Universe

21 pages, 2528 KiB

Open AccessArticle

On Dark Matter and Dark Energy in CCC+TL Cosmology

by Rajendra P. Gupta

Universe 2024, 10(6), 266; https://doi.org/10.3390/universe10060266 (registering DOI) - 18 Jun 2024

Viewed by 97

Relaxing the temporal constancy constraint on coupling constants in an expanding universe results in Friedmann equations containing terms that may be interpreted as dark energy and dark matter. When tired light (TL) was considered to complement the redshift due to the expanding universe, [...] Read more.

Relaxing the temporal constancy constraint on coupling constants in an expanding universe results in Friedmann equations containing terms that may be interpreted as dark energy and dark matter. When tired light (TL) was considered to complement the redshift due to the expanding universe, the resulting covarying coupling constants (CCC+TL) model not only fit the Type Ia supernovae data as precisely as the ΛCDM model, but also resolved concerns about the angular size of cosmic dawn galaxies observed by the James Webb Space Telescope. The model was recently shown to be compliant with the baryon acoustic oscillation features in the galaxy distribution and the cosmic microwave background (CMB). This paper demonstrates that dark energy and dark matter of the standard ΛCDM model are not arbitrary but can be derived from the CCC approach based on Dirac’s 1937 hypothesis. The energy densities associated with dark matter and dark energy turn out to be about the same in the ΛCDM and the CCC+TL models. However, the critical density in the new model can only account for the baryonic matter in the universe, raising concerns about how to account for observations requiring dark matter. We therefore analyze some key parameters of structure formation and show how they are affected in the absence of dark matter in the CCC+TL scenario. It requires reconsidering alternatives to dark matter to explain observations on gravitationally bound structures. The model’s consistency with the CMB power spectrum, BBN element abundances, and other critical observations is yet to be established. Full article

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

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1 pages, 117 KiB

Open AccessCorrection

Correction: Kim et al. Estimates of the Surface Magnetic Field Strength of Radio Pulsars. Universe 2023, 9, 334

by Universe Editorial Office

Universe 2024, 10(6), 265; https://doi.org/10.3390/universe10060265 (registering DOI) - 18 Jun 2024

Viewed by 42

Abstract

The Universe Editorial Office would like to make the following correction regarding the Academic Editor listed for this published paper [...] Full article

13 pages, 478 KiB

Open AccessArticle

Model-Independent Odderon Results Based on New TOTEM Data on Elastic Proton–Proton Collisions at 8 TeV

by Tamás Csörgő, Tamás Novák, Roman Pasechnik, András Ster and István Szanyi

Universe 2024, 10(6), 264; https://doi.org/10.3390/universe10060264 - 17 Jun 2024

Viewed by 157

Abstract

Evaluating the

H (x, s | p p)

scaling function of elastic proton–proton (

p p

) collisions from recent TOTEM data at

\sqrt{s} = 8

TeV and comparing it with the same function of elastic proton–antiproton ( [...] Read more.

Evaluating the

H (x, s | p p)

scaling function of elastic proton–proton (

p p

) collisions from recent TOTEM data at

\sqrt{s} = 8

TeV and comparing it with the same function of elastic proton–antiproton (

p \bar{p}

) data of the D0 collaboration at

\sqrt{s} = 1.96

TeV, we find, from this comparison alone, an at least 3.79

σ

signal of odderon exchange. If we combine this model-independently obtained result with that of a similar analysis but using TOTEM elastic

p p

scattering data at

\sqrt{s} = 7

TeV, which resulted in an at least 6.26

σ

signal, the combined significance of odderon exchange increases to at least 7.08

σ

. Further combinations of various datasets in the TeV energy range are detailed in the manuscript. Full article

(This article belongs to the Special Issue Multiparticle Dynamics)

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14 pages, 253 KiB

Open AccessArticle

Relations between Newtonian and Relativistic Cosmology

by Jaume de Haro

Universe 2024, 10(6), 263; https://doi.org/10.3390/universe10060263 - 16 Jun 2024

Viewed by 164

Abstract

We start with the cosmic Friedmann equations, where we adopt a novel perspective rooted in a Lagrangian formulation, grounded in Newtonian mechanics and the first law of thermodynamics. Our investigation operates under the assumption that the universe is populated by either a perfect [...] Read more.

We start with the cosmic Friedmann equations, where we adopt a novel perspective rooted in a Lagrangian formulation, grounded in Newtonian mechanics and the first law of thermodynamics. Our investigation operates under the assumption that the universe is populated by either a perfect fluid or a scalar field. By elucidating the intricate interplay between the Lagrangian formulation and the cosmic Friedmann equations, we uncover the fundamental principles governing the universe’s dynamics within the framework of these elemental constituents. In our concluding endeavor, we embark on the task of harmonizing the classical equations—namely, the conservation, Euler, and Poisson equations—with the principles of General Relativity. This undertaking seeks to extend these foundational equations to encompass the gravitational effects delineated by General Relativity, thus providing a comprehensive framework for understanding the behavior of matter and spacetime in the cosmic context. Full article

(This article belongs to the Special Issue The Large-Scale Structure of the Universe: Theory and Observation)

18 pages, 5090 KiB

Open AccessArticle

Altitude Heterogeneity of Magnetic Fields and Doppler Velocities in the Area of Seismic Source of a Strong Solar Flare from Data in Helium, Sodium, and Nickel Lines

by Ivan I. Yakovkin, Natalia I. Lozitska and Vsevolod G. Lozitsky

Universe 2024, 10(6), 262; https://doi.org/10.3390/universe10060262 - 14 Jun 2024

Viewed by 246

Abstract

Measurements of magnetic fields near seismic sources during solar flares are vital for understanding the dynamics of solar activity. We used spectropolarimetric observations of the X17.2/4B solar flare on 28 October 2003, over a wavelength interval of 43 Å, including the D3, D2, [...] Read more.

