Cross Sections for Coherent Elastic and Inelastic Neutrino-Nucleus Scattering
Improved Superscaling in Quasielastic Electron Scattering with Relativistic Effective Mass

Congratulations to Prof. Roger Penrose, Advisory Board member of Universe, for receiving the Nobel Prize in Physics 2020.

Journal Description

Universe

Universe is a peer-reviewed open access journal focused on principles and new discoveries in the universe. Universe is published monthly online by MDPI.

Open Access— free for readers, with article processing charges (APC) paid by authors or their institutions.
High Visibility: indexed within Scopus, SCIE (Web of Science), Astrophysics Data System, INSPIRE, CAPlus / SciFinder, Inspec, and other databases.
Journal Rank: JCR - Q2 (Astronomy & Astrophysics) / CiteScore - Q2 (General Physics and Astronomy)
Rapid Publication: manuscripts are peer-reviewed and a first decision is provided to authors approximately 19.3 days after submission; acceptance to publication is undertaken in 3.8 days (median values for papers published in this journal in the second half of 2022).
Recognition of Reviewers: reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.
Testimonials: See what our authors say about Universe.
Companion journal: Astronomy.

Impact Factor: 2.813 (2021); 5-Year Impact Factor: 2.442 (2021)

Imprint Information Journal Flyer Open Access ISSN: 2218-1997

Latest Articles

Open AccessReview

The Central Engine of GRB170817A and the Energy Budget Issue: Kerr Black Hole versus Neutron Star in a Multi-Messenger Analysis

Maurice H. P. M. van Putten

Universe 2023, 9(6), 279; https://doi.org/10.3390/universe9060279 (registering DOI) - 08 Jun 2023

Abstract

Upcoming LIGO–Virgo–KAGRA (LVK) observational runs offer new opportunities to probe the central engines of extreme transient events. Cosmological gamma-ray bursts (GRBs) and core-collapse supernovae (CC-SNe), in particular, are believed to be powered by compact objects, i.e., a neutron star (NS) or black hole [...] Read more.

E_{J}

. Per unit mass, this reaches a few percent in a rapidly rotating NS and tens of percent in a Kerr BH, respectively. Calorimetry by

E_{G W}

on a descending chirp may break the degeneracy between the two. We review this approach, anticipating new observational opportunities for planned LVK runs. GRB170817A is the first event revealing its central engine by a descending chirp in gravitational radiation. An accompanying energy output

E_{G W} ≃ 3.5 % M_{⊙} c^{2}

is observed during GRB170817A in the aftermath of the double neutron star merger GW170817. The progenitors of normal long GRBs, on the other hand, are the rare offspring of CC-SNe of type Ib/c. Yet, the extended emission to SGRBs (SGRBEEs) shares similar durations and the same Amati-relation of the prompt GRB emission of LGRBs, pointing to a common central engine. The central engine of these extreme transient events has, hitherto, eluded EM observations alone, even when including neutrino observations, as in SN1987A. The trigger signaling the birth of the compact object and the evolution powering these events is expected to be revealed by an accompanying GW signal, perhaps similar to that of GRB170817A. For GRB170817A,

E_{G W}

exceeds

E_{J}

in the initial hyper-massive neutron star (HMNS) produced in the immediate aftermath of GW170817. It identifies the spin-down of a Kerr BH of mass ∼

2.4 M_{⊙}

defined by the total mass of GW170817. This observation is realized in spectrograms generated by Butterfly matched filtering, a time-symmetric analysis with equal sensitivity to ascending and descending chirps, calibrated by signal injection experiments. It is implemented on a heterogeneous computing platform with synaptic parallel processing in F90/C++/C99 under bash. A statistical significance of

5.5 σ

is derived from multi-messenger event timing, based on a probability of false alarm (PFA) factored over a probability

p_{1} = 8.3 \times 10^{- 4}

by causality and a p-value

p_{2} = 4.9 \times 10^{- 5}

of consistency between H1 and L1 observations. For upcoming observations, this approach may be applied to similar emissions from SNIb/c and GRBs in the Local Universe, upon the mass-scaling of present results by the mass of their putative black hole-central engines. Full article

