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Rio de Janeiro, Brazil

Pereira F.I.M.,Observatorio Nacional
Nuclear Physics A | Year: 2011

We calculate the strange star properties in the framework of the Field Correlator Method. We find that for gluon condensate values G2 in the range 0.006-0.007 GeV4, which give a critical temperature Tc∼170 MeV at μc=0, the sequences of strange stars are compatible with some of the semi-empirical mass-radius relations and data obtained from astrophysical observations. © 2011 Elsevier B.V. Source

Pinheiro G.,Observatorio Nacional | Chan R.,Coordenacao de Astronomia e Astrofisica
General Relativity and Gravitation | Year: 2011

A new model is proposed to a collapsing star consisting of an initial inhomogeneous energy density and anisotropic pressure fluid with shear, radial heat flow and outgoing radiation. In previous papers one of us has always assumed an initial star with homogeneous energy density. The aim of this work is to generalize the previous models by introducing an initial inhomogeneous energy density and compare it to the initial homogeneous energy density collapse model. We will show the differences between these models in the evolution of all physical quantities that characterizes the gravitational collapse. The behavior of the energy density, pressure, mass, luminosity and the effective adiabatic index is analyzed. The pressure of the star, at the beginning of the collapse, is isotropic but due to the presence of the shear the pressure becomes more and more anisotropic. The black hole is never formed because the apparent horizon formation condition is never satisfied, in contrast of the previous model where a black hole is formed. An observer at infinity sees a radial point source radiating exponentially until reaches the time of maximum luminosity and suddenly the star turns off. In contrast of the former model where the luminosity also increases exponentially, reaching a maximum and after it decreases until the formation of the black hole. The effective adiabatic index is always positive without any discontinuity in contrast of the former model where there is a discontinuity around the time of maximum luminosity. The collapse is about three thousand times slower than in the case where the energy density is initially homogeneous. © 2010 Springer Science+Business Media, LLC. Source

Holanda R.F.L.,Observatorio Nacional
International Journal of Modern Physics D | Year: 2012

Constraints on the Hubble parameter, H0, via X-ray surface brightness and SunyaevZel'dovich effect (SZE) observations of the galaxy clusters depend on the validity of the cosmic distance duality relation (DD relation), η = DL(z)(1+z)-2/DA(z) = 1, where DL and DA are the luminosity distance and angular diameter distance (ADD), respectively. In this work, we argue that if the DD relation does not hold, the X-ray plus SZE technique furnishes a H 0* = H0η2. We use 25 ADD of galaxy clusters to obtain simultaneous constraints on H0 and possible violation of the DD relation in a flat ΛCDM model. Such a violation is parametrized by two functions: η(z) = 1 + η0z and η(z) = 1 + η0z/(1+z), where η0 is a constant parameter quantifying possible departures from the strict validity. Finally, by marginalizing on the η0 in both parametrizations, we obtain constraints on H0 regardless of the validity of the DD relation. For the linear and nonlinear η(z) functions, we obtain H0 = 75 -7 +7 km/s/Mpc and 75-7 +10 km/s/Mpc, respectively (without systematic errors). Our results support recent H 0 measurements by using X-ray and SZE observations of galaxy clusters which have taken the distance duality as valid. © World Scientific Publishing Company. Source

Pereira F.I.M.,Observatorio Nacional
Nuclear Physics A | Year: 2013

By using the recent nonperturbative equation of state of the quark-gluon plasma derived in the formalism of the Field Correlator Method, we investigate the bulk properties of the strange quark matter in β-equilibrium and with charge neutrality at T = p = 0. The results show that the stability of strange quark matter with respect to Fe56 is strongly dependent on the model parameters, namely, the gluon condensate G2 and the qq- interaction potential V1. A remarkable result is that the width of the stability window decreases as V1 increases, being maximum at V1 = 0 and nearly zero at V1 = 0.5GeV. For V1 in the range 0 ≤ V1 ≤ 0.5GeV, all values of G2 are lower than 0.006-0.007GeV4 obtained from comparison with lattice data at Tc(μ = 0) ~ 170MeV. Our results do not favor the possibilities for the existence of (either nonnegative or negative) absolutely stable strange quark matter. © 2012 Elsevier B.V.. Source

Costa F.E.M.,Observatorio Nacional
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

We discuss some cosmological consequences of a general model of coupled quintessence in which the phenomenological coupling between the cold dark matter and dark energy is a function of the cosmic scale factor ?(a). This class of models presents cosmological solutions in which the Universe is currently dominated by an exotic component, but will eventually be dominated by cold dark matter in the future. This dynamical behavior is considerably different from the standard ΛCDM evolution, and may alleviate some conflicts in reconciling the idea of the dark energy-dominated universe with observables in String/M theory. Finally, we investigate some observational features of this model and discuss some constraints on its parameters from current Type Ia supernovae (SNe Ia), baryonic acoustic oscillations (BAO) and cosmic microwave background (CMB) data. © 2010 The American Physical Society. Source

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