Universe

22 pages, 4740 KiB

Open AccessArticle

Determining the Scale Length and Height of the Milky Way’s Thick Disc Using RR Lyrae

by Roman Tkachenko, Katherine Vieira, Artem Lutsenko, Vladimir Korchagin and Giovanni Carraro

Universe 2025, 11(4), 132; https://doi.org/10.3390/universe11040132 - 17 Apr 2025

Viewed by 226

Using the RR Lyrae surveys Gaia DR3 Specific Objects Study, PanSTARRS1 and ASAS-SN-II, we determine the Milky Way’s thick disc scale length and scale height as well as the radial scale length of the galaxy’s inner halo. We use a Bayesian approach to [...] Read more.

Using the RR Lyrae surveys Gaia DR3 Specific Objects Study, PanSTARRS1 and ASAS-SN-II, we determine the Milky Way’s thick disc scale length and scale height as well as the radial scale length of the galaxy’s inner halo. We use a Bayesian approach to estimate these values using two independent techniques: Markov chain Monte Carlo sampling, and importance nested sampling. We consider two vertical density profiles for the thick disc. In the exponential model, the scale length of the thick disc is

h_{R} = {2.14}_{- 0.17}^{+ 0.19}

kpc, and its scale height is

h_{z} = {0.64}_{- 0.06}^{+ 0.06}

kpc. In the squared hyperbolic secant profile

s e c h^{2}

, those values are correspondingly

h_{R} = {2.10}_{- 0.17}^{+ 0.19}

kpc and

h_{z} = {1.02}_{- 0.08}^{+ 0.09}

kpc. The density distribution of the inner halo can be described as a power law function with the exponent

n = - {2.35}_{- 0.05}^{+ 0.05}

and flattening

q = {0.57}_{- 0.02}^{+ 0.02}

. We also estimate the halo to disc concentration ratio as

γ = {0.19}_{- 0.02}^{+ 0.02}

for the exponential disc and

γ = {0.32}_{- 0.03}^{+ 0.03}

for the

s e c h^{2}

disc. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—"Galaxies and Clusters")

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18 pages, 492 KiB

Open AccessArticle

Inflection Point Dynamics of Minimally Coupled Tachyonic Scalar Fields

by Jaskirat Kaur, S. D. Pathak, Maxim Khlopov and Manabendra Sharma

Universe 2025, 11(4), 131; https://doi.org/10.3390/universe11040131 - 14 Apr 2025

Viewed by 189

Abstract

In this paper, we explore the behavior of a minimally coupled tachyonic scalar field at an inflection point within an accelerating universe. We examine various cosmic expansion factors, including power-law, exponential, and a hybrid form combining power-law and exponential growth. For each of [...] Read more.

In this paper, we explore the behavior of a minimally coupled tachyonic scalar field at an inflection point within an accelerating universe. We examine various cosmic expansion factors, including power-law, exponential, and a hybrid form combining power-law and exponential growth. For each of these scenarios, we derive the corresponding potentials of the tachyonic scalar field. Subsequently, we calculate the inflection points of the spatially homogeneous tachyonic scalar field for these potentials. To further analyze the system, we employ dynamical system analysis techniques to identify equilibrium points and assess their stability. Full article

(This article belongs to the Special Issue The 10th Anniversary of Universe: Standard Cosmological Models, and Modified Gravity and Cosmological Tensions)

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17 pages, 3782 KiB

Open AccessArticle

Observability of Acausal and Uncorrelated Optical Quasar Pairs for Quantum-Mechanical Experiments

by Eric Steinbring

Universe 2025, 11(4), 130; https://doi.org/10.3390/universe11040130 - 13 Apr 2025

Viewed by 171

Abstract

Viewing high-redshift sources at near-opposite directions on the sky can ensure, using light-travel-time arguments, acausality between their emitted photons. One utility would be true random-number generation through sensing these via two independent telescopes that each flip a switch based on the latest-arrived colours; [...] Read more.

Viewing high-redshift sources at near-opposite directions on the sky can ensure, using light-travel-time arguments, acausality between their emitted photons. One utility would be true random-number generation through sensing these via two independent telescopes that each flip a switch based on the latest-arrived colours; for example, to autonomously control a quantum-mechanical (QM) experiment. Although demonstrated with distant quasars, those were not fully acausal pairs, which are restricted when simultaneously viewed from the ground at any single observatory. In optical light, such faint sources also require a large telescope aperture to avoid sampling assumptions when imaged at fast camera framerates: unsensed intrinsic correlations between them or equivalently correlated noise may ruin the expectation of pure randomness. One such case that could spoil a QM test is considered. Based on that, the allowed geometries and instrumental limits are modelled for any two ground-based sites, and their data are simulated. For comparison, an analysis of photometry from the Gemini twin 8 m telescopes is presented using the archival data of well-separated bright stars obtained with the instruments ‘Alopeke (on Gemini North in Hawai’i) and Zorro (on Gemini-South in Chile) simultaneously in two bands (centred at

562 nm

and

832 nm

) with 17 Hz framerate. No flux correlation is found; these results were used to calibrate an analytic model predicting where a search with a signal-to-noise over 50 at 50 Hz can be made using the same instrumentation. Finally, the software PDQ (Predict Different Quasars) is presented, which searches a large catalogue of known quasars, reporting those with a brightness and visibility suitable to verify acausal, uncorrelated photons at these limits. Full article

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

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15 pages, 1746 KiB

Open AccessArticle

The Impact of Stellar Initial Mass Function on the Epoch of Reionization: Insights from Semi-Analytic Galaxy Modeling

by Qingbo Ma, Lei Liu and Peiai Liu

Universe 2025, 11(4), 129; https://doi.org/10.3390/universe11040129 - 12 Apr 2025

Viewed by 254

Abstract

The adequate choice of stellar initial mass function (IMF) is crucial when studying high-z galaxy formation and the epoch of reionization (EoR) models. We employ the semi-analytical galaxy model L-Galaxies2020 and the dark matter simulation Millennium-II, in combination with the BPASS [...] Read more.

