The European Physical Journal C最新文献

There are several reasons to support the idea that entropy might be associated to gravity itself. In the absence of a quantum theory of gravity, classical estimators for the gravitational entropy have been proposed. Any viable description of the gravitational entropy should reproduce the Hawking–Bekenstein entropy at the event horizon of black holes. Furthermore, in any black hole transformation, these estimators must satisfy the second law of black hole thermodynamics. In this work, we analyze whether two entropy estimators, one based on the Weyl tensor and the other on the Bel–Robinson tensor, satisfy the second law in the transformation process from a Schwarzschild to a Reissner–Nordström black hole by the absorption of a charge test particle. We also address the inverse process. We show that depending whether the process is reversible or not, both estimators fulfill the second law.

In this work, we investigate the gravitational signatures of a nonlinear electromagnetic extension of the Reissner–Nordström solution. We conduct an analysis of light propagation, focusing on the photon sphere, shadow formation, and geodesic trajectories in this spacetime. The constraints on the parameter (xi ), which characterizes the nonlinear extension of the Reissner–Nordström black hole, are derived from observational data provided by the Event Horizon Telescope (EHT). The time delay effects are also considered. In the thermodynamic analysis, we examine the Hawking temperature, entropy, heat capacity, and the emission of Hawking radiation via the tunneling process. The remnant mass and evaporation time of the black hole at its final stage are estimated. In addition, we compute the quasinormal modes using the WKB approximation, taking into account the characteristic oscillations of the system under scalar, vector, and tensor perturbations. Additionally, the time-domain solution is analyzed for all these perturbations to examine their evolution over time.

This work discusses the influence of galaxy mergers in the evolution of a parabolic Lemaître–Tolman–Bondi (LTB) cosmology with simultaneous big bang endowed with two consecutive single fractal galaxy distributions systems possessing fractal dimension D. Based on recent empirical findings, it is assumed that the resulting galaxy mass from mergers can be expressed by a redshift dependent decaying power law. The proposed cosmological model modifies the relativistic fractal number counts distribution by including a merger rate evolution that estimates the model’s radial density. Numerical solutions for the first order small-merger-rate approximation (SMRA) are found and the results show that a fractal galaxy distribution having (D=1.5) in the range (0.1<z<1.0), and (D=0.5) for (1<z<6), as suggested by recent empirical findings, the SMRA allows consistent description of the model for a merger rate power law exponent up to (q=0.2) considering a fractal galaxy distribution starting from the Local Group. Consistent values were also found up to (q=2.5) and (z=7) from a scale smaller than the Local Supercluster. These results show that galaxy mergers can be successfully incorporated into the dynamics of a parabolic LTB fractal cosmology.

For the short hairs that have a significant impact only near the event horizon, studying their strong gravitational lensing effects is of great significance for revealing the properties of these hairs. In this study, we systematically investigated the strong gravitational lensing effects in the rotating short-haired black hole and constrained its hair parameter (Q_m). Specifically, (Q_m) causes the event horizon radius, photon – orbit radius, and impact parameter to be lower than those of the Kerr black hole. Regarding the lensing coefficients (bar{a}) and (bar{b}), as the spin parameter (a) increases, (bar{a}) shows an increasing trend, while (bar{b}) shows a decreasing trend. In the observational simulations of M87* and Sgr A*, the angular position and angular separation of the relativistic image increase with the increase of (a), while the magnification of the image shows an opposite trend. The existence of (Q_m) only intensifies these trends, while parameter k suppresses such tendencies. More importantly, the rotating short-haired black hole exhibits a significant difference in time delay compared to other black hole models. Especially in the simulation of M87*, the time delay deviation between the rotating short-haired black hole and the Kerr black hole, as well as the Kerr–Newman black hole, can reach dozens of hours. Through a comparative analysis with the observational data from the EHT, we effectively constrain the parameter space of the rotating short-haired black hole. The results indicate that this model has potential application prospects in explaining cosmic black hole phenomena and provides a possible theoretical basis for differentiating between different black hole models.

In this paper, the modified field equations in the context of Extended symmetric teleparallel gravity theory are considered for isotropic and charged compact stellar configuration. In order to obtain an exact solution to the modified field equations, we find an isotropy condition of the charge matter distribution in extended symmetric teleparallel gravity. After that, a regular and well-behaved electric field and a metric potential ansatz are used to solve this isotropy condition. Based on this specific choice, the obtained exact solution hence represents the characteristics and features of the stellar system such as charge, density, and pressure which are shown to be physically valid. The adiabatic stability condition as well as the influence of non-minimal coupling of non-metricity scalar and trace of energy–momentum tensor on stellar configuration have been performed. The present gravitational model investigates some known observed compact stars, viz. PSR J074 +6620, PSR J2215+5135, PSR J1748-2446ao, and GW190814 successfully by exploring mass-radius relationship enabling the model parameters. The present model describing the physical star with isotropic fluid and density of the order of (10^{14}) g/(hbox {cm}^3) is unable to account for the radii of the star falling in the mass gap region with respect to the parameters representing non-metricity and electric field. An increase in a model parameter representing the matter part in the non-minimal coupling justifies the existence of the compact stars in mass gap regions like GW190814 with predicted radii in the range [10.87, 12.49] km in the present isotropic charged stellar model in extended symmetric teleparallel gravity.

