordial Black Holes from Inflation)

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28 pages, 21146 KiB

Open AccessArticle

Combined Studies Approach to Rule Out Cosmological Models Which Are Based on Nonlinear Electrodynamics

by Ricardo García-Salcedo, Isidro Gómez-Vargas, Tame González, Vicent Martinez-Badenes and Israel Quiros

Universe 2024, 10(9), 353; https://doi.org/10.3390/universe10090353 - 4 Sep 2024

Viewed by 447

Abstract

We apply a combined study in order to investigate the dynamics of cosmological models incorporating nonlinear electrodynamics (NLED). The study is based on the simultaneous investigation of such fundamental aspects as stability and causality, complemented with a dynamical systems investigation of the involved [...] Read more.

We apply a combined study in order to investigate the dynamics of cosmological models incorporating nonlinear electrodynamics (NLED). The study is based on the simultaneous investigation of such fundamental aspects as stability and causality, complemented with a dynamical systems investigation of the involved models, as well as Bayesian inference for parameter estimation. We explore two specific NLED models: the power-law and the rational Lagrangian. We present the theoretical framework of NLED coupled with general relativity, followed by an analysis of the stability and causality of the various NLED Lagrangians. We then perform a detailed dynamical analysis to identify the ranges where these models are stable and causal. Our results show that the power-law Lagrangian model transitions through various cosmological phases, evolving from a Maxwell radiation-dominated state to a matter-dominated state. For the rational Lagrangian model, including the Maxwell term, stable and causal behavior is observed within specific parameter ranges, with critical points indicating the evolutionary pathways of the universe. To validate our theoretical findings, we perform Bayesian parameter estimation using a comprehensive set of observational data, including cosmic chronometers, baryon acoustic oscillation (BAO) measurements, and supernovae type Ia (SNeIa). The estimated parameters for both models align with the expected values for the current universe, particularly the matter density

Ω_{m}

and the Hubble parameter h. However, the parameters of the models are not tightly constrained within the prior ranges. Our combined studies approach rules out the mentioned models as an appropriate description of the cosmos. Our results highlight the need for further refinement and exploration of NLED-based cosmological models to fully integrate them into the standard cosmological framework. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')

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19 pages, 436 KiB

Open AccessReview

Different Aspects of Entropic Cosmology

by Shin’ichi Nojiri, Sergei D. Odintsov and Tanmoy Paul

Universe 2024, 10(9), 352; https://doi.org/10.3390/universe10090352 - 3 Sep 2024

Viewed by 426

Abstract

We provide a short review of the recent developments in entropic cosmology based on two thermodynamic laws of the apparent horizon, namely the first and the second laws of thermodynamics. The first law essentially provides the change in entropy of the apparent horizon [...] Read more.

We provide a short review of the recent developments in entropic cosmology based on two thermodynamic laws of the apparent horizon, namely the first and the second laws of thermodynamics. The first law essentially provides the change in entropy of the apparent horizon during the cosmic evolution of the universe; in particular, it is expressed by

T d S = - d (ρ V) + W d V

(where W is the work density and other quantities have their usual meanings). In this way, the first law actually links various theories of gravity with the entropy of the apparent horizon. This leads to a natural question—“What is the form of the horizon entropy corresponding to a general modified theory of gravity?”. The second law of horizon thermodynamics states that the change in total entropy (the sum of horizon entropy + matter fields’ entropy) with respect to cosmic time must be positive, where the matter fields behave like an open system characterised by a non-zero chemical potential. The second law of horizon thermodynamics importantly provides model-independent constraints on entropic parameters. Finally, we discuss the standpoint of entropic cosmology on inflation (or bounce), reheating and primordial gravitational waves from the perspective of a generalised entropy function. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')

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19 pages, 698 KiB

Open AccessArticle

On the Classical Limit of Freely Falling Quantum Particles, Quantum Corrections and the Emergence of the Equivalence Principle

by Juan A. Cañas, J. Bernal and A. Martín-Ruiz

Universe 2024, 10(9), 351; https://doi.org/10.3390/universe10090351 - 2 Sep 2024

Viewed by 511

Abstract

Quantum and classical mechanics are fundamentally different theories, but the correspondence principle states that quantum particles behave classically in the appropriate limit. For high-energy periodic quantum systems, the emergence of the classical description should be understood in a distributional sense, i.e., the quantum [...] Read more.

Quantum and classical mechanics are fundamentally different theories, but the correspondence principle states that quantum particles behave classically in the appropriate limit. For high-energy periodic quantum systems, the emergence of the classical description should be understood in a distributional sense, i.e., the quantum probability density approaches the classical distribution when the former is coarse-grained. Following a simple reformulation of this limit in the Fourier space, in this paper, we investigate the macroscopic behavior of freely falling quantum particles. To illustrate how the method works and to fix some ideas, we first successfully apply it to the case of a particle in a box. Next, we show that, for a particle bouncing under the gravity field, in the limit of a high quantum number, the leading term of the quantum distribution corresponds to the exact classical distribution plus sub-leading corrections, which we interpret as quantum corrections at the macroscopic level. Full article

(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)

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20 pages, 635 KiB

Open AccessArticle

About Jordan and Einstein Frames: A Study in Inflationary Magnetogenesis

by Joel Velásquez, Héctor J. Hortua and Leonardo Castañeda

Universe 2024, 10(9), 350; https://doi.org/10.3390/universe10090350 - 1 Sep 2024

Viewed by 380

Abstract

In this paper, we make a detailed side-by-side comparison between Jordan and Einstein frames in the context of cosmic magnetogenesis. We have computed the evolution of the vector potential in each frame along with some observables such as the spectral index and the [...] Read more.

In this paper, we make a detailed side-by-side comparison between Jordan and Einstein frames in the context of cosmic magnetogenesis. We have computed the evolution of the vector potential in each frame along with some observables such as the spectral index and the magnetic field amplitude. We found that contrary to the Einstein frame, the electric and magnetic energy densities in the Jordan Frame do not depend on any parameter associated with the scalar field. Furthermore, in the Einstein frame, and assuming scale invariance for the magnetic field, most of the total energy density contribution comes from the electric and magnetic densities. Finally, we show the ratio between magnetic field signals in both frames printed in the CMB. Full article

(This article belongs to the Special Issue The Nature of Dark Energy)

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