Universe

16 pages, 354 KiB

Open AccessReview

Dark Universe from QFT Mechanisms and Possible Experimental Probes

by A. Capolupo, S. Monda, G. Pisacane, A. Quaranta and R. Serao

Universe 2025, 11(5), 142; https://doi.org/10.3390/universe11050142 - 29 Apr 2025

We report the latest results on particle mixing in quantum field theory on curved spacetimes. We highlight possible connections with dark matter and dark energy. Furthermore, we present two indirect methods to observe these phenomena: one using non-relativistic neutrinos and the other employing [...] Read more.

We report the latest results on particle mixing in quantum field theory on curved spacetimes. We highlight possible connections with dark matter and dark energy. Furthermore, we present two indirect methods to observe these phenomena: one using non-relativistic neutrinos and the other employing an atomic analogue. Full article

(This article belongs to the Section Cosmology)

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23 pages, 10902 KiB

Open AccessArticle

Bayesian Analysis of Hybrid Neutron Star EOS Constraints Within an Instantaneous Nonlocal Chiral Quark Matter Model

by Alexander Ayriyan, David Blaschke, Juan Pablo Carlomagno, Gustavo A. Contrera and Ana Gabriela Grunfeld

Universe 2025, 11(5), 141; https://doi.org/10.3390/universe11050141 - 29 Apr 2025

Abstract

We present a physics-informed Bayesian analysis of equation of state constraints using observational data for masses, radii and tidal deformability of pulsars and a generic class of hybrid neutron star equation of state with color superconducting quark matter on the basis of a [...] Read more.

We present a physics-informed Bayesian analysis of equation of state constraints using observational data for masses, radii and tidal deformability of pulsars and a generic class of hybrid neutron star equation of state with color superconducting quark matter on the basis of a recently developed nonlocal chiral quark model. The nuclear matter phase is described within a relativistic density functional model of the DD2 class and the phase transition is obtained by a Maxwell construction. We find the region in the two-dimensional parameter space spanned by the vector meson coupling and the scalar diquark coupling, where three conditions are fulfilled: (1) the Maxwell construction can be performed, (2) the maximum mass of the hybrid neutron star is not smaller than 2.0

M_{⊙}

and (3) the onset density of the phase transition is not below the nuclear saturation density

n_{0} = 0.15

fm⁻³. The result of this study shows that the favorable neutron star equation of state has low onset masses for the occurrence of a color superconducting quark matter core between 0.5–0.7

M_{⊙}

and maximum masses in the range 2.15–2.22

M_{⊙}

. In the typical mass range of 1.2–2.0

M_{⊙}

, the radii of these stars are between 11.9 and 12.4 km, almost independent of the mass. In principle, hybrid stars would allow for larger maximum masses than provided by the hadronic reference equation of state. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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19 pages, 934 KiB

Open AccessArticle

Evidence of Gapless Superfluidity in MXB 1659-29 With and Without Late Time Cooling

by Valentin Allard and Nicolas Chamel

Universe 2025, 11(5), 140; https://doi.org/10.3390/universe11050140 - 27 Apr 2025

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Abstract

The interpretation of the thermal relaxation of some transiently accreting neutron stars in quasipersistent soft X-ray transients, especially MXB 1659-29, has been found to be challenging within the traditional deep crustal heating paradigm. Due to the pinning of quantized vortices, the neutron superfluid [...] Read more.

