[Show abstract][Hide abstract] ABSTRACT: We investigate the generation of electromagnetic and gravitational radiation
in the vicinity of a perturbed Schwarzschild black hole. The gravitational
perturbations and the electromagnetic field are studied by solving the
Teukolsky master equation with sources, which we take to be locally charged,
radially infalling, matter. Our results show that, in addition to the
gravitational wave generated as the matter falls into the black hole, there is
also a burst of electromagnetic radiation. This electromagnetic field has a
characteristic set of quasinormal frequencies, and the gravitational radiation
has the quasinormal frequencies of a Schwarzschild black hole. This scenario
allows us to compare the gravitational and electromagnetic signals that are
generated by a common source.
Full-text · Article · Oct 2014 · General Relativity and Gravitation
[Show abstract][Hide abstract] ABSTRACT: We perform numerical evolutions of cosmological scenarios using a standard
general relativistic code in spherical symmetry. We concentrate on two
different situations: initial matter distributions that are homogeneous and
isotropic, and perturbations to those that respect the spherical symmetry. As
matter models we consider the case of a pressureless perfect fluid, i.e. dust,
and the case of a real massive scalar field oscillating around the minimum of
the potential. Both types of matter have been considered as possible dark
matter candidates in the cosmology literature, dust being closely related to
the standard cold dark matter paradigm. We confirm that in the linear regime
the perturbations associated with these types of matter grow in essentially the
same way, the main difference being that in the case of a scalar field the
dynamics introduce a cut-off in the power spectrum of the density perturbations
at scales comparable with the Compton wavelength of the field. We also follow
the evolutions well beyond the linear regime showing that both models are able
to form structure. In particular we find that, once in the nonlinear regime,
perturbations collapse faster in a universe dominated by dust. This is expected
to delay the formation of the first structures in the scalar field dark matter
scenario with respect to the standard cold dark matter one.
[Show abstract][Hide abstract] ABSTRACT: Using the Green's function representation technique, the late time behavior
of localized scalar field distributions on Schwarzschild spacetimes is studied.
Assuming arbitrary initial data we perform a spectral analysis, computing the
amplitude of each excited quasi-bound mode without the necessity of performing
dynamical evolutions. The resulting superposition of modes is compared with a
traditional numerical evolution with excellent agreement; therefore, we have an
efficient way to determine final black hole wigs. The astrophysical relevance
of the quasi-bound modes is discussed in the context of scalar field dark
matter models and the axiverse.
Full-text · Article · Dec 2013 · Physical Review D
[Show abstract][Hide abstract] ABSTRACT: In this work, we model galactic haloes describing the dark matter as a non-zero pressure fluid and derive, not impose, a dark
matter equation of state by using observational data of the rotation curves of galaxies. In order to reach hydrostatic equilibrium,
as expected for the halo, it is mandatory that dark fluid's pressure should not be zero. The equation of state is obtained
by solving the matter-geometry system of equations assuming different dark matter density or rotational velocity profiles.
The resulting equations of state are, in general, different to a barotropic equation of state. The free parameters of the
equation of state are fixed by fitting the observed rotational velocities of a set of galaxies.
Full-text · Article · Jan 2013 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: The aim of these notes is to give a solid basis on the theory of gravitational waves
generation using the black hole perturbation description. We present an analytical and numerical point of view. In these notes we present the derivation of the Newman-Penrose equations by means of the null tetrad formalism, with particular attention to the derivation of the scalar functionwhich will describe the gravitational wave,
i.e, Ψ4. Next, we describe the case of a Schwarzschild background and show with great detail how to express the evolution equation in a way that can be dealt with in a numerical fashion. We show how to obtain the profiles of the gravitational waves and explain how to analyze several of their properties.
[Show abstract][Hide abstract] ABSTRACT: The dynamical evolution of boson stars in scalar-tensor theories of gravity
is considered in the physical (Jordan) frame. We focus on the study of
spontaneous and induced scalarization, for which we take as initial data
configurations on the well-known S-branch of a single boson star in general
relativity. We show that during the scalarization process a strong emission of
scalar radiation occurs. The new stable configurations (S-branch) of a single
boson star within a particular scalar-tensor theory are also presented.
[Show abstract][Hide abstract] ABSTRACT: We study the evolution of a massive scalar field surrounding a Schwarzschild
black hole and find configurations that can survive for arbitrarily long times,
provided the black hole or the scalar field mass is small enough. In
particular, both ultra-light scalar field dark matter around supermassive black
holes and axion-like scalar fields around primordial black holes can survive
for cosmological times. Moreover, these results are quite generic, in the sense
that fairly arbitrary initial data evolves, at late times, as a combination of
those long-lived configurations.