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

► Show Figures

Figure 1

Open AccessArticle

A Wheeler–DeWitt Quantum Approach to the Branch-Cut Gravitation with Ordering Parameters

Benno August Ludwig Bodmann

César Augusto Zen Vasconcellos

Peter Otto Hess Bechstedt

José Antonio de Freitas Pacheco

Dimiter Hadjimichef

Moisés Razeira

and

Gervásio Annes Degrazia

Universe 2023, 9(6), 278; https://doi.org/10.3390/universe9060278 (registering DOI) - 08 Jun 2023

Abstract

In this contribution to the Festschrift for Prof. Remo Ruffini, we investigate a formulation of quantum gravity using the Hořava–Lifshitz theory of gravity, which is General Relativity augmented by counter-terms to render the theory regularized. We are then led to the Wheeler–DeWitt (WDW) equation combined with the classical concepts of the branch-cut gravitation, which contemplates as a new scenario for the origin of the Universe, a smooth transition region between the contraction and expansion phases. Through the introduction of an energy-dependent effective potential, which describes the space-time curvature associated with the embedding geometry and its coupling with the cosmological constant and matter fields, solutions of the WDW equation for the wave function of the Universe are obtained. The Lagrangian density is quantized through the standard procedure of raising the Hamiltonian, the helix-like complex scale factor of branched gravitation as well as the corresponding conjugate momentum to the category of quantum operators. Ambiguities in the ordering of the quantum operators are overcome with the introduction of a set of ordering factors

α

, whose values are restricted, to make contact with similar approaches, to the integers

α = [0, 1, 2]

, allowing this way a broader class of solutions for the wave function of the Universe. In addition to a branched universe filled with underlying background vacuum energy, primordial matter and radiation, in order to connect with standard model calculations, we additionally supplement this formulation with baryon matter, dark matter and quintessence contributions. Finally, the boundary conditions for the wave function of the Universe are imposed by assuming the Bekenstein criterion. Our results indicate the consistency of a topological quantum leap, or alternatively a quantum tunneling, for the transition region of the early Universe in contrast to the classic branched cosmology view of a smooth transition. Full article

(This article belongs to the Special Issue Remo Ruffini Festschrift)

► Show Figures

Figure 1

Open AccessArticle

A Software for RFI Analysis of Radio Environment around Radio Telescope

and

Universe 2023, 9(6), 277; https://doi.org/10.3390/universe9060277 (registering DOI) - 08 Jun 2023

Abstract

Radio astronomy uses radio telescopes to detect very faint emissions from celestial objects. However, human-made radio frequency interference (RFI) is currently a common problem faced by most terrestrial radio telescopes, and it is getting worse with the development of the economy and technology. Therefore, it is essential to monitor and evaluate interference during the planning, construction, and operation stages of the radio telescope and protect the quiet radio environment around the radio astronomical site. In this paper, we present a software for an RFI analysis of the radio environment around the telescope. In this software, information has been collected, including the location of the site; the technical specifications, such as aperture and the frequency range of the radio telescopes; and the terrain around the site. The software and its modules are composed of telescope, geographic, and meteorological databases, and analysis modules of terrestrial and space-based RFI. Combined with the propagation characteristics of radio waves, we can analyze and evaluate RFI on the ground and in space around the radio telescope. The feasibility of the software has been proved by the experimental implementation of the propagation properties and RFI source estimation. With this software, efficient technical support can be expected for protecting the radio environment around the telescope, as well as improving site selection for planned radio astronomical facilities. Full article

(This article belongs to the Special Issue New Discoveries in Astronomical Data)

► Show Figures

Figure 1

Open AccessArticle

QCD Phase Structure and In-Medium Modifications of Meson Masses in Polyakov Linear-Sigma Model with Finite Isospin Asymmetry

Abdel Nasser Tawfik

Universe 2023, 9(6), 276; https://doi.org/10.3390/universe9060276 - 08 Jun 2023

Abstract

In the QCD-like effective model, the Polyakov linear-sigma model, the isospin sigma field (

{\bar{σ}}_{3} = f_{K^{\pm}} - f_{K^{0}}

) and the third generator of the matrix of the explicit symmetry breaking [ [...] Read more.