The adequate choice of stellar initial mass function (IMF) is crucial when studying high-z galaxy formation and the epoch of reionization (EoR) models. We employ the semi-analytical galaxy model L-Galaxies2020 and the dark matter simulation Millennium-II, in combination with the BPASS spectral model, to investigate the effects of different stellar IMFs on the properties of high-z galaxies and their ionizing photon budget during EoR. We find that different stellar IMFs lead to different SED of high-z galaxies, and thus different ultraviolet luminosity functions (UVLF) and budgets of ionizing photons for EoR. Specifically, at

z < 10

, the UVLF with Salpeter and Chabrier IMF models are closer to the observed results, while at

z > 10

, the ones with a Top-Heavy model are more consistent with the JWST observations. The increase in the upper limit of star mass within stellar IMF from

100 M_{⊙}

to

300 M_{⊙}

results in the increase in the UVLF and the ionizing photon number density. Full article

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19 pages, 846 KiB

Open AccessArticle

Scaling Invariance of Perturbations in k-Inflation Models

by Neven Bilić, Dragoljub D. Dimitrijević, Goran S. Djordjević, Milan Milošević and Marko Stojanović

Universe 2025, 11(4), 128; https://doi.org/10.3390/universe11040128 - 9 Apr 2025

Viewed by 153

Abstract

We study the background and perturbations in k-essence inflation models and show that a general k-essence exhibits a simple scaling property. In particular, we study two classes of k-inflation models with the potential characterized by an inflection point. We demonstrate [...] Read more.

We study the background and perturbations in k-essence inflation models and show that a general k-essence exhibits a simple scaling property. In particular, we study two classes of k-inflation models with the potential characterized by an inflection point. We demonstrate that these models enjoy scaling properties that could be used to redefine input parameters so that the perturbations spectra satisfy the correct normalization at the CMB pivot scale. The background and perturbation equations are integrated numerically for two specific models. 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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11 pages, 9181 KiB

Open AccessArticle

Extraplanar [C II] and Hα in the Edge-On Galaxy NGC 5775

by William T. Reach, Dario Fadda, Richard J. Rand and Gordon J. Stacey

Universe 2025, 11(4), 127; https://doi.org/10.3390/universe11040127 - 9 Apr 2025

Viewed by 170

Abstract

Spiral galaxies are thin and susceptible to being disrupted vertically. The largest star clusters, and nuclear starbursts, generate enough energy from winds and supernovae to send disk material to the halo. Observations of edge-on galaxies allow for the clearest view of vertical disruptions. [...] Read more.

Spiral galaxies are thin and susceptible to being disrupted vertically. The largest star clusters, and nuclear starbursts, generate enough energy from winds and supernovae to send disk material to the halo. Observations of edge-on galaxies allow for the clearest view of vertical disruptions. We present new observations of the nearby, edge-on galaxy NGC 5775 with SOFIA in [C II]

157.7 μ

m and archival images from Hubble in H

α

to search for extraplanar gas. The extraplanar [C II] extends 2 kpc from the midplane over much of the star-forming disk. The extraplanar [C II] at 2 kpc from the midplane approximately follows the rotation of the disk, with a lag of approximately 40 km

s^{- 1}

; this lag is similar to what has been previously reported in H

α

. Significant vertical extensions (to 3 kpc) are seen on the northeast side of the galaxy, potentially due to super star clusters in the NGC 5775 disk combined with gravitational interaction with the companion galaxy NGC 5774. The H

α

narrow-band image reveals a narrow plume that extends 7 kpc from the nucleus and is almost exactly perpendicular to the disk. The plume shape is similar to that seen from the comparable galaxy NGC 3628 and may arise from the nuclear starburst. Alternatively, the H

α

plume could be a relic of past activity. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—"Galaxies and Clusters")

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35 pages, 541 KiB

Open AccessArticle

Negation of the Smooth Poincare Conjecture in Dimension 4 and Negation of the Tsirelson’s Conjecture Shed Light on Quantum Gravity

by Jerzy Król and Torsten Asselmeyer-Maluga

Universe 2025, 11(4), 126; https://doi.org/10.3390/universe11040126 - 8 Apr 2025

Viewed by 151

Abstract

If spacetime is a physical object, it is conceivable that it loses its integrity or is destroyed in some way as a continuum in an abrupt process initiated in spacetime itself. An example is a gravitational collapse leading to a spacetime singularity, as [...] Read more.

If spacetime is a physical object, it is conceivable that it loses its integrity or is destroyed in some way as a continuum in an abrupt process initiated in spacetime itself. An example is a gravitational collapse leading to a spacetime singularity, as in the interior of a black hole. We find a conservative extension of quantum mechanics by quantum set theory over the singular domain and show that it is reconcilable with the special extension of spacetime 4-diffeomorphisms by automorphisms of Boolean models of set theory. The extension of quantum mechanics supports the random sequences of the quantum mechanical outcomes that can negate Tsirelson’s conjecture, whereas the extension of 4-diffeomorphisms indicates the role of exotic smooth 4-spheres as gravitational instantons. This leads to the negation of the smooth 4-dimensional Poincaré conjecture before its final resolution by mathematicians. We also discuss the case where the Poincaré conjecture would remain true. Full article

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

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13 pages, 292 KiB

Open AccessArticle

Polyadic Supersymmetry

by Steven Duplij

Universe 2025, 11(4), 125; https://doi.org/10.3390/universe11040125 - 8 Apr 2025

Viewed by 184

Abstract

We introduce a polyadic analog of supersymmetry by considering the polyadization procedure (proposed by the author) applied to the toy model of one-dimensional supersymmetric quantum mechanics. The supercharges are generalized to polyadic ones using the n-ary sigma matrices defined in earlier work. [...] Read more.

