Universe

39 pages, 23135 KiB

Open AccessArticle

Core to Cosmic Edge: SIMBA-C’s New Take on Abundance Profiles in the Intragroup Medium at z=0

by Aviv Padawer-Blatt, Zhiwei Shao, Renier T. Hough, Douglas Rennehan, Ruxin Barré, Vida Saeedzadeh, Arif Babul, Romeel Davé, Chiaki Kobayashi, Weiguang Cui, François Mernier and Ghassem Gozaliasl

Universe 2025, 11(2), 47; https://doi.org/10.3390/universe11020047 (registering DOI) - 1 Feb 2025

Abstract

We employ the simba-c cosmological simulation to study the impact of its upgraded chemical enrichment model (Chem5) on the distribution of metals in the intragroup medium (IGrM). We investigate the projected X-ray emission-weighted abundance profiles of key elements over two decades in halo [...] Read more.

We employ the simba-c cosmological simulation to study the impact of its upgraded chemical enrichment model (Chem5) on the distribution of metals in the intragroup medium (IGrM). We investigate the projected X-ray emission-weighted abundance profiles of key elements over two decades in halo mass (

10^{13} \leq M_{500} / M_{⊙} \leq 10^{15}

). Typically, simba-c generates lower-amplitude abundance profiles than simba with flatter cores, in better agreement with observations. For low-mass groups, both simulations over-enrich the IGrM with Si, S, Ca, and Fe compared to observations, a trend likely related to inadequate modeling of metal dispersal and mixing. We analyze the 3D mass-weighted abundance profiles, concluding that the lower simba-c IGrM abundances are primarily a consequence of fewer metals in the IGrM, driven by reduced metal yields in Chem5, and the removal of the instantaneous recycling of metals approximation employed by simba. Additionally, an increased IGrM mass in low-mass simba-c groups is likely triggered by changes to the AGN and stellar feedback models. Our study suggests that a more realistic chemical enrichment model broadly improves agreement with observations, but physically motivated sub-grid models for other key processes, like AGN and stellar feedback and turbulent diffusion, are required to realistically reproduce observed group environments. Full article

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

10 pages, 230 KiB

Open AccessArticle

A Small Cosmological Constant from a Large Number of Extra Dimensions

by Changjun Gao

Universe 2025, 11(2), 46; https://doi.org/10.3390/universe11020046 (registering DOI) - 1 Feb 2025

Abstract

In this article, we consider the 4 + n dimensional spacetimes among which one is the four dimensional physical Universe and the other is an n-dimensional sphere with constant radius in the framework of Lanczos-Lovelock gravity. We find that the curvature of extra [...] Read more.

In this article, we consider the 4 + n dimensional spacetimes among which one is the four dimensional physical Universe and the other is an n-dimensional sphere with constant radius in the framework of Lanczos-Lovelock gravity. We find that the curvature of extra dimensional sphere contributes a huge but negative energy density provided that its radius is sufficiently small, such as the scale of Planck length. Therefore, the huge positive vacuum energy, i.e., the large positive cosmological constant is exactly cancelled out by the curvature of extra sphere. In the mean time the higher order of Lanczos-Lovelock term contributes an observations-allowed small cosmological constant if the number of extra dimensions is sufficiently large, such as n ≈ 69. Full article

(This article belongs to the Special Issue Modified Gravity and Dark Energy Theories)

18 pages, 854 KiB

Open AccessArticle

Non-Keplerian Charged Accretion Disk Orbiting a Black Hole Pulsar

by Audrey Trova and Eva Hackmann

Universe 2025, 11(2), 45; https://doi.org/10.3390/universe11020045 (registering DOI) - 1 Feb 2025

Abstract

Recent studies have focused on how spinning black holes (BHs) within a binary system containing a strongly magnetized neutron star, then immersed in external magnetic fields, can acquire charge through mechanisms like the Wald process and how this charge could power pulsar-like electromagnetic [...] Read more.

