Search Results (37)

We investigate the dynamics of quantum coherence in an anisotropically expanding emergent universe, modeled by a Bianchi type I spacetime. In particular, our findings suggest that the presence of small anisotropic perturbations introduces a directional dependence in the behavior of quantum coherence. Notably, we identify the emergence of frozen coherence regimes when the $\{a_0,H_0,m,k\}$ parameters lie within particular ranges. The physical origin of these frozen regimes can be attributed to the suppression of mode mixing, which consequently leads to reduced particle creation. Full article

We present a novel approach for the construction of interior solutions for the Kerr metric, extending J. Ovalle’s foundational work through ellipsoidal coordinate transformations. By deriving a physically plausible interior solution that smoothly matches the Kerr exterior metric, we analyze the energy conditions across various rotation parameters. Our findings reveal anisotropic fluid properties and energy condition behaviors in specific space-time regions, providing insights into the strong-field regime of rotating black holes. The proposed solution offers a more realistic description of rotating black hole interiors, with implications for understanding compact astrophysical objects. Full article

In this article, we study the form of the deviation of geodesics (tidal forces) and the Raychaudhuri equation in a Schwarzschild–Finsler–Randers (SFR) spacetime which has been investigated in previous papers. This model is obtained by considering the structure of a Lorentz tangent bundle of spacetime and, in particular, the kind of the curvatures in generalized metric spaces where there is more than one curvature tensor, such as Finsler-like spacetimes. In these cases, the concept of the Raychaudhuri equation is extended with extra terms and degrees of freedom from the dependence on internal variables such as the velocity or an anisotropic vector field. Additionally, we investigate some consequences of the weak-field limit on the spacetime under consideration and study the Newtonian limit equations which include a generalization of the Poisson equation. Full article

Internal gravity waves (IGWs) in the middle atmosphere are the main source of mesoscale fluctuations of wind and temperature. The parameterization of IGWs and study of their climatology is necessary for the development of global atmospheric circulation models. In this review, we focus on the application of Radio Occultation (RO) observations for the retrieval of IGW parameters. (1) The simplest approach employs the retrieved temperature profiles. It is based on the fact that IGWs are highly anisotropic structures and can be accurately retrieved by RO. The basic assumption is that all the temperature fluctuations are caused by IGWs. The smoothed background temperature profile defines the the Brunt–Väisälä frequency, which, together with the temperature fluctuations, defines the IGW specific potential energy. Many studies have derived the distribution and climatology of potential energy, which is one of the most important characteristics of IGWs. (2) More detailed analysis of the temperature profiles is based on the derivation of the temperature fluctuation spectra. For saturated IGWs, the spectra must obey the power law with an exponent of $-3$. Such spectra are obtained by using Wave Optical (WO) processing. (3) More advanced analysis employs space–frequency analysis. It is based on phase-sensitive techniques like cross S- or wavelet transforms in order to identify propagating IGWs. (4) Another direction is the IGW parameter estimate from separate temperature profiles applying the stability condition in terms of the Richardson number. In this framework, a necessary condition is formulated that defines whether or not the temperature fluctuations can be related to IGW events. The temperature profile retrieval involves integral transforms and filtering that constitute the observation filter. (5) A simpler filter is implemented by the analysis of the RO amplitude fluctuation spectra, based on the diffraction theory in the framework of the phase screen and weak fluctuation approximations. The two spectral parameters, the external scale and the structural characteristic, define the specific potential energy. This approach allows the derivation of the spacial and seasonal distributions of IGW activity. We conclude that the success of IGW study by RO is stimulated by a large number of RO observations and advanced techniques based on Fourier and space–time analysis, physical equations describing IGWs, and diffraction theory. Full article

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 come from beyond the cosmological horizon. We analyze 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. Full article

We study the holographic dual of the two-dimensional non-relativistic field theory with anisotropic scaling from a symmetry perspective. We construct a new four-dimensional metric with two-dimensional global anisotropic scaling isometry. The four-dimensional spacetime is homogeneous and is a solution of Einstein gravity with quadratic-curvature extension. We consider this spacetime dual to the vacuum of the boundary field theory. By introducing a proper solution phase space, we find that the asymptotic symmetry of the gravity theory is the two-dimensional local anisotropic conformal symmetry, which recovers precisely the results from the dual non-relativistic field theory side. Full article