In the QCD-like effective model, the Polyakov linear-sigma model, the isospin sigma field (

{\bar{σ}}_{3} = f_{K^{\pm}} - f_{K^{0}}

) and the third generator of the matrix of the explicit symmetry breaking [

h_{3} = m_{a_{0}}^{2} (f_{K^{\pm}} - f_{K^{0}})

] are estimated in terms of the decay constants of the neutral (

f_{K}^{0}

) and charged Kaon (

f_{K^{\pm}}

) and the mass of

a_{0}

meson. Both quantities

{\bar{σ}}_{3}

and

h_{3}

are then evaluated, at finite baryon (

μ_{B}

), isospin chemical potential (

μ_{I}

), and temperature (T). Thereby, the dependence of the critical temperature on isospin chemical potential could be mapped out in the (

T - μ_{I}

) phase diagram In the QCD-like effective model, the Polyakov linear-sigma model, the isospin sigma field (

{\bar{σ}}_{3} = f_{K^{\pm}} - f_{K^{0}}

) and the third generator of the matrix of the explicit symmetry breaking [

h_{3} = m_{a_{0}}^{2} (f_{K^{\pm}} - f_{K^{0}})

] are estimated in terms of the decay constants of the neutral (

f_{K}^{0}

) and charged Kaon (

f_{K^{\pm}}

) and the mass of

a_{0}

meson. Both quantities

{\bar{σ}}_{3}

and

h_{3}

are then evaluated, at finite baryon (

μ_{B}

), isospin chemical potential (

μ_{I}

), and temperature (T). Thereby, the dependence of the critical temperature on isospin chemical potential could be mapped out in the (

T - μ_{I}

) phase diagram. The in-medium modifications of pseudoscalars (

J^{p c} = 0^{- +}

), scalars (

J^{p c} = 0^{+ +}

), vectors (

J^{p c} = 1^{- -}

), and axial-vectors (

J^{p c} = 1^{+ +}

) meson states are then analyzed in thermal and dense medium. We conclude that the QCD phase diagram (

T - μ_{I}

) is qualitatively similar to the (

T - μ_{B}

) phase diagram. We also conclude that both temperature and isospin chemical potential enhance the in-medium modifications of the meson states

a_{0}

σ

η^{'}

π

f_{0}

κ

η

, K,

ρ

ω

κ^{*}

ϕ

a_{1}

f_{1}

K^{*}

, and

f_{1}^{*}

. Regarding their chemical potential, at high temperatures the various meson states likely dissolve into colored partonic phase. In this limit, the meson masses form a universal bundle. Thus, we conclude that the increase in the chemical potential similar to temperature derives the colorless confined meson states into the colored deconfined parton phase. Full article

(This article belongs to the Special Issue Selected Papers from the 2nd International Electronic Conference on Universe (ECU 2023))

► Show Figures

Figure 1

Open AccessCommunication

The Neutrino Mass Problem: From Double Beta Decay to Cosmology

Osvaldo Civitarese

Universe 2023, 9(6), 275; https://doi.org/10.3390/universe9060275 - 07 Jun 2023

Abstract

The neutrino is perhaps the most elusive member of the particle zoo. The questions about its nature, namely: Dirac or Majorana, the value of its mass and the interactions with other particles, the number of its components including sterile species, are long standing ones and still remain to a large extent without conclusive answers. From the side of the nuclear structure and nuclear reactions, both theories and experiments, the need to elucidate these questions has, and still has, prompt crucial developments in the fields of double beta decay, double charge exchange and neutrino induced reactions. The measurements of neutrino flavor oscillation parameters contribute largely to restrict models with massless neutrinos. From the particle physics side, the possibilities to extend the standard model of electroweak interactions to incorporate a right-handed sector of the electroweak Lagrangian are directly linked to the adopted neutrino model. Here, I would like to address another aspect of the problem by asking the question of the neutrino mass mechanism in the cosmological context, and particularly about dark matter. 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)