[Show abstract][Hide abstract] ABSTRACT: During this talk, I will summarize our studies from the determination of the mass of the dark matter halo, based on the observations of the rotation curves of test particles or of the gravitational lensing. As I will show you, it is not uncommon that some studies on the nature of dark matter include extra assumption, some even on the very nature of the dark matter, what we want to determine!, that bais the studies and the results obtained from the observation and, in some cases, imply an inconsistent system altogether.
[Show abstract][Hide abstract] ABSTRACT: Classical scalar fields have been proposed as possible candidates for the
dark matter component of the universe. Given the fact that super-massive black
holes seem to exist at the center of most galaxies, in order to be a viable
candidate for the dark matter halo a scalar field configuration should be
stable in the presence of a central black hole, or at least be able to survive
for cosmological time-scales. In the present work we consider a scalar field as
a test field on a Schwarzschild background, and study under which conditions
one can obtain long-lived configurations. We present a detailed study of the
Klein-Gordon equation in the Schwarzschild spacetime, both from an analytical
and numerical point of view, and show that indeed there exist quasi-stationary
solutions that can remain surrounding a black hole for large time-scales.
Full-text · Article · Aug 2011 · Physical review D: Particles and fields
[Show abstract][Hide abstract] ABSTRACT: There are two observations of galaxies that can offer some insight into the nature of the dark matter (DM), namely the rotation curves and the gravitational lensing. While the first one can be studied using the Newtonian limit, the second one is completely relativistic. Each one separately can not determine the nature of DM, but both together give us key information about this open problem. In this work we use a static and spherically symmetric metric to model the DM halo in a galaxy or in a galaxy cluster. The metric contains two free functions, one associated with the distribution of mass and the other one with the gravitational potential. We use galactic, typical rotation curves to univocally determine the kinematics of the halos. We compute separately the mass functions for a perfect fluid and a scalar field, and demonstrate that both models can be fitted to the observations, though with different masses. We then employ lensing to discriminate between these models. This procedure represents a test of models using two measurements: rotation curves and lensing. Comment: 5 pages, 2 figures, accepted in Phys. Rev. D
Full-text · Article · Jun 2010 · Physical review D: Particles and fields
[Show abstract][Hide abstract] ABSTRACT: The main point of this work is to emphasize the fact that the estimation of the mass content of a dark matter halo must be done in a consistent way with the nature of the dark matter. In general relativity, depending on how dark matter nature is modelled, different expressions arise for the computation of the mass and the deflection of light. Here we present the comparison of the mass estimated by assuming two different dark matter models, the first one in which dark matter is modelled by a perfect fluid and the second in which it is modelled by a scalar field, but assuming they both reproduce the same rotation curve and discuss the consequences for the lensing observations.
No preview · Article · Jun 2010 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: We show that the phenomenon of spontaneous scalarization predicted in neutron
stars within the framework of scalar-tensor tensor theories of gravity, also
takes place in boson stars without including a self-interaction term for the
boson field (other than the mass term), contrary to what was claimed before.
The analysis is performed in the physical (Jordan) frame and is based on a 3+1
decomposition of spacetime assuming spherical symmetry.
Full-text · Article · Mar 2010 · Physical review D: Particles and fields
[Show abstract][Hide abstract] ABSTRACT: The perturbation equation in a Kerr background is written as a coupled system of one dimensional equations for the different modes in the time domain. Numerical simulations show that the dominant mode in the gravitational response is the one corresponding to the mode of the initial perturbation, allowing us to conjecture that the coupling among the modes has a weak influence in our system of equations. We conclude that by neglecting the coupling terms it can be obtained a one dimensional harmonic equation which indeed describes with good accuracy the gravitational response from the Kerr black hole with low spin, while only few couplings are necessary to describe a high spin one. This result may help to understand the structure of test fields in a Kerr background and even to generate accurate waveforms for various cases in an efficient manner. Comment: 14 pages, 3 figures
Full-text · Article · Feb 2010 · Physical review D: Particles and fields
[Show abstract][Hide abstract] ABSTRACT: We build a spherical halo model for galaxies using a general scalar-tensor theory of gravity in its Newtonian limit. The scalar field is described by a time-independent Klein-Gordon equation with a source that is coupled to the standard Poisson equation of Newtonian gravity. Our model, by construction, fits both the observed rotation velocities of stars in spirals and a typical luminosity profile. As a result, the form of the new Newtonian potential, the scalar field, and dark matter distribution in a galaxy are determined. Taking into account the constraints for the fundamental parameters of the theory (lambda,alpha), we analyze the influence of the scalar field in the dark matter distribution, resulting in shallow density profiles in galactic centers.