We introduce a polyadic analog of supersymmetry by considering the polyadization procedure (proposed by the author) applied to the toy model of one-dimensional supersymmetric quantum mechanics. The supercharges are generalized to polyadic ones using the n-ary sigma matrices defined in earlier work. In this way, polyadic analogs of supercharges and Hamiltonians take the cyclic shift block matrix form, and they are different from the N-extended and multigraded SQM. While constructing the corresponding supersymmetry as an n-ary Lie superalgebra (n is the arity of the initial associative multiplication), we have found new brackets with a reduced arity of

2 \leq m < n

and a related series of m-ary superalgebras (which is impossible for binary superalgebras). In the case of even reduced arity m, we obtain a tower of higher-order (as differential operators) even Hamiltonians, while for m odd we obtain a tower of higher-order odd supercharges, and the corresponding algebra consists of the odd sector only. Full article

13 pages, 1735 KiB

Open AccessArticle

Performance of Reduced Polarimetric Optical Switching Demodulation Technique in Different Spatio-Temporal Polarization Modulation Schemes

by Zhi Xu and Yue Zhong

Universe 2025, 11(4), 124; https://doi.org/10.3390/universe11040124 - 7 Apr 2025

Viewed by 131

Abstract

The beam exchange is a classical supplementary technique for spatio-temporal modulation in a dual-beam setup. In order to save time, the reduced polarimetric-optical-switching (RPOS) technique was propsed as an alternative technique. In this work, we revisit the assumptions of several formulas specifically constructed [...] Read more.

The beam exchange is a classical supplementary technique for spatio-temporal modulation in a dual-beam setup. In order to save time, the reduced polarimetric-optical-switching (RPOS) technique was propsed as an alternative technique. In this work, we revisit the assumptions of several formulas specifically constructed for this technique and evaluate their validity in different modulation schemes (e.g., dependent modulation), especially when reference measurements are acquired using specific Stokes signals. Subsequently, we compare the RPOS technique based on the most appropriate formula with the demodulation method based on the demodulation matrix by using synthesized observation data. The artificial observation takes into account the influence several factors have on the modulated intensities, including dark current, gain variation, atmospheric seeing fluctuations, and photon noise. Our numerical tests demonstrate that the RPOS technique has an advantage in mitigating the effects of atmospheric seeing fluctuations and gain variations between two beams. However, the selection of a specific Stokes signal for reference measurements has a notable impact on performance in minimizing the effect of photon noise. 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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15 pages, 828 KiB

Open AccessArticle

New Results of the Experiment to Search for Double Beta Decay of ¹⁰⁶Cd with Enriched ¹⁰⁶CdWO₄ Scintillator

by P. Belli, R. Bernabei, F. Cappella, V. Caracciolo, R. Cerulli, F. A. Danevich, A. Incicchitti, D. V. Kasperovych, V. R. Klavdiienko, V. V. Kobychev, A. Leoncini, V. Merlo, O. G. Polischuk and V. I. Tretyak

Universe 2025, 11(4), 123; https://doi.org/10.3390/universe11040123 - 7 Apr 2025

Viewed by 221

Abstract

In this article, we present current results of the experiment searching for double beta decay of ¹⁰⁶Cd with the help of an enriched ¹⁰⁶CdWO₄ crystal scintillator in coincidence with two CdWO₄ scintillation detectors. The experiment is carried out at [...] Read more.

In this article, we present current results of the experiment searching for double beta decay of ¹⁰⁶Cd with the help of an enriched ¹⁰⁶CdWO₄ crystal scintillator in coincidence with two CdWO₄ scintillation detectors. The experiment is carried out at the Gran Sasso underground laboratory of the National Institute for Nuclear Physics (LNGS INFN, Italy). After 1075 days of data-taking, no double-beta effects were observed. New half-life limits have been set for the different modes and channels of double beta processes in ¹⁰⁶Cd at the level of

lim T_{1 / 2} = 10^{20} - 10^{22}

years. Full article

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

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32 pages, 3983 KiB

Open AccessArticle

Parameter Estimation Precision with Geocentric Gravitational Wave Interferometers: Monochromatic Signals

by Manoel Felipe Sousa, Tabata Aira Ferreira and Massimo Tinto

Universe 2025, 11(4), 122; https://doi.org/10.3390/universe11040122 - 7 Apr 2025

Viewed by 235

Abstract

We present a Fisher information matrix study of the parameter estimation precision achievable by a class of future space-based, “mid-band”, gravitational wave interferometers observing monochromatic signals. The mid-band is the frequency region between that accessible by the Laser Interferometer Space Antenna (LISA) and [...] Read more.

We present a Fisher information matrix study of the parameter estimation precision achievable by a class of future space-based, “mid-band”, gravitational wave interferometers observing monochromatic signals. The mid-band is the frequency region between that accessible by the Laser Interferometer Space Antenna (LISA) and ground-based interferometers. We analyze monochromatic signals observed by the TianQin mission, gLISA (a LISA-like interferometer in a geosynchronous orbit) and a descoped gLISA mission, gLISA_d, characterized by an acceleration noise level that is three orders of magnitude worse than that of gLISA. We find that all three missions achieve their best angular source reconstruction precision in the higher part of their accessible frequency band, with an error box better than

10^{- 10}

sr in the frequency band [

10^{- 1}, 10

] Hz when observing a monochromatic gravitational wave signal of amplitude

h_{0} = 10^{- 21}

that is incoming from a given direction. In terms of their reconstructed frequencies and amplitudes, TianQin achieves its best precision values in both quantities in the frequency band [

