arXiv subjects

Tiberiu Harko

Publications and source records attributed to Tiberiu Harko.

221 records · Page 13Linked to original sources

arXiv2008

Generalized virial theorem in f(R) gravity

We generalize the virial theorem in f(R) modified gravity using the collisionless Boltzmann equation. We find supplementary geometric terms in the modified Einstein equation providing an effective contribution to the gravitational energy. The total virial mass is proportional to the effective mass associated with the new geometrical term, which may account for the well-known virial theorem mass discrepancy in clusters of galaxies. The model predicts that the geometric mass and its effects extend beyond the virial radius of the clusters. We also consider the behavior of the galaxy cluster velocity dispersion in f(R) models. Thus, the f(R) virial theorem can be an efficient tool in observationally testing the viability of this class of generalized gravity models.

gr-qc↗

arXiv2008

Wormhole geometries with conformal motions

Exact solutions of traversable wormholes were recently found under the assumption of spherical symmetry and the existence of a non-static conformal symmetry. In this paper, we verify that in the case of the conformally symmetric spacetimes with a non-static vector field generating the symmetry, the conformal factor $ψ$ can be physically interpreted in terms of a measurable quantity, namely, the tangential velocity of a massive test particle moving in a stable circular orbit in the spacetime. Physical properties of the rotational velocity of test particles and of the redshift of radiation emitted by ultra-relativistic particles rotating around these hypothetical general relativistic objects are further discussed. Finally, specific characteristics and properties of gravitational bremsstrahlung emitted by charged particles in geodesic motion in conformally symmetric wormhole geometries are also explored.

gr-qc↗

arXiv2008

Solar system tests of brane world models

The classical tests of general relativity (perihelion precession, deflection of light, and the radar echo delay) are considered for the Dadhich, Maartens, Papadopoulos and Rezania (DMPR) solution of the spherically symmetric static vacuum field equations in brane world models. For this solution the metric in the vacuum exterior to a brane world star is similar to the Reissner-Nordstrom form of classical general relativity, with the role of the charge played by the tidal effects arising from projections of the fifth dimension. The existing observational solar system data on the perihelion shift of Mercury, on the light bending around the Sun (obtained using long-baseline radio interferometry), and ranging to Mars using the Viking lander, constrain the numerical values of the bulk tidal parameter and of the brane tension.

gr-qc↗

arXiv2007

Conformally symmetric traversable wormholes

Exact solutions of traversable wormholes are found under the assumption of spherical symmetry and the existence of a {\it non-static} conformal symmetry, which presents a more systematic approach in searching for exact wormhole solutions. In this work, a wide variety of solutions are deduced by considering choices for the form function, a specific linear equation of state relating the energy density and the pressure anisotropy, and various phantom wormhole geometries are explored. A large class of solutions impose that the spatial distribution of the exotic matter is restricted to the throat neighborhood, with a cut-off of the stress-energy tensor at a finite junction interface, although asymptotically flat exact solutions are also found. Using the ``volume integral quantifier,'' it is found that the conformally symmetric phantom wormhole geometries may, in principle, be constructed by infinitesimally small amounts of averaged null energy condition violating matter. Considering the tidal acceleration traversability conditions for the phantom wormhole geometry, specific wormhole dimensions and the traversal velocity are also deduced.

gr-qc↗

arXiv2007

Extra force in $f(R)$ modified theories of gravity

The equation of motion for test particles in $f(R)$ modified theories of gravity is derived. By considering an explicit coupling between an arbitrary function of the scalar curvature, $R$, and the Lagrangian density of matter, it is shown that an extra force arises. This extra force is orthogonal to the four-velocity and the corresponding acceleration law is obtained in the weak field limit. Connections with MOND and with the Pioneer anomaly are further discussed.

gr-qc↗