Universe

17 pages, 2951 KB

Open AccessArticle

Measurement-Induced Nonlocality for Observers Near a Black Hole

by Adam Z. Kaczmarek, Dominik Szczȩśniak and Sabre Kais

Universe 2023, 9(4), 199; https://doi.org/10.3390/universe9040199 - 21 Apr 2023

Cited by 3 | Viewed by 2117

We present a systematic and complementary study of quantum correlations near a black hole by considering measurement-induced nonlocality (MIN). The quantum measure of interest is discussed for the fermionic, bosonic and mixed fermion–boson modes on equal footing with respect to the Hawking radiation. [...] Read more.

We present a systematic and complementary study of quantum correlations near a black hole by considering measurement-induced nonlocality (MIN). The quantum measure of interest is discussed for the fermionic, bosonic and mixed fermion–boson modes on equal footing with respect to the Hawking radiation. The obtained results show that in the infinite Hawking temperature limit, the physically accessible correlations do not vanish only in the fermionic case. However, the higher frequency modes can sustain correlations for the finite Hawking temperature, with mixed systems being more sensitive towards the increase in the fermionic frequencies than the bosonic ones. Since the MIN for the latter modes quickly diminishes, the increased frequency may be a way to maintain nonlocal correlations for the scenarios at the finite Hawking temperature. Full article

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

► Show Figures

Figure 1

25 pages, 654 KB

Open AccessArticle

The Finsler Spacetime Condition for (α,β)-Metrics and Their Isometries

by Nicoleta Voicu, Annamária Friedl-Szász, Elena Popovici-Popescu and Christian Pfeifer

Universe 2023, 9(4), 198; https://doi.org/10.3390/universe9040198 - 20 Apr 2023

Cited by 7 | Viewed by 2908

Abstract

For the general class of pseudo-Finsler spaces with

(α, β)

-metrics, we establish necessary and sufficient conditions such that these admit a Finsler spacetime structure. This means that the fundamental tensor has a Lorentzian signature on a conic subbundle of [...] Read more.

For the general class of pseudo-Finsler spaces with

(α, β)

-metrics, we establish necessary and sufficient conditions such that these admit a Finsler spacetime structure. This means that the fundamental tensor has a Lorentzian signature on a conic subbundle of the tangent bundle and thus the existence of a cone of future-pointing time-like vectors is ensured. The identified

(α, β)

-Finsler spacetimes are candidates for applications in gravitational physics. Moreover, we completely determine the relation between the isometries of an

(α, β)

-metric and the isometries of the underlying pseudo-Riemannian metric a; in particular, we list all

(α, β)

-metrics which admit isometries that are not isometries of a. Full article

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

► Show Figures

Figure 1

35 pages, 3145 KB

Open AccessReview

Fermionic Dark Matter: Physics, Astrophysics, and Cosmology

by C. R. Argüelles, E. A. Becerra-Vergara, J. A. Rueda and R. Ruffini

Universe 2023, 9(4), 197; https://doi.org/10.3390/universe9040197 - 20 Apr 2023

Cited by 23 | Viewed by 3204

Abstract

The nature of dark matter (DM) is one of the most relevant questions in modern astrophysics. We present a brief overview of recent results that inquire into the possible fermionic quantum nature of the DM particles, focusing mainly on the interconnection between the [...] Read more.

The nature of dark matter (DM) is one of the most relevant questions in modern astrophysics. We present a brief overview of recent results that inquire into the possible fermionic quantum nature of the DM particles, focusing mainly on the interconnection between the microphysics of the neutral fermions and the macrophysical structure of galactic halos, including their formation both in the linear and non-linear cosmological regimes. We discuss the general relativistic Ruffini–Argüelles–Rueda (RAR) model of fermionic DM in galaxies, its applications to the Milky Way, the possibility that the Galactic center harbors a DM core instead of a supermassive black hole (SMBH), the S-cluster stellar orbits with an in-depth analysis of the S2’s orbit including precession, the application of the RAR model to other galaxy types (dwarf, elliptic, big elliptic, and galaxy clusters), and universal galaxy relations. All the above focus on the model parameters’ constraints most relevant to the fermion mass. We also connect the RAR model fermions with particle physics DM candidates, self-interactions, and galactic observable constraints. The formation and stability of core–halo galactic structures predicted by the RAR model and their relations to warm DM cosmologies are also addressed. Finally, we provide a brief discussion of how gravitational lensing, dynamical friction, and the formation of SMBHs can also probe the DM’s nature. Full article

(This article belongs to the Special Issue Galactic Center with Gravity)

► Show Figures

Figure 1

13 pages, 412 KB

Open AccessPerspective

High-Energy Lepton Scattering and Nuclear Structure Issues

by Thomas W. Donnelly

Universe 2023, 9(4), 196; https://doi.org/10.3390/universe9040196 - 20 Apr 2023

Cited by 2 | Viewed by 1584

Abstract

High-energy lepton scattering constitutes the focus of this study. Developments are provided to motivate the basic choices of kinematic variables for the particular case of semi-inclusive electron scattering where these variables are devised to match well with the underlying dynamics to be expected [...] Read more.

High-energy lepton scattering constitutes the focus of this study. Developments are provided to motivate the basic choices of kinematic variables for the particular case of semi-inclusive electron scattering where these variables are devised to match well with the underlying dynamics to be expected for the general “nuclear landscape”. Various nuclear structure issues and other issues related to the nature of the electroweak currents at high energies are then discussed, as are some of the issues related to the different conditions occurring for electron scattering versus what is typically the case for charge-changing neutrino reactions. Full article

(This article belongs to the Special Issue Many Body Theory)

► Show Figures

Figure 1

17 pages, 463 KB

Open AccessArticle

Production of Cumulative Pions and Percolation of Strings

by Mikhail Aleksandrovich Braun

Universe 2023, 9(4), 195; https://doi.org/10.3390/universe9040195 - 19 Apr 2023

Cited by 1 | Viewed by 1198

Abstract

Production of pions in high-energy collisions with nuclei in the kinematics prohibited for free nucleons (“cumulative pions”) is studied in the fusing color string model. The model describes the so-called direct mechanism for cumulative production. The other (spectator) mechanism dominates in production of [...] Read more.