10^{- 2}, 4 \times 10^{- 1}

] Hz, with a frequency precision

σ_{f_{g w}} = 2 \times 10^{- 11}

Hz and an amplitude precision

σ_{h_{0}} = 2 \times 10^{- 24}

. gLISA matches these precisions in a frequency band slightly higher than that of TianQin, [

3 \times 10^{- 2}, 1

] Hz, as a consequence of its smaller arm length. gLISA_d, on the other hand, matches the performance of gLISA only over the narrower frequency region, [

7 \times 10^{- 1}, 1

] Hz, as a consequence of its higher acceleration noise at lower frequencies. The angular, frequency, and amplitude precisions as functions of the source sky location are then derived by assuming an average signal-to-noise ratio of 10 at a selected number of gravitational wave frequencies covering the operational bandwidth of TianQin and gLISA. Similar precision functions are then derived for gLISA_d by using the amplitudes resulting in the gLISA average SNR being equal to 10 at the selected frequencies. We find that, for any given source location, all three missions display a marked precision improvement in the three reconstructed parameters at higher gravitational wave frequencies. Full article

(This article belongs to the Special Issue Exploring Low-Frequency Gravitational Wave Sources: Waveforms, Detection and Sciences)

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15 pages, 565 KiB

Open AccessArticle

Scanning the Universe for Large-Scale Structures Using Gamma-Ray Bursts

by Istvan Horvath, Zsolt Bagoly, Lajos G. Balazs, Jon Hakkila, Bendeguz Koncz, Istvan I. Racz, Peter Veres and Sandor Pinter

Universe 2025, 11(4), 121; https://doi.org/10.3390/universe11040121 - 6 Apr 2025

Viewed by 235

Abstract

In the past few decades, large universal structures have been found that challenge the homogeneity and isotropy expected in standard cosmological models. The largest of these, identified as the Hercules–Corona Borealis Great Wall, was found in 2014 in the northern galactic hemisphere in [...] Read more.

In the past few decades, large universal structures have been found that challenge the homogeneity and isotropy expected in standard cosmological models. The largest of these, identified as the Hercules–Corona Borealis Great Wall, was found in 2014 in the northern galactic hemisphere in the redshift range of

1.6 \leq z \leq 2.1

. Subsequent studies used an increasing gamma-ray burst database to show that the cluster was unlikely to have been caused by statistical sampling uncertainties. This study re-examines burst clustering in the northern galactic hemisphere using a recently developed methodology. Evidence is provided that the Hercules–Corona Borealis Great Wall cluster is larger than previously thought, with members potentially spanning the redshift range of

0.33 \leq z \leq 2.43

. The extension of this cluster’s size does not appear to have been due to statistical variations or sampling biases. Full article

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13 pages, 885 KiB

Open AccessArticle

Machine Learning to Simulate Quantum Computing System Errors from Physical Observations

by Jie Feng, Xingchen Zhang, Guanhao Feng and Hong-Hao Zhang

Universe 2025, 11(4), 120; https://doi.org/10.3390/universe11040120 - 4 Apr 2025

Viewed by 251

Abstract

In the context of quantum computing, error correction remains a pivotal challenge, primarily due to imperfect gate operations and environmental interactions. This study introduces a machine learning-based method to simulate and analyze these errors. Utilizing a minimal scalable 2-Majorana-zero-mode (2-MZM) island model within [...] Read more.

In the context of quantum computing, error correction remains a pivotal challenge, primarily due to imperfect gate operations and environmental interactions. This study introduces a machine learning-based method to simulate and analyze these errors. Utilizing a minimal scalable 2-Majorana-zero-mode (2-MZM) island model within a one-dimensional p-wave topological superconductor, this research employs a detailed Hamiltonian approach combined with a bosonic thermal bath interaction. The analysis is conducted using the Pauli master equation and Monte Carlo simulations. This work’s novel contribution lies in applying Boosted Decision Tree with Gradient boosting (BDTG) and Multi-Layer Perceptron (MLP) machine learning techniques. These methods, trained on Monte Carlo simulation data, showed proficiency in predicting the evolution of error probabilities in the quantum system. The results indicate a significant potential for machine learning to offer a more efficient alternative for simulating quantum computing errors, thereby contributing to developing more robust quantum computing systems. Full article

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

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11 pages, 432 KiB

Open AccessArticle

Inclusive Neutrino and Antineutrino Scattering on the ¹²C Nucleus Within the Coherent Density Fluctuation Model

by Martin V. Ivanov and Anton N. Antonov

Universe 2025, 11(4), 119; https://doi.org/10.3390/universe11040119 - 4 Apr 2025

Viewed by 234

Abstract

We investigate quasielastic (anti)neutrino scattering on the ¹²C nucleus utilizing a novel scaling variable,

ψ^{*}

. This variable is derived from the interacting relativistic Fermi gas model, which incorporates both scalar and vector interactions, leading to a relativistic effective mass for [...] Read more.

We investigate quasielastic (anti)neutrino scattering on the ¹²C nucleus utilizing a novel scaling variable,

ψ^{*}

. This variable is derived from the interacting relativistic Fermi gas model, which incorporates both scalar and vector interactions, leading to a relativistic effective mass for the interacting nucleons. For inclusive lepton scattering from nuclei, we develop a new scaling function, denoted as

f^{QE} (ψ^{*})

, based on the coherent density fluctuation model (CDFM). This model serves as a natural extension of the relativistic Fermi gas (RFG) model applicable to finite nuclei. In this study, we compute theoretical predictions and compare them with experimental data from Miner

ν

a and T2K for inclusive (anti)neutrino cross-sections. The scaling function is derived within the CDFM framework, employing a relativistic effective mass of

m_{N}^{*} = 0.8 m_{N}

. The findings demonstrate a high degree of consistency with experimental data across all (anti)neutrino energy ranges. Full article

(This article belongs to the Special Issue Neutrino Insights: Peering into the Subatomic Universe)

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33 pages, 1207 KiB

Open AccessArticle

Running Vacuum and H⁴ Inflation

by Joan Solà Peracaula, Cristian Moreno-Pulido and Alex González-Fuentes

Universe 2025, 11(4), 118; https://doi.org/10.3390/universe11040118 - 2 Apr 2025

Viewed by 149

Abstract

Recent studies of QFT in cosmological spacetime indicate that the speeding up of the present universe may not just be associated with a rigid cosmological term but with a running one that evolves with the expansion rate

Λ = Λ (H)

[...] Read more.