► Show Figures

Figure 1

Open AccessArticle

A Novel, Low-Cost, Position-Sensitive Neutron Detector to Support Thick-Target Inverse Kinematics Experiments for Nuclear Data Measurements

and

Universe 2023, 9(6), 274; https://doi.org/10.3390/universe9060274 - 06 Jun 2023

Abstract

High quality nuclear data lie at the heart of accurately modelling stellar systems and terrestrial nuclear reactors. However, some key reaction cross sections have large uncertainties, which limit such models in predicting isotopic abundances and other aspects of stellar evolution, along with key operational parameters for nuclear reactors. Reactions involving neutrons are particularly difficult to measure experimentally in laboratories, not least due to the unique challenges involved when detecting neutrons. We present a new approach to measuring nuclear reactions involving neutrons by exploiting the Thick-Target Inverse Kinematics (TTIK) approach. For such measurements, a new detector called ATTIKUS (A Thick-Target Inverse Kinematics detector by Universities in Sheffield) is under construction. Here we present designs and Geant4 Monte-Carlo simulations of the detector. The simulations indicate that a neutron position reconstruction resolution of 10 cm is obtainable and demonstrate how this device could be applied to the ¹³C(α,n) reaction, which is considered to be the main neutron source for the s-process in low-mass Asymptotic Giant Branch stars. In the TTIK method, the emission position of the neutron (the nuclear interaction position in a gaseous target) is directly linked to the centre-of-mass energy of the reaction. Therefore, a position resolution will translate into an energy resolution, depending on the beam-target combination. The inverse reaction, ¹⁶O(n,α), causes a large uncertainty in calculating the effective neutron multiplication factor, K_eff in nuclear reactors, so improvements are required here. Full article

(This article belongs to the Special Issue Advances in Cosmology and Subatomic Particle Physics)

► Show Figures

Figure 1

Open AccessReview

The Zoo of Isolated Neutron Stars

Sergei B. Popov

Universe 2023, 9(6), 273; https://doi.org/10.3390/universe9060273 - 06 Jun 2023

Abstract

In this brief review, I summarize our basic knowledge about different types of isolated neutron stars. I discuss radio pulsars, central compact objects in supernova remnants, magnetars, nearby cooling neutron stars (also known as the magnificent seven), and sources of fast radio bursts. Several scenarios of magneto-rotational evolution are presented. Recent observational data, such as the discovery of long-period radio pulsars, require the non-trivial evolution of magnetic fields, the spin periods of neutron stars, or both. In some detail, I discuss different models of magnetic field decay and interactions of young neutron stars with fallback matter. Full article

(This article belongs to the Special Issue Selected Papers from the 2nd International Electronic Conference on Universe (ECU 2023))