Production of pions in high-energy collisions with nuclei in the kinematics prohibited for free nucleons (“cumulative pions”) is studied in the fusing color string model. The model describes the so-called direct mechanism for cumulative production. The other (spectator) mechanism dominates in production of cumulative protons, and is suppressed for pions. In the model, cumulative pions are generated by string fusion, which raises the maximal energy of produced partons above the level of the free nucleon kinematics. Momentum and multiplicity sum rules are used to determine the spectra in the deep fragmentation region. Predicted spectra of cumulative pions exponentially fall with the scaling variable x in the interval

1 < x < 3

with a slope between 5.1 and 5.6, which agrees well with the raw data obtained in the recent experiment at RHIC involving Cu–Au collisioins. However, the agreement is worse for the so-called unfolded data, presumably taking into account corrections due to the experimental setup and having rather a power-like form. Full article

(This article belongs to the Special Issue Relativistic Heavy Ion Collision)

► Show Figures

Figure 1

9 pages, 1615 KB

Open AccessArticle

Do White Holes Exist?

by Enrique Gaztanaga

Universe 2023, 9(4), 194; https://doi.org/10.3390/universe9040194 - 19 Apr 2023

Cited by 4 | Viewed by 3156

Abstract

In a paper published in 1939, Albert Einstein argued that Black Holes (BHs) did not exist “in the real world”. However, recent astronomical observations indicate otherwise. Does this mean that we should also expect White Holes (WHs) to exist in the real world? [...] Read more.

In a paper published in 1939, Albert Einstein argued that Black Holes (BHs) did not exist “in the real world”. However, recent astronomical observations indicate otherwise. Does this mean that we should also expect White Holes (WHs) to exist in the real world? In classical General Relativity (GR), a WH refers to the time reversed version of a collapsing BH solution that allows the crossing of the BH event horizon inside out. Such solution has been disputed as not possible because escaping an event horizon violates causality. Despite such objections, the Big Bang model is often understood as a WH (the reverse of a BH collapse). Does this mean that the Big Bang breaks causality? Recent measurements of cosmic acceleration indicate that our Big Bang solution is not really a WH, but a BH. Events decelerate when the expansion accelerates and this prevents the crossing of the event horizon from inside out. We present a general explanation of why this happens; the explanation resolves the above causality puzzle and indicates that such apparent WH solutions have a regular Schwarzschild BH exterior. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)

► Show Figures

Figure 1

19 pages, 561 KB

Open AccessEditor’s ChoiceReview

Elimination of QCD Renormalization Scale and Scheme Ambiguities

by Sheng-Quan Wang, Stanley J. Brodsky, Xing-Gang Wu, Jian-Ming Shen and Leonardo Di Giustino

Universe 2023, 9(4), 193; https://doi.org/10.3390/universe9040193 - 17 Apr 2023

Cited by 4 | Viewed by 2607

Abstract

The setting of the renormalization scale (

μ_{r}

) in the perturbative QCD (pQCD) is one of the crucial problems for achieving precise fixed-order pQCD predictions. The conventional prescription is to take its value as the typical momentum transfer Q in a [...] Read more.

The setting of the renormalization scale (

μ_{r}

) in the perturbative QCD (pQCD) is one of the crucial problems for achieving precise fixed-order pQCD predictions. The conventional prescription is to take its value as the typical momentum transfer Q in a given process, and theoretical uncertainties are then evaluated by varying it over an arbitrary range. The conventional scale-setting procedure introduces arbitrary scheme-and-scale ambiguities in fixed-order pQCD predictions. The principle of maximum conformality (PMC) provides a systematic way to eliminate the renormalization scheme-and-scale ambiguities. The PMC method has rigorous theoretical foundations; it satisfies the renormalization group invariance (RGI) and all of the self-consistency conditions derived from the renormalization group. The PMC has now been successfully applied to many physical processes. In this paper, we summarize recent PMC applications, including event shape observables and heavy quark pair production near the threshold region in

e^{+} e^{-}

annihilation and top-quark decay at hadronic colliders. In addition, estimating the contributions related to the uncalculated higher-order terms is also summarized. These results show that the major theoretical uncertainties caused by different choices of

μ_{r}

are eliminated, and the improved pQCD predictions are thus obtained, demonstrating the generality and applicability of the PMC. Full article

(This article belongs to the Special Issue The Quantum Chromodynamics: 50th Anniversary of the Discovery)

► Show Figures

Figure 1

17 pages, 3924 KB

Open AccessArticle

Constraining Young Hot Jupiter Occurrence Rate in Stellar Associations Using 2-min Cadence TESS Data

by Yuanqing Fang, Bo Ma, Chen Chen and Yongxin Wen

Universe 2023, 9(4), 192; https://doi.org/10.3390/universe9040192 - 17 Apr 2023

Cited by 4 | Viewed by 2523

Abstract

The characterization of young planet distribution is essential for our understanding of the early evolution of exoplanets. Here we conduct a systematic search for young planets from young open clusters and associations using the 2-min cadence TESS survey data. We obtain TESS light [...] Read more.

The characterization of young planet distribution is essential for our understanding of the early evolution of exoplanets. Here we conduct a systematic search for young planets from young open clusters and associations using the 2-min cadence TESS survey data. We obtain TESS light curves for a total of 1075 young stars, which are selected with the aid of Gaia data. There are a total of 16 possible transiting signals. After a thorough vetting process, some have been confirmed as planets, and others are likely caused by eclipsing binaries. The final sample contains six confirmed planets, of which one is a hot Jupiter. After accounting for survey completeness using a Monte Carlo simulation, we can put a 95% confidence level upper limit on the hot Jupiter (P < 10 days, Rp = 0.7–2.9 R_Jup) occurrence rate orbiting stars in young associations at <5.1% and a 68% confidence level upper limit at <2.5%. We estimate that a sample size of ∼5000 dwarf stars with 2-min cadence data will be needed to reach a 0.5% upper limit on the hot Jupiter occurrence rate, which is the typical hot Jupiter occurrence rate around main sequence stars. Thus, future studies with larger sample sizes are required to put more constraints on planet formation and evolution theories. Full article

(This article belongs to the Section Planetary Sciences)

► Show Figures

Figure 1

23 pages, 371 KB

Open AccessArticle

Modified Stückelberg Formalism: Free Massive Abelian 2-Form Theory in 4D

by A. K. Rao and R. P. Malik

Universe 2023, 9(4), 191; https://doi.org/10.3390/universe9040191 - 17 Apr 2023

Cited by 3 | Viewed by 1598

Abstract

We demonstrate that the celebrated Stückelberg formalism is modified in the case of a massive four (3 + 1)-dimensional (4D) Abelian 2-form theory due to the presence of a self-duality discrete symmetry in the theory. The latter symmetry entails upon the modified 4D [...] Read more.