Recent studies of QFT in cosmological spacetime indicate that the speeding up of the present universe may not just be associated with a rigid cosmological term but with a running one that evolves with the expansion rate

Λ = Λ (H)

. This running is inherited from the cosmic evolution of the vacuum energy density (VED),

ρ_{vac}

, which is sensitive to quantum effects in curved spacetime that ultimately trigger that running. The VED is a function of the Hubble rate and its time derivatives

ρ_{vac} = ρ_{vac} (H, \dot{H}, \ddot{H}, \dots)

. Two nearby points of cosmic evolution during the FLRW epoch are smoothly related as

δ ρ_{vac} \sim O (H^{2})

. In the very early universe, in contrast, the higher powers of the Hubble rate take over and bring about a period of fast inflation. They originate from quantum effects on the effective action of a vacuum, which we compute. Herein, we focus on the lowest possible power for inflation to occur:

H^{4}

. During the inflationary phase, H remains approximately constant and very large. Subsequently, the universe enters the usual FLRW radiation epoch. This new mechanism (‘RVM inflation’) is not based on any supplementary ‘inflaton’ field; it is fueled by pure QFT effects on the dynamical background and is different from Starobinsky’s inflation, in which H is never constant. Full article

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

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17 pages, 2736 KiB

Open AccessArticle

Using Machine Learning for Lunar Mineralogy-I: Hyperspectral Imaging of Volcanic Samples

by Fatemeh Fazel Hesar, Mojtaba Raouf, Peyman Soltani, Bernard Foing, Michiel J. A. de Dood and Fons J. Verbeek

Universe 2025, 11(4), 117; https://doi.org/10.3390/universe11040117 - 2 Apr 2025

Viewed by 208

Abstract

This study examines the mineral composition of volcanic samples similar to lunar materials, focusing on olivine and pyroxene. Using hyperspectral imaging (HSI) from 400 to 1000 nm, we created data cubes to analyze the reflectance characteristics of samples from Vulcano, a volcanically active [...] Read more.

This study examines the mineral composition of volcanic samples similar to lunar materials, focusing on olivine and pyroxene. Using hyperspectral imaging (HSI) from 400 to 1000 nm, we created data cubes to analyze the reflectance characteristics of samples from Vulcano, a volcanically active island in the Aeolian archipelago, north of Sicily, Italy, categorizing them into nine regions of interest (ROIs) and analyzing spectral data for each. We applied various unsupervised clustering algorithms, including K-Means, hierarchical clustering, Gaussian mixture models (GMMs), and spectral clustering, to classify the spectral profiles. Principal component analysis (PCA) revealed distinct spectral signatures associated with specific minerals, facilitating precise identification. The clustering performance varied by region, with K-Means achieving the highest silhouette score of 0.47, whereas GMMs performed poorly with a score of only 0.25. Non-negative matrix factorization (NMF) aided in identifying similarities among clusters across different methods and reference spectra for olivine and pyroxene. Hierarchical clustering emerged as the most reliable technique, achieving a 94% similarity with the olivine spectrum in one sample, whereas GMMs exhibited notable variability. Overall, the analysis indicated that both the hierarchical and K-Means methods yielded lower errors in total measurements, with K-Means demonstrating superior performance in estimated dispersion and clustering. Additionally, GMMs showed a higher root mean square error (RMSE) compared to the other models. The RMSE analysis confirmed K-Means as the most consistent algorithm across all samples, suggesting a predominance of olivine in the Vulcano region relative to pyroxene. This predominance is likely linked to historical formation conditions similar to volcanic processes on the Moon, where olivine-rich compositions are common in ancient lava flows and impact-melt rocks. These findings provide a deeper context for mineral distribution and formation processes in volcanic landscapes. Full article

(This article belongs to the Special Issue Planetary Radar Astronomy)

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26 pages, 5021 KiB

Open AccessArticle

Protoplanet and Proto-Brown Dwarf Clumps in Gravitationally Unstable Protoplanetary Disks of Various Metallicity

by Eduard Vorobyov and Carina Schoenhacker

Universe 2025, 11(4), 116; https://doi.org/10.3390/universe11040116 - 2 Apr 2025

Viewed by 241

Abstract

Gravitational fragmentation of a protoplanetary disk is considered a possible mechanism for the formation of planets and brown dwarfs. In this process, transitory objects are formed that are known as clumps, which are compact gas–dust condensations with a size of several astronomical units. [...] Read more.