The general-relativistic (GR) magnetohydrodynamic (MHD) equations for a conductive plasma fluid are derived and discussed in the curved spacetime described by Thorne’s metric tensor, i.e., a family of cosmological models with inherent anisotropy due to the existence of an ambient, large-scale magnetic field. In this framework, it is examined whether the magnetized plasma fluid that drives the evolution of such a model can be subsequently excited by a transient, plane-polarized gravitational wave (GW) or not. To do so, we consider the associated set of the perturbed equations of motion and integrate them numerically in order to study the evolution of instabilities triggered by the GW propagation. In particular, we examine to what extend perturbations of the electric and/or the magnetic field can be amplified due to a potential energy transfer from the GW to the electromagnetic (EM) degrees of freedom. The evolution of the perturbed quantities depends on four free parameters, namely, the conductivity of the fluid, $\sigma$; the speed of sound square, $\frac{1}{3}<{\left(\frac{C_s}{c}\right)}^2\equiv \gamma <1$, which in this model may serve also as a measure of the inherent anisotropy; the GW frequency, ${\omega }_g$; and the associated angle of propagation with respect to the direction of the magnetic field, $\theta$. We find that GW propagation in the anisotropic magnetized medium under consideration does excite several MHD modes; in other words, there is energy transfer from the gravitational to the EM degrees of freedom that can result in the acceleration of charged particles at the spot and in the subsequent damping of the GW. Full article

In the present paper, we focused on exploring the possibility of providing a new class of exact solutions for viable anisotropic stellar systems by means of the massive Brans–Dicke (BD) theory of gravity. In this respect, we used the decoupling of gravitational sources by minimal geometric deformation (MGD) ($e^{-\eta }=\Psi +\beta h$) for compact stellar objects in the realm of embedding class-one space-time to study anisotropic solutions for matter sources through the modified Einstein field equations. For this purpose, we used the ansatz for $\Psi$ relating to the prominent, well-known and well-behaved Finch–Skea model via Karmarkar condition, and the determination scheme for deformation function $h\left(r\right)$ was proposed via mimic requirement on radial pressure component: ${\theta }_1^1\left(r\right)=p_r\left(r\right)$ and matter density: ${\theta }_0^0\left(r\right)=\rho \left(r\right)$ for the anisotropic sector. Moreover, we analyzed the main physical highlights of the anisotropic celestial object by executing several physical tests for the case ${\theta }_1^1\left(r\right)=p_r\left(r\right)$. We have clearly shown how the parameters $\alpha$, $\beta$ and ${\omega }_{BD}$ introduced by massive BD gravity via the MGD approach incorporating the anisotropic profile of the matter distribution have an immense effect on many physical parameters of compact bodies such as LMC X-4, LMC X-4, Her X-1, 4U 1820-30, 4U 1608-52, SAX J1808.4–658 and many others that can be fitted. Full article

Self-similar cosmological solutions correspond to spacetimes that admit a homothetic symmetry. The physical properties of self-similar solutions can describe important eras of the cosmological evolution. Recently, self-similar cosmological solutions were derived for symmetric teleparallel $f\left(Q\right)$-theory with different types of connections. In this work, we study the stability properties of the self-similar cosmological solutions in order to investigate the effects of the different connections on the stability properties of the cosmic history. For the background geometry, we consider the isotropic Friedmann–Lemaître–Robertson–Walker space and the anisotropic and homogeneous Bianchi I space, for which we investigate the stability properties of Kasner-like universes. Full article

We carried out a detailed group classification of the potential in Klein–Gordon equation in anisotropic Riemannian manifolds. Specifically, we consider the Klein–Gordon equations for the four-dimensional anisotropic and homogeneous spacetimes of Bianchi I, Bianchi III and Bianchi V. We derive all the closed-form expressions for the potential function where the equation admits Lie and Noether symmetries. We apply previous results which connect the Lie symmetries of the differential equation with the collineations of the Riemannian space which defines the Laplace operator, and we solve the classification problem in a systematic way. Full article

In this article, we consider an anisotropic viscous cosmological model having LRS Bianchi type I spacetime with $f\left(Q\right)$ gravity. We investigate the modified $f\left(Q\right)$ gravity with form $f\left(Q\right)=\alpha Q^2+\beta$, where Q is the non-metricity scalar and $\alpha$, $\beta$ are the positive constants. From the modified Einstein’s field equation having the viscosity coefficient $\xi \left(t\right)={\xi }_{0}H$, the scale factor is derived as $a\left(t\right)=2sinh\left(\frac{m+2}{6}\sqrt{\frac{{\xi }_0}{\alpha (2m+1)}}t\right)$. We apply the observational constraints on the apparent magnitude $m\left(z\right)$ using the ${\chi }^2$ test formula with the observational data set such as JLA, Union 2.1 compilation and obtained the best approximate values of the model parameters $m,\alpha ,H_0,{\xi }_0$. We find a transit universe which is accelerating at late times. We also examined the bulk viscosity equation of state (EoS) parameter ${\omega }_v$ and derived its current value satisfying ${\omega }_v<-1/3$, which shows the dark energy dominating universe evolution having a cosmological constant, phantom, and super-phantom evolution stages. It tends to the $\Lambda$ cold dark matter ($\Lambda$CDM) value (${\omega }_v=-1$) at late times. We also estimate the current age of the universe as $t_0\approx 13.6$ Gyrs and analyze the statefinder parameters with $(s,r)\to (0,1)$ as $t\to \infty$. Full article