We demonstrate that the celebrated Stückelberg formalism is modified in the case of a massive four (3 + 1)-dimensional (4D) Abelian 2-form theory due to the presence of a self-duality discrete symmetry in the theory. The latter symmetry entails upon the modified 4D massive Abelian 2-form gauge theory to become a massive model of Hodge theory within the framework of Becchi–Rouet–Stora–Tyutin (BRST) formalism where there is the existence of a set of (anti-)co-BRST transformations corresponding to the usual nilpotent (anti-)BRST transformations. The latter exist in any arbitrary dimension of spacetime for the usual Stückelberg-modified massive Abelian 2-form gauge theory. The modification in the Stückelberg technique is backed by the precise mathematical arguments from the differential geometry where the exterior derivative and Hodge duality operator play the decisive roles. The modified version of the Stückelberg technique remains invariant under the discrete duality transformations which also establish a precise and deep connection between the off-shell nilpotent (anti-)BRST and (anti-)co-BRST transformations. We have clarified a simple trick of using the equations of motion to remove the higher derivative terms in the appropriate Lagrangian densities so that our 4D theory can become consistent. Full article

(This article belongs to the Section Field Theory)

10 pages, 348 KB

Open AccessArticle

Can a Rotating Black Hole Be Overspun in Seven Dimensions?

by Sanjar Shaymatov, Bobomurat Ahmedov and Eldor Karimbaev

Universe 2023, 9(4), 190; https://doi.org/10.3390/universe9040190 - 17 Apr 2023

Cited by 1 | Viewed by 1417

Abstract

Five-dimensional rotating black holes with two rotations could be overspun except for a single rotation, whereas a black hole in six dimensions always obeys the weak cosmic censorship conjecture (WCCC) in the weak form even for linear particle accretion. In this paper, we [...] Read more.

Five-dimensional rotating black holes with two rotations could be overspun except for a single rotation, whereas a black hole in six dimensions always obeys the weak cosmic censorship conjecture (WCCC) in the weak form even for linear particle accretion. In this paper, we investigate the overspinning of a seven-dimensional rotating black hole with three rotation parameters. It is shown that a black hole in the seven dimensions cannot be similarly overspun, thereby obeying the WCCC even under linear particle accretion. It turns out that a black hole always respects the weak cosmic censorship conjecture in seven dimensions. Full article

(This article belongs to the Section Gravitation)

► Show Figures

Figure 1

11 pages, 280 KB

Open AccessArticle

Modeling Clock Comparison Experiments to Test Special Relativity

by Xiao-Yu Lu, Jin-Shu Huang, Cong-Bin Liu, Xiu-Mei Xu, Jin-Bing Cheng, Wan Chang, Yu-Yu Zhou and Ya-Jie Wang

Universe 2023, 9(4), 189; https://doi.org/10.3390/universe9040189 - 17 Apr 2023

Cited by 2 | Viewed by 1916

Abstract

The clock comparison experiments to test special relativity mainly include the Michelson–Morley experiment, Kennedy–Thorndike experiment, Ives–Stilwell experiment and the comparison experiment of atomic clocks in two locations. These experiments can be roughly classified as the comparison of two types of clocks: optical clocks [...] Read more.

The clock comparison experiments to test special relativity mainly include the Michelson–Morley experiment, Kennedy–Thorndike experiment, Ives–Stilwell experiment and the comparison experiment of atomic clocks in two locations. These experiments can be roughly classified as the comparison of two types of clocks: optical clocks and atomic clocks. Through the comparison of such clocks, Lorentz invariance breaking parameters in the RMS framework can be tested. However, in such experiments, the structural effects of optical clocks have been fully considered, yet the structural effects of atomic clocks have not been carefully studied. Based on this, this paper analyzes the structural effects of atomic clocks in detail and divides the experiments into six types: the comparison of two atomic clocks, two optical clocks, and atomic clocks and optical clocks placed in different and the same locations. Finally, correction parameters for the experimental measurements are given. Full article

(This article belongs to the Section Gravitation)

32 pages, 886 KB

Open AccessArticle

Cosmic-Time Quantum Mechanics and the Passage-of-Time Problem

by Marek Czachor

Universe 2023, 9(4), 188; https://doi.org/10.3390/universe9040188 - 16 Apr 2023

Cited by 2 | Viewed by 2538

Abstract

A new dynamical paradigm merging quantum dynamics with cosmology is discussed. We distinguish between a universe and its background space-time. The universe here is the subset of space-time defined by

Ψ_{τ} (x) \neq 0

, where [...] Read more.

A new dynamical paradigm merging quantum dynamics with cosmology is discussed. We distinguish between a universe and its background space-time. The universe here is the subset of space-time defined by

Ψ_{τ} (x) \neq 0

, where

Ψ_{τ} (x)

is a solution of a Schrödinger equation, x is a point in n-dimensional Minkowski space, and

τ \geq 0

is a dimensionless ‘cosmic-time’ evolution parameter. We derive the form of the Schrödinger equation and show that an empty universe is described by a

Ψ_{τ} (x)

that propagates towards the future inside some future-cone

V_{+}

. The resulting dynamical semigroup is unitary, i.e.,

\int_{V_{+}} d^{4} x {| Ψ_{τ} (x) |}^{2} = 1

for

τ \geq 0

. The initial condition

Ψ_{0} (x)

is not localized at

x = 0

. Rather, it satisfies the boundary condition

Ψ_{0} (x) = 0

for

x \notin V_{+}

. For

n = 1 + 3

the support of

Ψ_{τ} (x)

is bounded from the past by the ‘gap hyperboloid’

