General Relativity and Gravitation最新文献

In this short note we investigate canonical formalism for General Relativity which is formulated with the metric (f^{ab}=(-g)^alpha g^{ab}). We find corresponding Hamiltonian and we show that constraint structure is the same as in the standard formulation. We also analyze another model when the spatial part of metric (h^{ij}) is related with the new one by relation (a^{ij}=(det h_{ij})^beta h^{ij}) and we argue that it corresponds to the gauge fixed version of the General Relativity formulated with the metric (f^{ab}=(-g)^alpha g^{ab}).

We consider two Unruh-DeWitt detectors interacting with a massless, minimally coupled scalar field in a ((1+1)) dimensional Reissner-Nordström black hole spacetime. In particular, one of the detectors, corresponding to Alice, is moving along an outgoing null trajectory. While the other detector carried by Bob is static. With this set-up, we investigate the entangling condition and the measure of the entanglement, concurrence, in the nonextremal and extremal scenarios. Our observations suggest, as expected, a qualitative similarity in characteristics of the entanglement between these two scenarios. However, we find quantitative differences between the nonextremal and extremal concurrences for a broad range of black hole charges. With moderately large detector transition energy, the extremal background always accounts for the larger entanglement than the nonextremal one. In contrast, with low detector transition energy, entanglement on the nonextremal background can be greater. Therefore, by adjusting the detector transition energy, one can perceive optimum entanglement from either the extremal or the nonextremal background.

This paper discusses several functional analytic issues relevant for field theories in the context of the Hamiltonian formulation for a free, massless, scalar field defined on a closed interval of the real line. The fields that we use belong to a Sobolev space with a scalar product. As we show this choice is useful because it leads to an explicit representation of the points in the fibers of the phase space (the cotangent bundle of the configuration space). The dynamical role of the boundary of the spatial manifold where the fields are defined is analyzed.

In a recent series of papers new exact analytical interior spacetimes sourced by stationary rigidly rotating cylinders of fluids have been displayed. A fluid with an axially directed pressure has been first considered, then a perfect fluid, followed by a fluid with an azimuthally directed pressure, and, finally, by a fluid where the pressure is radially oriented. The perfect fluid configuration has subsequently been extended to the case of differential rotation. In the present paper, three different cases of anisotropic pressure analogous to those studied for rigidly rotating motion are considered in turn for differentially rotating fluids. General methods for generating mathematical solutions to the field equations and physically well-behaved examples are displayed for the axial and azimuthal pressure cases. As regards radial pressure fluids, four classes of solutions naturally emerge from the corresponding Einstein’s equations, among which one class, after being fully integrated, exhibits physically well-behaved solutions.

We outline the essential features of the adiabatic theory and demonstrate how test particle motion in general relativity may be investigated using it. The theory relies on adiabatic invariants and vector elements of the orbits. We derive a specific representation of the Kerr metric in harmonic coordinates, which enables us to obtain a general formula for the perihelion shift of test particles orbiting on the non-equatorial plane of a rotating central object. This proves the applicability of the adiabatic theory in Einstein’s gravity. We demonstrate that, for the individual effects of the gravitational source mass and angular momentum up to the second order, the principle of superposition is satisfied. We show that, in addition to its simplicity, the adiabatic theory produces correct results that, in the limiting cases, correspond to the ones reported in the literature.

This paper is a part of a series devoted to the Euclidean-hyperboloidal foliation method introduced by the authors for investigating the global existence problem associated with nonlinear systems of coupled wave-Klein–Gordon equations with small data. This method was developed especially for investigating the initial value problem for the Einstein-massive field system in wave gauge. Here, we study the (fourth-order) field equations of f(R) modified gravity and investigate the global dynamical behavior of the gravitational field in the near-Minkowski regime. We establish the existence of a globally hyperbolic Cauchy development approaching Minkowski spacetime (in spacelike, null, and timelike directions), when the initial data set is sufficiently close to an asymptotically Euclidean and spacelike hypersurface in Minkowski spacetime. We cast the (fourth-order) f(R)-field equations in the form of a second-order wave-Klein–Gordon system, which has an analogous structure to the Einstein-massive field system but, in addition, involves a (possibly small) effective mass parameter. We establish the nonlinear stability of the Minkowski spacetime in the context of f(R) gravity, when the integrand f(R) in the action functional can be taken to be arbitrarily close to the integrand R of the standard Hilbert–Einstein action.