In this work, we have invoked (A_4) modular symmetry in the left-right symmetric linear seesaw model. Interestingly, such modular symmetry restricts the proliferation of flavon fields, and as a result, the predictability of the model is enhanced. The fermion sector of the model comprises quarks, leptons, and a sterile fermion in each generation, while the scalar sector consists of Higgs doublets and bidoublets. We investigate numerically various Yukawa coupling coefficients, the neutrino masses and mixing parameters in our intended model and predictions become consistent with the (3sigma ) range of current neutrino oscillation data. We also studied the non-unitarity, effects on lepton flavor violation in our model and evolution of lepton asymmetry to explain the current baryon asymmetry of the universe.

A long-standing debate on Gibbons–Hawking (GH) decoherence centers on its obscure thermal nature. In this work, we investigate the robustness of quantum Fisher information (QFI) and local quantum uncertainty (LQU) in the presence of GH decoherence, using free-falling Unruh–DeWitt (UDW) detectors in de Sitter spacetime (dS-ST). The UDW detectors interact with a massless scalar field in dS-ST and are modeled as open quantum systems, with the field serving as the environment, described by a master equation that outlines their evolution. Our analysis investigates the roles of energy spacing, GH temperature, initial state preparation, and various de Sitter-invariant vacuum sectors on the optimization of QFI and LQU. We find that the optimal values of QFI and LQU depend on the selected de Sitter-invariant vacuum sector and increase with larger energy spacing. Our findings reveal that QFI exhibits resilience to GH decoherence, maintaining a pronounced local peak across a broader range of parameters. This robustness can be further enhanced through strategic initial state preparation and increased energy spacing, resulting in a higher maximum QFI value even under significant environmental decoherence. Our results underscore the critical role of GH thermality in governing QFI and LQU, offering valuable insights for advances in relativistic quantum metrology (RQM).

In recent years, the modeling of compact astrophysical objects (COs) has garnered significant attention from various research groups, particularly in efforts to determine their stable structures. This interest has been further amplified by the incorporation of dark energy as an additional source within the relativistic interior geometries of these stellar objects. In this paper, we present new structural properties of anisotropic, static, and spherically symmetric compact stars, characterized by a two-fluid distribution comprising ordinary baryonic matter and dark energy, within the framework of the gravity theory f(R). We derive a novel class of exact analytical solutions to the modified field equations by employing the well-established Tolman–Buchdahl solutions as seed ansatz for the (g_{tt}) and (g_{rr}) metric potentials, in conjunction with a linear dark energy equation of state. The unknown parameters involved in these seed solutions, along with the dark energy coupling factor (omega ), are determined by a smooth matching of the interior and exterior regions in the hypersurface of the boundary. To analyze the physical viability of our model, we apply it to the compact star PSR (J1614-2230), using the widely studied and cosmologically consistent (f(R)=R+chi Big (R^{2}+eta R^{3}Big )) gravity model. The obtained space-time geometry is assessed on the basis of several physical constraints, including the regularity of metric components, the viability of matter variables, the validity of state parameters and energy conditions, and various stability factors. Additionally, we examine the mass-radius profile, compactness, and surface redshift to ensure the model’s physical acceptability. Notably, our analysis reveals that the maximum allowable mass and compactness of the proposed dark energy star model exceed observational data, providing strong implications within this modified gravity framework. This suggests that such a model has the potential to surpass conventional observational predictions. In conclusion, our results confirm that the proposed solutions are physically viable and realistic, effectively mimicking a stable ultra-compact dark-energy star. These findings offer new insights into the relativistic stellar framework, highlighting the intricate interplay between extended gravity theories and a two-fluid distribution.

The standard procedure to explain the accelerated expansion of the Universe is to assume the existence of an exotic component with negative pressure, generically called dark energy. Here, we propose a new accelerating flat cosmology without dark energy, driven by the negative creation pressure of a reduced relativistic gas (RRG). When the hybrid dark matter of the RRG is identified with cold dark matter, it describes the so-called CCDM cosmology whose dynamics is equivalent to the standard (Lambda )CDM model at both the background and perturbative levels (linear and nonlinear). This effect is quantified by the creation parameter (alpha ). However, when the pressure from the RRG slightly changes the dynamics of the universe, as measured by a parameter b, the model departs slightly from the standard (Lambda )CDM cosmology. Therefore, this two-parametric model ((alpha , b)) describes a new scenario whose dynamics is different but close to the late-time scenarios predicted by CCDM and (Lambda )CDM models. The free parameters of the RRG model with creation are constrained based on SNe Ia data (Pantheon+SH0ES) and also using H(z) from cosmic clocks. In principle, this mild distinction in comparison with both CCDM or (Lambda )CDM may help alleviate some cosmological problems plaguing the current standard cosmology.

We present results for single axial-vector and scalar meson pole contributions to the hadronic light-by-light scattering (HLbL) part of the muon’s anomalous magnetic moment. In the dispersive approach to these quantities (in narrow width approximation) the central inputs are the corresponding space-like electromagnetic transition form factors. We determine these directly using a functional approach to QCD by Dyson–Schwinger and Bethe–Salpeter equations in the very same setup we used previously to determine pseudo-scalar meson exchange ((pi ), (eta ) and (eta ')) as well as meson ((pi ) and K) box contributions. Particular care is taken to preserve gauge invariance and to comply with short distance constraints in both the form factors and the HLbL tensor. Our result for the contributions from a tower of axial-vector states including short distance constraints is (a_mu ^{text {HLbL}}[text {AV-tower+SDC}] = 24.8 ,(6.1) times 10^{-11}). For the combined contributions from (f_0(980), a_0(980), f_0(1370)) and (a_0(1450)) we find (a_mu ^{text {HLbL}}[text {scalar}] = -1.6 ,(5) times 10^{-11}).