The interpretation of the thermal relaxation of some transiently accreting neutron stars in quasipersistent soft X-ray transients, especially MXB 1659-29, has been found to be challenging within the traditional deep crustal heating paradigm. Due to the pinning of quantized vortices, the neutron superfluid is not expected to remain at rest in the crust, as was generally assumed. We have recently shown that for sufficiently large relative superflows, the neutron superfluid could become gapless. This dynamical phase could naturally explain the late-time cooling of MXB 1659-29. However, the interpretation of the last observation of MXB 1659-29 in 2013 before its second accretion phase in 2015 remains debated, with some spectral fits being consistent with no further temperature decline. Here, we revisit the cooling of this neutron star considering the different fits. New simulations of the crust cooling are performed, accounting for neutron diffusion and allowing for gapless superfluidity. In all cases, gapless superfluidity is found to provide the best fit to observations. Full article

(This article belongs to the Special Issue Challenges and Future Directions in Neutron Star Research)

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10 pages, 246 KiB

Open AccessArticle

Is Planckian Discreteness Observable in Cosmology?

by Gabriel R. Bengochea, Gabriel León and Alejandro Perez

Universe 2025, 11(5), 139; https://doi.org/10.3390/universe11050139 - 27 Apr 2025

Viewed by 129

Abstract

A Planck scale inflationary era—in a quantum gravity theory predicting discreteness of quantum geometry at the fundamental scale—produces the scale-invariant spectrum of inhomogeneities with a very small tensor-to-scalar ratio of perturbations and a hot big bang leading to a natural dark matter genesis [...] Read more.

A Planck scale inflationary era—in a quantum gravity theory predicting discreteness of quantum geometry at the fundamental scale—produces the scale-invariant spectrum of inhomogeneities with a very small tensor-to-scalar ratio of perturbations and a hot big bang leading to a natural dark matter genesis scenario. Here, we evoke the possibility that some of the major puzzles in cosmology would have an explanation rooted in quantum gravity. Full article

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

10 pages, 399 KiB

Open AccessArticle

Correlating the 0νββ-Decay Amplitudes of ¹³⁶Xe with the Ordinary Muon Capture (OMC) Rates of ¹³⁶Ba

by Aagrah Agnihotri, Vikas Kumar and Jouni Suhonen

Universe 2025, 11(5), 138; https://doi.org/10.3390/universe11050138 - 27 Apr 2025

Viewed by 115

Abstract

The potential correlation between the ordinary muon capture (OMC) on ¹³⁶Ba and

0 ν β β

decay of ¹³⁶Xe is explored. For this, we compute

0 ν β β

-decay amplitudes for intermediate states in ¹³⁶Cs below 1 MeV of [...] Read more.

The potential correlation between the ordinary muon capture (OMC) on ¹³⁶Ba and

0 ν β β

decay of ¹³⁶Xe is explored. For this, we compute

0 ν β β

-decay amplitudes for intermediate states in ¹³⁶Cs below 1 MeV of excitation and for angular-momentum values

J \leq 5

by using the proton–neutron quasiparticle random-phase approximation (pnQRPA) and nuclear shell model (NSM). We compare these amplitudes with the corresponding OMC rates, computed in a previous Universe article (Universe 2023, 9, 270) for the same energy and angular-momentum ranges. The obtained results suggest that an extension of the present analysis to a wider energy and angular-momentum region could be highly beneficial for probing the

0 ν β β

-decay nuclear matrix elements using experimental data on OMC rates to intermediate states of

0 ν β β

decays. Full article

(This article belongs to the Special Issue Exploring Double Beta Decay: Probing Fundamental Properties of Neutrinos and Beyond)

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21 pages, 350 KiB

Open AccessArticle

Time-like Extra Dimensions: Quantum Nonlocality, Spin, and Tsirelson Bound

by Mohammad Furquan, Tejinder P. Singh and P Samuel Wesley

Universe 2025, 11(5), 137; https://doi.org/10.3390/universe11050137 - 27 Apr 2025

Viewed by 250

Abstract

The

E_{8} \otimes E_{8}

octonionic theory of unification suggests that our universe is six-dimensional and that the two extra dimensions are time-like. These time-like extra dimensions, in principle, offer an explanation of the quantum nonlocality puzzle, also known as the EPR [...] Read more.