► Show Figures

Figure 1

Open AccessReview

The Warm Inflation Story

Arjun Berera

Universe 2023, 9(6), 272; https://doi.org/10.3390/universe9060272 - 06 Jun 2023

Abstract

Warm inflation has normalized two ideas in cosmology, that in the early universe the initial primordial density perturbations generally could be of classical rather than quantum origin and that during inflation, particle production from interactions amongst quantum field, and its backreaction effects, can occur concurrent with inflationary expansion. When we first introduced these ideas, both were met with resistance, but today they are widely accepted as possibilities with many models and applications based on them, which is an indication of the widespread influence of warm inflation. Open quantum field theory, which has been utilized in studies of warm inflation, is by now a relevant subject in cosmology, in part due to this early work. In this review I first discuss the basic warm inflation dynamics. I then outline how to compute warm inflation dynamics from first-principles quantum field theory (QFT) and in particular how a dissipative term arises. Warm inflation models can have an inflaton mass bigger than the Hubble scale and the inflaton field excursion can remain sub-Planckian, thus overcoming the most prohibitive problems of inflation model building. I discuss the early period of my work in developing warm inflation that helped me arrive at these important features of its dynamics. Inflationary cosmology today is immersed in hypothetical models, which by now are acting as a diversion from reaching any endgame in this field. I discuss better ways to approach model selection and give necessary requirements for a well constrained and predictive inflation model. A few warm inflation models are pointed out that could be developed to this extent. I discuss how, at this stage, more progress would be made in this subject by taking a broader view on the possible early universe solutions that include not just inflation but the diverse range of options. Full article

(This article belongs to the Special Issue Warm Inflation)

Open AccessArticle

Recurrent Large Sunspot Structures and 27-Day Component of Solar Activity as Proxies to Axis-Nonsymmetry

Alexander Shapoval

and

Mikhail Shnirman

Universe 2023, 9(6), 271; https://doi.org/10.3390/universe9060271 - 05 Jun 2023

Abstract

The purpose of this paper is to design tools that quantify the structure of the nonsymmetrical component of the solar magnetic field. With the Fourier transform and the machine learning identification of recurrent objects, we define the 27-day component of solar proxies and recurrent large sunspot structures (ReLaSS), respectively. These two closely related characteristics are established to represent different components of the asymmetry of the solar magnetic field. We derive that the 27-day component and ReLaSS have anticorrelated since 1970 after dozens of years of a strong correlation. The persistence of the correlation sign during few solar cycles reflects yet unknown regularities of solar activity. The contribution of both proxies to the nonsymmetry of solar activity is shown to be lower in 1990–2010 than ∼100 years earlier. This property may be the trace of the asymmetry at the scales that are longer than the centennial Gleissberg cycle. Full article

(This article belongs to the Section Solar System)

► Show Figures

Figure 1

Open AccessArticle

Ordinary Muon Capture on ¹³⁶Ba: Comparative Study Using the Shell Model and pnQRPA

and

Universe 2023, 9(6), 270; https://doi.org/10.3390/universe9060270 - 05 Jun 2023

Abstract

In this work, we present a study of ordinary muon capture (OMC) on

^{136}

Ba, the daughter nucleus of

^{136}

Xe double beta decay (DBD). The OMC rates at low-lying nuclear states (below 1 MeV of excitation energy) in

^{136}

Cs are assessed [...] Read more.

In this work, we present a study of ordinary muon capture (OMC) on

^{136}

Ba, the daughter nucleus of

^{136}

Xe double beta decay (DBD). The OMC rates at low-lying nuclear states (below 1 MeV of excitation energy) in

^{136}

Cs are assessed by using both the interacting shell model (ISM) and proton–neutron quasiparticle random-phase approximation (pnQRPA). We also add chiral two-body (2BC) meson-exchange currents and use an exact Dirac wave function for the captured s-orbital muon. OMC can be viewed as a complementary probe of the wave functions in

^{136}

Cs, the intermediate nucleus of the

^{136}

Xe DBD. At the same time, OMC can be considered a powerful probe of the effective values of weak axial-type couplings in a 100 MeV momentum exchange region, which is relevant for neutrinoless DBD. The present work represents the first attempt to compare the ISM and pnQRPA results for OMC on a heavy nucleus while also including the exact muon wave function and the 2BC. The sensitivity estimates of the current and future neutrinoless DBD experiments will clearly benefit from future OMC measurements taken using OMC calculations similar to the one presented here. 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)

► Show Figures

Figure 1

Open AccessEditorial

Editorial for the Special Issue “Torsion-Gravity and Spinors in Fundamental Theoretical Physics”

Luca Fabbri

Universe 2023, 9(6), 269; https://doi.org/10.3390/universe9060269 - 04 Jun 2023

Abstract

The Einsteinian theory of gravitation is among the best-established theories ever conceived in physics [...] Full article

