54,233 research outputs found
Exotic Compact Objects and How to Quench their Ergoregion Instability
Gravitational-wave astronomy can give us access to the structure of black
holes, potentially probing microscopic or even Planckian corrections at the
horizon scale, as those predicted by some quantum-gravity models of exotic
compact objects. A generic feature of these models is the replacement of the
horizon by a reflective surface. Objects with these properties are prone to the
so-called ergoregion instability when they spin sufficiently fast. We
investigate in detail a simple model consisting of scalar perturbations of a
Kerr geometry with a reflective surface near the horizon. The instability
depends on the spin, on the compactness, and on the reflectivity at the
surface. The instability time scale increases only logarithmically in the
black-hole limit and, for a perfectly reflecting object, this is not enough to
prevent the instability from occurring on dynamical time scales. However, we
find that an absorption rate at the surface as small as 0.4% (reflectivity
coefficient as large as ) is sufficient to quench the
instability completely. Our results suggest that exotic compact objects are not
necessarily ruled out by the ergoregion instability.Comment: v3: 14 pages, 9 figures; further clarifications added, new appendix
on the superspinar case, results unchanged. Accepted in Phys. Rev.
An instability of unitary quantum dynamics
Instabilities of equilibrium quantum mechanics are common and
well-understood. They are manifested for example in phase transitions, where a
quantum system becomes so sensitive to perturbations that a symmetry can be
spontaneously broken. Here, we consider the possibility that the time evolution
governing quantum dynamics may be similarly subject to an instability, at which
its unitarity spontaneously breaks down owing to an extreme sensitivity towards
perturbations. We find that indeed such an instability exists, and we explore
its immediate consequences. Interpretations of the results both in terms of
extreme sensitivity to the influence of environmental degrees of freedom, and
in terms of a possible fundamental violation of unitarity are discussed.Comment: 11 pages, 2 figures; Conference proceedings DICE 201
Casimir repulsion between metallic objects in vacuum
We give an example of a geometry in which two metallic objects in vacuum
experience a repulsive Casimir force. The geometry consists of an elongated
metal particle centered above a metal plate with a hole. We prove that this
geometry has a repulsive regime using a symmetry argument and confirm it with
numerical calculations for both perfect and realistic metals. The system does
not support stable levitation, as the particle is unstable to displacements
away from the symmetry axis.Comment: 4 pages, 4 figures; added references, replaced Fig.
The Lagrangian description of aperiodic flows: a case study of the Kuroshio Current
This article reviews several recently developed Lagrangian tools and shows
how their combined use succeeds in obtaining a detailed description of purely
advective transport events in general aperiodic flows. In particular, because
of the climate impact of ocean transport processes, we illustrate a 2D
application on altimeter data sets over the area of the Kuroshio Current,
although the proposed techniques are general and applicable to arbitrary time
dependent aperiodic flows. The first challenge for describing transport in
aperiodical time dependent flows is obtaining a representation of the phase
portrait where the most relevant dynamical features may be identified. This
representation is accomplished by using global Lagrangian descriptors that when
applied for instance to the altimeter data sets retrieve over the ocean surface
a phase portrait where the geometry of interconnected dynamical systems is
visible. The phase portrait picture is essential because it evinces which
transport routes are acting on the whole flow. Once these routes are roughly
recognised it is possible to complete a detailed description by the direct
computation of the finite time stable and unstable manifolds of special
hyperbolic trajectories that act as organising centres of the flow.Comment: 40 pages, 24 figure
Dynamics of the Innermost Accretion Flows Around Compact Objects: Magnetosphere-Disc Interface, Global Oscillations and Instabilities
We study global non-axisymmetric oscillation modes and instabilities in
magnetosphere- disc systems, as expected in neutron star X-ray binaries and
possibly also in accreting black hole systems. Our two-dimensional
magnetosphere-disc model consists of a Keplerian disc in contact with an
uniformly rotating magnetosphere with low plasma density. Two types of global
overstable modes exist in such systems, the interface modes and the disc
inertial-acoustic modes. We examine various physical effects and parameters
that influence the properties of these oscillation modes, particularly their
growth rates, including the magnetosphere field configuration, the velocity and
density contrasts across the magnetosphere-disc interface, the rotation profile
(with Newtonian or General Relativistic potential), the sound speed and
magnetic field of the disc. The interface modes are driven unstable by
Rayleigh-Taylor and Kelvin-Helmholtz in- stabilities, but can be stabilized by