The

E_{8} \otimes E_{8}

octonionic theory of unification suggests that our universe is six-dimensional and that the two extra dimensions are time-like. These time-like extra dimensions, in principle, offer an explanation of the quantum nonlocality puzzle, also known as the EPR paradox. Quantum systems access all six dimensions, whereas classical systems such as detectors experience only four dimensions. Therefore, correlated quantum events that are time-like separated in 6D can appear to be space-like separated and, hence, nonlocal, when projected to 4D. Our lack of awareness of the extra time-like dimensions creates the illusion of nonlocality, whereas, in reality, the communication obeys special relativity and is local. Bell inequalities continue to be violated because quantum correlations continue to hold. In principle, this idea can be tested experimentally. We develop our analysis after first constructing the Dirac equation in 6D using quaternions and using the equation to derive spin matrices in 6D and then in 4D. We also show that the Tsirelson bound of the CHSH inequality can in principle be violated in 6D. 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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24 pages, 392 KiB

Open AccessArticle

Group-Theoretical Classification of Orientable Objects and Particle Phenomenology

by Dmitry M. Gitman and Aleksey L. Shelepin

Universe 2025, 11(5), 136; https://doi.org/10.3390/universe11050136 - 25 Apr 2025

Viewed by 59

Abstract

The quantum description of relativistic orientable objects by a scalar field on the Poincaré group is considered. The position of the relativistic orientable object in Minkowski space is completely determined by the position of a body-fixed reference frame with respect to the position [...] Read more.

The quantum description of relativistic orientable objects by a scalar field on the Poincaré group is considered. The position of the relativistic orientable object in Minkowski space is completely determined by the position of a body-fixed reference frame with respect to the position of the space-fixed reference frame, so that all the positions can be specified by elements q of the Poincaré group. Relativistic orientable objects are described by scalar wave functions

f (q)

, where the arguments

q = (x, z)

consist of space–time points x and of orientation variables z from

S L (2, C)

matrices. We introduce and study the double-sided representation

T (g) f (q) = f (g_{l}^{- 1} q g_{r})

,

g = (g_{l}, g_{r}) \in M,

of the group

M

. Here, the left multiplication by

g_{l}^{- 1}

corresponds to a change in a space-fixed reference frame, whereas the right multiplication by

g_{r}

corresponds to a change in a body-fixed reference frame. On this basis, we develop a classification of orientable objects and draw attention to the possibility of connecting these results with particle phenomenology. In particular, we demonstrate how one may identify fields described by polynomials in z with known elementary particles of spins 0,

\frac{1}{2}

, and 1. The developed classification does not contradict the phenomenology of elementary particles and, in some cases, even provides a group-theoretic explanation for it. Full article

(This article belongs to the Section Field Theory)

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18 pages, 4236 KiB

Open AccessArticle

Deep-Learning-Based Solar Flare Prediction Model: The Influence of the Magnetic Field Height

by Lei Hu, Zhongqin Chen, Long Xu and Xin Huang

Universe 2025, 11(5), 135; https://doi.org/10.3390/universe11050135 - 24 Apr 2025

Viewed by 154

Abstract

Solar flares, caused by magnetic field reconnection in the sun’s atmosphere, are intense bursts of electromagnetic radiation that can disrupt the Earth’s space environment, affecting communication systems, GPSs, and satellites. Traditional physics-based methods for solar flare forecasting have utilized the statistical relationships between [...] Read more.

Solar flares, caused by magnetic field reconnection in the sun’s atmosphere, are intense bursts of electromagnetic radiation that can disrupt the Earth’s space environment, affecting communication systems, GPSs, and satellites. Traditional physics-based methods for solar flare forecasting have utilized the statistical relationships between solar activity indicators, such as sunspots and magnetic field properties, employing techniques like Poisson distributions and discriminant analysis to estimate probabilities and identify critical parameters. While these methods provide valuable insights, limitations in predictive accuracy have driven the integration of deep learning approaches. With the accumulation of solar observation data and the development of data-driven algorithms, deep learning methods have been widely used to build solar flare prediction models. Most research has focused on designing or selecting the right deep network for the task. However, the influence of the magnetic field height on deep-learning-based prediction models has not been studied. This paper investigates how different magnetic field heights affect solar flare prediction performance. Active regions were observed using HMI magnetograms from 2010 to 2019. The magnetic field heights were stratified to create a database of active regions, and deep neural networks like AlexNet, ResNet-18, and SqueezeNet were used to evaluate prediction performance. The results show that predictions at around 7200 km above the photosphere outperform other heights, aligning with physical method analysis. At this altitude, the average AUC of the predictions from the three models reaches 0.788. Full article