Measurements of magnetic fields near seismic sources during solar flares are vital for understanding the dynamics of solar activity. We used spectropolarimetric observations of the X17.2/4B solar flare on 28 October 2003, over a wavelength interval of 43 Å, including the D3, D2, D1, and Ni I 5892.88 Å lines, to analyze the Stokes I ± V profiles. Effective magnetic fields within 0.5–1.5 kG were measured in the D1, D2, and D3 lines at different flare locations, with the photospheric Ni I 5892.88 Å line showing a weaker field of below 0.5 kG. The D3 line showed rapid plasma descents of up to 11 km/s, in contrast to the slower velocities within 2.3 km/s observed in other lines. The differing amplitudes in the I + V and I − V profiles indicated potential non-Zeeman polarization effects. Secondary Stokes V peaks were also detected up to 8 Å from the D3 emission core. Significant altitudinal inhomogeneity in the magnetic field strengths was detected, possibly indicating the local magnetic collapse, facilitating the Lorentz-force driven mechanism of the seismic source excitation. Full article

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

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15 pages, 378 KiB

Open AccessArticle

On the Effects of Non-Metricity in an Averaged Universe

by Anish Agashe and Sai Madhav Modumudi

Universe 2024, 10(6), 261; https://doi.org/10.3390/universe10060261 - 13 Jun 2024

Viewed by 208

Abstract

In the covariant averaging scheme of macroscopic gravity, the process of averaging breaks the metricity of geometry. We reinterpret the back-reaction within macroscopic gravity in terms of the non-metricity of averaged geometry. This interpretation extends the effect of back-reaction beyond mere dynamics to [...] Read more.

In the covariant averaging scheme of macroscopic gravity, the process of averaging breaks the metricity of geometry. We reinterpret the back-reaction within macroscopic gravity in terms of the non-metricity of averaged geometry. This interpretation extends the effect of back-reaction beyond mere dynamics to the kinematics of geodesic bundles. With a 1 + 3 decomposition of the spacetime, we analyse how geometric flows are modified by deriving the Raychaudhuri and Sachs equations. We also present the modified forms of Gauss and Codazzi equations. Finally, we derive an expression for the angular diameter distance in the Friedmann Lemaître Robertson Walker universe and show that non-metricity modifies it only through the Hubble parameter. Thus, we caution against overestimating the influence of back-reaction on the distances. Full article

(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)

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41 pages, 5093 KiB

Open AccessReview

A Short History of the First 50 Years: From the GRB Prompt Emission and Afterglow Discoveries to the Multimessenger Era

by Filippo Frontera

Universe 2024, 10(6), 260; https://doi.org/10.3390/universe10060260 - 12 Jun 2024

Viewed by 220

Abstract

More than fifty years have elapsed from the first discovery of gamma-ray bursts (GRBs) with American Vela satellites, and more than twenty-five years from the discovery with the BeppoSAX satellite of the first X-ray afterglow of a GRB. Thanks to the afterglow discovery [...] Read more.

More than fifty years have elapsed from the first discovery of gamma-ray bursts (GRBs) with American Vela satellites, and more than twenty-five years from the discovery with the BeppoSAX satellite of the first X-ray afterglow of a GRB. Thanks to the afterglow discovery and to the possibility given to the optical and radio astronomers to discover the GRB optical counterparts, the long-time mystery about the origin of these events has been solved. Now we know that GRBs are huge explosions, mainly ultra relativistic jets, in galaxies at cosmological distances. Starting from the first GRB detection with the Vela satellites, I will review the story of these discoveries, those obtained with BeppoSAX, the contribution to GRBs by other satellites and ground experiments, among them being Venera, Compton Gamma Ray Observatory, HETE-2, Swift, Fermi, AGILE, MAGIC, H.E.S.S., which were, and some of them are still, very important for the study of GRB properties. Then, I will review the main results obtained thus far and the still open problems and prospects of GRB astronomy. Full article

(This article belongs to the Special Issue GRBs Phenomenology, Models and Applications: A Beginner Guide)

14 pages, 15013 KiB

Open AccessReview

Theoretical Perspectives on Viscous Nature of Strongly Interacting Systems

by Kinkar Saha

Universe 2024, 10(6), 259; https://doi.org/10.3390/universe10060259 - 11 Jun 2024

Viewed by 298

Abstract

Matter prevailing during the early stages of the Universe or under extreme conditions in high-energy heavy-ion experiments supposedly possesses a rich phase structure. During the evolution of such a system, the complicated pictures of transitions among various phases are studied as part of [...] Read more.

Matter prevailing during the early stages of the Universe or under extreme conditions in high-energy heavy-ion experiments supposedly possesses a rich phase structure. During the evolution of such a system, the complicated pictures of transitions among various phases are studied as part of hydrodynamics. This system, on most occasions, is considered to be non-viscous. However, various theoretical studies reveal the importance of incorporating viscous effects into the analysis. Here, the paper discusses the behavioral patterns of transport coefficients with varying temperatures and chemical potentials to obtain a qualitative, if not quantitative, picture of the same. Discussions are also shared regarding their impacts on such an exotic system for different energies, as explored in the experimental domain. This theoretical analysis, made using the structure of the Polyakov–Nambu–Jona-Lasinio (PNJL) model with a 2+1-flavor quark–antiquark system reveals important aspects of the inclusion of viscous effects in the hydrodynamic studies of QGP. Full article

(This article belongs to the Special Issue Exploring Quark Matter under Extreme Scenarios of Temperature and Density)

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13 pages, 945 KiB

Open AccessArticle

Nonsingular, Lump-like, Scalar Compact Objects in (2 + 1)-Dimensional Einstein Gravity

by Roberto V. Maluf, Gerardo Mora-Pérez, Gonzalo J. Olmo and Diego Rubiera-Garcia

Universe 2024, 10(6), 258; https://doi.org/10.3390/universe10060258 - 11 Jun 2024

Viewed by 276

Abstract

We study the space-time geometry generated by coupling a free scalar field with a noncanonical kinetic term to general relativity in

(2 + 1)

dimensions. After identifying a family of scalar Lagrangians that yield exact analytical solutions in static and circularly [...] Read more.