Gravitational fragmentation of a protoplanetary disk is considered a possible mechanism for the formation of planets and brown dwarfs. In this process, transitory objects are formed that are known as clumps, which are compact gas–dust condensations with a size of several astronomical units. The contraction of these clumps to planetary sizes via the dissociation of molecular hydrogen or tidal downsizing can ultimately lead to planet or brown dwarf formation. Here, we present a comprehensive numerical and statistical study of the clump properties in protoplanetary disks formed from cloud cores of similar mass (0.9–1.0

M_{⊙}

). We focus on possible differences in their characteristics depending on the metallicity of the parental disk. We show that notable differences can be expected in the clump characteristics in terms of their number, internal energetics, mass, and distance to the star. For all metallicities considered, the propensity to forming planets or brown dwarfs via disk fragmentation is challenged by large amounts of gravitationally unbound clumps. We conclude that giant planet formation via disk fragmentation is possible down to 1/100 solar metallicity but it should be a rare outcome. Brown dwarf formation via disk fragmentation is possible only down to 1/10 solar metallicity. Our results stand for similar masses of the central star on the order of the Sun. Full article

(This article belongs to the Section Planetary Sciences)

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12 pages, 574 KiB

Open AccessArticle

Probing Dark Photons Through Gravitational Decoupling of Mass-State Oscillations in Interstellar Media

by Bo Zhang and Cui-Bai Luo

Universe 2025, 11(4), 115; https://doi.org/10.3390/universe11040115 - 1 Apr 2025

Viewed by 200

Abstract

We propose a novel mechanism for photon–dark photon mass-state oscillations mediated by gravitational separation during propagation through the interstellar medium. This phenomenon establishes a new avenue for the detection of dark matter. By analyzing gravitational lensing data from quasars, we investigate the sensitivity [...] Read more.

We propose a novel mechanism for photon–dark photon mass-state oscillations mediated by gravitational separation during propagation through the interstellar medium. This phenomenon establishes a new avenue for the detection of dark matter. By analyzing gravitational lensing data from quasars, we investigate the sensitivity of this approach to dark photons. Our analysis demonstrates constraints of

ε < 10^{- 2}

in the dark photon mass range of

1.7 \times 10^{- 14}

eV to

5.4 \times 10^{- 14}

eV. Furthermore, we propose potential applications of this mechanism to astrophysical systems with strong gravitational fields, such as neutron stars and black hole accretion disks. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—"Galaxies and Clusters")

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16 pages, 473 KiB

Open AccessCommunication

Scaling Behavior and Phases of Nonlinear Sigma Model on Real Stiefel Manifolds Near Two Dimensions

by Alexandre M. Gavrilik and Andriy V. Nazarenko

Universe 2025, 11(4), 114; https://doi.org/10.3390/universe11040114 - 31 Mar 2025

Viewed by 215

Abstract

For a quasi-two-dimensional nonlinear sigma model on the real Stiefel manifolds with a generalized (anisotropic) metric, the equations of a two-charge renormalization group (RG) for the homothety and anisotropy of the metric as effective couplings are obtained in a one-loop approximation. Normal coordinates [...] Read more.

For a quasi-two-dimensional nonlinear sigma model on the real Stiefel manifolds with a generalized (anisotropic) metric, the equations of a two-charge renormalization group (RG) for the homothety and anisotropy of the metric as effective couplings are obtained in a one-loop approximation. Normal coordinates and the curvature tensor are exploited for the renormalization of the metric. The RG trajectories are investigated and the presence of a fixed point common to four critical lines or four phases (tetracritical point) in the general case, or its absence in the case of an Abelian structure group, is established. For the tetracritical point, the critical exponents are evaluated and compared with those known earlier for a simpler particular case. Full article

(This article belongs to the Section Field Theory)

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16 pages, 6001 KiB

Open AccessArticle

Numerical Evaluation of a Soliton Pair with Long-Range Interaction

by Joachim Wabnig, Josef Resch, Dominik Theuerkauf, Fabian Anmasser and Manfried Faber

Universe 2025, 11(4), 113; https://doi.org/10.3390/universe11040113 - 30 Mar 2025

Viewed by 135

Abstract

We determine the interaction energy of electric monopole pairs, sources and sinks of a Coulombic field. These charges are represented by topological solitons of finite size and mass, described by a field of SO(3) rotations without any divergences. Such monopoles feel, at large [...] Read more.

We determine the interaction energy of electric monopole pairs, sources and sinks of a Coulombic field. These charges are represented by topological solitons of finite size and mass, described by a field of SO(3) rotations without any divergences. Such monopoles feel, at large distances, a pure Coulombic interaction. A crucial test for the physical interpretation of these monopoles is a classical running of the charge at small distances, expected from the finite soliton size. We investigate in detail a first observation of the increase in the effective charge at distances of a few soliton radii in this purely Coulombic system and compare it with the running of the coupling in perturbative QED. Full article

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

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16 pages, 1077 KiB

Open AccessArticle

Non-Singular “Gauss” Black Hole from Non-Locality

by Jens Boos

Universe 2025, 11(4), 112; https://doi.org/10.3390/universe11040112 - 29 Mar 2025

Viewed by 132

Abstract

Cutting out an infinite tube around

r = 0

formally removes the Schwarzschild singularity, but without a physical mechanism, this procedure seems ad hoc and artificial. In this paper, we provide justification for such a mechanism by means of non-locality. Motivated by the [...] Read more.

Cutting out an infinite tube around

r = 0

formally removes the Schwarzschild singularity, but without a physical mechanism, this procedure seems ad hoc and artificial. In this paper, we provide justification for such a mechanism by means of non-locality. Motivated by the Gauss law, we define a suitable radius variable as the inverse of a regular non-local potential, and use this variable to model a non-singular black hole. The resulting geometry has a de Sitter core, and for generic values of the regulator, there is no inner horizon, saving this model from potential issues via mass inflation. An outer horizon only exists for masses above a critical threshold, thereby reproducing the conjectured “mass gap” for black holes in non-local theories. The geometry’s density and pressure terms decrease exponentially, thereby rendering it an almost-exact vacuum solution of the Einstein equations outside of astrophysical black holes. Its thermodynamic properties resemble those of the Hayward black hole, with the notable exception that for critical mass, the horizon radius is zero. Full article

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32 pages, 10802 KiB

Open AccessArticle

Shadow Analysis of an Approximate Rotating Black Hole Solution with Weakly Coupled Global Monopole Charge

by Mohsen Fathi

Universe 2025, 11(4), 111; https://doi.org/10.3390/universe11040111 - 27 Mar 2025

Viewed by 155

Abstract

In this paper, we investigate the shadow properties of a rotating black hole with a weakly coupled global monopole charge using a modified Newman–Janis algorithm. This study explores how these charge and rotational effects shape the black hole’s shadow, causal structure, and ergoregions, [...] Read more.

