General Relativity and Gravitation最新文献

In this paper we study the higher dimensional (left( N > 4right) ) homogeneous and isotropic perfect fluid spacetimes in Einstein–Gauss–Bonnet (EGB) gravity. We solve the modified field equations with higher order curvature terms to determine the evolution of the scale factor. We transparently show that this scale factor cannot become smaller than a finite minimum positive value which depends on the dimension and equation of state. This bound completely eliminates any curvature singularities in homogeneous and isotropic spacetimes, where the scale factor must tend to zero. This is a unique property of EGB gravity which, despite being ghost-free and having quasi-linear field equations like general relativity, allows for the violation of singularity theorems. This phenomenon, thus, gives a natural way to dynamically construct regular black holes via higher dimensional continual gravitational collapse.

The null-surface formulation (NSF) of general relativity differs from the usual approach by treating the spacetime metric as a derivable quantity instead of regarding it as fundamental. The NSF has two mathematically equivalent interpretations: (a) Light rays leave a spacetime point and intersect null-infinity to form a ‘light-cone cut,’ which encodes the properties of the spacetime; (b) At null-infinity, angular coordinates (Bondi coordinates) label past light cones. Being null surfaces, these past light cones will satisfy the NSF field equations, the solution of which will provide a description of spacetime. In an earlier work, the present authors gave an exact solution for the NSF field equations in 2+1 dimensions, showing how the solution directly linked the two NSF interpretations. The present paper expands on that work by constructing the corresponding (3+1)-dimensional solution and then, as in 2+1 dimensions, linking the two interpretations so as to illustrate their equivalence. The functions relevant to the 3+1 NSF are calculated, and the field equations are shown to be satisfied. This is the first time that a nontrivial (3+1)-dimensional NSF solution has been found and its properties examined.

We mainly analyze the effective dynamics of scalar particles trapped in a two-dimensional static de Sitter (dS(_{2})) spacetime from the rainbow gravity perspective. For this purpose, in the framework of gravity’s rainbow, we exactly solve the covariant Klein-Gordon equation to reach the wave function identifying relativistic dynamics of scalar particles as well as generate the corresponding energy spectrum, oscillation frequency, particle creation rate, and transmission probability, analyze them thoroughly, and highlight how the results vary depending on the selected rainbow gravity scenario. We found that the investigation implies quite amazing results. One of them is that while the General Theory of Relativity (GTR) does not give physically measurable energy values for the system we are examining, the rainbow approach of gravity allows us to obtain a real energy spectrum. Another conclusion is that if the test particles are confined to a two-dimensional geometry, the particle creation process can be observed even if the space-time model is static. The previous studies in literature indicate that the production of particles occurs in an expanding space-time or depends on the existence of a strong exterior electric field. From this point of view, our study presents a remarkable result for the creation process of relativistic scalar particles.

We study the entanglement entropy in the context of supersymmetric quantum cosmology, focusing on a pair of TAUB-type universes within the microsuperspace sector ((R_{2}rightarrow 0)). Starting from the Lagrangian of a supergravity theory with (mathcal {N}=1) in (D=4), we construct the quantum Hamiltonian and obtain solutions to the Wheeler-DeWitt equation restricted to the subspace defined by first-order fermionic constraints. The resulting solutions take the form of four-component spinor-like wavefunctions, allowing a natural interpretation in terms of internal degrees of freedom. This spinorial structure enables the construction of an entangled state between two identical wavefunctions, formulated as a bilinear combination of the spin states, analogous to the bipartite entanglement of two electrons. We then compute the entanglement entropy between two such universes, each described by spinorial wavefunctions. The analysis reveals that the entropy is maximized for specific combinations of the Misner variables (Omega _{i}) and (beta _{pm i}), with (i=I,II) labeling each universe. We interpret these maxima as configurations of maximal quantum correlation determined by the geometric size and anisotropy of the universes. The role of anisotropy in modulating the entanglement is also elucidated.

We study the properties of a nonlinear magnetic-charged black hole in the presence of a phantom global monopole. By incorporating nonlinear electrodynamics (NLE) and exotic scalar fields, we derive an exact black hole solution and analyze its geometric structure, causal properties, and thermodynamic behavior. We examine how the presence of a phantom global monopole modifies the black hole’s Hawking temperature, entropy, and stability conditions, revealing significant changes in its phase structure. Additionally, we investigate the geodesic motion of test particles. The quasinormal mode (QNM) spectrum is computed using the WKB approximation and Pöschl-Teller potential method, providing insights into the perturbative stability of the system. Furthermore, we analyze the grey-body factors that characterize radiation emission, highlighting their dependence on black hole parameters. Our findings indicate that the interplay between phantom energy, NLE, and global monopoles introduces observable deviations in strong-field astrophysical phenomena. These results offer potential signatures for testing modified gravity theories and contribute to a deeper understanding of black hole physics in exotic field environments.