(This article belongs to the Special Issue Torsion-Gravity and Spinors in Fundamental Theoretical Physics)

Open AccessEssay

Finding My Drumbeat: Applying Lessons Learned from Remo Ruffini to Understanding Astrophysical Transients

Chris Fryer

Universe 2023, 9(6), 268; https://doi.org/10.3390/universe9060268 - 04 Jun 2023

Abstract

As with many fields from fashion to politics, science is susceptible to “bandwagon”-driven research where an idea becomes increasingly popular, garnering a growing amount of “scientific” support. Bandwagons allow scientists to converge on a solution, but when the prevailing bandwagon is incorrect or too simple, this rigid mentality makes it very difficult for scientists to find the right track. True scientific innovation often occurs through scientists willing to march to the beat of their own drum. Using examples in the field of astrophysical transients, this paper demonstrates the importance of supporting scientists in their quest to develop their own personal drumbeat. Full article

(This article belongs to the Special Issue Remo Ruffini Festschrift)

► Show Figures

Figure 1

Open AccessArticle

Magnetized Black Holes: Interplay between Charge and Rotation

Vladimír Karas

and

Zdeněk Stuchlík

Universe 2023, 9(6), 267; https://doi.org/10.3390/universe9060267 - 03 Jun 2023

Abstract

Already in the cornerstone works on astrophysical black holes published as early as in the 1970s, Ruffini and collaborators have revealed the potential importance of an intricate interaction between the effects of strong gravitational and electromagnetic fields. Close to the event horizon of the black hole, magnetic and electric lines of force become distorted and dragged even in a purely electro-vacuum system. Moreover, as the plasma effects inevitably arise in any astrophysically realistic environment, particles of different electric charges can separate from each other, become accelerated away from the black hole or accreted onto it, and contribute to the net electric charge of the black hole. From the point of principle, the case of super-strong magnetic fields is of particular interest, as the electromagnetic field can act as a source of gravity and influence spacetime geometry. In a brief celebratory note, we revisit aspects of rotation and charge within the framework of exact (asymptotically non-flat) solutions of mutually coupled Einstein–Maxwell equations that describe magnetized, rotating black holes. Full article

(This article belongs to the Special Issue Remo Ruffini Festschrift)

► Show Figures

Figure 1

Open AccessArticle

Spacetime as a Complex Network and the Cosmological Constant Problem

Alexander Nesterov

Universe 2023, 9(6), 266; https://doi.org/10.3390/universe9060266 - 02 Jun 2023

Abstract

We propose a promising model of discrete spacetime based on nonassociative geometry and complex networks. Our approach treats space as a simplicial 3-complex (or complex network), built from “atoms” of spacetime and entangled states forming n-dimensional simplices ( [...] Read more.

n = 1, 2, 3

). At large scales, a highly connected network is a coarse, discrete representation of a smooth spacetime. We show that, for high temperatures, the network describes disconnected discrete space. At the Planck temperature, the system experiences phase transition, and for low temperatures, the space becomes a triangulated discrete space. We show that the cosmological constant depends on the Universe’s topology. The “foamy” structure, analogous to Wheeler’s “spacetime foam”, significantly contributes to the effective cosmological constant, which is determined by the Euler characteristic of the Universe. Full article

(This article belongs to the Special Issue Open Issues in the Dark Universe and the Cosmological Constant Problem)

► Show Figures

Figure 1

Open AccessArticle

Study of Hidden-Charm and Hidden-Bottom Pentaquark Resonances in the Quark Model

and

Universe 2023, 9(6), 265; https://doi.org/10.3390/universe9060265 - 31 May 2023

Abstract

Inspired by the LHCb observation of hidden-charm pentaquarks, i.e.,

P_{c} (4312)

P_{c} (4440)

, and

P_{c} (4457)

in the

J / ψ p

invariant mass spectrum, a calculation of the [...] Read more.