In this paper, we investigate the shadow properties of a rotating black hole with a weakly coupled global monopole charge using a modified Newman–Janis algorithm. This study explores how these charge and rotational effects shape the black hole’s shadow, causal structure, and ergoregions, with implications for distinguishing it from Kerr-like solutions. Analysis of null geodesics reveals observable features that may constrain the global monopole charge and weak coupling parameters within nonminimal gravity frameworks. Observational data from M87* and Sgr A* constrain the global monopole charge and coupling constant to

0 \leq γ ≲ 0.036

and

- 0.2 ≲ α \leq 0

, respectively. Full article

(This article belongs to the Section Gravitation)

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9 pages, 239 KiB

Open AccessArticle

Knot Probability of Random Magnetic Field Lines

by Anda Xiong, Shangbin Yang and Xin Liu

Universe 2025, 11(4), 110; https://doi.org/10.3390/universe11040110 - 25 Mar 2025

Viewed by 221

Abstract

In this paper, we apply several latest results from statistical physics on the probability and energy of knotting to study the knotted field lines in solar corona. Since the solar magnetic field in small scale can be seen as nearly random, by assuming [...] Read more.

In this paper, we apply several latest results from statistical physics on the probability and energy of knotting to study the knotted field lines in solar corona. Since the solar magnetic field in small scale can be seen as nearly random, by assuming that the magnetic field lines behave similarly to random loops, we find the probability P of certain knot type K for the field line knotting as a function to the distance L between the foot-points of sunspots, which is

P_{K} (L) = C_{K} L^{2 α_{K}} exp (\frac{- L^{2}}{β})

. From the equation, we find that the variety of knot type increases with the distance. Since knotting is the topological resemblance to magnetic helicity, which is an invariant for ideal MHD, our result enriches the understanding of the probability of magnetic helicity as well as field line structure in active regions. Based on the relation between knotting and magnetic energy, we provide support to the high variety of field line knot types. Full article

(This article belongs to the Section Solar and Stellar Physics)

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25 pages, 3100 KiB

Open AccessArticle

Black Holes and de Sitter Space as Time Mirrors

by Anatoly Svidzinsky

Universe 2025, 11(4), 109; https://doi.org/10.3390/universe11040109 - 25 Mar 2025

Viewed by 238

Abstract

It is usually assumed that matter disappears together with the spacetime at the center of a Schwarzschild black hole (BH). Here, we find that if we impose a boundary condition that the field does not disappear at the BH center (that is, field [...] Read more.

It is usually assumed that matter disappears together with the spacetime at the center of a Schwarzschild black hole (BH). Here, we find that if we impose a boundary condition that the field does not disappear at the BH center (that is, field flux into the singularity vanishes), the BH acts as a time mirror that totally reflects the infalling light and matter outside the BH. Namely, the reflected field propagates backward in time, passes the event horizon and moves away from the BH. In this case, a BH can be used as a time machine that allows us to send a signal into the past. We also show that de Sitter spacetime acts as a time mirror provided particles do not disappear from the spacetime at

r = \infty

. Full article

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

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13 pages, 6448 KiB

Open AccessArticle

Nonlinear Calibration Method for FMG Line-of-Sight Magnetic Field

by Ziyao Hu, Kaifan Ji, Xianyong Bai, Yuanyong Deng, Jiangtao Su, Jingjing Guo, Suo Liu and Xiao Yang

Universe 2025, 11(4), 108; https://doi.org/10.3390/universe11040108 - 24 Mar 2025

Viewed by 188

Abstract

This study is to correct magnetic saturation and wavelength shift in Full-disk Magnetograph (FMG) solar magnetic field measurements on the Advanced Space-based Solar Observatory (ASO-S) satellite. Due to its single-wavelength polarization data limitations, currently, FMG relies on linear calibration. We propose a residual [...] Read more.

This study is to correct magnetic saturation and wavelength shift in Full-disk Magnetograph (FMG) solar magnetic field measurements on the Advanced Space-based Solar Observatory (ASO-S) satellite. Due to its single-wavelength polarization data limitations, currently, FMG relies on linear calibration. We propose a residual network model to output a line-of-sight (LOS) magnetic field which is trained with HMI LOS magnetic fields as target, and FMG Stokes I, V data and LOS velocity components as inputs. Compared to traditional methods, our model achieves lower MAE, RMSE, and improved consistency with the target, while also demonstrating robustness to wavelength shift, offering more accurate magnetic field measurements. 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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19 pages, 1881 KiB

Open AccessReview

Some Questions and Challenges in Measurements of Solar Magnetic Fields

by Hongqi Zhang, Jiangtao Su, Xiaofan Wang, Xingming Bao, Yu Liu, Yingzi Sun and Mingyu Zhao

Universe 2025, 11(4), 107; https://doi.org/10.3390/universe11040107 - 24 Mar 2025

Viewed by 167

Abstract

Huairou Solar Observing Station of the National Astronomical Observatories of Chinese Academy of Sciences has been in operation since 1987. During its operation, successful observations of the solar vector magnetic field have been conducted. On the basis of the achievements at Huairou, we [...] Read more.