We study the space-time geometry generated by coupling a free scalar field with a noncanonical kinetic term to general relativity in

(2 + 1)

dimensions. After identifying a family of scalar Lagrangians that yield exact analytical solutions in static and circularly symmetric scenarios, we classify the various types of solutions and focus on a branch that yields asymptotically flat geometries. We show that the solutions within such a branch can be divided in two types, namely naked singularities and nonsingular objects without a center. In the latter, the energy density is localized around a maximum and vanishes only at infinity and at an inner boundary. This boundary has vanishing curvatures and cannot be reached by any time-like or null geodesic in finite affine time. This allows us to consistently interpret such solutions as nonsingular, lump-like, static compact scalar objects whose eventual extension to the

(3 + 1)

-dimensional context could provide structures of astrophysical interest. Full article

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

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21 pages, 334 KiB

Open AccessArticle

Gravitational Wormholes

by Mengqi Lu, Jiayue Yang and Robert B. Mann

Universe 2024, 10(6), 257; https://doi.org/10.3390/universe10060257 - 10 Jun 2024

Viewed by 339

Abstract

Spacetime wormholes are evidently an essential component of the construction of a time machine. Within the context of general relativity, such objects require, for their formation, exotic matter—matter that violates at least one of the standard energy conditions. Here, we explore the possibility [...] Read more.

Spacetime wormholes are evidently an essential component of the construction of a time machine. Within the context of general relativity, such objects require, for their formation, exotic matter—matter that violates at least one of the standard energy conditions. Here, we explore the possibility that higher-curvature gravity theories might permit the construction of a wormhole without any matter at all. In particular, we consider the simplest form of a generalized quasi topological theory in four spacetime dimensions, known as Einsteinian Cubic Gravity. This theory has a number of promising features that make it an interesting phenomenological competitor to general relativity, including having non-hairy generalizations of the Schwarzschild black hole and linearized equations of second order around maximally symmetric backgrounds. By matching series solutions near the horizon and at large distances, we find evidence that strong asymptotically AdS wormhole solutions can be constructed, with strong curvature effects ensuring that the wormhole throat can exist. Full article

(This article belongs to the Special Issue The Physics of Time Travel)

19 pages, 6411 KiB

Open AccessReview

The Hearth of the World: The Sun before Astrophysics

by Gábor Kutrovátz

Universe 2024, 10(6), 256; https://doi.org/10.3390/universe10060256 - 7 Jun 2024

Viewed by 255

Abstract

This paper presents a historical overview of conceptions about the Sun in Western astronomical and cosmological traditions before the advent of spectroscopy and astrophysics. Rather than studying general cultural ideas, we focus on the concepts developed by astronomers or by natural philosophers impacting [...] Read more.

This paper presents a historical overview of conceptions about the Sun in Western astronomical and cosmological traditions before the advent of spectroscopy and astrophysics. Rather than studying general cultural ideas, we focus on the concepts developed by astronomers or by natural philosophers impacting astronomy. The ideas we investigate, from the works of Plato and Aristotle to William Herschel and his contemporaries, do not line up into a continuous and integrated narrative, since the nature of the Sun was not a genuine scientific topic before the nineteenth century. However, the question recurringly arose as embedded in cosmological and physical contexts. By outlining this heterogeneous story that spreads from transcendence to materiality, from metaphysics to physics, from divinity to solar inhabitants, we receive insight into some major themes and trends both in the general development of astronomical and cosmological thought and in the prehistory of modern solar science. Full article

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

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12 pages, 1285 KiB

Open AccessCommunication

The Pulsar Timing Array Signal from Infrared Regions of Scalar-Induced Gravitational Waves

by Qin Fei

Universe 2024, 10(6), 255; https://doi.org/10.3390/universe10060255 - 7 Jun 2024

Viewed by 328

Abstract

The common-spectrum process, characterized by the Hellings–Downs angular correlation and observed by pulsar timing array collaborations, such as NANOGrav, PPTA, EPTA, and CPTA, can be explained by the scalar-induced gravitational waves (SIGWs). The energy density of SIGWs exhibits universal behavior in the infrared [...] Read more.

The common-spectrum process, characterized by the Hellings–Downs angular correlation and observed by pulsar timing array collaborations, such as NANOGrav, PPTA, EPTA, and CPTA, can be explained by the scalar-induced gravitational waves (SIGWs). The energy density of SIGWs exhibits universal behavior in the infrared regions. Utilizing a broken power law parameterization for the primordial curvature power spectrum, we clarify the PTA signal through the infrared characteristics of the SIGWs, using Bayesian analysis to provide posterior distributions. Bayesian factors emphasize the statistical preference for the SIGW model over explanations involving supermassive black hole binaries. Full article

(This article belongs to the Section Cosmology)

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39 pages, 1533 KiB

Open AccessReview

The Correlation Luminosity-Velocity Dispersion of Galaxies and Active Galactic Nuclei

by Mauro D’Onofrio, Paola Marziani, Cesare Chiosi and Castalia Alenka Negrete

Universe 2024, 10(6), 254; https://doi.org/10.3390/universe10060254 - 4 Jun 2024

Viewed by 381

Abstract

In this work we discuss the correlation between luminosity L and velocity dispersion

σ

observed in different astrophysical contexts, in particular that of early-type galaxies (ETGs; Faber–Jackson (FJ) law) and that of active galactic nuclei (AGN). Our data for the ETGs confirm the [...] Read more.