(This article belongs to the Special Issue Measurements, Observations and Theoretical Studies on the Solar Magnetic Field—Celebrating the 40th Anniversary of the Huairou Solar Observing Station)

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17 pages, 277 KiB

Open AccessArticle

On-Shell Calculation of Low-Energy Photon–Photon Scattering

by Barry R. Holstein

Universe 2025, 11(5), 134; https://doi.org/10.3390/universe11050134 - 24 Apr 2025

Viewed by 137

Abstract

Although photon–photon scattering does not exist at the tree level, this is no longer the case at loop order and was first calculated by Euler and Heisenberg. The existence of this phenomenon has now been confirmed experimentally by the ATLAS collaboration and plays [...] Read more.

Although photon–photon scattering does not exist at the tree level, this is no longer the case at loop order and was first calculated by Euler and Heisenberg. The existence of this phenomenon has now been confirmed experimentally by the ATLAS collaboration and plays a small but important role in the calculation of

g_{μ}

-2. We show how the low-energy form of the

γ γ

scattering amplitude can be determined via causal (on-shell) methods using Compton scattering helicity amplitudes as input for the case of charged S = 0, S =

1 / 2

, and S = 1 intermediate state fields. Full article

(This article belongs to the Section Field Theory)

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18 pages, 431 KiB

Open AccessArticle

Reconciling the Waiting Time Peaks Variations of Repeating FRBs with an Eccentric Neutron Star–White Dwarf Binary

by Hao-Yan Chen

Universe 2025, 11(5), 133; https://doi.org/10.3390/universe11050133 - 22 Apr 2025

Viewed by 148

Abstract

Fast radio bursts (FRBs) are luminous radio transients with millisecond duration. For some active repeaters, such as FRBs 20121102A and 20201124A, more than a thousand bursts have been detected by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The waiting time (WT) distributions of [...] Read more.

Fast radio bursts (FRBs) are luminous radio transients with millisecond duration. For some active repeaters, such as FRBs 20121102A and 20201124A, more than a thousand bursts have been detected by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The waiting time (WT) distributions of both repeaters, defined as the time intervals between adjacent (detected) bursts, exhibit a bimodal structure well-fitted by two log-normal functions. Notably, the time scales of the long-duration WT peaks for both repeaters show a decreasing trend over time. These similar burst features suggest that there may be a common physical mechanism for FRBs 20121102A and 20201124A. In this paper, we revisit the neutron star (NS)–white dwarf (WD) binary model with an eccentric orbit to account for the observed changes in the long-duration WT peaks. According to our model, the shortening of the WT peaks corresponds to the orbital period decay of the NS-WD binary. We consider two mass transfer modes, namely, stable and unstable mass transfer, to examine how the orbital period evolves. Our findings reveal distinct evolutionary pathways for the two repeaters: for FRB 20121102A, the NS-WD binary likely undergoes a combination of common envelope (CE) ejection and Roche lobe overflow, whereas for FRB 20201124A the system may experience multiple CE ejections. These findings warrant further validation through follow-up observations. Full article

(This article belongs to the Special Issue New Insights into High-Energy Astrophysics, Galaxies, and Cosmology—Celebrating the 10th Anniversary of the Re-establishment of the Department of Astronomy at Xiamen University (2012–2022))

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Universe, Volume 11, Issue 5 (May 2025) – 10 articles

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