43 research outputs found
Constraining
No full textWe discuss spherically symmetric dynamical systems in the framework of a general model of gravity, i.e. , where is a dimensional quantity in squared length units [L]. We initially assume that the internal structure of such systems is governed by the Krori–Barua ansatz, alongside the presence of fluid anisotropy. By employing astrophysical observations obtained from the pulsar SAX J1748.9-2021, derived from bursting X-ray binaries located within globular clusters, we determine that is approximately equal to km. In particular, the model is capable of producing stable configurations for SAX J1748.9-2021, encompassing both its geometric and physical characteristics. We show that, within the framework of gravity, the Krori–Barua ansatz establishes semi-analytical connections between the radial () and tangential () pressures, and the density (). These relations are described as and . In this context, and denote the sound speeds in the radial and tangential directions, respectively. Meanwhile, pertains to the surface density, and is derived from the model parameters. These connections are consistent with the equations of state derived from the best-fit solutions identified in the ongoing investigation. Notably, within the framework of gravity where is negative, the maximum compactness, denoted as C, is inherently limited to values that do not exceed the Buchdahl limit. This contrasts with general relativity or gravity with positive, where the compactness has the potential to asymptotically reach the black hole threshold (). The model predictions suggest a central density that largely exceeds the saturation nuclear density, which is g/cm. Also the surface density surpasses . We obtain a mass-radius diagram, corresponding to the boundary density, which is consistent with other observational data
Charged spherically symmetric black holes in scalar-tensor Gauss–Bonnet gravity
No full textWe derive a novel class of four-dimensional black hole (BH) solutions in Gauss–Bonnet (GB) gravity coupled with a scalar field in presence of Maxwell electrodynamics. In order to derive such solutions, we assume the ansatz for metric potentials. Due to the choice of the ansatz of the metric, the Reissner Nordström gauge potential cannot be recovered because of the presence of higher-order terms which are not allowed to be vanishing. Moreover, the scalar field is not allowed to vanish. If it vanishes, a function of the solution results undefined. Furthermore, it is possible to show that the electric field is of higher-order in the monopole expansion: this fact explicitly comes from the contribution of the scalar field. Therefore, we can conclude that the GB scalar field acts as non-linear electrodynamics creating monopoles, quadrupoles, etc in the metric potentials. We compute the invariants associated with the BHs and show that, when compared to Schwarzschild or Reissner–Nordström space-times, they have a soft singularity. Also, it is possible to demonstrate that these BHs give rise to three horizons in AdS space-time and two horizons in dS space-time. Finally, thermodynamic quantities can be derived and we show that the solution can be stable or unstable depending on a critical value of the temperature
Spinning (A)dS black holes with slow-rotation approximation in dynamical Chern-Simons modified gravity
No full textOne of the most crucial areas of gravity research, after the direct
observation of gravitational waves, is the possible modification of General
Relativity at ultraviolet and infrared scales. In particular, the possibility
of parity violation should be considered in strong field regime. The
Chern-Simons gravity takes into account parity violation in strong gravity
regime. For all conformally flat spacetimes and spacetimes with a maximally
symmetric two-dimensional subspace, Chern-Simons gravity is identical to
General Relativity. Specifically, the Anti-de Sitter (A)dS-Kerr/Kerr black hole
is not a solution for Chern-Simons gravity. The slow-rotating BH and the
quadratic order in spin solutions are some of the known solutions to quadratic
order in spin and they are rotating solutions in the frame of dynamical
Chern-Simons gravity.
In the present study, for the (A)dS slow-rotating situation (correct to the
first order in spin), we derive the linear perturbation equations controlling
the metric and the dynamical Chern-Simons field equation corrected to the
linear order in spin and to the second order in the Chern-Simons coupling
parameter. We show that the black hole of the (A)dS-Kerr solution is stronger
(i.e. more compact and energetic) than the Kerr black hole solution and the
reason for this feature comes form contributions at Planck scales. Moreover, we
calculate the thermodynamical quantities related to this black hole. Finally,
we calculate the geodesic equation and derive the effective potential of the
black hole.Comment: 15 pages 3 figures, will appear in Phys. Rev
Momentum in Teleparallel Equivalent of General Relativity
No full textA new exact solution describing a general stationary and axisymmetric object of the gravitational field in the framework of teleparallel equivalent of general relativity TEGR is derived. The solution is characterized by three parameters "the gravitational mass M, the rotation a, and the NUT L." The vierbein field is axially symmetric, and the associated metric gives the Kerr-Taub-NUT spacetime. Calculation of the total energy using two different methods, the gravitational energy momentum and the Riemannian connection 1-form Γ α β , is carried out. It is shown that the two methods give the same results of energy and momentum. The value of energy is shown to depend on the mass M and the NUT parameter L. If L is vanishing, then the total energy reduced to the energy of Kerr black hole
Kerr-Taub-NUT General Frame, Energy, and Momentum in Teleparallel Equivalent of General Relativity
No full textA new exact solution describing a general stationary and axisymmetric object of the gravitational field in the framework of teleparallel equivalent of general relativity (TEGR) is derived. The solution is characterized by three parameters “the gravitational mass M, the rotation a, and the NUT L.” The vierbein field is axially symmetric, and the associated metric gives the Kerr-Taub-NUT spacetime. Calculation of the total energy using two different methods, the gravitational energy momentum and the Riemannian connection 1-form Γα̃β, is carried out. It is shown that the two methods give the same results of energy and momentum. The value of energy is shown to depend on the mass M and the NUT parameter L. If L is vanishing, then the total energy reduced to the energy of Kerr black hole
Regularization of f(T) Gravity Theories and Local Lorentz Transformation
No full textWe regularized the field equations of f(T) gravity theories such that the effect of local Lorentz transformation (LLT), in the case of spherical symmetry, is removed. A “general tetrad field,” with an arbitrary function of radial coordinate preserving spherical symmetry, is provided. We split that tetrad field into two matrices; the first represents a LLT, which contains an arbitrary function, and the second matrix represents a proper tetrad field which is a solution to the field equations of f(T) gravitational theory (which are not invariant under LLT). This “general tetrad field” is then applied to the regularized field equations of f(T). We show that the effect of the arbitrary function which is involved in the LLT invariably disappears
Axially Symmetric-dS Solution in Teleparallel f(T) Gravity Theories
No full textWe apply a tetrad field with six unknown functions to Einstein field equations. Exact vacuum solution, which represents axially symmetric-dS spacetime, is derived. We multiply the tetrad field of the derived solution by a local Lorentz transformation which involves a generalization of the angle ϕ and get a new tetrad field. Using this tetrad, we get a differential equation from the scalar torsion T=TαμνSαμν. Solving this differential equation we obtain a solution to the f(T) gravity theories under certain conditions on the form of f(T) and its first derivatives. Finally, we calculate the scalars of Riemann Christoffel tensor, Ricci tensor, Ricci scalar, torsion tensor, and its contraction to explain the singularities associated with this solution
Schwarzschild solution in extended teleparallel gravity
No full textA tetrad field with two unknown functions of the radial coordinate and an angle Φ (the polar angle ϕ times a function of the radial coordinate), is applied to the field equation of the modified theory of gravity. An exact vacuum solution is derived; its scalar torsion, , is constant. When the angle Φ coincides with the polar angle ϕ, the derived solution will be a solution only for the linear form of the f(T) gravitational theory