In this work we discuss the correlation between luminosity L and velocity dispersion

σ

observed in different astrophysical contexts, in particular that of early-type galaxies (ETGs; Faber–Jackson (FJ) law) and that of active galactic nuclei (AGN). Our data for the ETGs confirm the bending of the FJ at high masses and the existence of similar curvatures in the projections of the Fundamental Plane (FP) approximately at the mass scale of ∼

10^{10} M_{⊙}

. We provide an explanation for such curvatures and for the presence of the Zone of Exclusion (ZoE) in these diagrams. The new prospected theory for the FJ law introduces a new framework to understand galaxy evolution in line with the hierarchical structure of the Universe. The classic analysis carried out for a class of type 1 AGN accreting gas at very high rates, confirms that a FJ law of the form

L = L_{0} σ^{4}

is roughly consistent with the observations, with a slope quite similar to that of ETGs. We discuss the physics behind the FJ law for the AGN in different contexts and also examine the biases affecting both the luminosity and the velocity dispersion, paying particular attention to the effects induced by the spherical symmetry of the emitting sources on the accuracy of the luminosity estimates. Full article

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

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10 pages, 263 KiB

Open AccessArticle

Cosmic Strings from Thermal Inflation

by Robert Brandenberger and Aline Favero

Universe 2024, 10(6), 253; https://doi.org/10.3390/universe10060253 - 4 Jun 2024

Viewed by 320

Abstract

Thermal inflation was proposed as a mechanism to dilute the density of cosmological moduli. Thermal inflation is driven by a complex scalar field possessing a large vacuum expectation value and a very flat potential, called a “flaton”. Such a model admits cosmic string [...] Read more.

Thermal inflation was proposed as a mechanism to dilute the density of cosmological moduli. Thermal inflation is driven by a complex scalar field possessing a large vacuum expectation value and a very flat potential, called a “flaton”. Such a model admits cosmic string solutions, and a network of such strings will inevitably form in the symmetry breaking phase transition at the end of the period of thermal inflation. We discuss the differences of these strings compared to the strings which form in the Abelian Higgs model. Specifically, we find that the upper bound on the symmetry breaking scale is parametrically lower than in the case of Abelian Higgs strings, and that the lower cutoff on the string loop distribution is determined by cusp annihilation rather than by gravitational radiation (for the value of the transition temperature proposed in the original work on thermal inflation). Full article

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

12 pages, 1322 KiB

Open AccessArticle

Improved Statistical Analysis for the Neutrinoless Double-Beta Decay Matrix Element of ¹³⁶Xe

by Mihai Horoi

Universe 2024, 10(6), 252; https://doi.org/10.3390/universe10060252 - 4 Jun 2024

Viewed by 235

Abstract

Neutrinoless double beta decay nuclear matrix element (

M_{0 ν}

) for ¹³⁶Xe was recently analyzed using a statistical approach (Phys. Rev. C 107, 045501 (2023)). In the analysis, three initial shell model effective Hamiltonians were randomly altered, and their [...] Read more.

Neutrinoless double beta decay nuclear matrix element (

M_{0 ν}

) for ¹³⁶Xe was recently analyzed using a statistical approach (Phys. Rev. C 107, 045501 (2023)). In the analysis, three initial shell model effective Hamiltonians were randomly altered, and their results for 23 measured observables were used to infer credibility for the

M_{0 ν}

nuclear matrix element (NME) based on a Bayesian Model Averaging approach. In that analysis, a reasonable Gamow-Teller quenching factor of 0.7 was assumed for each starting effective Hamiltonian. Given that the result of the statistical analysis was sensible to this choice, we are here improving that analysis by assuming that the Gamow-Teller quenching factor is also randomly chosen within reasonabe limits for all three starting Hamiltonians. The outcomes are slightly higher expectation values and uncertainties for the

M_{0 ν}

NME. Full article

(This article belongs to the Special Issue Recent Advances in Double Beta Decay Investigations: In Honor of Prof. Sabin Stoica at His 70th Anniversary)

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13 pages, 511 KiB

Open AccessCommunication

Constraints on the Primordial Curvature Power Spectrum and Reheating Temperature from the NANOGrav 15-Year Dataset

by Qin Fei

Universe 2024, 10(6), 251; https://doi.org/10.3390/universe10060251 - 4 Jun 2024

Viewed by 187

Abstract

The stochastic signal observed by collaborations such as NANOGrav, PPTA, EPTA +InPTA, and CPTA may originate from gravitational waves induced by primordial curvature perturbations during inflation. This study investigates small-scale properties of inflation and reheating, assuming a log-normal form for the power spectrum [...] Read more.