Inspired by the LHCb observation of hidden-charm pentaquarks, i.e.,

P_{c} (4312)

P_{c} (4440)

, and

P_{c} (4457)

in the

J / ψ p

invariant mass spectrum, a calculation of the

J / ψ p

scattering cross-section was performed using the quark-delocalization color screening model. The results show that

P_{c} (4312)

can be identified as a hidden-charm molecular state

Σ_{c} D

with

J^{P} = {\frac{1}{2}}^{-}

. The two-peak structure can be reproduced around 4450 MeV, which corresponds to

P_{c} (4440)

and

P_{c} (4457)

. They are the resonances molecular states

Σ_{c} D^{*}

J^{P} = {\frac{1}{2}}^{-}

and

J^{P} = {\frac{3}{2}}^{-}

. Moreover, the

Σ_{c}^{*} D^{*}

of both

J^{P} = {\frac{1}{2}}^{-}

and

J^{P} = {\frac{3}{2}}^{-}

are possible molecular pentaquarks. Moreover, in the same theoretical frame, the calculation is extended to the

P_{c}

-like molecular pentaquarks, denoted as

P_{b}

. Several hidden-bottom pentaquarks with masses above 11 GeV and narrow widths were obtained. All of these heavy pentaquarks are worth exploring in future experiments. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—High Energy Nuclear and Particle Physics)

► Show Figures

Figure 1

Open AccessArticle

Even and Odd Self-Similar Solutions of the Diffusion Equation for Infinite Horizon

László Mátyás

and

Imre Ferenc Barna

Universe 2023, 9(6), 264; https://doi.org/10.3390/universe9060264 - 31 May 2023

Abstract

In the description of transport phenomena, diffusion represents an important aspect. In certain cases, the diffusion may appear together with convection. In this paper, we study the diffusion equation with the self-similar Ansatz. With an appropriate change of variables, we have found an original new type of solution of the diffusion equation for infinite horizon. We derive novel even solutions of diffusion equation for the boundary conditions presented. For completeness, the odd solutions are also mentioned as well, as part of the previous works. We have found a countable set of even and odd solutions, of which linear combinations also fulfill the diffusion equation. Finally, the diffusion equation with a constant source term is discussed, which also has even and odd solutions. Full article

(This article belongs to the Special Issue Zimányi School – Heavy Ion Physics)

► Show Figures

Figure 1

Open AccessCommunication

On the Possibility of Observing Negative Shapiro-like Delay Using Michelson–Morley-Type Experiments

Arunava Bhadra

Ramil N. Izmailov

and

Kamal K. Nandi

Universe 2023, 9(6), 263; https://doi.org/10.3390/universe9060263 - 31 May 2023

Abstract

The possibility of observing negative Shapiro-like gravitational time delay (or time advancement) due to the Earth’s gravity employing interferometric experiments on the Earth’s surface is discussed. It is suggested that such a measurement may be realized in the near future with the help [...] Read more.

(This article belongs to the Special Issue Advances in Cosmology and Subatomic Particle Physics)

► Show Figures

Figure 1

Open AccessArticle

Running Vacuum in the Universe: Phenomenological Status in Light of the Latest Observations, and Its Impact on the σ₈ and H₀ Tensions

Joan Solà Peracaula

Adrià Gómez-Valent

Javier de Cruz Pérez

and

Cristian Moreno-Pulido

Universe 2023, 9(6), 262; https://doi.org/10.3390/universe9060262 - 30 May 2023

Abstract

A substantial body of phenomenological and theoretical work over the last few years strengthens the possibility that the vacuum energy density (VED) of the universe is dynamical, and in particular that it adopts the ‘running vacuum model’ (RVM) form, in which the VED [...] Read more.