ℓ^{2} \sqrt{τ} = c^{2} t^{2} - x^{2}

, where ℓ is a fundamental length. Consequently, the points located between the hyperboloid and the light cone

c^{2} t^{2} - x^{2} = 0

satisfy

Ψ_{τ} (x) = 0

, and thus do not belong to the universe. As

τ

grows, the gap between the support of

Ψ_{τ} (x)

and the light cone increases. The past thus literally disappears. Unitarity of the dynamical semigroup implies that the universe becomes localized in a finite-thickness future-neighbourhood of

ℓ^{2} \sqrt{τ} = c^{2} t^{2} - x^{2}

, simultaneously spreading along the hyperboloid. Effectively, for large

τ

the subset occupied by the universe resembles a part of the gap hyperboloid itself, but its thickness

Δ_{τ}

is non-zero for finite

τ

. Finite

Δ_{τ}

implies that the three-dimensional volume of the universe is finite as well. An approximate radius of the universe,

r_{τ}

, grows with

τ

due to

Δ_{τ} r_{τ}^{3} = Δ_{0} r_{0}^{3}

and

Δ_{τ} \to 0

. The propagation of

Ψ_{τ} (x)

through space-time matches an intuitive picture of the passage of time. What we regard as the Minkowski-space classical time can be identified with

c t_{τ} = \int d^{4} x x^{0} {| Ψ_{τ} (x) |}^{2}

, so

t_{τ}

grows with

τ

as a consequence of the Ehrenfest theorem, and its present uncertainty can be identified with the Planck time. Assuming that at present values of

τ

(corresponding to 13–14 billion years)

Δ_{τ}

and

r_{τ}

are of the order of the Planck length and the Hubble radius, we estimate that the analogous thickness

Δ_{0}

of the support of

Ψ_{0} (x)

is of the order of 1 AU, and

r_{0}^{3} \sim {(c t_{H})}^{3} \times 10^{- 44}

. The estimates imply that the initial volume of the universe was finite and its uncertainty in time was several minutes. Next, we generalize the formalism in a way that incorporates interactions with matter. We are guided by the correspondence principle with quantum mechanics, which should be asymptotically reconstructed for the present values of

τ

. We argue that Hamiltonians corresponding to the present values of

τ

approximately describe quantum mechanics in a conformally Minkowskian space-time. The conformal factor is directly related to

| Ψ_{τ} {(x) |}^{2}

. As a by-product of the construction, we arrive at a new formulation of conformal invariance of

m \neq 0

fields. Full article

(This article belongs to the Special Issue Approaches towards Quantum Foundations)

► Show Figures

Figure 1

14 pages, 320 KB

Open AccessReview

DeWitt Boundary Condition in One-Loop Quantum Cosmology

by Giampiero Esposito

Universe 2023, 9(4), 187; https://doi.org/10.3390/universe9040187 - 14 Apr 2023

Cited by 4 | Viewed by 1534

Abstract

DeWitt’s suggestion that the wave function of the universe should vanish at the classical Big Bang singularity is considered here within the framework of one-loop quantum cosmology. For pure gravity at one loop about a flat four-dimensional background bounded by a 3-sphere, three [...] Read more.

DeWitt’s suggestion that the wave function of the universe should vanish at the classical Big Bang singularity is considered here within the framework of one-loop quantum cosmology. For pure gravity at one loop about a flat four-dimensional background bounded by a 3-sphere, three choices of boundary conditions are considered: vanishing of the linearized magnetic curvature when only transverse-traceless gravitational modes are quantized; a one-parameter family of mixed boundary conditions for gravitational and ghost modes; and diffeomorphism-invariant boundary conditions for metric perturbations and ghost modes. A positive

ζ (0)

value in these cases ensures that, when the three-sphere boundary approaches zero, the resulting one-loop wave function approaches zero. This property may be interpreted by saying that, in the limit of small three-geometry, the resulting one-loop wave function describes a singularity-free universe. This property holds for one-loop functional integrals, which are not necessarily equivalent to solutions of the quantum constraint equations. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

17 pages, 1510 KB

Open AccessArticle

Solar Sail Trajectories to Earth’s Trojan Asteroids

by Alessandro A. Quarta and Giovanni Mengali

Universe 2023, 9(4), 186; https://doi.org/10.3390/universe9040186 - 14 Apr 2023

Cited by 2 | Viewed by 2095

Abstract

The recent discovery of Earth’s second Trojan asteroid (2020 XL

_{5}

), which will remain in the vicinity of the Sun–[Earth+Moon] triangular Lagrangian point

L_{4}

for at least 4000 years, has attracted the attention of the scientific community as a remarkable example [...] Read more.

The recent discovery of Earth’s second Trojan asteroid (2020 XL

_{5}

), which will remain in the vicinity of the Sun–[Earth+Moon] triangular Lagrangian point

L_{4}

for at least 4000 years, has attracted the attention of the scientific community as a remarkable example of those elusive objects that are the witnesses of the first phase of our Solar System. The possibility that an Earth’s Trojan asteroid (ETa) may represent a pristine record of the initial conditions of the Solar System formation makes these small objects an interesting target for a robotic exploration mission. This paper analyzes orbit-to-orbit Earth–ETa transfer trajectories of an interplanetary spacecraft propelled by a solar sail. In the last decade, some pioneering space missions have confirmed the feasibility and potentiality of the solar sail concept as a propellantless propulsion system able to convert the solar radiation pressure in a continuous thrust by means of a large, lightweight and highly reflective surface. Using the state-of-the-art level of solar sail technology, this paper studies the performance of a solar-sail-based transfer trajectory toward an ETa from an optimal viewpoint and with a parametric approach. Full article

(This article belongs to the Special Issue Space Missions to Small Bodies: Results and Future Activities)

► Show Figures

Figure 1

19 pages, 3246 KB

Open AccessArticle

Quantum Fractionary Cosmology: K-Essence Theory

by J. Socorro and J. Juan Rosales

Universe 2023, 9(4), 185; https://doi.org/10.3390/universe9040185 - 13 Apr 2023

Cited by 15 | Viewed by 1975

Abstract

Using a particular form of the quantum K-essence scalar field, we show that in the quantum formalism, a fractional differential equation in the scalar field variable, for some epochs in the Friedmann–Lema

\hat{ı}

tre–Robertson–Walker (FLRW) model (radiation and inflation-like epochs, for example), [...] Read more.