The stochastic signal observed by collaborations such as NANOGrav, PPTA, EPTA +InPTA, and CPTA may originate from gravitational waves induced by primordial curvature perturbations during inflation. This study investigates small-scale properties of inflation and reheating, assuming a log-normal form for the power spectrum of the primordial curvature and a reheating phase equation of state

w = 1 / 9

. Inflation and reheating scenarios are thoroughly examined using Bayesian methods applied to the NANOGrav 15-year dataset. The analysis establishes constraints on the reheating temperature, suggesting

T_{rh} ≳ 0.1 Gev

, consistent with Big Bang nucleosynthesis constraints. Additionally, the NANOGrav 15-year dataset requires the amplitude (A∼0.1) and width (

Δ ≲ 0.001

) of the primordial curvature power spectrum to be within specific ranges. A notable turning point in the energy density of scalar-induced gravitational waves occurs due to a change in the equation of state w. This turning point signifies a transition from the reheating epoch to radiation domination. Further observations of scalar-induced gravitational waves could provide insights into the precise timing of this transition, enhancing our understanding of early Universe dynamics. Full article

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18 pages, 448 KiB

Open AccessArticle

Rotation Matrix of a Charged Symmetrical Body: One-Parameter Family of Solutions in Elementary Functions

by Alexei A. Deriglazov

Universe 2024, 10(6), 250; https://doi.org/10.3390/universe10060250 - 3 Jun 2024

Viewed by 156

Abstract

Euler–Poisson equations of a charged symmetrical body in external constant and homogeneous electric and magnetic fields are deduced starting from the variational problem, where the body is considered as a system of charged point particles subject to holonomic constraints. The final equations are [...] Read more.

Euler–Poisson equations of a charged symmetrical body in external constant and homogeneous electric and magnetic fields are deduced starting from the variational problem, where the body is considered as a system of charged point particles subject to holonomic constraints. The final equations are written for the center-of-mass coordinate, rotation matrix and angular velocity. A general solution to the equations of motion is obtained for the case of a charged ball. For the case of a symmetrical charged body (solenoid), the task of obtaining the general solution is reduced to the problem of a one-dimensional cubic pseudo-oscillator. In addition, we present a one-parametric family of solutions to the problem in elementary functions. Full article

8 pages, 502 KiB

Open AccessArticle

On the Role of the Tail Term in Electromagnetic Radiation Reaction

by Zdeněk Stuchlík, Martin Kološ, Arman Tursunov and Dmitri Gal’tsov

Universe 2024, 10(6), 249; https://doi.org/10.3390/universe10060249 - 3 Jun 2024

Viewed by 169

Abstract

In a recent study devoted to the influence of electromagnetic radiation reaction on the motion of radiating charged particles in magnetized black hole spacetimes the authors claim that the tail term cannot be neglected in the complete DeWitt–Brehme equation, putting into doubt the [...] Read more.

In a recent study devoted to the influence of electromagnetic radiation reaction on the motion of radiating charged particles in magnetized black hole spacetimes the authors claim that the tail term cannot be neglected in the complete DeWitt–Brehme equation, putting into doubt the previous papers where such an approximation was used. Here, we demonstrate by using simple dimensional arguments that such a statement is misleading in many astrophysically relevant situations. In the case of relativistic electrons moving around a stellar-mass black hole, the tail term is ignorable if a magnetic field of at least a few Gauss is present.On the other hand, in different situations, the tail term can be relevant, as demonstrated in the case of orbital widening, where it can even amplify the effect. Full article

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

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29 pages, 1891 KiB

Open AccessReview

Decoding the Nature of Coherent Radio Emission in Pulsars I: Observational Constraints

by Dipanjan Mitra, Rahul Basu and George I. Melikizde

Universe 2024, 10(6), 248; https://doi.org/10.3390/universe10060248 - 3 Jun 2024

Viewed by 272

Abstract

Radio observations from normal pulsars indicate that the coherent radio emission is excited by curvature radiation from charge bunches. In this review, we provide a systematic description of the various observational constraints on the radio emission mechanism. We have discussed the presence of [...] Read more.

Radio observations from normal pulsars indicate that the coherent radio emission is excited by curvature radiation from charge bunches. In this review, we provide a systematic description of the various observational constraints on the radio emission mechanism. We have discussed the presence of highly polarized time samples where the polarization position angle follow two orthogonal well-defined tracks across the profile that closely match the rotating vector model in an identical manner. The observations also show the presence of circular polarization, with both the right and left handed circular polarization seen across the profile. Other constraints on the emission mechanism are provided by the detailed measurements of the spectral index variation across the profile window, where the central part of the profile, corresponding to the core component, has a steeper spectrum than the surrounding cones. Finally, the detailed measurements of the subpulse drifting behaviour can be explained by considering the presence of non-dipolar field on the stellar surface and the formation of the partially screened gap (PSG) above the polar cap region. The PSG gives rise to a non-stationary plasma flow that has a multi-component nature, consisting of highly energetic primary particles, secondary pair plasma, and iron ions discharged from the surface, with large fragmentation resulting in dense plasma clouds and lower-density inter-cloud regions. The physical properties of the outflowing plasma and the observational constraints lead us to consider coherent curvature radiation as the most viable explanation for the emission mechanism in normal pulsars, where propagation effects due to adiabatic walking and refraction are largely inconsequential. Full article

(This article belongs to the Special Issue A New Horizon of Pulsar and Neutron Star: The 55-Year Anniversary)

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10 pages, 679 KiB

Open AccessReview

Neutrino Masses and Right-Handed Weak Currents Studied by Neutrino-Less ββ-Decay Detectors

by Saori Umehara and Hiroyasu Ejiri

Universe 2024, 10(6), 247; https://doi.org/10.3390/universe10060247 - 3 Jun 2024

Viewed by 261

Abstract

Detecting neutrino-less double beta (

0 ν β β

) decay with high-sensitivity

0 ν β β

detectors is of current interest for studying the Majorana neutrino’s nature, the neutrino mass (

ν

-mass), right-handed weak currents (RHCs), and others beyond the Standard [...] Read more.

