22 research outputs found
Scalar field confinement as a model for accreting systems
We investigate the possibility to localize scalar field configurations as a
model for black hole accretion. We analyze and resolve difficulties encountered
when localizing scalar fields in General Relativity. We illustrate this ability
with a simple spherically symmetric model which can be used to study features
of accreting shells around a black hole. This is accomplished by prescribing a
scalar field with a coordinate dependent potential. Numerical solutions to the
Einstein-Klein-Gordon equations are shown, where a scalar filed is indeed
confined within a region surrounding a black hole. The resulting spacetime can
be described in terms of simple harmonic time dependence.Comment: 18 pages; accepted for publication in Classical and Quantum Gravit
Matter sources interacting with a black hole: dynamics and observable signatures
Dynamical systems involving black holes are one of the most promising sources of detectable gravitational waves. Additionally, one expects strong electromagnetic signals whenever matter sources are present. In this dissertation, we study different astrophysical scenarios pertaining the interaction of matter with a black hole. We first investigate the possibility to localize scalar field configurations surrounding a (dynamic) black hole. The analytical study is illustrated by performing numerical simulations that show the evolution of a Klein-Gordon-like scalar field shell surrounding a black hole. Second, we present a method to estimate the gravitational wave frequency at the end of the inspiral phase of a compact binary. This method is applied to study the possibility of a neutron star’s tidal disruption occurring before plunging into the companion black hole, and to provide a way of improving gravitational wave data analysis when using match filtering techniques. Last, we study the effects of a black hole merger on a circumbinary disk. We consider separately the effects of central mass reduction (due to the energy loss through gravitational waves) and black hole recoil (due to asymmetric emission of gravitational radiation), presenting possibly detectable electromagnetic signatures
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
In recent years, numerical simulations with Gaussian initial states have
demonstrated the existence of a quantum bounce in loop quantum cosmology in
various models. A key issue pertaining to the robustness of the bounce and the
associated physics is to understand the quantum evolution for more general
initial states which may depart significantly from Gaussianity and may have no
well defined peakedness properties. The analysis of such states, including
squeezed and highly non-Gaussian states, has been computationally challenging
until now. In this manuscript, we overcome these challenges by using the
Chimera scheme for the spatially flat, homogeneous and isotropic model sourced
with a massless scalar field. We demonstrate that the quantum bounce in this
model occurs even for states which are highly squeezed or are non-Gaussian with
multiple peaks and with little resemblance to semi-classical states. The
existence of the bounce is found to be robust, being independent of the
properties of the states. The evolution of squeezed and non-Gaussian states
turns out to be qualitatively similar to that of Gaussian states, and satisfies
strong constraints on the growth of the relative fluctuations across the
bounce. We also compare the results from the effective dynamics and find that,
although it captures the qualitative aspects of the evolution for squeezed and
highly non-Gaussian states, it always underestimates the bounce volume. We show
that various properties of the evolution, such as the energy density at the
bounce, are in excellent agreement with the predictions from an exactly
solvable loop quantum cosmological model for arbitrary states.Comment: 26 pages, 16 figures. v2: Discussion of the main results expande
Schwarzschild black holes can wear scalar wigs
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.Comment: 5 pages, 3 figures. Accepted for publication in Physical Review
Letter
â„“-boson stars
We present new, fully nonlinear numerical solutions to the static, spherically symmetric Einstein-Klein-Gordon system for a collection of an arbitrary odd number N of complex scalar fields with an internal symmetry and no self-interactions. These solutions, which we dub -boson stars, are parametrized by an angular momentum number â„“ =(N - 1)/2, an excitation number n, and a continuous parameter representing the amplitude of the fields. They are regular at every point and possess a finite total mass. For â„“ = 0the standard spherically symmetric boson stars are recovered. We determine their generalizations for â„“ ≤ 0, and show that they give rise to a large class of new static configurations which might have a much larger compactness ratio than stars.Fil: Alcubierre, Miguel. Universidad Nacional Autónoma de México. Instituto de Ciencias Nucleares; MéxicoFil: Barranco, Juan. Universidad de Guanajuato; MéxicoFil: Bernal, Argelia. Universidad de Guanajuato; MéxicoFil: Degollado, Juan Carlos. Universidad Nacional Autónoma de México; MéxicoFil: Diez Tejedor, Alberto. Universidad de Guanajuato; MéxicoFil: Megevand Politano, Miguel Federico. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Córdoba. Instituto de FÃsica Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de FÃsica Enrique Gaviola; ArgentinaFil: Núñez, DarÃo. Universidad Nacional Autónoma de México. Instituto de Ciencias Nucleares; MéxicoFil: Sarbach, Olivier. Universidad Michoacana de San Nicolás de Hidalgo; Méxic
