165 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
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
Stable and self-consistent compact star models in teleparallel gravity
No full textIn the framework of Teleparallel Gravity, we derive a charged non-vacuum solution for a physically symmetric tetrad field with two unknown functions of radial coordinate. The field equations result in a closed-form adopting particular metric potentials and a suitable anisotropy function combined with the charge. Under these circumstances, it is possible to obtain a set of configurations compatible with observed pulsars. Specifically, boundary conditions for the interior spacetime are applied to the exterior Reissner–Nordström metric to constrain the radial pressure that has to vanish through the boundary. Starting from these considerations, we are able to fix the model parameters. The pulsar , with estimated mass and radius km is used to test numerically the model. The stability is studied, through the causality conditions and adiabatic index, adopting the Tolman–Oppenheimer–Volkov equation. The mass–radius (M, R) relation is derived. Furthermore, the compatibility of the model with other observed pulsars is also studied. We reasonably conclude that the model can represent realistic compact objects
Anisotropic compact stars in
No full textWe derive a new interior solution for stellar compact objects in gravity assuming a differential relation to constrain the Ricci curvature scalar. To this aim, we consider specific forms for the radial component of the metric and the first derivative of . After, the time component of the metric potential and the form of function are derived. From these results, it is possible to obtain the radial and tangential components of pressure and the density. The resulting interior solution represents a physically motivated anisotropic neutron star model. It is possible to match it with a boundary exterior solution. From this matching, the components of metric potentials can be rewritten in terms of a compactness parameter C which has to be for physical consistency. Other physical conditions for real stellar objects are taken into account according to the solution. We show that the model accurately bypasses conditions like the finiteness of radial and tangential pressures, and energy density at the center of the star, the positivity of these components through the stellar structure, and the negativity of the gradients. These conditions are satisfied if the energy-conditions hold. Moreover, we study the stability of the model by showing that Tolman–Oppenheimer–Volkoff equation is at hydrostatic equilibrium. The solution is matched with observational data of millisecond pulsars with a withe dwarf companion and pulsars presenting thermonuclear bursts
Charged anti-de Sitter BTZ black holes in Maxwell-f(T) gravity
No full text Inspired by the Bañados, Teitelboim and Zanelli (BTZ) formalism, we discuss the Maxwell-[Formula: see text] gravity in [Formula: see text] dimensions. The main task is to derive exact solutions for a special form of [Formula: see text], with [Formula: see text] being the torsion scalar of Weitzenböck geometry. To this end, a triad field is applied to the equations of motion of charged [Formula: see text] and sets of circularly symmetric noncharged and charged solutions have been derived. We show that, in the charged case, the monopole-like and the [Formula: see text] terms are linked by a correlative constant despite the known results in teleparallel geometry and its extensions.[Formula: see text] Furthermore, it is possible to show that the event horizon is not identical with the Cauchy horizon due to such a constant. The singularities and the horizons of these black holes are examined: they are new and have no analogue in the literature due to the fact that their curvature singularities are soft. We calculate the energy content of these solutions by using the general vector form of the energy–momentum within the framework of [Formula: see text] gravity. Finally, some thermodynamical quantities, like entropy and Hawking temperature, are derived. </jats:p
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
D-dimensional charged Anti-de-Sitter black holes in f (T) gravity
No full textAbstract We present a D-dimensional charged Anti-de-Sitter black hole solutions in f (T) gravity, where f (T) = T + βT 2 and D ≥ 4. These solutions are characterized by flat or cylindrical horizons. The interesting feature of these solutions is the existence of inseparable electric monopole and quadrupole terms in the potential which share related momenta, in contrast with most of the known charged black hole solutions in General Relativity and its extensions. Furthermore, these solutions have curvature singularities which are milder than those of the known charged black hole solutions in General Relativity and Teleparallel Gravity. This feature can be shown by calculating some invariants of curvature and torsion tensors. Furthermore, we calculate the total energy of these black holes using the energy-momentum tensor. Finally, we show that these charged black hole solutions violate the first law of thermodynamics in agreement with previous results
Black Hole Solution in f(R,G) Gravitational Theory Coupled with Scalar Field
No full textIn this work, we explore a class of spherically symmetric black hole (BH) solutions within the framework of modified gravity, focusing on a non-ghost-free f(R,G) theory coupled to a scalar field. We present a novel black hole geometry that arises as a deformation of the Schwarzschild solution and analyze its physical and thermodynamic properties. Our results show that the model satisfies stability conditions, with the Ricci scalar R, as well as its first and second derivatives, remaining positive throughout the spacetime. The solution admits multiple horizons and exhibits strong curvature singularities compared to those in general relativity. Furthermore, it supports a non-trivial scalar field potential. A comprehensive thermodynamic analysis is performed, including evaluations of the entropy, temperature, heat capacity, and quasi-local energy. We find that the black hole exhibits thermodynamic stability within certain ranges of model parameters. In addition, we investigate geodesic deviation and derive the conditions necessary for stability within the f(R,G) gravitational framework
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