Using a particular form of the quantum K-essence scalar field, we show that in the quantum formalism, a fractional differential equation in the scalar field variable, for some epochs in the Friedmann–Lema

\hat{ı}

tre–Robertson–Walker (FLRW) model (radiation and inflation-like epochs, for example), appears naturally. In the classical analysis, the kinetic energy of scalar fields can falsify the standard matter in the sense that we obtain the time behavior for the scale factor in all scenarios of our Universe by using the Hamiltonian formalism, where the results are analogous to those obtained by an algebraic procedure in the Einstein field equations with standard matter. In the case of the quantum Wheeler–DeWitt (WDW) equation for the scalar field

ϕ

, a fractional differential equation of order

β = \frac{2 α}{2 α - 1}

is obtained. This fractional equation belongs to different intervals, depending on the value of the barotropic parameter; that is to say, when

ω_{X} \in [0, 1]

, the order belongs to the interval

1 \leq β \leq 2

, and when

ω_{X} \in [- 1, 0)

, the order belongs to the interval

0 < β \leq 1

. The corresponding quantum solutions are also given. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)

► Show Figures

Figure 1

22 pages, 366 KB

Open AccessArticle

Interpolation Formulas for Asymptotically Safe Cosmology

by Sandor Nagy and Kornel Sailer

Universe 2023, 9(4), 184; https://doi.org/10.3390/universe9040184 - 12 Apr 2023

Cited by 1 | Viewed by 1505

Abstract

Simple interpolation formulas are proposed for the description of the renormalization group (RG) scale dependences of the gravitational couplings in the framework of the 2-parameters Einstein-Hilbert (EH) theory of gravity and applied to a simple, analytically solvable, spatially homogeneous and isotropic, spatially flat [...] Read more.

Simple interpolation formulas are proposed for the description of the renormalization group (RG) scale dependences of the gravitational couplings in the framework of the 2-parameters Einstein-Hilbert (EH) theory of gravity and applied to a simple, analytically solvable, spatially homogeneous and isotropic, spatially flat model universe. The analytical solution is found in two schemes incorporating different methods of the determination of the conversion rule

k (t)

of the RG scale k to the cosmological time t. In the case of the discussed model these schemes turn out to yield identical cosmological evolution. Explicit analytical formulas are found for the conversion rule

k (t)

as well as for the characteristic time scales

t_{G}

and

t_{Λ} > t_{G}

corresponding to the dynamical energy scales

k_{G}

and

k_{Λ}

, respectively, arising form the RG analysis of the EH theory. It is shown that there exists a model-dependent time scale

t_{d}

(

t_{G} \leq t_{d} < t_{Λ}

) at which the accelerating expansion changes to the decelerating one. It is shown that the evolution runs from a well-identified cosmological fixed point to another one. As a by-product we show that the entropy of the system decreases monotonically in the interval

0 < t \leq t_{Λ}

due to the quantum effects. Full article

(This article belongs to the Section Mathematical Physics)

18 pages, 1413 KB

Open AccessArticle

The Neutron Mean Life and Big Bang Nucleosynthesis

by Tsung-Han Yeh, Keith A. Olive and Brian D. Fields

Universe 2023, 9(4), 183; https://doi.org/10.3390/universe9040183 - 12 Apr 2023

Cited by 6 | Viewed by 2887

Abstract

We explore the effect of neutron lifetime and its uncertainty on standard big bang nucleosynthesis (BBN). BBN describes the cosmic production of the light nuclides,

^{1} H

, D,

^{3} H

+

^{3} He

,

^{4} He

, and

^{7} Li

+ [...] Read more.

We explore the effect of neutron lifetime and its uncertainty on standard big bang nucleosynthesis (BBN). BBN describes the cosmic production of the light nuclides,

^{1} H

, D,

^{3} H

+

^{3} He

,

^{4} He

, and

^{7} Li

+

^{7} Be

, in the first minutes of cosmic time. The neutron mean life

τ_{n}

has two roles in modern BBN calculations: (1) it normalizes the matrix element for weak

n \leftrightarrow p

interconversions, and (2) it sets the rate of free neutron decay after the weak interactions freeze-out. We review the history of the interplay between

τ_{n}

measurements and BBN, and present a study of the sensitivity of the light element abundances to the modern neutron lifetime measurements. We find that

τ_{n}

uncertainties dominate the predicted

^{4} He

error budget, but these theory errors remain smaller than the uncertainties in

^{4} He

observations, even with the dispersion in recent neutron lifetime measurements. For the other light element predictions,

τ_{n}

contributes negligibly to their error budget. Turning the problem around, we combine present BBN and cosmic microwave background (CMB) determinations of the cosmic baryon density to predict a “cosmologically preferred” mean life of

τ_{n} (BBN + CMB) = 870 \pm 16 s

, which is consistent with experimental mean life determinations. We show that if future astronomical and cosmological helium observations can reach an uncertainty of

σ_{obs} (Y_{p}) = 0.001

in the

^{4} He

mass fraction

Y_{p}

, this could begin to discriminate between the mean life determinations. Full article

(This article belongs to the Special Issue Neutron Lifetime)

► Show Figures

Figure 1

28 pages, 1834 KB

Open AccessReview

Proton Electric Charge Radius from Lepton Scattering

by Weizhi Xiong and Chao Peng

Universe 2023, 9(4), 182; https://doi.org/10.3390/universe9040182 - 12 Apr 2023

Cited by 13 | Viewed by 3365

Abstract

A proton is a bound state of a strong interaction, governed by Quantum Chromodynamics (QCD). The electric charge radius of a proton, denoted by

r_{E}^{p}

, characterizes the spatial distribution of its electric charge carried by the quarks. It is an [...] Read more.

