Dario Nunez

Universidad Nacional Autónoma de México, Ciudad de México, The Federal District, Mexico

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Publications (45)60.81 Total impact

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    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.
    10/2014;
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    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.
    09/2014;
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    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.
    12/2013; 89(8).
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    ABSTRACT: We obtain a dark matter equation of state by using the rotation curves of galaxies only. Dark matter is modeled as a fluid component with pressure and we show that, within the Newtonian limit, the resulting equation of state is compatible with existing cosmological bounds. An upper bound on the central pressure of the dark matter is obtained, within the full general relativistic treatment, by demanding that the dark matter fulfills the positive energy condition.
    01/2013;
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    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.
    Physical review D: Particles and fields 07/2012; 86(10).
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    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.
    Physical Review Letters 07/2012; 109(8). · 7.73 Impact Factor
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    ABSTRACT: We summarize our studies on the determination of the mass of the dark matter halo, based on observations of rotation curves of test particles or of the gravitational lensing. As we show, it is not uncommon that some studies on the nature of dark matter include extra assumptions, some even on the very nature of the dark matter, what we want to determine!, and that bias the studies and the results obtained from the observation and, in some cases, imply an inconsistent system altogether.
    11/2011;
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    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.
    Physical review D: Particles and fields 08/2011; 84(8).
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    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
    Physical review D: Particles and fields 06/2010;
  • Alma X González-Morales, Darío Núñez
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    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.
    Journal of Physics Conference Series 06/2010; 229(1):012041.
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    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.
    Physical review D: Particles and fields 03/2010;
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    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
    Physical review D: Particles and fields 02/2010;
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    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.
    Physical review D: Particles and fields 03/2009;
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    Jose A. de Diego, Dario Nunez
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    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.
    08/2008;
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    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.
    General Relativity and Gravitation 08/2008; 40(8):1705-1729. · 1.90 Impact Factor
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    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 2sigma bounds for the abundance of dark matter: 0.112
    Journal of Cosmology and Astroparticle Physics - JCAP. 01/2008; 5(05).
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    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.
    Journal of Physics Conference Series 08/2007;
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    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.
    General Relativity and Gravitation 07/2007; · 1.90 Impact Factor
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    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 [3] 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 [3], 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.
    06/2007;
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    ABSTRACT: Motivated by the possible conflict between the Navarro-Frenk-White(NFW) model predictions for the dark matter contents of galactic systems and its correlation with baryonic surface density, we will explore an alternative paradigm for the description of dark matter halos. Such an alternative emerges from Tsallis' non-extensive thermodynamics applied to self-gravitating systems and leads to the so-called ``stellar polytrope'' (SP) model. We consider that this could be a better approach to real structures rather than the isothermal model, given the fact that the first one takes into account the non-extensivity of energy and entropy present in these type of systems characterized by long-range interactions. We compare a halo based on the Navarro-Frenk-White (NFW) and one which follows the SP description. Analyzing the dark matter contents estimated by means of global physical parameters of galactic disks, obtained from a sample of actual galaxies, with the ones of the unobserved dark matter halos, we conclude that the SP model is favored over the NFW model in such a comparison.
    Journal of Cosmology and Astroparticle Physics 06/2006; · 6.04 Impact Factor

Publication Stats

327 Citations
60.81 Total Impact Points

Institutions

  • 1998–2013
    • Universidad Nacional Autónoma de México
      • Institute of Nuclear Science
      Ciudad de México, The Federal District, Mexico
  • 2012
    • Universidad de Guanajuato
      • Department of Physics
      Ciudad Guanajuato, Guanajuato, Mexico
  • 2006
    • Max Planck Institute for Astrophysics
      Arching, Bavaria, Germany
  • 2002
    • William Penn University
      Worcester, Massachusetts, United States