Recent studies have focused on how spinning black holes (BHs) within a binary system containing a strongly magnetized neutron star, then immersed in external magnetic fields, can acquire charge through mechanisms like the Wald process and how this charge could power pulsar-like electromagnetic radiation. Those objects called “Black hole pulsar” mimic the behaviour of a traditional pulsar, and they can generate electromagnetic fields, such as magnetic dipoles. Charged particles within an accretion disk around the black hole would then be influenced not only by the gravitational forces but also by electromagnetic forces, leading to different geometries and dynamics. In this context, we focus here on the interplay of the magnetic dipole and the accretion disk. We construct the equilibrium structures of non-conducting charged perfect fluids orbiting Kerr black holes under the influence of a dipole magnetic field aligned with the rotation axis of the BH. The dynamics of the accretion disk in such a system are shaped by a complex interplay between the non-uniform, non-Keplerian angular momentum distribution, the black hole’s induced magnetic dipole, and the fluid’s charge. We show how these factors jointly influence key properties of the disk, such as its geometry, aspect ratio, size, and rest mass density. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

28 pages, 406 KiB

Open AccessArticle

The Flipped SU(5)×U(1) Model from Four-Dimensional Strings

by Ignatios Antoniadis and John Rizos

Universe 2025, 11(2), 44; https://doi.org/10.3390/universe11020044 - 29 Jan 2025

Abstract

We review the construction of a flipped

S U (5) \times U (1)

model in the context of the Free-Fermionic Formulation of four-dimensional strings and present its main phenomenological consequences in particle physics and cosmology. Full article

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

10 pages, 1846 KiB

Open AccessArticle

Random Asymmetric Jets Driven by Black-Hole Hyperaccretion in Gamma-Ray Bursts

by Zi-Ou Yang, Yan-Qing Qi and Tong Liu

Universe 2025, 11(2), 43; https://doi.org/10.3390/universe11020043 - 28 Jan 2025

Abstract

The relativistic jets of gamma-ray bursts (GRBs) might be powered by a black-hole (BH) hyperaccretion system. The inherent asymmetry in these jets generates recoil forces, inducing oscillations and positional deviations of the BH from equilibrium. In this study, we explore the influence of [...] Read more.

The relativistic jets of gamma-ray bursts (GRBs) might be powered by a black-hole (BH) hyperaccretion system. The inherent asymmetry in these jets generates recoil forces, inducing oscillations and positional deviations of the BH from equilibrium. In this study, we explore the influence of different initial BH mass, spin, and mass accretion rate, as well as their evolutions on the dynamical properties of BH under the effect of asymmetric jets. Our results reveal that the initial mass and accretion rate significantly impact the BH’s acceleration, velocity, and displacement, while the different initial spin plays a negligible role in shaping the overall dynamical evolution. Additionally, we calculate the gravitational wave (GW) strains associated with the asymmetric jets, finding that the resulting GW signals are too weak to be detected, even for nearby GRBs. These findings provide critical insights into the dynamical response of BHs to asymmetric jets and the associated GW radiation, advancing our understanding of BH physics in GRBs. Full article

34 pages, 460 KiB

Open AccessArticle

Gauge-Invariant Perturbation Theory on the Schwarzschild Background Spacetime Part II: Even-Mode Perturbations

by Kouji Nakamura

Universe 2025, 11(2), 42; https://doi.org/10.3390/universe11020042 - 27 Jan 2025

Abstract

This is the Part II paper of our series of papers on a gauge-invariant perturbation theory on the Schwarzschild background spacetime. After reviewing our general framework of the gauge-invariant perturbation theory and the proposal of gauge-invariant treatments for

l = 0, 1

[...] Read more.

This is the Part II paper of our series of papers on a gauge-invariant perturbation theory on the Schwarzschild background spacetime. After reviewing our general framework of the gauge-invariant perturbation theory and the proposal of gauge-invariant treatments for

l = 0, 1

-mode perturbations on the Schwarzschild background spacetime in the Part I paper, we examine the linearized Einstein equations for even-mode perturbations. We discuss the strategy to solve the linearized Einstein equations for these even-mode perturbations including

l = 0, 1

modes. Furthermore, we explicitly derive the

l = 0, 1

-mode solutions to the linearized Einstein equations in both the vacuum and the non-vacuum cases. We show that the solutions for

l = 0

-mode perturbations includes the additional Schwarzschild mass parameter perturbation, which is physically reasonable. Then, we conclude that our proposal of the resolution of the

l = 0, 1

-mode problem is physically reasonable due to the realization of the additional Schwarzschild mass parameter perturbation and the Kerr parameter perturbation in the Part I paper. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

14 pages, 448 KiB

Open AccessArticle

Constraining the Milky Way’s Dispersion Measure Using FRB and X-Ray Data

by Jiale Wang, Zheng Zhou, Xiaochuan Jiang and Taotao Fang

Universe 2025, 11(2), 41; https://doi.org/10.3390/universe11020041 - 27 Jan 2025

Abstract

The dispersion measures (DMs) of the fast radio bursts (FRBs) are a valuable tool to probe the baryonic content of the intergalactic medium and the circumgalactic medium of the intervening galaxies along the sightlines. However, interpreting the DMs is complicated by the contribution [...] Read more.

