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Norman Cruz

Publications and source records attributed to Norman Cruz.

65 records · Page 4Linked to original sources
arXiv2005

Viscous dark energy and phantom evolution

In order to study if the bulk viscosity may induce a big rip singularity on the flat FRW cosmologies, we investigate dissipative processes in the universe within the framework of the standard Eckart theory of relativistic irreversible thermodynamics, and in the full causal Israel–Stewart-Hiscock theory. We have found cosmological solutions which exhibit, under certain constraints, a big rip singularity. We show that the negative pressure generated by the bulk viscosity cannot avoid that the dark energy of the universe to be phantom energy.

hep-th↗
arXiv2004

The geodesic structure of the Schwarzschild Anti-de Sitter black hole

In the present work we found the geodesic structure of an AdS black hole. By means of a detailed analyze of the corresponding effective potentials for particles and photon, we found all the possible motions which are allowed by the energy levels. Radial and non radial trajectories were exactly evaluated for both geodesics. The founded orbits were plotted in order to have a direct visualization of the allowed motions. We show that the geodesic structure of this black hole presents new type of motions not allowed by the Schwarzschild spacetime.

gr-qc↗
arXiv2004

Static circularly symmetric perfect fluid solutions with an exterior BTZ metric

In this work we study static perfect fluid stars in 2+1 dimensions with an exterior BTZ spacetime. We found the general expression for the metric coefficients as a function of the density and pressure of the fluid. We found the conditions to have regularity at the origin throughout the analysis of a set of linearly independent invariants. We also obtain an exact solution of the Einstein equations, with the corresponding equation of state $p=p(ρ)$, which is regular at the origin.

gr-qc↗
arXiv2004

On the thermal description of the BTZ black holes

We investigate the limitations on the thermal description of three dimensional BTZ black holes. We derive on physical grounds three basic mass scales that are relevant to characterize these limitations. The Planck mass in 2+1 dimensions indicate the limits where the black hole can emit Hawking's radiation. We show that the back reaction is meaningless for spinless BTZ black hole. For stationary BTZ black holes the nearly extreme case is analyzed showing that may occur a break down of its description as a thermal object.

hep-th↗
arXiv2002

Hydrostatic Equilibrium of a Perfect Fluid Sphere with Exterior Higher-Dimensional Schwarzschild Spacetime

We discuss the question of how the number of dimensions of space and time can influence the equilibrium configurations of stars. We find that dimensionality does increase the effect of mass but not the contribution of the pressure, which is the same in any dimension. In the presence of a (positive) cosmological constant the condition of hydrostatic equilibrium imposes a lower limit on mass and matter density. We show how this limit depends on the number of dimensions and suggest that $Λ> 0$ is more effective in 4D than in higher dimensions. We obtain a general limit for the degree of compactification (gravitational potential on the boundary) of perfect fluid stars in $D$-dimensions. We argue that the effects of gravity are stronger in 4D than in any other number of dimensions. The generality of the results is also discussed.

gr-qc↗
arXiv2001

Closed universes can satisfy the holographic principle in three dimensions

We examine in details Friedmann-Robertson-Walker models in 2+1 dimensions in order to investigate the cosmic holographic principle suggested by Fischler and Susskind. Our results are rigorously derived differing from the previous one found by Wang and Abdalla. We discuss the erroneous assumptions done in this work. The matter content of the models is composed of a perfect fluid, with a $γ$-law equation of state. We found that closed universes satisfy the holographic principle only for exotic matter with a negative pressure. We also analyze the case of a collapsing flat universe.

hep-th↗
arXiv2000

Cosmological scaling solutions of minimally coupled scalar fields in three dimensions

We examine Friedmann-Robertson-Walker models in three spacetime dimensions. The matter content of the models is composed of a perfect fluid, with a $γ$-law equation of state, and a homogeneous scalar field minimally coupled to gravity with a self-interacting potential whose energy density red-shifts as $a^{-2 ν}$, where a denotes the scale factor. Cosmological solutions are presented for different range of values of $γ$ and $ν$. The potential required to agree with the above red-shift for the scalar field energy density is also calculated.

gr-qc↗
arXiv1998

A Matter Scalar Field in a Closed Universe

We investigate the possibility that the matter of the universe has a significant component (the quintessence component) determined by the equation of state $p=wρ$, with $w<0$. Here, we find conditions under which a closed model may look like a flat Friedmann-Robertson-Walker universe at low redshift. We study this problem in Einstein's general relativity and Brans-Dicke theories. In both cases we obtain explicit expressions for the quintessence scalar potential $V(Q)$, and the angular size as a function of the redshift.

astro-ph↗
arXiv1994

Stellar Equilibrium in 2+1 Dimensions

The hydrostatic equilibrium of a $2+1$ dimensional perfect fluid star in asymptotically anti-de Sitter space is discussed. The interior geometry matches the exterior $2+1$ black-hole solution. An upper mass limit is found, analogous to Buchdahl's theorem in 3+1, and the possibility of collapse is discussed. The case of a uniform matter density is solved exactly and a new interior solution is presented.

gr-qc↗
arXiv1994

Geodesic Structure of the 2 + 1 Black Hole

Null and timelike geodesics around a 2+1 black hole are determined. Complete geodesics of both types exist in the rotating black-hole background, but not in the spinless case. Upper and lower bounds for the radial size of the orbits are given in all cases and the possibility of passing from one black hole exterior spacetime to another is discussed using the Penrose diagrams. An analysis of particle motions by means of effective potentials and orbit graphs are also included.

gr-qc↗