arXiv subjects

Stefano Viaggiu

Publications and source records attributed to Stefano Viaggiu.

44 records · Page 3Linked to original sources

arXiv2009

A Lemaitre-Tolman-Friedmann Universe without Dark Energy

We build a simple cosmological model by means of a parabolic Lemaître-Tolman-Bondi (LTB) metric up to a redshift $z\simeq 0.4$, an hyperbolic Friedmann metric for $z\sim 0.4$ up to the scale where dimming galaxies are observed ($z\sim 1.4$) and a bulk spatially flat metric up to the last scattering surface. Following Wiltshire, by taking into account the different rate of clocks for an observer at the centre of a parabolic LTB spacetime with respect to a one in the hyperbolic Friedmann metric, an "apparent" negative deceleration parameter is perceived by the observer at the centre of LTB, provided that all the regularity conditions are imposed and the past null sections of the LTB and the hyperbolic Friedmann metrics are identified. As a result, a first order Hubble law emerges at low redshifts. A parameter $K$ arises driving the deceleration parameter perceived by the central LTB observer. Finally, we obtain that a negative value for the deceleration parameter is compatible with the observed energy-density at our present epoch.

gr-qc↗

arXiv2009

Modeling usual and unusual anisotropic spheres

In this paper, we study anisotropic spheres built from known static spherical solutions. In particular, we are interested in the physical consequences of a "small" departure from a physically sensible configuration. The obtained solutions smoothly depend on free parameters. By setting these parameters to zero, the starting seed solution is regained. We apply our procedure in detail by taking as seed solutions the Florides metrics, and the Tolman IV solution. We show that the chosen Tolman IV, and also Heint IIa Durg IV,V perfect fluid solutions, can be used to generate a class of parametric solutions where the anisotropic factor has features recalling boson stars. This is an indication that boson stars could emerge by "perturbing" appropriately a perfect fluid solution (at least for the seed metrics considered). Finally, starting with Tolman IV, Heint IIa and Durg IV,V solutions, we build anisotropic gravastar-like sources with the appropriate boundary conditions.

gr-qc↗

arXiv2007

Rigidly rotating dust solutions depending upon harmonic functions

We write down the relevant field equations for a stationary axially symmetric rigidly rotating dust source in such a way that the general solution depends upon the solution of an elliptic equation and upon harmonic functions. Starting with the dipole Bonnor solution, we built an asymptotically flat solution with two curvature singularities on the rotational axis with diverging mass. Apart from the two point singularities on the axis, the metric is regular everywhere. Finally, we study a non-asymptotically flat solution with NUT charge and a massless ring singularity, but with a well-defined mass-energy expression.

gr-qc↗

arXiv2006

Interior Kerr solutions with the Newman-Janis algorithm starting with static physically reasonable space-times

We present a simple approach for obtaining Kerr interior solutions with the help of the Newman-Janis algorithm (NJA) starting with static space-times describing physically sensible interior Schwarzschild solutions. In this context, the Darmois-Israel (DI) junction conditions are analyzed. Starting from the incompressible Schwarzschild solution, a class of Kerr interior solutions is presented, together with a discussion of the slowly rotating limit. The energy conditions are discussed for the solutions so obtained. Finally, the NJA algorithm is applied to the static, anisotropic, conformally flat solutions found by Stewart leading to interior Kerr solutions with oblate spheroidal boundary surfaces.

gr-qc↗

arXiv2005

Vacuum Solutions of Einstein's Equations in Parabolic Coordinates

We present a simple method to obtain vacuum solutions of Einstein's equations in parabolic coordinates starting from ones with cylindrical symmetries. Furthermore, a generalization of the method to a more general situation is given together with a discussion of the possible relations between our method and the Belinsky-Zakharov soliton-generating solutions.

gr-qc↗

arXiv2004

A novel derivation for Kerr metric in Papapetrou gauge

We present a simple novel derivation, ab initio, of the equations appropriate for stationary axisymmetric spacetimes using the Papapetrou form of the metric (Papapetrou gauge). It is shown that using coordinates which preserve the Papapetrou gauge three separated solutions of the Ernst equations appear in the case of Kerr metric. In this context a parameter arises which represents topological defects induced by an infinite static string along the z axis. Finally, we discuss a simple solution that may be derived from the Kerr ansatz.

gr-qc↗

arXiv2004

Black Hole Entropy and Dilatations

Dilatations by means of a constant factor can be seen in a double way: as a simple change of units length or as a conformal mapping of the starting spacetime into a ``stretched'' one with the same units length. The numerical value of the black hole entropy depends on the interpretations made for the stretched manifold. Further, we study the possibility to choose an unusual ``mass dependent'' normalization for the timelike Killing vector for a Kerr black hole with and without a cosmic string.

gr-qc↗

arXiv2003

A technique for generating new solutions of Einstein's equations

We present a simple technique for generating new solutions of Einstein's equations using such function transformations that leave the field equations in the Ernst form. In this context we recover all the known covariant transformations of Ernst equations and we find the role of the analytic ones. Finally we obtain a new asymptotically flat solution starting from the Kerr solutions.

gr-qc↗