The weak cosmic censorship conjecture states that the black hole singularity is hidden inside the event horizon of the black hole, making it impossible for an external observer to measure. In this study, we investigate the weak cosmic censorship conjecture test of dark matter halo-black hole systems in both the cold dark matter model and ultralight dark matter model scenarios, with the aim of gaining insights into the influence of dark matter particles on the weak cosmic censorship conjecture. By examining the particle incident on an extremely or nearly extremal dark matter-black hole, as well as the scattering of a scalar field by an extreme or near-extreme dark matter-black hole. Our model calculations (based on second-order iterative approximation) indicate that under extremal conditions, the weak cosmic censorship conjecture may be violated in test particle scenarios; however, under near-extremal conditions, the relevant effects are all second-order small quantities, making it uncertain whether the conjecture is violated. Due to the influence of second-order small effects, the current approximation treatment cannot provide definitive conclusions. To obtain definitive conclusions, future studies need to incorporate self-gravitational effects, backreaction effects, and other important factors into more comprehensive theoretical models and conduct in-depth analysis, which is beyond the scope of this work. For the scalar field case, our model also suggests the possibility of the conjecture being violated under extremal conditions. In contrast, under near-extremal conditions, within the framework of our current approximation, the results are consistent with the view that the weak cosmic censorship conjecture holds. However, it must be emphasized that our research is based on second-order iterative approximation analysis, and whether the weak cosmic censorship conjecture would be violated if back-reaction effects and higher-order iterative corrections are fully considered still requires further in-depth research. This study contributes to deepening our understanding of the complex interaction mechanisms between dark matter and black holes.

Starting from the eigenvalue equation for the mass of a black hole derived by Mäkelä and Repo, we show that, by reparametrizing the radial coordinate and the wave function, it can be rewritten as the eigenvalue equation of a quantum harmonic oscillator. We then study the interior of a Schwarzschild black hole using two quantization approaches. In the standard quantization, the area and mass spectra are discrete, characterized by a quantum number (n), but the wave function is not square-integrable, limiting its physical interpretation. In contrast, a minimal-uncertainty quantization approach yields an area spectrum that grows as (n^2), and consequently the mass (M) also increases. In this framework, the wave function is finite and square-integrable, with convergence requiring that the deformation parameter (beta ) be regulated by a discrete quantum number (m). The wave function exhibits quantum tunneling connecting the black hole interior with both its exterior and a white hole region, effects that disappear in the limit (beta rightarrow 0). These results demonstrate how minimal-length effects both regularize the wave function and modify the semiclassical structure of the black hole.

The Schwarzschild-Melvin spacetime is an exact solution of the Einstein electrovacuum equations describing a black hole immersed in a magnetic field which is asymptotically aligned with the (z-)axis. It plays an important role in our understanding of the interplay between geometry and matter, and is often used as a proxy for astrophysical environments. Here, we construct the scalar counterpart to the Schwarzschild-Melvin spacetime: a non-asymptotically flat black hole geometry with an everywhere regular scalar field whose gradient is asymptotically aligned with the (z-)axis.

In this study, we investigate the thermodynamic law of accelerating and rotating black hole described by rotating C-metric, as well as holography properties in Nariai limit, which are related to Nariai-CFT and Kerr-CFT correspondence. In order to achieve this goal we define a regularized Komar mass with physical interpretation of varying the horizon area from spinless limit to general case, and derive the first law based on this construction through covariant phase space formalism. Serving for potential future studies, we also reduce the model to a 2-dimensional JT-type action and discuss some of its properties.

In this paper, we derive the entropy of Reissner-Nordström (RN) and Kerr black holes using the Hawking–Gibbons path integral method. We determine the periodicity of the Euclidean time coordinate using two approaches: first, by analyzing the near-horizon geometry, and second, by applying the Chern–Gauss–Bonnet (CGB) theorem. For non-extremal cases, both these methods yield a consistent and unique periodicity, which in turn leads to a well-defined expression for the entropy. In contrast, the extremal case exhibits a crucial difference. The absence of a conical structure in the near-horizon geometry implies that the periodicity of the Euclidean time is no longer uniquely fixed within the Hawking–Gibbons framework. The CGB theorem also fails to constrain the periodicity, as the corresponding Euler characteristic vanishes. As a result, the entropy cannot be uniquely determined using either method.