We examine the model of two-dimensional quadratic gravity as a consequence of symmetry breaking within the framework of background field (BF) theory. This theory is essentially an extension of BF theory, introducing an additional polynomial term that operates on both the gauge and background fields. We analyze the theory using the Dirac and Faddeev–Jackiw procedures, determining the form of the gauge transformation, the full structure of the constraints, the counting of degrees of freedom, and the generalized Faddeev–Jackiw brackets. Additionally, we demonstrate the coincidence of the Faddeev–Jackiw and Dirac’s brackets. Finally, we provide some remarks and discuss prospects.

In this study, authors investigate a novel framework incorporating exponential electrodynamics and d-dimensional energy-dependent massive gravity. Our focus lies in demonstrating that a nonlinear electromagnetic field, acting as the gravitational source, leads to an accelerated expansion of the universe. Employing massive gravity’s rainbow as a catalyst for cosmic evolution, the big bang singularity inherent in the model is successfully eliminated. This analysis reveals that the synergy between the magnetic universe and massive gravity’s rainbow is the underlying cause of the observed accelerated cosmic expansion. The classical stability during the deceleration phase and ensure the causality of the model are established. Additionally, we delve into 4-dimensional energy-dependent cosmology. Furthermore, approximations for key cosmological parameters such as the running of the spectral index, tensor-to-scalar ratio, and, the spectral index are presented. These estimations closely align with observational data from PLANCK and WMAP, providing valuable insights into the compatibility of the proposed model with empirical evidence.

In this article, we attempt to explore the dark sector of the universe i.e. dark matter and dark energy, where the dark energy components are related to the modified f(Q) Lagrangian, particularly a power law function (f(Q)= gamma left( frac{Q}{Q_0}right) ^n), while the dark matter component is described by the Extended Bose–Einstein Condensate (EBEC) equation of state for dark matter, specifically, (p = alpha rho + beta rho ^2). We find the corresponding Friedmann-like equations and the continuity equation for both dark components along with an interacting term, specifically (mathcal {Q} = 3b^2H rho ), which signifies the energy exchange between the dark sector of the universe. Further, we derive the analytical expression of the Hubble function, and then we find the best-fit values of free parameters utilizing the Bayesian analysis to estimate the posterior probability and the Markov Chain Monte Carlo (MCMC) sampling technique corresponding to CC+Pantheon+SH0ES samples. In addition, to examine the robustness of our MCMC analysis, we perform a statistical assessment using the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC). Further from the evolutionary profile of the deceleration parameter and the energy density, we obtain a transition from the decelerated epoch to the accelerated expansion phase, with the present deceleration parameter value as (q(z=0)=q_0=-0.56^{+0.04}_{-0.03}) ((68 %) confidence limit), that is quite consistent with cosmological observations. In addition, we find the expected positive behavior of the effective energy density. Finally, by examining the sound speed parameter, we find that the assumed theoretical f(Q) model is thermodynamically stable.

Hawking’s black hole area theorem was proven using the null energy condition (NEC), a pointwise condition violated by quantum fields. The violation of the NEC is usually cited as the reason that black hole evaporation is allowed in the context of semiclassical gravity. Here we provide two generalizations of the classical black hole area theorem: first, a proof of the original theorem with an averaged condition, the weakest possible energy condition to prove the theorem using focusing of null geodesics. Second, a proof of an area-type result that allows for the shrinking of the black hole horizon but provides a bound on it. This bound can be translated to a bound on the black hole evaporation rate using a condition inspired from quantum energy inequalities. Finally, we show how our bound can be applied to two cases that violate classical energy conditions.