General Relativity and Quantum Cosmology (gr-qc)

Abstract: We investigate the quasinormal mode and greybody factor of Bardeen black holes with a string clouds by WKB approximation and verify them by Prony algorithm. We found that the imaginary part of the quasinormal modes spectra is always negative and the perturbation does not increase with the time, indicating that the system is stable under scalar field perturbation. Moreover, the string parameter $a$ has a dramatically impact on the frequency and decay rate of the waveforms. In addition, the greybody factor becomes larger when $a$ and $\lambda$ increase while $q$ and $l$ decreases. The parameter $\lambda$ and $l$ have a big effect on the tails. Especially, when $l=0$, a de Sitter phase appears at the tail.

Abstract: In this overview we discuss the prospects for a first detection of an isotropic gravitational wave background with earth-based interferometric detectors. Furthermore, we focus on how correlated noise sources could endanger such a detection with current generation of detectors. Finally, we project how correlated noise could significantly impede the potential of the future detector, the Einstein Telescope, in its search for an isotropic gravitational wave background. The triangular configuration of three (almost) co-located detectors, makes the Einstein Telescope especially prone to correlated noise sources.

Abstract: We consider the motion of test particles around a $Reissner-Nordstr\"{o}m$ black hole immersed into a strong external magnetic field modifying the spacetime structure. When the particles are neutral, their dynamics are nonintegrable because the magnetic field acts as a gravitational effect, which destroys the existence of a fourth motion constant in the $Reissner-Nordstr\"{o}m$ spacetime. A time-transformed explicit symplectic integrator is used to show that the motion of neutral particles can be chaotic under some circumstances. When test particles have electric charges, their motions are subject to an electromagnetic field surrounding the black hole as well as the gravitational forces from the black hole and the magnetic field. It is found that increasing both the magnetic field and the particle energy or decreasing the particle angular momentum can strengthen the degree of chaos regardless of whether the particles are neutral or charged. The effect of varying the black hole positive charge on the dynamical transition from order to chaos is associated with the electric charges of particles. The dynamical transition of neutral particles has no sensitive dependence on a change of the black hole charge. An increase of the black hole charge weakens the chaoticity of positive charged particles, whereas enhances the chaoticity of negative charged particles. With the magnitude of particle charge increasing, chaos always gets stronger.

Abstract: In this paper, we have studied Bianchi type I space-time in the presence of domain walls in the context of $f(G)$ theory of gravitation. Field equations are solved by using the special form of deceleration parameter. It is also assumed that expansion is proportional to the shear scalar of the model. Some physical parameters are discussed in detail.

Abstract: We investigate possible astronomical manifestations of space-time anisotropy. The homogeneous vacuum Kasner solution was chosen as a reference anisotropic cosmological model because there are no effects caused by inhomogeneity in this simple model with a constant degree of anisotropy. This anisotropy cannot become weak. The study of its geodesic structure made it possible to clarify the properties of this space-time. It showed that the degree of manifestation of anisotropy varies significantly depending on the travel time of the light from the observed object. For nearby objects, for which it does not exceed half the age of the universe, the manifestations of anisotropy are very small. Distant objects show more pronounced manifestations, for example, in the distribution of objects over the sky and over photometric distances. These effects for each of the individual objects decrease with time, but in general, the manifestations of anisotropy in the Kasner space-time remain constant due to the fact that new sources emerging from beyond the cosmological horizon.We analyse observable signatures of the Kasner-type anisotropy and compare it to observations. These effects were not found in astronomical observations, including the study of the CMB. We can assume that the Universe has always been isotropic or almost isotropic since the recombination era. This does not exclude the possibility of its significant anisotropy at the moment of the Big Bang followed by rapid isotropization during the inflationary epoch.

Abstract: We find static fluid solutions of Einstein and pure Lovelock equations with $p_r=0$, $p_t=k\rho$, which could be possible models for the interior of a Buchdahl-like star. Buchdahl star is a limiting stellar configuration without a horizon whose formation does not need any exotic matter.

Abstract: We consider the Little Rip (LR), Pseudo Rip (PR) and bounce cosmological models in the Friedmann-Robertson-Walker (FRW) metric with nonzero spatial curvature. We describe the evolution of the universe using a generalized equation of state in the presence of a viscous fluid. The conditions of the occurrence of the LR, PR and bounce were obtained from the point of view of the parameters of the generalized equation of state for the cosmic dark fluid, taking into account the spatial curvature. The analytical expressions for the spatial curvature were obtained. Asymptotic cases of the early and late universe are considered. A method of Darboux transformation was proposed in the case of models of an accelerating universe with viscosity.

Abstract: Supposing the existence of Dark Fluid with a Chaplygin-like equation of state $p=-B/\rho$ (CDF) as a cosmic background, we obtain a static spherically-symmetric black hole (BH) solution to the Einstein gravitational equations. We study the $P-V$ critical behavior of AdS BH surrounded by the CDF in the extended phase space where the cosmological constant appears as pressure, and our results show the existence of the Van der Waals like small/large BH phase transition. Also, it is found that such a BH displays a first-order low/high-$\Phi$ BH phase transition and admits the same criticality with van der Waals liquid/gas system in the non-extended phase space, where the normalization factor $q$ is considered as a thermodynamic variable, while the cosmological constant being fixed. In both $P-V$ and the newly proposed $q-\Phi$ phase spaces, we calculate the BH equations of state and then numerically study the corresponding critical quantities. Moreover, the critical exponents are derived and the results show the universal class of the scaling behavior of thermodynamic quantities near criticality. Finally, we study the shadow thermodynamics of AdS BHs surrounded by the CDF. We find that, there exists a positive correlation between the shadow radius and the event horizon radius in our case. By analyzing the temperature and heat capacity curves under the shadow context, we discover that the shadow radius can replace the event horizon radius to demonstrate the BH phase transition process, and the changes of the shadow radius can serve as order parameters for the small/large BH phase transition, indicating that the shadow radius could give us a glimpse into the BH phase structure from the observational point of view.