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

ℓ-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 ℓ =(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 ℓ = 0the standard spherically symmetric boson stars are recovered. We determine their generalizations for ℓ ≤ 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 Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Núñez, Darío. Universidad Nacional Autónoma de México. Instituto de Ciencias Nucleares; MéxicoFil: Sarbach, Olivier. Universidad Michoacana de San Nicolás de Hidalgo; Méxic

Dynamical evolutions of ℓ-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 Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Núñez, Darío. 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 $\mu$ 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 $a$ extremely close to 1. Among others, the paper derives a model problem for the equation which supports the instability of the field down to $a/M \approx 0.97$.Comment: Updated version, after minor change