Detecting neutrino-less double beta (

0 ν β β

) decay with high-sensitivity

0 ν β β

detectors is of current interest for studying the Majorana neutrino’s nature, the neutrino mass (

ν

-mass), right-handed weak currents (RHCs), and others beyond the Standard Model. Many experimental groups have studied

0 ν β β

decay with

ν

-mass sensitivities on the order of 100 meV and RHC sensitivities on the order of 10

^{- 9}

–10

^{- 6}

, but no clear

0 ν β β

signals have been observed so far in these

ν

-mass and RHC regions. Thus, several experimental groups are developing higher-sensitivity detectors to explore a smaller

ν

-mass region around 15–50 meV, which corresponds to the inverted hierarchy

ν

-mass, and smaller RHC regions on the order of 10

^{- 10}

–10

^{- 7}

in the near future. Nuclear matrix elements (NMEs) for

ν

-mass and RHC processes are crucial for extracting the

ν

-mass and RHCs of particle physics interest from

0 ν β β

experiments. This report briefly reviews detector sensitivities and upper limits on the

ν

-mass and right-handed currents for several current

0 ν β β

detectors and the

ν

-mass and RHC sensitivities expected for some near-future ones. Full article

(This article belongs to the Special Issue Recent Advances in Double Beta Decay Investigations: In Honor of Prof. Sabin Stoica at His 70th Anniversary)

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18 pages, 6374 KiB

Open AccessArticle

New Parametrization of the Dark-Energy Equation of State with a Single Parameter

by Jainendra Kumar Singh, Preeti Singh, Emmanuel N. Saridakis, Shynaray Myrzakul and Harshna Balhara

Universe 2024, 10(6), 246; https://doi.org/10.3390/universe10060246 - 1 Jun 2024

Viewed by 241

Abstract

We propose a novel dark-energy equation-of-state parametrization, with a single parameter

η

that quantifies the deviation from ΛCDM cosmology. We first confront the scenario with various datasets, from the Hubble function (OHD), Pantheon, baryon acoustic oscillations (BAO), and their joint observations, and we [...] Read more.

We propose a novel dark-energy equation-of-state parametrization, with a single parameter

η

that quantifies the deviation from ΛCDM cosmology. We first confront the scenario with various datasets, from the Hubble function (OHD), Pantheon, baryon acoustic oscillations (BAO), and their joint observations, and we show that

η

has a preference for a non-zero value, namely, a deviation from ΛCDM cosmology is favored, although the zero value is marginally inside the 1

σ

confidence level. However, we find that the present Hubble function value acquires a higher value, namely,

H_{0} = 66 . 624_{- 0.013}^{+ 0.011}

Km s

^{- 1}

Mpc

^{- 1}

, which implies that the

H_{0}

tension can be partially alleviated. Additionally, we perform a cosmographic analysis, showing that the universe transits from deceleration to acceleration in the recent cosmological past; nevertheless, in the future, it will not result in a de Sitter phase since it exhibits a second transition from acceleration to deceleration. Finally, we perform the statefinder analysis. The scenario behaves similarly to the ΛCDM paradigm at high redshifts, while the deviation becomes significant at late and recent times and especially in the future. Full article

(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)

9 pages, 696 KiB

Open AccessCommunication

X-ray Redshifts for Obscured Active Galactic Nuclei with AXIS Deep and Intermediate Surveys

by Alessandro Peca, Nico Cappelluti, Stefano Marchesi, Edmund Hodges-Kluck and Adi Foord

Universe 2024, 10(6), 245; https://doi.org/10.3390/universe10060245 - 1 Jun 2024

Viewed by 256

Abstract

This study presents the capabilities of the AXIS telescope in estimating redshifts from X-ray spectra alone (X-ray redshifts, XZs). Through extensive simulations, we establish that AXIS observations enable reliable XZ estimates for more than 5500 obscured active galactic nuclei (AGNs) up to redshift [...] Read more.

This study presents the capabilities of the AXIS telescope in estimating redshifts from X-ray spectra alone (X-ray redshifts, XZs). Through extensive simulations, we establish that AXIS observations enable reliable XZ estimates for more than 5500 obscured active galactic nuclei (AGNs) up to redshift

z \sim 6

in the proposed deep (7 Ms) and intermediate (375 ks) surveys. Notably, at least 1600 of them are expected to be in the Compton-thick regime (

log N_{H} / {cm}^{- 2} \geq 24

), underscoring the pivotal role of AXIS in sampling these elusive objects that continue to be poorly understood. XZs provide an efficient alternative for optical/infrared faint sources, overcoming the need for time-consuming spectroscopy, the potential limitations of photometric redshifts, and potential issues related to multi-band counterpart association. This approach will significantly enhance the accuracy of constraints on the X-ray luminosity function and obscured AGN fractions up to high redshifts. This white paper is part of a series commissioned for the AXIS Probe Concept Mission; additional AXIS white papers can be found at the AXIS website. Full article

(This article belongs to the Section Galaxies and Clusters)

21 pages, 1043 KiB

Open AccessArticle

Charmonium Transport in Heavy-Ion Collisions at the LHC

by Biaogang Wu and Ralf Rapp

Universe 2024, 10(6), 244; https://doi.org/10.3390/universe10060244 - 31 May 2024

Viewed by 256

Abstract

We provide an update on our semi-classical transport approach for quarkonium production in high-energy heavy-ion collisions, focusing on

J / ψ

and

ψ (2 S)

mesons in 5.02 TeV Pb-Pb collisions at the Large Hadron Collider (LHC) at both forward and [...] Read more.

