arXiv subjects

J. C. N. de Araujo

Publications and source records attributed to J. C. N. de Araujo.

At least 37 records · Page 2Linked to original sources

arXiv2013

Ondas gravitacionales y objetos compactos (Gravitational waves and compact objects)

It is presented a brief review on gravitational waves (GWs). It is shown how the wave equation is obtained from Einstein's equations and how many and how are the polarization modes of these waves. It is discussed the reasons why GWs sources should be of astrophysical or cosmological origin. Thus, it is discussed what would be the most likely sources of GWs to be detected by the detectors of GWs currently in operation and those that should be operational in the future, emphasizing in particular the sources involving compact objects. The compact objects such as neutron stars, black holes and binary systems involving compact stars can be important sources of GWs. Last but not least, it is discussed the GWs astrophysics that is already possible to do, in particular involving the compact objects.

astro-ph.HE↗

arXiv2012

A Recipe to Probe Alternative Theories of Gravitation via N-body Numerical Simulations. I. Spiral Galaxies

A way to probe alternative theories of gravitation is to study if they could account for the structures of the universe. We then modified the well-known Gadget-2 code to probe alternative theories of gravitation through galactic dynamics. As an application, we simulate the evolution of spiral galaxies to probe alternative theories of gravitation whose weak field limits have a Yukawa-like gravitational potential. These simulations show that galactic dynamics can be used to constrain the parameters associated with alternative theories of gravitation. It is worth stressing that the recipe given in the present study can be applied to any other alternative theory of gravitation in which the superposition principle is valid.

astro-ph.CO↗

arXiv2012

Gravitational Waves in Braneworld Scenarios with AdS Background

In this paper, we investigate gravitational waves as metric perturbations around a general warped 5-dimensional background. We find an analytical solution in Randall-Sundrum braneworld model and analyze the implications of braneworld models in the gravitational waves propagation.

gr-qc↗

arXiv2011

A Geostationary Gravitational Wave Interferometer (GEOGRAWI)

We propose a Geostationary Gravitational Wave Interferometer (GEOGRAWI) mission concept for making observations in the sub-Hertz band. GEOGRAWI is expected to meet some of LISA's science goals in the lower part of its accessible frequency band ($10^{-4} - 2 \times 10^{-2}$ Hz), and to outperform them by a large margin in the higher-part of it ($2 \times 10^{-2} - 10$ Hz). As a consequence of its Earth-bound orbit, GEOGRAWI is significantly less expensive than the interplanetary LISA mission and could be either an entirely US mission or managed and operated by NASA in partnership with the Brazilian Space Agency.

astro-ph.IM↗

arXiv2011

Coalescence Rate of Supermassive Black Hole Binaries Derived from Cosmological Simulations: Detection Rates for LISA and ET

The coalescence history of massive black holes has been derived from cosmological simulations, in which the evolution of those objects and that of the host galaxies are followed in a consistent way. The present study indicates that supermassive black holes having masses greater than $\sim 10^{9} M_{\odot}$ underwent up to 500 merger events along their history. The derived coalescence rate per comoving volume and per mass interval permitted to obtain an estimate of the expected detection rate distribution of gravitational wave signals ("ring-down") along frequencies accessible by the planned interferometers either in space (LISA) or in the ground (Einstein). For LISA, in its original configuration, a total detection rate of about $15 yr^{-1}$ is predicted for events having a signal-to-noise ratio equal to 10, expected to occur mainly in the frequency range $4-9 mHz$. For the Einstein gravitational wave telescope, one event each 14 months down to one event each 4 years is expected with a signal-to-noise ratio of 5, occurring mainly in the frequency interval $10-20 Hz$. The detection of these gravitational signals and their distribution in frequency would be in the future an important tool able to discriminate among different scenarios explaining the origin of supermassive black holes.

astro-ph.CO↗

arXiv2010

Probing Brownstein-Moffat Gravity via Numerical Simulations

In the standard scenario of the Newtonian gravity, a late-type galaxy (i.e., a spiral galaxy) is well described by a disk and a bulge embedded in a halo mainly composed by dark matter. In Brownstein-Moffat gravity, there is a claim that late-type galaxy systems would not need to have halos, avoiding as a result the dark matter problem, i.e., a modified gravity (non-Newtonian) would account for the galactic structure with no need of dark matter. In the present paper, we probe this claim via numerical simulations. Instead of using a "static galaxy," where the centrifugal equilibrium is usually adopted, we probe the Brownstein-Moffat gravity dynamically via numerical $N$-body simulations.

astro-ph.IM↗

arXiv2009

Probing Yukawian Gravitational Potential by Numerical Simulations. II. Elliptical Galaxies