A proton is a bound state of a strong interaction, governed by Quantum Chromodynamics (QCD). The electric charge radius of a proton, denoted by

r_{E}^{p}

, characterizes the spatial distribution of its electric charge carried by the quarks. It is an important input for bound-state Quantum Electrodynamic (QED) calculations of the hydrogen atomic energy levels. However, physicists have been puzzled by the large discrepancy between

r_{E}^{p}

measurements from muonic hydrogen spectroscopy and those from

e p

elastic scattering and ordinary hydrogen spectroscopy for over a decade. Tremendous efforts, both theoretical and experimental, have been dedicated to providing various insights into this puzzle, but certain issues still remain unresolved, particularly in the field of lepton scatterings. This review will focus on lepton-scattering measurements of

r_{E}^{p}

, recent theoretical and experimental developments in this field, as well as future experiments using this technique. Full article

(This article belongs to the Special Issue The Quantum Chromodynamics: 50th Anniversary of the Discovery)

► Show Figures

Figure 1

18 pages, 685 KB

Open AccessArticle

Effective f(R) Actions for Modified Loop Quantum Cosmologies via Order Reduction

by Ana Rita Ribeiro, Daniele Vernieri and Francisco S. N. Lobo

Universe 2023, 9(4), 181; https://doi.org/10.3390/universe9040181 - 11 Apr 2023

Cited by 5 | Viewed by 2028

Abstract

General Relativity is an extremely successful theory, at least for weak gravitational fields; however, it breaks down at very high energies, such as in correspondence to the initial singularity. Quantum Gravity is expected to provide more physical insights in relation to this open [...] Read more.

General Relativity is an extremely successful theory, at least for weak gravitational fields; however, it breaks down at very high energies, such as in correspondence to the initial singularity. Quantum Gravity is expected to provide more physical insights in relation to this open question. Indeed, one alternative scenario to the Big Bang, that manages to completely avoid the singularity, is offered by Loop Quantum Cosmology (LQC), which predicts that the Universe undergoes a collapse to an expansion through a bounce. In this work, we use metric

f (R)

gravity to reproduce the modified Friedmann equations which have been obtained in the context of modified loop quantum cosmologies. To achieve this, we apply an order reduction method to the

f (R)

field equations, and obtain covariant effective actions that lead to a bounce, for specific models of modified LQC, considering a massless scalar field. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)

► Show Figures

Figure 1

27 pages, 896 KB

Open AccessReview

Neutron Lifetime Anomaly and Mirror Matter Theory

by Wanpeng Tan

Universe 2023, 9(4), 180; https://doi.org/10.3390/universe9040180 - 11 Apr 2023

Cited by 10 | Viewed by 3232

Abstract

This paper reviews the puzzles in modern neutron lifetime measurements and related unitarity issues in the CKM matrix. It is not a comprehensive and unbiased compilation of all historic data and studies, but rather a focus on compelling evidence leading to new physics. [...] Read more.

This paper reviews the puzzles in modern neutron lifetime measurements and related unitarity issues in the CKM matrix. It is not a comprehensive and unbiased compilation of all historic data and studies, but rather a focus on compelling evidence leading to new physics. In particular, the largely overlooked nuances of different techniques applied in material and magnetic trap experiments are clarified. Further detailed analysis shows that the “beam” approach of neutron lifetime measurements is likely to give the “true”

β

-decay lifetime, while discrepancies in “bottle” measurements indicate new physics at play. The most feasible solution to these puzzles is a newly proposed ordinary-mirror neutron (

n - n^{'}

) oscillation model under the framework of mirror matter theory. This phenomenological model is reviewed and introduced, and its explanations of the neutron lifetime anomaly and possible non-unitarity of the CKM matrix are presented. Most importantly, various new experimental proposals, especially lifetime measurements with small/narrow magnetic traps or under super-strong magnetic fields, are discussed in order to test the surprisingly large anomalous signals that are uniquely predicted by this new

n - n^{'}

oscillation model. Full article

(This article belongs to the Special Issue Neutron Lifetime)

► Show Figures

Figure 1

15 pages, 2961 KB

Open AccessArticle

Parameter Study of Geomagnetic Storms and Associated Phenomena: CME Speed De-Projection vs. In Situ Data

by Rositsa Miteva, Mohamed Nedal, Susan W. Samwel and Manuela Temmer

Universe 2023, 9(4), 179; https://doi.org/10.3390/universe9040179 - 8 Apr 2023

Cited by 3 | Viewed by 2656

Abstract

In this study, we give correlations between the geomagnetic storm (GS) intensity and parameters of solar and interplanetary (IP) phenomena. We also perform 3D geometry reconstructions of geo-effective coronal mass ejections (CMEs) using the recently developed PyThea framework and compare on-sky and de-projected [...] Read more.

In this study, we give correlations between the geomagnetic storm (GS) intensity and parameters of solar and interplanetary (IP) phenomena. We also perform 3D geometry reconstructions of geo-effective coronal mass ejections (CMEs) using the recently developed PyThea framework and compare on-sky and de-projected parameter values, focusing on the reliability of the de-projection capabilities. We utilize spheroid, ellipsoid and graduated cylindrical shell models. In addition, we collected a number of parameters of the GS-associated phenomena. A large variation in 3D de-projections is obtained for the CME speeds depending on the selected model for CME reconstruction and observer subjectivity. A combination of fast speed and frontal orientation of the magnetic structure upon its arrival at the terrestrial magnetosphere proves to be the best indicator for the GS strength. More reliable estimations of geometry and directivity, in addition to de-projected speeds, are important for GS forecasting in operational space weather schemes. Full article

(This article belongs to the Special Issue The Solar Origin of Space Weather)

► Show Figures

Figure 1

21 pages, 2683 KB

Open AccessReview

A Concise Review on Some Higgs-Related New Physics Models in Light of Current Experiments

by Lei Wang, Jin Min Yang, Yang Zhang, Pengxuan Zhu and Rui Zhu

Universe 2023, 9(4), 178; https://doi.org/10.3390/universe9040178 - 4 Apr 2023

Cited by 12 | Viewed by 2793

Abstract

The Higgs boson may serve as a portal to new physics beyond the standard model (BSM), which is implied by the theoretical naturalness or experimental anomalies. This review aims to briefly survey some typical Higgs-related BSM models. First, for the theories to solve [...] Read more.