The dispersion measures (DMs) of the fast radio bursts (FRBs) are a valuable tool to probe the baryonic content of the intergalactic medium and the circumgalactic medium of the intervening galaxies along the sightlines. However, interpreting the DMs is complicated by the contribution of the hot gas in and around our Milky Way. This study examines the relationship between

D M_{MW}

, derived from localized FRBs, and the Galaxy’s hot gas, using X-ray absorption and emission data from O vii and O viii. We find evidence for a positive correlation between

D M_{MW}

and O vii absorption, reflecting contributions from both the disk and halo components. This conclusion is supported by two lines of evidence: (1) no correlation between

D M_{MW}

and O vii/O viii emission, which primarily traces dense disk regions; and (2) the comparison with electron density models, where

D M_{MW}

aligns with models that incorporate both disk and halo components but significantly exceeds predictions from pure disk-only models, emphasizing the halo’s role. Furthermore, the lack of correlation with O viii absorption suggests that the primary temperature of the Galaxy’s hot gas is likely around

2 \times 10^{6}

K or less, as traced by O vii absorption, while gas at higher temperatures (∼3–5 ×

10^{6}

K) is present but less abundant. Our findings provide insights into the Milky Way’s gas distribution and improve

D M_{MW}

estimates for future cosmological studies. Full article

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

► Show Figures

Figure 1

16 pages, 421 KiB

Open AccessArticle

The Gaussian-Drude Lens: A Dusty Plasma Model Applicable to Observations Across the Electromagnetic Spectrum

by Adam Rogers

Universe 2025, 11(2), 40; https://doi.org/10.3390/universe11020040 - 26 Jan 2025

Abstract

When radiation from a background source passes through a cloud of cold plasma, diverging lensing occurs if the source and observer are well-aligned. Unlike gravitational lensing, plasma lensing is dispersive, increasing in strength with wavelength. The Drude model is a generalization of cold [...] Read more.

When radiation from a background source passes through a cloud of cold plasma, diverging lensing occurs if the source and observer are well-aligned. Unlike gravitational lensing, plasma lensing is dispersive, increasing in strength with wavelength. The Drude model is a generalization of cold plasma, including absorbing dielectric dust described by a complex index of refraction. The Drude lens is only dispersive for wavelengths shorter than the dust characteristic scale (

λ ≪ λ_{d}

). At sufficient photon energy, the dust particles act like refractive clouds. For longer wavelengths

λ ≫ λ_{d}

, the optical properties of the Drude lens are constant, unique behavior compared to the predictions of the cold plasma lens. Thus, cold plasma lenses can be distinguished from Drude lenses using multi-band observations. The Drude medium extends the applicability of all previous tools, from gravitational and plasma lensing, to describe scattering phenomena in the X-ray regime. Full article

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

► Show Figures

Figure 1

62 pages, 1814 KiB

Open AccessArticle

Gauge-Invariant Perturbation Theory on the Schwarzschild Background Spacetime Part I: Formulation and Odd-Mode Perturbations

by Kouji Nakamura

Universe 2025, 11(2), 39; https://doi.org/10.3390/universe11020039 - 26 Jan 2025

Abstract

This article is Part I of our series of full papers on a gauge-invariant “linear” perturbation theory on the Schwarzschild background spacetime which was briefly reported in our short papers by the present author in 2021. We first review our general framework of [...] Read more.