Since the Newtonian gravitation is largely used to model with success the structures of the universe, such as galaxies and clusters of galaxies, for example, a way to probe and constrain alternative theories, in the weak field limit, is to apply them to model the structures of the universe. We then modified the well known Gadget-2 code to probe alternative theories of gravitation through galactic dynamics. In particular, we modified the Gadget-2 code to probe alternatives theories whose weak field limits have a Yukawa-like gravitational potential. As a first application of this modified Gadget-2 code we simulate the evolution of elliptical galaxies. These simulations show that galactic dynamics can be used to constrain the parameters associated with alternative theories of gravitation.

astro-ph.CO↗

arXiv2009

Gravitational wave background from neutron star phase transition for a new class of equation of state

We study the generation of a stochastic gravitational wave (GW) background produced by a population of neutron stars (NSs) which go over a hadron-quark phase transition in its inner shells. We obtain, for example, that the NS phase transition, in cold dark matter scenarios, could generate a stochastic GW background with a maximum amplitude of $h_{\rm BG} \sim 10^{-24}$, in the frequency band $\simeq 20-2000 {\rm Hz}$ for stars forming at redshifts of up to $z\simeq 20.$ We study the possibility of detection of this isotropic GW background by correlating signals of a pair of `advanced' LIGO observatories.

gr-qc↗

arXiv2009

Probing Yukawian gravitational potential by numerical simulations. I. Changing N-body codes

In the weak field limit general relativity reduces, as is well known, to the Newtonian gravitation. Alternative theories of gravity, however, do not necessarily reduce to Newtonian gravitation; some of them, for example, reduce to Yukawa-like potentials instead of the Newtonian potential. Since the Newtonian gravitation is largely used to model with success the structures of the universe, such as for example galaxies and clusters of galaxies, a way to probe and constrain alternative theories, in the weak field limit, is to apply them to model the structures of the universe. In the present study, we consider how to probe Yukawa-like potentials using N-body numerical simulations.

astro-ph.GA↗

arXiv2009

Hydrodynamical Collapse of Neutron Stars due to Hadron-Quark Phase Transition

We present studies of the collapse of neutron stars that undergo a hadron-quark phase transition. A spherical Lagrangian hydrodynamic code has been written. As initial condition we take different neutron star configurations taking into account its density, energy density and pressure distribution. The phase transition is imposed at different evolution times. We have found that a significant amount of matter on the surface can be ejected while the remaining star rings in the fundamental and first pressure modes.

gr-qc↗

arXiv2009

Gravitational wave background from neutron star phase transition

We study the generation of a stochastic gravitational wave (GW) background produced by a population of neutron stars (NSs) which go over a hadron-quark phase transition in its inner shells. We obtain, for example, that the NS phase transition, in cold dark matter scenarios, could generate a stochastic GW background with a maximum amplitude of $h_{\rm BG} \sim 10^{-24}$, in the frequency band $ν_{\rm{obs}} \simeq 20-2000 {\rm Hz}$ for stars forming at redshifts of up to $z\simeq 20.$ We study the possibility of detection of this isotropic GW background by correlating signals of a pair of Advanced LIGO observatories.

gr-qc↗

arXiv2007

The conservation of energy-momentum and the mass for the graviton

In this work we give special attention to the bimetric theory of gravitation with massive gravitons proposed by Visser in 1998. In his theory, a prior background metric is necessary to take in account the massive term. Although in the great part of the astrophysical studies the Minkowski metric is the best choice to the background metric, it is not possible to consider this metric in cosmology. In order to keep the Minkowski metric as background in this case, we suggest an interpretation of the energy-momentum conservation in Visser's theory, which is in accordance with the equivalence principle and recovers naturally the special relativity in the absence of gravitational sources. Although we do not present a general proof of our hypothesis we show its validity in the simple case of a plane and dust-dominated universe, in which the `massive term' appears like an extra contribution for the energy density.

gr-qc↗

arXiv2007

A solution for galactic disks with Yukawian gravitational potential

We present a new solution for the rotation curves of galactic disks with gravitational potential of the Yukawa type. We follow the technique employed by Toomre in 1963 in the study of galactic disks in the Newtonian theory. This new solution allows an easy comparison between the Newtonian solution and the Yukawian one. Therefore, constraints on the parameters of theories of gravitation can be imposed, which in the weak field limit reduce to Yukawian potentials. We then apply our formulae to the study of rotation curves for a zero-thickness exponential disk and compare it with the Newtonian case studied by Freeman in 1970. As an application of the mathematical tool developed here, we show that in any theory of gravity with a massive graviton (this means a gravitational potential of the Yukawa type), a strong limit can be imposed on the mass (m_g) of this particle. For example, in order to obtain a galactic disk with a scale length of b ~ 10 kpc, we should have a massive graviton of m_g << 10^{-59} g. This result is much more restrictive than those inferred from solar system observations.

