Coordenacao de Astronomia e Astrofisica

Rio de Janeiro, Brazil

Coordenacao de Astronomia e Astrofisica

Rio de Janeiro, Brazil
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Bengaly C.A.P.,Coordenacao de Astronomia e Astrofisica
Journal of Cosmology and Astroparticle Physics | Year: 2016

We evaluate the local variance of the Hubble Constant H0 with low-z Type Ia Supernovae (SNe). Our analyses are performed using a hemispherical comparison method in order to test whether taking the bulk flow motion into account can reconcile the measurement of the Hubble Constant H0 from standard candles (H0 = 73.8±2.4 km s-1 Mpc -1) with that of the Planck's Cosmic Microwave Background data (H0 = 67.8 ± 0.9km s-1 Mpc-1). We obtain that H0 ranges from 68.9±0.5 km s-1 Mpc-1 to 71.2±0.7 km s-1 Mpc-1 through the celestial sphere (1σ uncertainty), implying a Hubble Constant maximal variance of δH0 = (2.30±0.86) km s-1 Mpc-1 towards the (l,b) = (315°,27°) direction. Interestingly, this result agrees with the bulk flow direction estimates found in the literature, as well as previous evaluations of the H0 variance due to the presence of nearby inhomogeneities. We assess the statistical significance of this result with different prescriptions of Monte Carlo simulations, obtaining moderate statistical significance, i.e., 68.7% confidence level (CL) for such variance. Furthermore, we test the hypothesis of a higher H0 value in the presence of a bulk flow velocity dipole, finding some evidence for this result which, however, cannot be claimed to be significant due to the current large uncertainty in the SNe distance modulus. Then, we conclude that the tension between different H0 determinations can plausibly be caused to the bulk flow motion of the local Universe, even though the current incompleteness of the SNe data set, both in terms of celestial coverage and distance uncertainties, does not allow a high statistical significance for these results or a definitive conclusion about this issue. © 2016 IOP Publishing Ltd and Sissa Medialab srl .

Santos B.,Coordenacao de Astronomia e Astrofisica | Devi N.C.,Coordenacao de Astronomia e Astrofisica | Devi N.C.,National Autonomous University of Mexico | Alcaniz J.S.,Coordenacao de Astronomia e Astrofisica
Physical Review D | Year: 2017

We use the most recent type Ia supernovae (SNe Ia) observations to perform a statistical comparison between the standard ΛCDM model and its extensions [wCDM and w(z)CDM] and some alternative cosmologies: namely, the Dvali-Gabadadze-Porrati (DGP) model, a power-law f(R) scenario in the metric formalism and an example of vacuum decay [Λ(t)CDM] cosmology in which the dilution of pressureless matter is attenuated with respect to the usual a-3 scaling due to the interaction of the dark matter and dark energy fields. We perform a Bayesian model selection analysis using the MultiNest algorithm. To obtain the posterior distribution for the parameters of each model, we use the joint light-curve analysis (JLA) SNe Ia compilation containing 740 events in the interval 0.01

Chan R.,Coordenacao de Astronomia e Astrofisica | Da Silva M.F.A.,State University of Rio de Janeiro | Brandt C.F.C.,State University of Rio de Janeiro
International Journal of Modern Physics D | Year: 2014

We study the evolution of an anisotropic shear-free fluid with heat flux and kinematic self-similarity of the second kind. We found a class of solution to the Einstein field equations by assuming that the part of the tangential pressure which is explicitly time-dependent of the fluid is zero and that the fluid moves along timelike geodesics. The energy conditions, geometrical and physical properties of the solutions are studied. The energy conditions are all satisfied at the beginning of the collapse but when the system approaches the singularity the energy conditions are violated, allowing for the appearance of an attractive phantom energy. We have found that, depending on the self-similar parameter α and the geometrical radius, they may represent a naked singularity. We speculate that the apparent horizon disappears due to the emergence of exotic energy at the end of the collapse, or due to the characteristics of null acceleration systems as shown by recent work. © 2014 World Scientific Publishing Company.

Goldoni O.,State University of Rio de Janeiro | Da Silva M.F.A.,State University of Rio de Janeiro | Pinheiro G.,Federal University of Rio de Janeiro | Chan R.,Coordenacao de Astronomia e Astrofisica
International Journal of Modern Physics D | Year: 2014

In this paper, we have studied nonstationary radiative spherically symmetric spacetime, in general covariant theory (U(1) extension) of Hořava-Lifshitz (HL) gravity without the projectability condition and in the infrared (IR) limit. The Newtonian prepotential φ was assumed null. We have shown that there is not the analogue of the Vaidya's solution in the Hořava-Lifshitz Theory (HLT), as we know in the General Relativity Theory (GRT). Therefore, we conclude that the gauge field A should interact with the null radiation field of the Vaidya's spacetime in the HLT. © 2014 World Scientific Publishing Company.