This article is Part I of our series of full papers on a gauge-invariant “linear” perturbation theory on the Schwarzschild background spacetime which was briefly reported in our short papers by the present author in 2021. We first review our general framework of the gauge-invariant perturbation theory, which can be easily extended to the “higher-order” perturbation theory. When we apply this general framework to perturbations on the Schwarzschild background spacetime, gauge-invariant treatments of

l = 0, 1

mode perturbations are required. On the other hand, in the current consensus on the perturbations of the Schwarzschild spacetime, gauge-invariant treatments for

l = 0, 1

modes are difficult if we keep the reconstruction of the original metric perturbations in our mind. Due to this situation, we propose a strategy of a gauge-invariant treatment of

l = 0, 1

mode perturbations through the decomposition of the metric perturbations by singular harmonic functions at once and the regularization of these singularities through the imposition of the boundary conditions to the Einstein equations. Following this proposal, we derive the linearized Einstein equations for any modes of

l \geq 0

in a gauge-invariant manner. We discuss the solutions to the odd-mode perturbation equations in the linearized Einstein equations and show that these perturbations include the Kerr parameter perturbation in these odd-mode perturbations, which is physically reasonable. In the Part II and Part III papers of this series of papers, we will show that the even-mode solutions to the linearized Einstein equations obtained through our proposal are also physically reasonable. Then, we conclude that our proposal of a gauge-invariant treatment for

l = 0, 1

-mode perturbations is also physically reasonable. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

12 pages, 784 KiB

Open AccessArticle

Thermal Profile of Accretion Disk Around Black Hole in 4D Einstein–Gauss–Bonnet Gravity

by Odilbek Kholmuminov, Bakhtiyor Narzilloev and Bobomurat Ahmedov

Universe 2025, 11(2), 38; https://doi.org/10.3390/universe11020038 - 26 Jan 2025

Abstract

In this study, we investigate the properties of a thin accretion disk around a static spherically symmetric black hole in 4D Einstein–Gauss–Bonnet gravity, with an additional coupling constant,

α

, appearing in the spacetime metric. Using the Novikov–Thorne accretion disk model, we examine [...] Read more.

In this study, we investigate the properties of a thin accretion disk around a static spherically symmetric black hole in 4D Einstein–Gauss–Bonnet gravity, with an additional coupling constant,

α

, appearing in the spacetime metric. Using the Novikov–Thorne accretion disk model, we examine the thermal properties of the disk, finding that increasing

α

reduces the energy, angular momentum, and effective potential of a test particle orbiting the black hole. We demonstrate that

α

can mimic the spin of a Kerr black hole in general relativity up to

a ≃

0.23 M for the maximum value of

α

. Our analysis of the thermal radiation flux shows that larger

α

values increase the flux and shift its maximum towards the central black hole, while far from the black hole, the solution recovers the Schwarzschild limit. The impact of

α

on the radiative efficiency of the disk is weak but can slightly alter it. Assuming black-body radiation, we observe that the disk’s temperature peaks near its inner edge and is higher for larger

α

values. Lastly, the electromagnetic spectra reveal that the disk’s luminosity is lower in Einstein–Gauss–Bonnet gravity compared to general relativity, with the peak luminosity shifting toward higher frequencies, corresponding to the soft X-ray band as

α

increases. Full article

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

► Show Figures

Figure 1

12 pages, 1136 KiB

Open AccessArticle

Prediction of Individual Halo Concentrations Across Cosmic Time Using Neural Networks

by Tianchi Zhang, Tianxiang Mao, Wenxiao Xu and Guan Li

Universe 2025, 11(2), 37; https://doi.org/10.3390/universe11020037 - 26 Jan 2025

Abstract

The concentration of dark matter haloes is closely linked to their mass accretion history. We utilize the halo mass accretion histories from large cosmological N-body simulations as inputs for our neural networks, which we train to predict the concentration of individual haloes [...] Read more.

The concentration of dark matter haloes is closely linked to their mass accretion history. We utilize the halo mass accretion histories from large cosmological N-body simulations as inputs for our neural networks, which we train to predict the concentration of individual haloes at a given redshift. The trained model performs effectively in other cosmological simulations, achieving the root mean square error between the actual and predicted concentrations that significantly lower than that of the model by Zhao et al. and Giocoli et al. at any redshift. This model serves as a valuable tool for rapidly predicting halo concentrations at specified redshifts in large cosmological simulations. Full article

(This article belongs to the Special Issue Advances in Studies of Galaxies at High Redshift)

► Show Figures

Figure 1

12 pages, 449 KiB

Open AccessArticle

Hadronic Molecules with Four Charm or Beauty Quarks

by Wen-Ying Liu and Hua-Xing Chen

Universe 2025, 11(2), 36; https://doi.org/10.3390/universe11020036 - 24 Jan 2025

Abstract

We apply extended local hidden-gauge formalism to study meson–meson interactions with the quark constituents

c c \bar{c} \bar{c}

,

c c \bar{c} \bar{b} / \bar{c} \bar{c} c b

, [...] Read more.