We provide an update on our semi-classical transport approach for quarkonium production in high-energy heavy-ion collisions, focusing on

J / ψ

and

ψ (2 S)

mesons in 5.02 TeV Pb-Pb collisions at the Large Hadron Collider (LHC) at both forward and mid-rapidity. In particular, we employ the most recent charm-production cross sections reported in pp collisions, which are pivotal for the magnitude of the regeneration contribution, and their modifications due to cold-nuclear-matter (CNM) effects. Multi-differential observables are calculated in terms of nuclear modification factors as a function of centrality, transverse momentum, and rapidity, including the contributions from feeddown from bottom hadron decays. For our predictions for

ψ (2 S)

production, the mechanism of sequential regeneration relative to the more strongly bound

J / ψ

meson plays an important role in interpreting recent ALICE data. Full article

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

12 pages, 577 KiB

Open AccessCommunication

Search for R-Parity-Violation-Induced Charged Lepton Flavor Violation at Future Lepton Colliders

by Xunye Cai, Jingshu Li, Ran Ding, Meng Lu, Zhengyun You and Qiang Li

Universe 2024, 10(6), 243; https://doi.org/10.3390/universe10060243 - 31 May 2024

Viewed by 252

Abstract

Interest in searches for Charged Lepton Flavor Violation (CLFV) has continued in the past few decades since the observation of CLFV would indicate a new physics Beyond the Standard Model (BSM). As several future lepton colliders with high luminosity have been proposed, the [...] Read more.

Interest in searches for Charged Lepton Flavor Violation (CLFV) has continued in the past few decades since the observation of CLFV would indicate a new physics Beyond the Standard Model (BSM). As several future lepton colliders with high luminosity have been proposed, the search for CLFV will reach an unprecedented level of precision. Many BSM models allow CLFV processes at the tree level, such as the R-parity-violating (RPV) Minimal Supersymmetric Standard Model (MSSM), which is a good choice for benchmarking. In this paper, we perform a detailed fast Monte Carlo simulation study on RPV-induced CLFV processes at future lepton colliders, including a 240

GeV

circular electron positron collider (CEPC) and a 6 or 14

TeV

Muon Collider. As a result, we found that the upper limits on the

τ

-related RPV couplings will be significantly improved, while several new limits on RPV couplings can be set, which are inaccessible by low-energy experiments. Full article

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

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9 pages, 852 KiB

Open AccessArticle

Optical Quasi-Periodic Oscillation of Blazar PKS 1440-389 in the TESS Light Curve

by He Lu, Tingfeng Yi, Yanke Tang, Junjie Wang, Shun Zhang, Liang Wang, Yutong Chen, Yuncai Shen, Liang Dong and Yangwei Zhang

Universe 2024, 10(6), 242; https://doi.org/10.3390/universe10060242 - 31 May 2024

Viewed by 254

Abstract

We report the results of time series analysis of blazar PKS 1440-389, observed by the Transiting Exoplanet Survey Satellite (TESS) in two sectors. We find that the source has a quasi-periodic oscillation (QPO) of about 3.1 days for sector 11 and around 3.7 [...] Read more.

We report the results of time series analysis of blazar PKS 1440-389, observed by the Transiting Exoplanet Survey Satellite (TESS) in two sectors. We find that the source has a quasi-periodic oscillation (QPO) of about 3.1 days for sector 11 and around 3.7 days for sector 38 in the optical band. We use two methods to assess the QPO and its confidence level: Lomb–Scargle periodogram and weighted wavelet Z-transforms. We explore various potential explanations for these rapid quasi-periodic variations and propose that their source most likely resides within the innermost region of the accretion disk. Within this framework, we estimate the mass of the central black hole of this blazar. We obtain black hole masses of 6.65 × 10⁸

M_{⊙}

(Schwarzschild black hole) and 4.22 × 10⁹

M_{⊙}

(maximally rotating Kerr black hole), with a main period of 3.7 days. Finally, we utilize the kink instability model to explain the QPO. Full article

(This article belongs to the Special Issue Blazar Bursts: Theory and Observation)

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21 pages, 596 KiB

Open AccessArticle

Mechanisms for Producing Primordial Black Holes from Inflationary Models beyond Fine-Tuning

by Ioanna Stamou

Universe 2024, 10(6), 241; https://doi.org/10.3390/universe10060241 - 30 May 2024

Viewed by 219

Abstract

In this study, we present an analysis of the fine-tuning required in various inflationary models in order to explain the production of Primordial Black Holes (PBHs). We specifically examine the degree of fine-tuning necessary in two prominent single-field inflationary models: those with an [...] Read more.

In this study, we present an analysis of the fine-tuning required in various inflationary models in order to explain the production of Primordial Black Holes (PBHs). We specifically examine the degree of fine-tuning necessary in two prominent single-field inflationary models: those with an inflection point and those with step-like features in the potential. Our findings indicate that models with step-like features generally require less fine-tuning compared to those with an inflection point, making them more viable for consistent PBH production. An interesting outcome of these models is that, in addition to improved fine-tuning, they may also predict low-frequency signals that can be detected by pulsar timing array (PTA) collaborations. Additionally, we extend our analysis to multifield inflationary models to assess whether the integration of additional fields can further alleviate the fine-tuning demands. The study also explores the role of a spectator field and its impact on the fine-tuning process. Our results indicate that although mechanisms involving a spectator field can circumvent the issue of fine-tuning parameters for PBH production, both multifield models and models with step-like features present promising alternatives. While fine-tuning involves multiple considerations, our primary objective is to evaluate various inflationary models to identify the one that most naturally explains the formation of PBHs. Hence, this study introduces a novel approach by categorizing existing PBH mechanisms, paving the way for subsequent research to prioritize models that minimize the need for extensive fine-tuning. Full article

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25 pages, 661 KiB

Open AccessArticle

Renormalization Analysis of Magnetohydrodynamics: Two-Loop Approximation

by Michal Hnatič, Tomáš Lučivjanský, Lukáš Mižišin, Yurii Molotkov and Andrei Ovsiannikov

Universe 2024, 10(6), 240; https://doi.org/10.3390/universe10060240 - 30 May 2024

Viewed by 240

Abstract

We investigate the stochastic version of the paradigmatic model of magnetohydrodynamic turbulence. The model can be interpreted as an active vector admixture subject to advective processes governed by turbulent flow. The back influence on fluid dynamics is explicitly taken into account. The velocity [...] Read more.