Huairou Solar Observing Station of the National Astronomical Observatories of Chinese Academy of Sciences has been in operation since 1987. During its operation, successful observations of the solar vector magnetic field have been conducted. On the basis of the achievements at Huairou, we analyze the methods of observing the solar magnetic field, including discussions of the approximation of the transfer theory of the solar magnetic field in the atmosphere, wide field of view polarized observation, and some questions on the inversion of solar magnetic field data. We also present relevant challenges for further research. 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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25 pages, 7124 KiB

Open AccessArticle

Observations of the Formation of a Proto-Spot in a Pre-Existing Field Environment

by Mariarita Murabito, Ilaria Ermolli, Salvo L. Guglielmino, Paolo Romano and Fabrizio Giorgi

Universe 2025, 11(4), 106; https://doi.org/10.3390/universe11040106 - 22 Mar 2025

Viewed by 163

Abstract

Bipolar emerging flux regions (EFRs) form active regions (ARs) that generally evolve into a pre-existing magnetic environment in the solar atmosphere. Reconfiguration of the small- and large-scale magnetic connectivities is invoked to explain a plethora of energy-release phenomena observed at the sites of [...] Read more.

Bipolar emerging flux regions (EFRs) form active regions (ARs) that generally evolve into a pre-existing magnetic environment in the solar atmosphere. Reconfiguration of the small- and large-scale magnetic connectivities is invoked to explain a plethora of energy-release phenomena observed at the sites of EFRs. These include brightening events, surges, and jets, whose triggers and relationships are still unclear. In this context, we study the formation of a proto-spot in AR NOAA 11462 by analyzing spectropolarimetric and spectroscopic measurements taken by the Interferometric Bidimensional Spectrometer along the Fe I 630.2 nm and Ca II 854.2 nm lines on 17 April 2012. We complement these high-resolution data with simultaneous SDO satellite observations. The proto-spot forms from magnetic flux and emerges into the photosphere, coalescing following plasma flows in its surroundings. The chromospheric and higher atmosphere observations show that flux emergence occurs in a pre-existing magnetic environment, with small- and large-scale coronal arcades that seemingly shape the proto-spot formation in the upper atmospheric layers. In addition, in the chromosphere, we observe an arch filament system and repeated intense brightening events and surges, likely due to magnetic interactions of the new flux with the pre-existing overlying coronal field. These phenomena have been observed since the early stages of the new flux emergence. Full article

(This article belongs to the Section Solar and Stellar Physics)

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52 pages, 10192 KiB

Open AccessReview

Broad Observational Perspectives Achieved by the Accreting White Dwarf Sciences in the XMM-Newton and Chandra Eras

by Şölen Balman, Marina Orio and Gerardo J. M. Luna

Universe 2025, 11(4), 105; https://doi.org/10.3390/universe11040105 - 21 Mar 2025

Viewed by 616

Abstract

Accreting white dwarf binaries (AWDs) comprise cataclysmic variables (CVs), symbiotics, AM CVns, and other related systems that host a primary white dwarf (WD) accreting from a main sequence or evolved companion star. AWDs are a product of close binary evolution; thus, they are [...] Read more.

Accreting white dwarf binaries (AWDs) comprise cataclysmic variables (CVs), symbiotics, AM CVns, and other related systems that host a primary white dwarf (WD) accreting from a main sequence or evolved companion star. AWDs are a product of close binary evolution; thus, they are important for understanding the evolution and population of X-ray binaries in the Milky Way and other galaxies. AWDs are essential for studying astrophysical plasmas under different conditions along with accretion physics and processes, transient events, matter ejection and outflows, compact binary evolution, mergers, angular momentum loss mechanisms, and nuclear processes leading to explosions. AWDs are also closely related to other objects in the late stages of stellar evolution, with other accreting objects in compact binaries, and even share common phenomena with young stellar objects, active galactic nuclei, quasars, and supernova remnants. As X-ray astronomy came to a climax with the start of the Chandra and XMM-Newton missions owing to their unprecedented instrumentation, new excellent imaging capabilities, good time resolution, and X-ray grating technologies allowed immense advancement in many aspects of astronomy and astrophysics. In this review, we lay out a panorama of developments on the study of AWDs that have been accomplished and have been made possible by these two observatories; we summarize the key observational achievements and the challenges ahead. Full article

(This article belongs to the Special Issue X-ray Astronomy Steps into the 21st Century: Chandra and XMM-Newton at 25)

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7 pages, 227 KiB

Open AccessCommunication

Are the Galaxies with Indefinitely Flat Circular Velocities Located Inside Large Dark Matter Haloes?

by Man Ho Chan, Yangzhanhao Zhang and Antonino Del Popolo

Universe 2025, 11(4), 104; https://doi.org/10.3390/universe11040104 - 21 Mar 2025

Viewed by 212

Abstract

A recent study using weak gravitational lensing revealed that the rotation curves of some isolated galaxies are found at a very large distance from their galactic centres. This may provide strong evidence supporting Modified Newtonian Dynamics (MOND) and challenging the standard cold dark [...] Read more.

A recent study using weak gravitational lensing revealed that the rotation curves of some isolated galaxies are found at a very large distance from their galactic centres. This may provide strong evidence supporting Modified Newtonian Dynamics (MOND) and challenging the standard cold dark matter model. In this article, we propose the possibility that these isolated galaxies are possibly located at the centres of corresponding large dark matter haloes. Using the standard gravitational framework of galaxy groups and galaxy clusters, we show that this scenario can reproduce the observed rotation curves, provided the existence of corresponding hot gas haloes extending beyond 1 Mpc. Therefore, MOND may not be the only solution to the observed rotation curves and the cold dark matter interpretation still remains viable. Full article

(This article belongs to the Special Issue The 10th Anniversary of Universe: Standard Cosmological Models, and Modified Gravity and Cosmological Tensions)

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Universe, Volume 11, Issue 4 (April 2025) – 29 articles

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