Dynamical evolutions of â„“-boson stars in spherical symmetry
In previous work, we have found new static, spherically symmetric boson star solutions which generalize the standard boson stars (BSs) by allowing a particular superposition of scalar fields in which each of the fields is characterized by a fixed value of its non-vanishing angular momentum number. We call such solutions 'boson stars'. Here, we perform a series of fully non-linear dynamical simulations of perturbed BSs in order to study their stability, and the final fate of unstable configurations. We show that for each value of, the configuration of maximum mass separates the parameter space into stable and unstable regions. Stable configurations, when perturbed, oscillate around the unperturbed solution and very slowly return to a stationary configuration. Unstable configurations, in contrast, can have three different final states: collapse to a black hole, migration to the stable branch, or explosion (dissipation) to infinity. Just as it happens with BSs, migration to the stable branch or dissipation to infinity depends on the sign of the total binding energy of the star: bound unstable stars collapse to black holes or migrate to the stable branch, whereas unbound unstable stars either collapse to a black hole or explode to infinity. Thus, the parameter allows us to construct a new set of stable configurations. All our simulations are performed in spherical symmetry, leaving a more detailed stability analysis including non-spherical perturbations for future work.Fil: Alcubierre, Miguel. Universidad Nacional Autónoma de México. Instituto de Ciencias Nucleares; MéxicoFil: Barranco, Juan. Universidad de Guanajuato; MéxicoFil: Bernal, Argelia. Universidad de Guanajuato; MéxicoFil: Degollado, Juan Carlos. Universidad Nacional Autónoma de México; MéxicoFil: Diez Tejedor, Alberto. Universidad de Guanajuato; MéxicoFil: Megevand Politano, Miguel Federico. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Córdoba. Instituto de FÃsica Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de FÃsica Enrique Gaviola; ArgentinaFil: Núñez, DarÃo. Universidad Nacional Autónoma de México. Instituto de Ciencias Nucleares; MéxicoFil: Sarbach, Olivier. Universidad Michoacana de San Nicolás de Hidalgo; Méxic
Perturbed disks get shocked. Binary black hole merger effects on accretion disks
The merger process of a binary black hole system can have a strong impact on
a circumbinary disk. In the present work we study the effect of both central
mass reduction (due to the energy loss through gravitational waves) and a
possible black hole recoil (due to asymmetric emission of gravitational
radiation). For the mass reduction case and recoil directed along the disk's
angular momentum, oscillations are induced in the disk which then modulate the
internal energy and bremsstrahlung luminosities. On the other hand, when the
recoil direction has a component orthogonal to the disk's angular momentum, the
disk's dynamics are strongly impacted, giving rise to relativistic shocks. The
shock heating leaves its signature in our proxies for radiation, the total
internal energy and bremsstrahlung luminosity. Interestingly, for cases where
the kick velocity is below the smallest orbital velocity in the disk (a likely
scenario in real AGN), we observe a common, characteristic pattern in the
internal energy of the disk. Variations in kick velocity simply provide a phase
offset in the characteristic pattern implying that observations of such a
signature could yield a measure of the kick velocity through electromagnetic
signals alone.Comment: 10 pages, 13 figures. v2: Minor changes, version to be published in
PR
Are black holes a serious threat to scalar field dark matter models?
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.Comment: 34 pages, 13 figure
Stability study of a model for the Klein-Gordon equation in Kerr spacetime
The current early stage in the investigation of the stability of the Kerr
metric is characterized by the study of appropriate model problems.
Particularly interesting is the problem of the stability of the solutions of
the Klein-Gordon equation, describing the propagation of a scalar field of mass
in the background of a rotating black hole. Rigorous results proof the
stability of the reduced, by separation in the azimuth angle in Boyer-Lindquist
coordinates, field for sufficiently large masses. Some, but not all, numerical
investigations find instability of the reduced field for rotational parameters
extremely close to 1. Among others, the paper derives a model problem for
the equation which supports the instability of the field down to .Comment: Updated version, after minor change