We investigate the stochastic version of the paradigmatic model of magnetohydrodynamic turbulence. The model can be interpreted as an active vector admixture subject to advective processes governed by turbulent flow. The back influence on fluid dynamics is explicitly taken into account. The velocity field is generated through a fully developed turbulent flow taking into account the violation of spatial parity, which is introduced through the helicity parameter

ρ

. We consider a generalized setup in which parameter A is introduced in model formulation, which is associated with the interaction part of the model, and its actual value represents different physical systems. The model is analyzed by means of the field-theoretic renormalization group. The calculation is performed using

ε

-expansion, where

ε

is the deviation from the Kolmogorov scaling. Two-loop numerical calculations of the renormalization constant associated with the renormalization of the magnetic field are presented. Full article

(This article belongs to the Section Field Theory)

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21 pages, 462 KiB

Open AccessArticle

Inflaton Decay in No-Scale Supergravity and Starobinsky-like Models

by Yohei Ema, Marcos A. G. Garcia, Wenqi Ke, Keith A. Olive and Sarunas Verner

Universe 2024, 10(6), 239; https://doi.org/10.3390/universe10060239 - 30 May 2024

Cited by 1 | Viewed by 182

Abstract

We consider the decay of the inflaton in Starobinsky-like models arising from either an

R + R^{2}

theory of gravity or

N = 1

no-scale supergravity models. If Standard Model matter is simply introduced to the

R + R^{2}

theory, the [...] Read more.

We consider the decay of the inflaton in Starobinsky-like models arising from either an

R + R^{2}

theory of gravity or

N = 1

no-scale supergravity models. If Standard Model matter is simply introduced to the

R + R^{2}

theory, the inflaton (which appears when the theory is conformally transformed into the Einstein frame) couples to matter predominantly in Standard Model Higgs kinetic terms. This will typically lead to a reheating temperature of ∼3 ×

10^{9}

GeV. However, if the Standard Model Higgs is conformally coupled to curvature, the decay rate may be suppressed and vanishes for conformal coupling

ξ = 1 / 6

. Nevertheless, the inflaton decays through the conformal anomaly, leading to a reheating temperature of the order of

10^{8}

GeV. The Starobinsky potential may also arise in no-scale supergravity. In this case, the inflaton decays if there is a direct coupling of the inflaton to matter in the superpotential or to gauge fields through the gauge kinetic function. We also discuss the relation between the theories and demonstrate the correspondence between the no-scale models and the conformally coupled

R + R^{2}

theory (with

ξ = 1 / 6

). Full article

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

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25 pages, 990 KiB

Open AccessReview

Supernova Neutrinos: Flavour Conversion Mechanisms and New Physics Scenarios

by Manibrata Sen

Universe 2024, 10(6), 238; https://doi.org/10.3390/universe10060238 - 30 May 2024

Viewed by 242

Abstract

A core-collapse supernova (SN) releases almost all of its energy in the form of neutrinos, which provide a unique opportunity to probe the working machinery of an SN. These sites are prone to neutrino–neutrino refractive effects, which can lead to fascinating collective flavour [...] Read more.

A core-collapse supernova (SN) releases almost all of its energy in the form of neutrinos, which provide a unique opportunity to probe the working machinery of an SN. These sites are prone to neutrino–neutrino refractive effects, which can lead to fascinating collective flavour oscillations among neutrinos. This causes rapid neutrino flavour conversions deep inside the SN even for suppressed mixing angles, with intriguing consequences for the explosion mechanism as well as nucleosynthesis. We review the physics of collective oscillations of neutrinos—both slow and fast—along with the well-known resonant flavour conversion effects and discuss the current state-of-the-art of the field. Furthermore, we discuss how neutrinos from an SN can be used to probe novel particle physics properties, extreme values of which are otherwise inaccessible in laboratories. Full article

(This article belongs to the Special Issue Neutrinos across Different Energy Scales)

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20 pages, 972 KiB

Open AccessArticle

Tracking Supermassive Black Hole Mergers from kpc to sub-pc Scales with AXIS

by Adi Foord, Nico Cappelluti, Tingting Liu, Marta Volonteri, Melanie Habouzit, Fabio Pacucci, Stefano Marchesi, Nianyi Chen, Tiziana Di Matteo, Labani Mallick and Michael Koss

Universe 2024, 10(6), 237; https://doi.org/10.3390/universe10060237 - 28 May 2024

Cited by 2 | Viewed by 350

Abstract

We present an analysis showcasing how the Advanced X-ray Imaging Satellite (AXIS), a proposed NASA Probe-class mission, will significantly increase our understanding of supermassive black holes undergoing mergers—from kpc to sub-pc scales. In particular, the AXIS point spread function, field of view, and [...] Read more.

We present an analysis showcasing how the Advanced X-ray Imaging Satellite (AXIS), a proposed NASA Probe-class mission, will significantly increase our understanding of supermassive black holes undergoing mergers—from kpc to sub-pc scales. In particular, the AXIS point spread function, field of view, and effective area are expected to result in (1) the detection of hundreds to thousands of new dual AGNs across the redshift range

0 < z < 5

and (2) blind searches for binary AGNs that are exhibiting merger signatures in their light curves and spectra. AXIS will detect some of the highest-redshift dual AGNs to date, over a large range of physical separations. The large sample of AGN pairs detected by AXIS (over a magnitude more than currently known) will result in the first X-ray study that quantifies the frequency of dual AGNs as a function of redshift up to

z = 4

. Full article

(This article belongs to the Section Galaxies and Clusters)

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