The Higgs boson may serve as a portal to new physics beyond the standard model (BSM), which is implied by the theoretical naturalness or experimental anomalies. This review aims to briefly survey some typical Higgs-related BSM models. First, for the theories to solve the hierarchy problem, the two exemplary theories, the low energy supersymmetry (focusing on the minimal supersymmetric model) and the little Higgs theory, are discussed. For the phenomenological models without addressing the hierarchy problem, we choose the two-Higgs-doublet models (2HDMs) to emphatically elucidate their phenomenological power in explaining current measurements of muon

g - 2

, the W-boson mass and the dark matter (DM) data. For the singlet extensions, which are motivated by the cosmic phase transition and the DM issue, we illustrate the singlet-extended standard model (xSM) and the singlet-extended 2HDM (2HDM+S), emphasizing the vacuum stability. In the decade since the discovery of the Higgs boson, these theories have remained the typical candidates of new physics, which will be intensively studied in future theoretical and experimental research. Full article

(This article belongs to the Special Issue Higgs and BSM Physics: 10th Anniversary of the Discovery of the Higgs Boson)

► Show Figures

Figure 1

15 pages, 3192 KB

Open AccessArticle

Search for DA White Dwarf Binary Candidates from LAMOST DR7

by Hua-Hui Yan, Jing-Kun Zhao, Wei-Bin Shi, Jin-Cheng Guo, Liang Wang, Zhen-Xin Lei and Gang Zhao

Universe 2023, 9(4), 177; https://doi.org/10.3390/universe9040177 - 3 Apr 2023

Cited by 1 | Viewed by 2118

Abstract

We obtained spectroscopy data for 761 Degenerate A (DA)white dwarfs (WDs) with multiple LAMOST observations. The radial velocity (RV) of each spectrum was calculated using the cross-correlation function method (CCF), and 60 DA WD binary candidates were selected based on the variation of [...] Read more.

We obtained spectroscopy data for 761 Degenerate A (DA)white dwarfs (WDs) with multiple LAMOST observations. The radial velocity (RV) of each spectrum was calculated using the cross-correlation function method (CCF), and 60 DA WD binary candidates were selected based on the variation of the RV. Then, the atmosphere parameter

T_{e f f}

,

log g

, and the mass of these DA WDs were estimated by the Balmer line fitting method and interpolation in theoretical evolution tracks, respectively. Our parameters are consistent with those from SDSS and Gaia for the common stars. No evident difference in the mass distribution of binary candidates compared with total DA WDs was found. We surmise these DA WD binary candidates are mainly composed of two WDs. With the Zwicky Transient Facility (ZTF) data, we obtained the light curve periods of two targets with significant light curve periods in the DA WD binary candidates. For the spectra with anomalous CCF curves or with large errors in their RV calculations, we re-certified their spectral types by visual review. Based on their spectral features, we found 11 DA + M-type binaries and four cataclysmic variables (CVs). The light curve period of one CV was obtained with ZTF data. Full article

(This article belongs to the Special Issue Advances in Astrophysics and Cosmology – in Memory of Prof. Tan Lu)

► Show Figures

Figure 1

24 pages, 389 KB

Open AccessArticle

Dynamically Implementing the

\bar{μ}

-Scheme in Cosmological and Spherically Symmetric Models in an Extended Phase Space Model

by Kristina Giesel and Hongguang Liu

Universe 2023, 9(4), 176; https://doi.org/10.3390/universe9040176 - 3 Apr 2023

Cited by 1 | Viewed by 1586

Abstract

We consider an extended phase space formulation for cosmological and spherically symmetric models in which the choice of a given

\bar{μ}

-scheme can be implemented dynamically. These models are constructed in the context of the relational formalism by using a canonical transformation [...] Read more.

We consider an extended phase space formulation for cosmological and spherically symmetric models in which the choice of a given

\bar{μ}

-scheme can be implemented dynamically. These models are constructed in the context of the relational formalism by using a canonical transformation on the extended phase space, which provides a Kuchař decomposition of the extended phase space. The resulting model can be understood as a gauge-unfixed model of a given

\bar{μ}

-scheme. We use this formalism to investigate the restrictions to the allowed

\bar{μ}

-scheme from this perspective and discuss the differences in the cosmological and spherically symmetric case. This method can be useful, for example, to obtain a

\bar{μ}

-scheme in a top-down derivation from full LQG to symmetry-reduced effective models, where, for some models, only the

μ_{0}

-scheme has been obtained thus far. Full article

(This article belongs to the Special Issue Loop Quantum Gravity: A Themed Issue in Honor of Prof. Abhay Ashtekar)

18 pages, 570 KB

Open AccessArticle

Estimate of SMBH Spin for Narrow-Line Seyfert 1 Galaxies

by Mikhail Piotrovich, Stanislava Buliga and Tinatin Natsvlishvili

Universe 2023, 9(4), 175; https://doi.org/10.3390/universe9040175 - 3 Apr 2023

Cited by 4 | Viewed by 1164

Abstract

We estimated the spin values of the supermassive black holes (SMBHs) of the active galactic nuclei (AGN) for a large set of Narrow Line Seyfert 1 (NLS1) galaxies assuming the inclination angle between the line of sight and the axis of the accretion [...] Read more.

We estimated the spin values of the supermassive black holes (SMBHs) of the active galactic nuclei (AGN) for a large set of Narrow Line Seyfert 1 (NLS1) galaxies assuming the inclination angle between the line of sight and the axis of the accretion disk to be approximately 45 degrees. We found that for these objects the spin values are on average less than for the Seyfert 1 galaxies that we studied previously. In addition, we found that the dependencies of the spin on the bolometric luminosity and the SMBH mass are two to three times stronger that for Seyfert 1 galaxies, which could mean that at early stages of evolution NLS1 galaxies either have a low accretion rate or chaotic accretion, while at later stages they have standard disk accretion, which very effectively increases the spin value. Full article

(This article belongs to the Section Galaxies and Clusters)

► Show Figures

Figure 1

23 pages, 10179 KB

Open AccessReview

Recent Quarkonia Measurements in Small Systems at RHIC and LHC Energies

by Krista L. Smith

Universe 2023, 9(4), 174; https://doi.org/10.3390/universe9040174 - 3 Apr 2023

Viewed by 1541

Abstract

Heavy-ion research at the Relativistic Heavy Ion Collider (RHIC) during the first decade of data collection, approximately during the years 2000–2010, was primarily focused on the study of Au+Au collisions. The search for evidence of quark-gluon plasma (QGP), a state of matter where [...] Read more.

