Conserved charges of black holes in Weyl and Einstein-Gauss-Bonnet gravities

An off-shell generalization of the Abbott-Deser-Tekin (ADT) conserved charge was recently proposed by Kim et al. They achieved this by introducing off-shell Noether currents and potentials. In this paper, we construct the crucial off-shell Noether current by the variation of the Bianchi identity for the expression of motion equation, with the help of the property of Killing vector. Our Noether current, which contains an additional term that is just one half of the Lie derivative of a surface term with respect to the Killing vector, takes a different form in comparison with the one in their work. Then we employ the generalized formulation to calculate the quasi-local conserved charges for the most general charged spherically symmetric and the dyonic rotating black holes with AdS asymptotics in four-dimensional conformal Weyl gravity, as well as the charged spherically symmetric black holes in arbitrary dimensional Einstein-Gauss-Bonnet gravity coupled to Maxwell or nonlinear electrodynamics in AdS spacetime. Our results confirm those through other methods in the literature.Comment: 21 Pages, no figures, references adde

Off-shell Noether current and conserved charge in Horndeski theory

We derive the off-shell Noether current and potential in the context of Horndeski theory, which is the most general scalar-tensor theory with a Lagrangian containing derivatives up to second order while yielding at most to second-order equations of motion in four dimensions. Then the formulation of conserved charges is proposed on basis of the off-shell Noether potential and the surface term got from the variation of the Lagrangian. As an application, we calculate the conserved charges of black holes in a scalar-tensor theory with non-minimal coupling between derivatives of the scalar field and the Einstein tensor.Comment: 19 pages, no figures, to appear in PL

Mass and angular momentum of charged rotating G\"{o}del black holes in five-dimensional minimal supergravity

In this paper, for the sake of providing a concrete comparison between the usual Abbott-Deser-Tekin (ADT) formalism and its off-shell extension, as well as comparing the latter with the Barnich-Brandt-Compere (BBC) approach, we carry out these methods to compute the mass and angular momentum of the rotating charged G\"{o}del black holes in five-dimensional minimal supergravity. We first present the off-shell ADT potential of the supergravity theories in arbitrary odd dimensions, which is consistent with the superpotential via the BBC approach. Then the off-shell generalized ADT method is applied to evaluate the mass and angular momentum of the G\"{o}del-type black holes by including the contribution from the gauge field. Finally, we strictly obey the rules of the original ADT formalism to incorporate the contribution from the gauge field within the potential. With the help of the modified potential, we try to seek for appropriate reference backgrounds to produce the mass and angular momentum. It is observed that the ADT formalism has to incorporate the contribution from the matter fields to yield physical charges for the G\"{o}del-type black holes.Comment: 15 Pages, NO figure

A brief note on field equations in generalized theories of gravity

In the work (arXiv:1109.3846 [gr-qc]), to obtain a simple and economic formulation of field equations of generalised theories of gravity described by the Lagrangian $\sqrt{-g}L\big(g^{\alpha\beta},R_{\mu\nu\rho\sigma}\big)$, the key equality $\big(\partial L/\partial g^{\mu\nu}\big)_{R_{\alpha\beta\kappa\omega}} =2P_{\mu}^{~\lambda\rho\sigma}R_{\nu\lambda\rho\sigma}$ was derived. In this short note, it is demonstrated that such an equality can be directly derived from an off-shell Noether current associated with an arbitrary vector field. As a byproduct, a generalized Bianchi identity related to the divergence for the expression of field equations is obtained. The results reveal that using the Noether current to determine field equations even can avoid calculating the derivative of the Lagrangian density with respect to the metric. On the basis of the above, we further propose a method to derive the field equations of motion from the Noether current, and then this method is extended to the modified gravities with the Lagrangian including additional terms of the covariant derivatives of the Riemann tensor. Both the expression for field equations and the Noether potential associated with such theories are given.Comment: 15 pages, no figure

Constructing p,np,n-forms from pp-forms via the Hodge star operator and the exterior derivative

In this paper, we aim to explore the properties and applications on the operators consisting of the Hodge star operator together with the exterior derivative, whose action on an arbitrary $p$-form field in $n$-dimensional spacetimes makes its form degree remain invariant. Such operations are able to generate a variety of $p$-forms with the even-order derivatives of the $p$-form. To do this, we first investigate the properties of the operators, such as the Laplace-de Rham operator, the codifferential and their combinations, as well as the applications of the operators in the construction of conserved currents. On basis of two general p-forms, then we construct a general n-form with higher-order derivatives. Finally, we propose that such an n-form could be applied to define a generalized Lagrangian with respect to a p-form field according to the fact that it incudes the ordinary Lagrangians for the $p$-form and scalar fields as special cases.Comment: 23 pages, two tables, accepted by Communications in Theoretical Physic

Helicity hardens the gas

A screw generally works better than a nail, or a complicated rope knot better than a simple one, in fastening solid matter, but a gas is more tameless. However, a flow itself has a physical quantity, helicity, measuring the screwing strength of the velocity field and the degree of the knottedness of the vorticity ropes. It is shown that helicity favors the partition of energy to the vortical modes, compared to others such as the dilatation and pressure modes of turbulence; that is, helicity stiffens the flow, with nontrivial implications for aerodynamics, such as aeroacoustics, and conducting fluids, among others