gr-qc↗

arXiv2006

Determining the neutron star equation of state using the narrow-band gravitational wave detector Schenberg

We briefly review the properties of quasi-normal modes of neutron stars and black holes. We analyse the consequences of a possible detection of such modes via the gravitational waves associated with them, especially addressing our study to the Brazilian spherical antenna, on which a possible detection would occur at 3.0-3.4 kHz. A question related to any putative gravitational wave detection concerns the source that produces it. We argue that, since the characteristic damping times for the gravitational waves of neutron stars and black holes are different, a detection can distinguish between them, and also distinguish the neutron stars oscillating modes. Moreover, since the source can be identified by its characteristic damping time, we are able to extract information about the neutron star or black hole. This information would lead, for example, to a strong constraint in the nuclear matter equation of state, namely the compression modulus should be K=220 MeV.

astro-ph↗

arXiv2005

Star formation rate density and the stochastic background of gravitational waves

There is in the literature a number of papers addressing the stochastic background of gravitational waves (GWs) generated by an ensemble of astrophysical sources. The main ingredient in such studies is the so called star formation rate density (SFRD), which gives the density of stars formed per unit time. Some authors argue, however, that there is, in the equation that determines the amplitude of the stochastic background of GWs, an additional (1+z) term dividing the SFRD, which would account for the effect of cosmic expansion onto the time variable. We argue here that the inclusion of this additional term is wrong. In order to clarify where the inclusion of the (1+z) term is really necessary, we briefly discuss the calculation of event rates in the study of GRBs (gamma ray bursts) from cosmological origin.

astro-ph↗

arXiv2005

The dark energy-dominated Universe

In this paper we investigate the epochs in which the Universe started accelerating and when it began to become dark energy-dominated (i.e., the dynamics of the expansion of the Universe dominated by the dark energy). We provide analytic expressions to calculate the redshifts of these epochs as a function of density parameters. Moreover, we review and discuss cosmological models with a dark energy component, which can have an interesting characteristic, namely, they never stop accelerating. This holds even if the Universe is at present time either flat, open, or closed. If the dark energy is the cosmological constant the Universe will eventually end up undergoing an exponentially expansion phase, and the total density parameter converging to $Ω=1$. This is exactly what is considered in inflationary scenario to generate the initial conditions for the big bang. One can then argue that the Universe begun with an inflationary phase and will end up with another inflationary phase. Thus, it follows that in both the early and the late Universe $Ω\to 1$. We also discuss the above issues in the context of the XCDM parametrization.

astro-ph↗

arXiv2005

Can black hole MACHO binaries be detected by the Brazilian spherical antenna?

Different studies show that dark matter of non-baryonic origin might exist. There have been experimental evidences that at least one form of dark matter has been detected through microlensing effects. This form of dark matter is named MACHOs (Massive Astrophysical Compact Halo Objects). The MACHO collaboration estimated that the masses of these objects are to be in the range 0.15-0.95 solar masses, where the most probable mass is of 0.5 solar masses. Some authors argue that MACHOs could be black holes, and that they could form binary systems, BHMACHO binaries. As is well known binary systems are sources of gravitational waves. The Brazilian spherical antenna will operate in the frequency band of 3.0-3.4 kHz, sensitive to binaries of a pair of 0.5 solar mass black holes just before coalescing. In the present work we study the detectability of these putative BHMACHO binaries by the Brazilian spherical antenna Mario Schenberg.

astro-ph↗

arXiv2005

Detectability of f-mode Unstable Neutron Stars by the Schenberg Spherical Antenna

The Brazilian spherical antenna (Schenberg) is planned to detect high frequency gravitational waves (GWs) ranging from 3.0 kHz to 3.4 kHz. There is a host of astrophysical sources capable of being detected by the Brazilian antenna, namely: core collapse in supernova events; (proto)neutron stars undergoing hydrodynamical instability; f-mode unstable neutron stars, caused by quakes and oscillations; excitation of the first quadrupole normal mode of 4-9 solar mass black holes; coalescence of neutron stars and/or black holes; exotic sources such as bosonic or strange matter stars rotating at 1.6 kHz; and inspiralling of mini black hole binaries. We here address our study in particular to the neutron stars, which could well become f-mode unstable producing therefore GWs. We estimate, for this particular source of GWs, the event rates that in principle can be detected by Schenberg and by the Dutch Mini-Grail antenna.

gr-qc↗