Heavy-ion research at the Relativistic Heavy Ion Collider (RHIC) during the first decade of data collection, approximately during the years 2000–2010, was primarily focused on the study of Au+Au collisions. The search for evidence of quark-gluon plasma (QGP), a state of matter where quarks and gluons become unbound within a high energy density environment, which was at the forefront of research efforts. However, studies of the azimuthal anisotropy parameter

v_{2}

in p/d+Pb collisions from the Large Hadron Collider (LHC) yielded results consistent with the hydrodynamic flow, one of the signatures of quark-gluon plasma formation in heavy-ion collisions. Since the publication of these findings, the field of heavy-ion physics has made subsequent measurements in small system collisions to study cold nuclear matter effects as well as look for additional evidence of hot nuclear matter effects. Quarkonia, a bound state of a

c \bar{c}

or

b \bar{b}

pair, has often been used to probe a wide range of nuclear effects in both large and small collision systems. Here we will review recent quarkonia measurements in small system collisions at RHIC and LHC energies and summarize the experimental conclusions. Full article

(This article belongs to the Special Issue Jet and Heavy Flavor Production)

► Show Figures

Figure 1

11 pages, 293 KB

Open AccessArticle

Scattering in Algebraic Approach to Quantum Theory—Jordan Algebras

by Albert Schwarz

Universe 2023, 9(4), 173; https://doi.org/10.3390/universe9040173 - 31 Mar 2023

Cited by 2 | Viewed by 1504

Abstract

Using the geometric approach, we formulate a quantum theory in terms of Jordan algebras. We analyze the notion of a (quasi)particle (=elementary excitation of translation-invariant stationary state) and the scattering of (quasi)particles in this framework. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Field Theory)

27 pages, 1045 KB

Open AccessArticle

Evolution of Cosmological Parameters and Fundamental Constants in a Flat Quintessence Cosmology: A Dynamical Alternative to ΛCDM

by Rodger I. Thompson

Universe 2023, 9(4), 172; https://doi.org/10.3390/universe9040172 - 31 Mar 2023

Cited by 2 | Viewed by 1933

Abstract

The primary purpose of this work is the provision of accurate, analytic, evolutionary templates for cosmological parameters and fundamental constants in a dynamical cosmology. A flat quintessence cosmology with a dark energy potential that has the mathematical form of the Higgs potential is [...] Read more.

The primary purpose of this work is the provision of accurate, analytic, evolutionary templates for cosmological parameters and fundamental constants in a dynamical cosmology. A flat quintessence cosmology with a dark energy potential that has the mathematical form of the Higgs potential is the specific cosmology and potential addressed in this work. These templates, based on the physics of the cosmology and potential, are intended to replace those parameterizations currently used to determine the likelihoods of dynamical cosmologies. Acknowledging that, unlike

Λ

CDM, the evolutions are dependent on both the specific cosmology and the dark energy potential, the templates are referred to as specific cosmology and potential (SCP) templates. The requirements set for the SCP templates are that they must be accurate, analytic functions of an observable, such as the scale factor or redshift. This is achieved through the utilization of a modified beta function formalism that is based on a physically motivated dark energy potential to calculate the beta function. The methodology developed here is designed to be adaptable to other cosmologies and dark energy potentials. The SCP templates are essential tools in determining the relative likelihoods of a range of dynamical cosmologies and potentials. The ultimate purpose is the determination of whether dark energy is dynamical or static in a quantitative manner. It is suggested that the SCP templates calculated in this work can serve as fiducial dynamical templates in the same manner as

Λ

CDM serves for static dark energy. Full article

(This article belongs to the Section Cosmology)

► Show Figures

Figure 1

15 pages, 536 KB

Open AccessArticle

Mission Target: Exotic Multiquark Hadrons—Sharpened Blades

by Wolfgang Lucha

Universe 2023, 9(4), 171; https://doi.org/10.3390/universe9040171 - 31 Mar 2023

Cited by 3 | Viewed by 2839

Abstract

Motivated by recent experimental progress in establishing the likely existence of (variants of) exotic hadrons, predicted to be formed by the strong interactions, various proposed concepts and ideas are compiled in an attempt to draft a coherent picture of the achievable improvement in [...] Read more.

Motivated by recent experimental progress in establishing the likely existence of (variants of) exotic hadrons, predicted to be formed by the strong interactions, various proposed concepts and ideas are compiled in an attempt to draft a coherent picture of the achievable improvement in the theoretical interpretation of exotic hadrons in terms of the underlying quantum field theory of strong interactions. Full article

(This article belongs to the Special Issue The Quantum Chromodynamics: 50th Anniversary of the Discovery)

► Show Figures

Figure 1

11 pages, 301 KB

Open AccessCommunication

Study of Isothermal Compressibility and Speed of Sound in the Hadronic Matter Formed in Heavy-Ion Collision Using Unified Formalism

by Shubhangi Jain, Rohit Gupta and Satyajit Jena

Universe 2023, 9(4), 170; https://doi.org/10.3390/universe9040170 - 30 Mar 2023

Cited by 3 | Viewed by 1579

Abstract

The thermodynamical quantities and response functions are useful to describe the particle production in heavy-ion collisions as they reveal crucial information about the produced system. While the study of isothermal compressibility provides an inference about the viscosity of the medium, speed of sound [...] Read more.

The thermodynamical quantities and response functions are useful to describe the particle production in heavy-ion collisions as they reveal crucial information about the produced system. While the study of isothermal compressibility provides an inference about the viscosity of the medium, speed of sound helps in understanding the equation of state. With an aim towards understanding the system produced in the heavy-ion collision, we have made an attempt to study isothermal compressibility and speed of sound as function of charged particle multiplicity in heavy-ion collisions at

\sqrt{s_{N N}}

=

2.76

TeV,

5.02

TeV, and

5.44

TeV using unified formalism. Full article

(This article belongs to the Special Issue Collectivity in High-Energy Proton-Proton and Heavy-Ion Collisions)

► Show Figures

Figure 1

Journal Menu

Journal Browser

Universe, Volume 9, Issue 4 (April 2023) – 44 articles

Further Information

Guidelines

MDPI Initiatives

Follow MDPI