the toroidal field (through magnetic tension) and disc differential rotation
(through finite vorticity). General relativity increases their growth rates by
modifying the disc vorticity outside the magnetosphere boundary. The interface
modes may also be affected by wave absorption associated with corotation
resonance in the disc. In the presence of a magnetosphere, the
inertial-acoustic modes are effectively trapped at the innermost region of the
relativistic disc just outside the interface. They are driven unstable by wave
absorption at the corotation resonance, but can be stabilized by modest disc
magnetic fields. The overstable oscillation modes studied in this paper have
characteristic properties that make them possible candidates for the
quasi-periodic oscillations observed in X-ray binaries.Comment: 18 pages, 9 figures, MNRAS accepte
Dynamics of a linear magnetic "microswimmer molecule"
In analogy to nanoscopic molecules that are composed of individual atoms, we
consider an active "microswimmer molecule". It is built up from three
individual magnetic colloidal microswimmers that are connected by harmonic
springs and hydrodynamically interact with each other. In the ground state,
they form a linear straight molecule. We analyze the relaxation dynamics for
perturbations of this straight configuration. As a central result, with
increasing self-propulsion, we observe an oscillatory instability in accord
with a subcritical Hopf bifurcation scenario. It is accompanied by a
corkscrew-like swimming trajectory of increasing radius. Our results can be
tested experimentally, using for instance magnetic self-propelled Janus
particles, supposably linked by DNA molecules.Comment: 6 pages, 8 figure
Observable Properties of Orbits in Exact Bumpy Spacetimes
We explore the properties of test-particle orbits in "bumpy" spacetimes -
stationary, reflection-symmetric, asymptotically flat solutions of Einstein
equations that have a non-Kerr (anomalous) higher-order multipole-moment
structure but can be tuned arbitrarily close to the Kerr metric. Future
detectors should observe gravitational waves generated during inspirals of
compact objects into supermassive central bodies. If the central body deviates
from the Kerr metric, this will manifest itself in the emitted waves. Here, we
explore some of the features of orbits in non-Kerr spacetimes that might lead
to observable signatures. As a basis for this analysis, we use a family of
exact solutions proposed by Manko & Novikov which deviate from the Kerr metric
in the quadrupole and higher moments, but we also compare our results to other
work in the literature. We examine isolating integrals of the orbits and find
that the majority of geodesic orbits have an approximate fourth constant of the
motion (in addition to the energy, angular momentum and rest mass) and the
resulting orbits are tri-periodic to high precision. We also find that this
fourth integral can be lost for certain orbits in some oblately deformed
Manko-Novikov spacetimes. However, compact objects will probably not end up on
these chaotic orbits in nature. We compute the location of the innermost stable
circular orbit (ISCO) and find that the behavior of orbtis near the ISCO can be
qualitatively different depending on whether the ISCO is determined by the
onset of an instability in the radial or vertical direction. Finally, we
compute periapsis and orbital-plane precessions for nearly circular and nearly
equatorial orbits in both the strong and weak field, and discuss weak-field
precessions for eccentric equatorial orbits.Comment: 42 pages, 20 figures, accepted by Phys. Rev. D, v2 has minor changes
to make it consistent with published versio
Anisotropic stars as ultracompact objects in General Relativity
Anisotropic stresses are ubiquitous in nature, but their modeling in General
Relativity is poorly understood and frame dependent. We introduce the first
study on the dynamical properties of anisotropic self-gravitating fluids in a
covariant framework. Our description is particularly useful in the context of
tests of the black hole paradigm, wherein ultracompact objects are used as
black hole mimickers but otherwise lack a proper theoretical framework. We show
that: (i) anisotropic stars can be as compact and as massive as black holes,
even for very small anisotropy parameters; (ii) the nonlinear dynamics of the
1+1 system is in good agreement with linearized calculations, and shows that
configurations below the maximum mass are nonlinearly stable; (iii) strongly
anisotropic stars have vanishing tidal Love numbers in the black-hole limit;
(iv) their formation will usually be accompanied by gravitational-wave echoes
at late times.Comment: 7+2 pages, 6 figures; v2: include extra material (general covariant
framework for anisotropic fluids in General Relativity without symmetries and
code validation); to appear in PR
Deterministic Chaos in Quantum Field Theory
We discuss the necessity and demonstrate the validity of introduction the
notion of deterministic chaos in quantum field theory. Brief review of the
existing approaches to this problem is given. We compare proposed chaos
criterion for quantum fields with existing ones. Its consequences in particle
physics are also discussed
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