We apply extended local hidden-gauge formalism to study meson–meson interactions with the quark constituents

c c \bar{c} \bar{c}

,

c c \bar{c} \bar{b} / \bar{c} \bar{c} c b

,

c c \bar{b} \bar{b} / \bar{c} \bar{c} b b

,

b b \bar{c} \bar{b} / \bar{b} \bar{b} c b

, and

b b \bar{b} \bar{b}

, in which the exchanged mesons are the fully heavy vector mesons

J / ψ

,

B_{c}^{*}

and

Υ

. We solve the coupled-channel Bethe–Salpeter equation to derive two poles in the

b b \bar{c} \bar{b}

system and two poles in the

c c \bar{c} \bar{b}

system. There are also four charge-conjugated poles in the

\bar{b} \bar{b} c b

and

\bar{c} \bar{c} c b

systems. In the

b b \bar{c} \bar{b}

system, one pole corresponds to a sub-threshold bound state when the cutoff momentum is set to

Λ > 850 MeV

. The other pole in this system corresponds to a sub-threshold bound state when

Λ > 1100 MeV

. In the

c c \bar{c} \bar{b}

system, the two poles correspond to sub-threshold bound states only when

Λ > 1550 MeV

and

Λ > 2650 MeV

. This makes them difficult to identify as deeply bound hadronic molecules. We propose investigating the two poles of the

b b \bar{c} \bar{b}

system in the

μ^{+} μ^{-} B_{c}^{-}

channel at the LHC. Full article

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

13 pages, 879 KiB

Open AccessArticle

Impact of Proton–Proton Collisions on the Cosmic-Ray Spectrum in Giant Clouds

by Hao-Ran Ge and Ruo-Yu Liu

Universe 2025, 11(2), 35; https://doi.org/10.3390/universe11020035 - 23 Jan 2025

Abstract

Gamma-ray production by proton–proton (

p p

) inelastic collisions plays an important role in searching for cosmic-ray (CR) accelerators. Understanding the pionic gamma-ray production associated with giant molecular/atomic clouds is thus crucial to identify this process. In this work, we study the [...] Read more.

Gamma-ray production by proton–proton (

p p

) inelastic collisions plays an important role in searching for cosmic-ray (CR) accelerators. Understanding the pionic gamma-ray production associated with giant molecular/atomic clouds is thus crucial to identify this process. In this work, we study the feedback of the pionic gamma-ray production to the CR distribution, by considering collision-induced energy loss on cosmic-ray protons in the dense core region of molecular clouds (MCs). We try to introduce a Monte Carlo simulation framework to quantify this effect, and present a detailed analysis to evaluate how

p p

collisions harden the cosmic-ray proton spectrum and the resulting gamma-ray spectrum in giant clouds. Full article

► Show Figures

Figure 1

12 pages, 252 KiB

Open AccessArticle

The Geometric Proca–Weyl Field as a Candidate for Dark Matter

by Mauro Duarte, Fábio Dahia and Carlos Romero

Universe 2025, 11(2), 34; https://doi.org/10.3390/universe11020034 - 22 Jan 2025

Abstract

We consider the Weyl invariant theory of gravity as an alternative approach to the problem of the origin of dark matter. According to this theory, the geometric Weyl 1-form effectively behaves as a Proca field. In this work, our starting point is to [...] Read more.

We consider the Weyl invariant theory of gravity as an alternative approach to the problem of the origin of dark matter. According to this theory, the geometric Weyl 1-form effectively behaves as a Proca field. In this work, our starting point is to consider the existence of a gas of Weyl–Proca particles in a Bose–Einstein condensate and investigate its behavior in a cosmological context. The results obtained show that, for appropriate values of the free parameter of the model, the Weyl field behaves approximately as a dust fluid in the matter-dominated era as expected for a dark matter candidate. Full article

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

18 pages, 767 KiB

Open AccessArticle

Anisotropic Compact Stars in General Relativity: An Exact Self-Bound Analytical Solution for Stellar Systems

by Sunil Kumar Maurya, Ghulam Mustafa, Faisal Javed, Saibal Ray, Assmaa Abd-Elmonem and Neissrien Alhubieshi

Universe 2025, 11(2), 33; https://doi.org/10.3390/universe11020033 - 21 Jan 2025

Abstract

In the present work, we investigate anisotropic compact stars under the general relativistic platform. Following a novel technique, we found an exact self-bound analytical stellar solution. The obtained solution was matched on the spherical surface of the boundary to the Schwarzschild metric in [...] Read more.