Chan R.,Coordenacao de Astronomia e Astrofisica | da Silva M.F.A.,State University of Rio de Janeiro | Rocha P.,State University of Rio de Janeiro | Rocha P.,Ist Instituto Superior Of Tecnologia Of Paracambi
General Relativity and Gravitation | Year: 2011

Dynamical models of prototype gravastars made of anisotropic dark energy are constructed, in which an infinitely thin spherical shell of a perfect fluid with the equation of state p = (1 - γ)σ divides the whole spacetime into two regions, the internal region filled with a dark energy fluid, and the external Schwarzschild region. The models represent "bounded excursion" stable gravastars, where the thin shell is oscillating between two finite radii, while in other cases they collapse until the formation of black holes. Here we show, for the first time in the literature, a model of gravastar and formation of black hole with both interior and thin shell constituted exclusively of dark energy. Besides, the sign of the parameter of anisotropy (pt - pr) seems to be relevant to the gravastar formation. The formation is favored when the tangential pressure is greater than the radial pressure, at least in the neighborhood of the isotropic case (ω = -1). © 2011 Springer Science+Business Media, LLC.

Chan R.,Coordenacao de Astronomia e Astrofisica | Da Silva M.F.A.,State University of Rio de Janeiro
Journal of Cosmology and Astroparticle Physics | Year: 2010

In recent work we physically interpreted a special gravastar solution characterized by a zero Schwarzschild mass. In fact, in that case, none gravastar was formed and the shell collapsed without forming an event horizon, originating what we called a massive nongravitational object. This object has two components of non zero mass but the exterior spacetime is de Sitter. One of the component is a massive thin shell and the other one is de Sitter spacetime inside. The total mass of this object is zero Schwarzschild mass, which characterizes an exterior vacuum spacetime. Here, we extend this study to the case where we have a charged shell. Now, the exterior is a Reissner- Nordström spacetime and, depending on the parameter w = 1 - γ of the equation of state of the shell, and the charge, a gravastar structure can be formed. We have found that the presence of the charge contributes to the stability of the gravastar, if the charge is greater than a critical value. Otherwise, a massive non-gravitational object is formed for small charges. © 2010 IOP Publishing Ltd and SISSA.

Pinheiro G.,Observatorio Nacional | Pinheiro G.,State University of Rio de Janeiro | Chan R.,Coordenacao de Astronomia e Astrofisica
General Relativity and Gravitation | Year: 2013

We present here a new shear free model for the gravitational collapse of a spherically symmetric charged body. We propose a dissipative contraction with radiation emitted outwards represented by the Vaidya-Reissner-Nordström metric. The Einstein field equations, using the junction conditions and an ansatz, are integrated numerically. A check of the energy conditions is also performed. We obtain that the charge delays the Reissner-Nordström black hole formation and it can even prevent the collapse. © 2012 Springer Science+Business Media New York.

Goldoni O.,State University of Rio de Janeiro | da Silva M.F.A.,State University of Rio de Janeiro | Chan R.,Coordenacao de Astronomia e Astrofisica
General Relativity and Gravitation | Year: 2016

In this paper, we have studied non stationary dust spherically symmetric spacetime, in general covariant theory [U(1) extension] of the Hořava–Lifshitz gravity with the minimally coupling and non-minimum coupling with matter, in the post-newtonian approximation in the infrared limit. The Newtonian prepotential $$\varphi$$φ was assumed null. The aim of this work is to know if we can have the same spacetime, as we know in the General Relativity Theory (GRT), in Hořava–Lifshitz Theory (HLT) in this limit. We have shown that there is not an analogy of the dust solution in HLT with the minimally coupling, as in GRT. Using non-minimum coupling with matter, we have shown that the solution admits a process of gravitational collapse, leaving a singularity at the end. This solution has, qualitatively, the same temporal behaviour as the dust collapse in GRT. However, we have also found a second possible solution, representing a bounce behavior that is not found in GRT. © 2016, Springer Science+Business Media New York.

Chan R.,Coordenacao de Astronomia e Astrofisica | Junqueira S.,Observatorio Nacional
Astronomy and Astrophysics | Year: 2014

We present the results of several detailed numerical N-body simulations of the dynamical interactions of two equal-mass disk galaxies. Both galaxies are embedded in spherical halos of dark matter and contain central bulges. Our analysis of the dynamical evolution of the binary system focuses on the morphological evolution of the stellar distribution of the disks. The satellite galaxy has coplanar or polar disk orientation in relation to the disk of the primary galaxy and their initial orbits are prograde eccentric (e = 0.1, e = 0.4 or e = 0.7). Both galaxies have mass and size similar to the Milky Way. We show that the merger of the two disk galaxies, depending on the relative orientation of the disks, can yield either a disk or lenticular remnant, instead of an elliptical one. These are the first reported simulations that show the formation of S0-like galaxies from protracted binary galaxy interactions. Additionally, we demonstrate that the time to merger increases linearly with the initial apocentric distance between the galaxies, and decreases with the initial orbital eccentricity. We also show that the tidal forces of the disks excite transient m = 1 and m = 2 wave modes, that is, lopsidedness, spiral arms, and bars. However, after the merging of the disks, these larger instabilities fade completely, and the remnant is thicker and more extended than the original disks. The maximum relative amplitude of these waves is at most about 15 times higher than the control case. The m = 2 wave mode is generated mainly by tidal interaction in the outer region of the disks. The m = 1 wave mode depends mostly on the interaction of the inner part of the disks, producing an off-centering effect of the wave mode center relative to the center of mass of the disk. These characteristics produce a time lag among the maximum formation of these two wave modes. Finally, the disk settles down quickly after the merger, in less than one outer disk rotation period. © 2014 ESO.

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.