arXiv subjects

Jackson Levi Said

Publications and source records attributed to Jackson Levi Said.

118 records · Page 7Linked to original sources

arXiv2016

Stability of the flat FLRW metric in $f(T)$ gravity

In this paper, we investigate the stability of the flat FLRW metric in $f(T)$ gravity. This is achieved by analysing the small perturbations, $δ$ about the Hubble parameter and the matter energy density, $δ_\text{m}$. We find that $δ\propto \dot{H}/H$ and $δ_{\text{m}} \propto H$. Since the Hubble parameter depends on the function $f(T)$, two models were considered (A) the power-law model $f(T) = α(-T)^n$, and (B) the exponential model $f(T) = αT_0 \left(1 - \exp \left[-p \sqrt{\dfrac{T}{T_0}}\right]\right)$, where the parameters $n$ and $p$ were chosen to give comparable physical results. For the parameters considered, it was found that the solutions are stable with vanishing $δ$ and decaying then constant $δ_{\text{m}}$, meaning that the matter perturbations persist during late times.

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arXiv2016

Growth factor in $f(T,\mathcal{T})$ gravity

We investigate the growth factor for sub-horizon modes during late times in $f(T,\mathcal{T})$ gravity, where $T$ is the torsion scalar and $\mathcal{T}$ is the trace of the stress-energy tensor. This is achieved by obtaining the modified Mészáros equation, which describes the evolution of the perturbations of the matter energy density, and obtaining numerical results. Such results are obtained by solving the modified continuity equation and analysing the behaviour of the solutions of the latter using various constraints on the integration constants. Furthermore, the role of the anisotropic term $π^{S}$ is investigated.

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arXiv2016

Gravitomagnetic effects in quadratic gravity with a scalar field

The two gravitomagnetic effects which influence bodies orbiting around a gravitational source are the geodetic effect and the Lense-Thirring effect. The former describes the precession angle of the axis of a spinning gyroscope while in orbit around a nonrotating gravitational source whereas the latter provides a correction for this angle in the case of a spinning source. In this paper we derive the relevant equations in quadratic gravity and relate them to their equivalents in general relativity. Starting with an investigation into Kepler's third law in quadratic gravity with a scalar field, the effects of an axisymmetric and rotating gravitational source on an orbiting body in a circular, equatorial orbit are introduced.

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arXiv2016

Solar System tests in $f(T)$ gravity

We investigate the four solar system tests of gravity - perihelion precession, light bending, Shapiro time delay, gravitational redshift - in $f(T)$ gravity. In particular, we investigate the solution derived by Ruggiero and Radicella, Phys. Rev. D 91, 104014 (2015), for a nondiagonal vierbein field for a polynomial $f(T) = T + αT^n$, where $α$ is a constant and $|n| \neq 1$. In this paper, we derive the solutions for each test, in which Weinberg's, Bodenner and Will's, Cattani et al. and Rindler and Ishak's methods are applied, Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity (Wiley, New York, 1972); Am. J. Phys. 71 (2003); Phys. Rev. D 87, 047503 (2013); Phys. Rev. D 76, 043006 (2007). We set a constraint on alpha for $n$ = 2, 3 by using data available from literature.

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arXiv2014

Gravitomagnetic effects in conformal gravity

Gravitomagnetic effects are characterized by two phenomena: first, the geodetic effect which describes the precession of the spin of a gyroscope in a free orbit around a massive object, second, the Lense-Thirring effect which describes the precession of the orbital plane about a rotating source mass. We calculate both these effects in the fourth-order theory of conformal Weyl gravity for the test case of circular orbits. We show that for the geodetic effect a linear term arises which may be interesting for high radial orbits, whereas for the Lense-Thirring effect the additional term has a diminishing effect for most orbits. Circular orbits are also considered in general leading up to a generalization of Kepler's third law.

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arXiv2013

Charged Cylindrical Black Holes in Conformal Gravity

Considering cylindrical topology we present the static solution for a charged black hole in conformal gravity. We show that unlike the general relativistic case there are two different solutions, both including a factor that when set to zero recovers the familiar static charged black string solution in Einstein's theory. This factor gives rise to a linear term in the potential that also features in the neutral case and may have significant ramifications for particle trajectories.

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arXiv2012

Exact Static Cylindrical Solution to Conformal Weyl Gravity

We present the exact exterior solution for a static and neutral cylindrically symmetric source in locally conformal invariant Weyl gravity. As a special case the general relativity analogue still can be attained, however only as a sub-family of solutions. Our solution contains a linear term that would thus result in a potential that grows linearly over large distances. This may have implications for exotic astrophysical structures as well as matter fields on the extremely small scale.

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arXiv2012

Cosmological Effects on Black Hole Formation

The formation of cosmological black holes is investigated using the functional Schroedinger equation as observed by an asymptotic observer, assuming a spherical domain wall collapse process. The mass formula of the Sultana-Dyer black hole is derived using Israel's domain wall mathematical framework. This is used to examine the semi-classical and quantum nature of the collapsing domain wall in the general scale of an arbitrary scale factor, while ignoring evaporation and back reaction mass losses. Particular FLRW scale factors are then explored for black hole horizon formation times, all yielding the same over-all classical result, namely that an arbitrary amount of time is required for an asymptotic observer to register the formation of this surface.

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arXiv2011

Rotating Charged Cylindrical Black Holes as Particle Accelerators

It has recently been pointed out that arbitrary center-of-mass energies may be obtained for particle collisions near the horizon of an extremal Kerr black hole. We investigate this mechanism in cylindrical topology. In particular we consider the center-of-mass energies of a cylindrical black hole with an extremal rotation and charge parameter. The geodesics are first derived with a rotating charged cylindrical black hole producing the background gravitational field. Finally the center-of-mass is determined for this background and its extremal limit is taken.

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arXiv2011

The Generalized Uncertainty Principle in f(R) Gravity for a Charged Black Hole

Using f (R) gravity in the Palatini formularism, the metric for a charged spherically symmetric black hole is derived, taking the Ricci scalar curvature to be constant. The generalized uncertainty principle is then used to calculate the temperature of the resulting black hole, through this the entropy is found correcting the Bekenstein-Hawking entropy in this case. Using the entropy the tunneling probability and heat capacity are calculated up to the order of the Planck length, which produces an extra factor that becomes important as black holes become small, such as in the case of mini black holes.

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