The core of this manuscript is to conduct a broad investigation into the features of static matter configurations with hyperbolical symmetry, which might possibly serve as formation of corresponding spacetime within the limits of f(R,T,Q) gravity, where (Q ≡ R${}_{\alpha \sigma }$T${}^{\alpha \sigma }$). We recognize that such matter distributions can be anisotropic in pressure, with just two primary stresses unequal and a negative energy density. Usually, negative matter densities are suggested in extreme cosmological and astrophysical situations, particularly with regard to quantum occurrences that might occur within the horizon. Eventually, we construct a generic formalism that allows every static hyperbolically symmetric (HS) fluid solution to be expressed with respect to two generating functions (GFs). Full article

We highlight here the fact that the distantly observed luminosity of a spherically symmetric compact star radiating thermal radiation isotropically is higher by a factor of ${(1+z_{\mathrm{b}})}^2$ compared to the corresponding flat space-time case, where $z_{\mathrm{b}}$ is the surface gravitational redshift of the compact star. In particular, we emphasize that if the thermal radiation is indeed emitted isotropically along the respective normal directions at each point, this factor of increment ${(1+z_{\mathrm{b}})}^2$ remains unchanged even if the compact object would lie within its photon sphere. Since a canonical neutron star has $z_{\mathrm{b}}\approx 0.1$, the actual X-ray luminosity from the neutron star surface could be $\sim 20\%$ higher than what would be interpreted by ignoring the general relativistic effects described here. For a static compact object, supported by only isotropic pressure, compactness is limited by the Buchdahl limit $z_{\mathrm{b}}<2.0$. However, for compact objects supported by anisotropic pressure, $z_{\mathrm{b}}$ could be even higher ($z_{\mathrm{b}}<5.211$). In addition, in principle, there could be ultra-compact objects having $z_{\mathrm{b}}\gg 1$. Accordingly, the general relativistic effects described here might be quite important for studies of thermal radiation from some ultra-compact objects. Full article

We investigate exact and analytic solutions for the field equations in the teleparallel dark energy model, where the physical space is described by the locally rotational symmetric Bianchi I, Bianchi III and Kantowski-Sachs geometries. We make use of the property that a point-like Lagrangian exists for the description of the field equations, and variational symmetries are applied for the construction of invariant functions and conservation laws. The latter are used for the derivation of new analytic solutions for the classical field equations and exact function forms for the wavefunction in the quantum limit. Full article

We investigate electromagnetic, gravitational, and plasma-related perturbations to the first order on homogeneous and hypersurface orthogonal locally rotationally symmetric (LRS) class II spacetimes. Due to the anisotropic nature of the studied backgrounds, we are able to include a non-zero magnetic field to the zeroth order. As a result of this inclusion, we find interesting interactions between the electromagnetic and gravitational variables already of the first order in the perturbations. The equations governing these perturbations are found by using the Ricci identities, the Bianchi identities, Einstein’s field equations, Maxwell’s equations, particle conservation, and a form of energy-momentum conservation for the plasma components. Using a $1+1+2$ covariant split of spacetime, the studied quantities and equations are decomposed with respect to the preferred directions on the background spacetimes. After linearizing the decomposed equations around an LRS background, performing a harmonic decomposition, and imposing the cold magnetohydrodynamic (MHD) limit with a finite electrical resistivity, the system is then reduced to a set of ordinary differential equations in time and some constraints. On solving for some of the harmonic coefficients in terms of the others, the system is found to decouple into two closed and independent subsectors. Through numerical calculations, we then observe some mechanisms for generating magnetic field perturbations, showing some traits similar to previous works using Friedmann–Lemaître–Robertson–Walker (FLRW) backgrounds. Furthermore, beat-like patterns are observed in the short wave length limit due to interference between gravitational waves and plasmonic modes. Full article