Full-text · Article · Mar 2009 · Physical review D: Particles and fields
[Show abstract][Hide abstract] ABSTRACT: Analysis of the radio-metric data from Pioneer 10 and 11 spacecrafts has indicated the presence of an unmodeled acceleration starting at 20 AU, which has become known as the Pioneer anomaly. The nature of this acceleration is uncertain. In this paper we give a description of the effect and review some relevant mechanisms proposed to explain the observed anomaly. We also discuss on some future projects to investigate this phenomenon.
[Show abstract][Hide abstract] ABSTRACT: We present expressions for the energy, linear momentum and angular momentum carried away from an isolated system by gravitational
radiation based on spin-weighted spherical harmonics decomposition of the Weyl scalar Ψ
4. We also show that the expressions derived are equivalent to the common expressions obtained when using a framework based
on perturbations of a Schwazschild background. The main idea is to collect together all the different expressions in a uniform
and consistent way. The formulae presented here are directly applicable to the calculation of the radiated energy, linear
momentum and angular momentum starting from the gravitational waveforms which are typically extracted from numerical simulations.
Full-text · Article · Aug 2008 · General Relativity and Gravitation
[Show abstract][Hide abstract] ABSTRACT: We derive an expression for the entropy of a dark matter halo described using a Navarro-Frenk-White model with a core. The comparison of this entropy with that of dark matter in the freeze-out era allows us to constrain the parameter space in mSUGRA models. Moreover, combining these constraints with the ones obtained from the usual abundance criterion and demanding that these criteria be consistent with the 2σ bounds for the abundance of dark matter: 0.112≤ΩDMh2≤0.122, we are able to clearly identify validity regions among the values of tanβ, which is one of the parameters of the mSUGRA model. We found that for the regions of the parameter space explored, small values of tanβ are not favored; only for are the two criteria significantly consistent. In the region where the two criteria are consistent we also found a lower bound for the neutralino mass, mχ≥141 GeV.
Preview · Article · May 2008 · Journal of Cosmology and Astroparticle Physics
[Show abstract][Hide abstract] ABSTRACT: We computed flat rotation curves from scalar-tensor theories in their weak field limit. Our model, by construction, fits a flat rotation profile for velocities of stars. As a result, the form of the scalar field potential and DM distribution in a galaxy are determined. By taking into account the constraints for the fundamental parameters of the theory $(\lambda, \alpha)$, it is possible to obtain analytical results for the density profiles. For positive and negative values of $\alpha$, the DM matter profile is as cuspy as NFW's.
Full-text · Article · Aug 2007 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: Several interesting astrophysical phenomena are symmetric with respect to the rotation axis, like the head-on collision of compact bodies, the collapse and/or accretion of fields with a large variety of geometries, or some forms of gravitational waves. Most current numerical relativity codes, however, can not take advantage of these symmetries due to the fact that singularities in the adapted coordinates, either at the origin or at the axis of symmetry, rapidly cause the simulation to crash. Because of this regularity problem it has become common practice to use full-blown Cartesian three-dimensional codes to simulate axi-symmetric systems. In this work we follow a recent idea idea of Rinne and Stewart and present a simple procedure to regularize the equations both in spherical and axi-symmetric spaces. We explicitly show the regularity of the evolution equations, describe the corresponding numerical code, and present several examples clearly showing the regularity of our evolutions.
Full-text · Article · Jul 2007 · General Relativity and Gravitation
[Show abstract][Hide abstract] ABSTRACT: We explore the use of two criteria to constrain the allowed parameter space in mSUGRA models; both criteria are based in the calculation of the present density of neutralinos chi0 as Dark Matter in the Universe. The first one is the usual ``abundance'' criterion that requieres that present neutralino relic density complies with 0.0945 < OmegaCDMh2 < 0.1287, which are the 2sigma bounds according to WMAP. To calculate the relic density we use the public numerical code micrOMEGAS. The second criterion is the original idea presented in  that basically applies the microcanonical definition of entropy to a weakly interacting and self-gravitating gas, and then evaluate the change in entropy per particle of this gas between the freeze-out era and present day virialized structures. An ``entropy consistency'' criterion emerges by comparing theoretical and empirical estimates of this entropy. One of the objetives of the work is to analyze the joint application of both criteria, already done in , to see if their results, using approximations for the calculations of the relic density, agree with the results coming from the exact numerical results of micrOMEGAS. The main objetive of the work is to use this method to constrain the parameter space in mSUGRA models that are inputs for the calculations of micrOMEGAS, and thus to get some bounds on the predictions for the SUSY spectra.