In the present work, we investigate anisotropic compact stars under the general relativistic platform. Following a novel technique, we found an exact self-bound analytical stellar solution. The obtained solution was matched on the spherical surface of the boundary to the Schwarzschild metric in order to find the expressions of the constants involved in the neutral system. We conducted various critical investigations such as on the variations in pressures, density, energy conditions, pressure–density ratios, the velocity of sounds, and gravitational potentials within stellar objects. We also conducted a stability analysis of the models using the cracking concepts and the adiabatic index. The values of the constant parameters were taken corresponding to the secondary components of GW190814 to validate the physical viability of our solution. This study provides fruitful results that are physically viable and, hence, satisfactory. Full article

► Show Figures

Figure 1

51 pages, 679 KiB

Open AccessArticle

Approximate SU(5) Fine Structure Constants

by Holger B. Nielsen

Universe 2025, 11(2), 32; https://doi.org/10.3390/universe11020032 - 21 Jan 2025

Abstract

We fit the three finestructure constants of the Standard Model, in which the first approximation of theoretically estimable parameters include (1) a “unified scale”, turning out not equal to the Planck scale and thus only estimable by a very speculative story, the second [...] Read more.

We fit the three finestructure constants of the Standard Model, in which the first approximation of theoretically estimable parameters include (1) a “unified scale”, turning out not equal to the Planck scale and thus only estimable by a very speculative story, the second includes (2) a “number of layers” being a priori the number of families, and the third is (3) a unified coupling related to a critical coupling on a lattice. So formally, we postdict the three fine structure constants! In the philosophy of our model, there is a physical lattice theory with link variables taking values in a (or in the various) “small” representation(s) of the standard model Group. We argue for that these representations function in the first approximation based on the theory of a genuine

S U (5)

theory. Next, we take into account fluctuation of the gauge fields in the lattice and obtain a correction to the a priori

S U (5)

approximation, because of course the link fluctuations not corresponding to any standard model Lie algebra, but only to the SU(5), do not exist. The model is a development of our old anti-grand-unification model having as its genuine gauge group, close to fundamental scale, a cross-product of the standard model group

S (U (3) \times U (2))

with itself, with there being one Cartesian product factor for each family. In our old works, we included the hypothesis of the “multiple point criticallity principle”, which here effectively means the coupling constants are critical on the lattice. Counted relative to the Higgs scale, we suggest in our sense that the“unified scale” (where the deviations between the inverse fine structure constants deviate by quantum fluctuations being only from standard model groups, not SU(5)) makes up the 2/3rd power of the Planck scale relative to the Higgs scale or the topquarkmass scale. Full article

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

► Show Figures

Figure 1

16 pages, 571 KiB

Open AccessArticle

Imaging Analyses for Pion and Kaon Sources in Relativistic Heavy-Ion Collisions in a Multiphase Transport Model

by Shi-Yao Wang, Yan-Yu Ren and Wei-Ning Zhang

Universe 2025, 11(2), 31; https://doi.org/10.3390/universe11020031 - 21 Jan 2025

Abstract

In this paper, we describe the study of one- and three-dimension pion and kaon source functions for chaotic and partially coherent sources in Pb-Pb central collisions at

\sqrt{s_{N N}} = 2.76

TeV using the AMPT model. The characteristic source function quantities are [...] Read more.

In this paper, we describe the study of one- and three-dimension pion and kaon source functions for chaotic and partially coherent sources in Pb-Pb central collisions at

\sqrt{s_{N N}} = 2.76

TeV using the AMPT model. The characteristic source function quantities are calculated and compared with the results obtained by fitting the two-boson correlation functions using the Gaussian source formula. It was found that the imaging results are approximately consistent with the results of the Gaussian source formula fits. The partially coherent pion sources exhibit a high degree of coherence. However, the kaon pairs with high transverse momenta are emitted with a high degree of chaos. Full article

(This article belongs to the Special Issue Search for New Physics at the LHC and Future Colliders)

► Show Figures

Figure 1

21 pages, 3397 KiB

Open AccessArticle

A Scale Invariant Fully Conformal Cosmological Model and Its Support by Astrophysical Data

by Richard Dvorsky

Universe 2025, 11(2), 30; https://doi.org/10.3390/universe11020030 - 21 Jan 2025

Abstract

According to general relativity, the cosmological redshift may be caused by other mechanisms than the source moving away from the observer. It can occur on a global scale, similar to the gravitational redshift near massive stars. In principle, these are differences in the [...] Read more.