δ ρ_{vac} (H) \sim ν_{eff} m_{Pl}^{2} O (H^{2})

, where H is the Hubble rate and

ν_{eff}

is a (small) free parameter. This dynamical scenario is grounded on recent studies of quantum field theory (QFT) in curved spacetime and also on string theory. It turns out that what we call the ‘cosmological constant’,

Λ

, is no longer a rigid parameter but the nearly sustained value of

8 π G (H) ρ_{vac} (H)

around any given epoch

H (t)

, where

G (H)

is the gravitational coupling, which can also be very mildly running (logarithmically). Of particular interest is the possibility suggested in past works that such a running may help to cure the cosmological tensions afflicting the

Λ

CDM. In the current study, we reanalyze the RVM in full and we find it becomes further buttressed. Using modern cosmological data, namely a compilation of the latest SNIa+BAO+

H (z)

+LSS+CMB observations, we probe to what extent the RVM provides a quality fit better than the concordance

Λ

CDM model, with particular emphasis on its impact on the

σ_{8}

and

H_{0}

tensions. We utilize the Einstein–Boltzmann system solver CLASS and the Monte Carlo sampler MontePython for the statistical analysis, as well as the statistical DIC criterion to compare the running vacuum against the rigid vacuum (

ν_{eff} = 0

). On fundamental grounds,

ν_{eff}

receives contributions from all the quantized matter fields in FLRW spacetime. We show that with a tiny amount of vacuum dynamics (

ν_{eff} ≪ 1

) the global fit can improve significantly with respect to the

Λ

CDM and the mentioned tensions may subside to inconspicuous levels. Full article

(This article belongs to the Special Issue Modified Gravity Approaches to the Tensions of ΛCDM)

► Show Figures

Figure 1

Open AccessCommunication

Diagnostics of Flare Loop Parameters in Shrinkage and Ascent Stages Using Radio, X-ray, and UV Emission

Valery Zaitsev

and

Alexander Stepanov

Universe 2023, 9(6), 261; https://doi.org/10.3390/universe9060261 - 30 May 2023

Abstract

We propose the diagnostics of plasma parameters in flare loops using the data of multi-wavelength observations in both shrinkage and expansion phases of the loops. The approach of a flare loop as an equivalent electric circuit is applied. We show that depending on plasma loop parameters, the shrinkage may be accompanied by an increase in the electric current in the loop rather than a decrease. The number density, temperature, electric current, radius, loop-top altitude, and loop volume are determined for the flare events on 16 April 2002 and 24 August 2002. Full article

(This article belongs to the Special Issue Solar Radio Emissions)

► Show Figures

Figure 1

Open AccessArticle

Neutron Stars in the Context of f(

T, T

) Gravity

Clésio E. Mota

Luis C. N. Santos

Franciele M. da Silva

César V. Flores

Iarley P. Lobo

and

Valdir B. Bezerra

Universe 2023, 9(6), 260; https://doi.org/10.3390/universe9060260 - 29 May 2023

Abstract

In this work, we investigate the existence of neutron stars (NS) in the framework of

f (T, T)

gravity, where

T

is the torsion tensor and

T

is the trace of the energy–momentum tensor. The hydrostatic equilibrium equations are obtained, [...] Read more.

In this work, we investigate the existence of neutron stars (NS) in the framework of

f (T, T)

gravity, where

T

is the torsion tensor and

T

is the trace of the energy–momentum tensor. The hydrostatic equilibrium equations are obtained, however, with p and

ρ

quantities passed on by effective quantities

\bar{p}

and

\bar{ρ}

, whose mass–radius diagrams are obtained using modern equations of state (EoS) of nuclear matter derived from relativistic mean field models and compared with the ones computed by the Tolman–Oppenheimer–Volkoff (TOV) equations. Substantial changes in the mass–radius profiles of NS are obtained even for small changes in the free parameter of this modified theory. The results indicate that the use of

f (T, T)

gravity in the study of NS provides good results for the masses and radii of some important astrophysical objects, as, for example, the NS of low-mass X-ray binary in NGC 6397, the millisecond pulsar PSR J0740+6620 and the GW170817 event. In addition, radii results inferred from the Lead Radius Experiment (PREX-2) can also be described for certain parameter values. Full article

(This article belongs to the Special Issue Advances and Limitations of Modified Gravity)

► Show Figures