According to general relativity, the cosmological redshift may be caused by other mechanisms than the source moving away from the observer. It can occur on a global scale, similar to the gravitational redshift near massive stars. In principle, these are differences in the time-dependent global metric field between the source in the past and the observer in the present. In this paper we attempt a new interpretation of the simple solution of Einstein’s equations within a fully conformal metric for the case of a time-independent energy-momentum tensor. The scaling factor here acts identically on all four space-time coordinates and the speed of light is independent of the conformal time. The fully conformal metric is interpreted here as a universal geometric background which is scale invariant and acts universally on all objects, including gauges and clocks, regardless of their dimensions and internal interactions. The associated scale invariant exponential expansion is thus only relative and all observers at different times are completely equal. The model introduces the concept of the appearent age of the universe, which is the limiting consequence of time dilation into the past, and corresponds to the present value of the age of the universe H⁻¹ according to the standard model. This appearent age is the same for all observers, and the Hubble constant is thus a true universal constant, invariant to time translations. The motivation of this work was to test the possibility of the above cosmological redshift mechanism in confrontation with astrophysical data. Probably the most important consequence is the generalized formulation and interpretation of the Hubble-Lemaître law z(r) = (e^Hr^/c − 1), which shows good agreement with astrophysical data even for the most distant supernovae. Confronting the conformal metric model with some astrophysical data shows an interesting agreement with the observed spatial distribution of astrophysical sources such as γ-ray bursts and quasars. On a cosmological scale, the above fully conformal metric naturally determines the global energy density, spatial flatness, and solves the horizon problem and Olbers’ paradox in infinite spacetime. Full article

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

► Show Figures

Figure 1

19 pages, 703 KiB

Open AccessArticle

Surface and Curvature Tensions of Cold, Dense Quark Matter: A Term-by-Term Analysis Within the Nambu–Jona–Lasinio Model

by Ana Gabriela Grunfeld, María Florencia Izzo Villafañe and Germán Lugones

Universe 2025, 11(2), 29; https://doi.org/10.3390/universe11020029 - 21 Jan 2025

Abstract

In this paper, we conduct a thorough investigation of the surface and curvature tensions,

σ

and

γ

, of three-flavor cold quark matter using the Nambu–Jona–Lasinio (NJL) model with vector interactions. Our approach ensures both local and global electric charge neutrality, as well [...] Read more.

In this paper, we conduct a thorough investigation of the surface and curvature tensions,

σ

and

γ

, of three-flavor cold quark matter using the Nambu–Jona–Lasinio (NJL) model with vector interactions. Our approach ensures both local and global electric charge neutrality, as well as chemical equilibrium under weak interactions. By employing the multiple reflection expansion formalism to account for finite size effects, we explore the impact of specific input parameters, particularly the vector coupling constant ratio

η_{V}

, the radius R of quark matter droplets, as well as the charge-per-baryon ratio

ξ

of the finite size configurations. We focus on the role of the contributions of each term of the NJL Lagrangian to the surface and curvature tensions in the mean field approximation. We find that the total surface tension exhibits two different density regimes: it remains roughly constant at around

100 MeV {fm}^{- 2}

up to approximately 2–4 times the nuclear saturation density, and beyond this point, it becomes a steeply increasing function of

n_{B}

. The total surface and curvature tensions are relatively insensitive to variations in R but are affected by changes in

ξ

and

η_{V}

. We observe that the largest contribution to

σ

and

γ

comes from the regularized divergent term, making these quantities significantly higher than those obtained within the MIT bag model. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

► Show Figures

Figure 1

Journal Description

Universe

Latest Articles

Journal Menu

Journal Browser

Highly Accessed Articles

Latest Books

E-Mail Alert

News

Topics

Conferences

Special Issues

Topical Collections

Further Information

Guidelines

MDPI Initiatives

Follow MDPI