40 research outputs found
Charged fermions and strong cosmic censorship
No full textIt was recently shown that the Strong Cosmic Censorship conjecture might be violated for near-extremally-charged black holes in de Sitter space. Here, we extend our study to charged fermionic fields in the exterior of Reissner-Nordstrom-de Sitter black holes. We identify three families of modes; one related to the photon sphere, a second related to the de Sitter horizon and a third which dominates near extremality. We show that for near-extremally-charged black holes there is a critical fermionic charge below which Strong Cosmic Censorship may potentially be violated. Surprisingly enough, as one approaches extremality even more, violation of Strong Cosmic Censorship may occur even beyond the critical fermionic charge. (C) 2019 The Author. Published by Elsevier B.V
Gravitational waves from extreme-mass-ratio systems in astrophysical environments
No full textWe establish a generic, fully-relativistic formalism to study
gravitational-wave emission by extreme-mass-ratio systems in
spherically-symmetric, non-vacuum black-hole spacetimes. The potential
applications to astrophysical setups range from black holes accreting baryonic
matter to those within axionic clouds and dark matter environments, allowing to
assess the impact of the galactic potential, of accretion, gravitational drag
and halo feedback on the generation and propagation of gravitational-waves. We
apply our methods to a black hole within a halo of matter. We find fluid modes
imparted to the gravitational-wave signal (a clear evidence of the black hole
fundamental mode instability) and the tantalizing possibility to infer galactic
properties from gravitational-wave measurements by sensitive, low-frequency
detectors.Comment: v2. Supplemental Material added, minor improvements. Accepted for
publication in Physical Review Letter
Black holes in galaxies: environmental impact on gravitational-wave generation and propagation
No full textWe introduce a family of solutions of Einstein's gravity minimally coupled to an anisotropic fluid, describing asymptotically flat black holes with "hair"and a regular horizon. These spacetimes can describe the geometry of galaxies harboring supermassive black holes, and are extensions of Einstein clusters to include horizons. They are useful to constrain the environment surrounding astrophysical black holes, using electromagnetic or gravitational-wave observations. We compute the main properties of the geometry, including the corrections to the ringdown stage induced by the external matter and fluxes by orbiting particles. The leading order effect to these corrections is a gravitational-redshift, but gravitational-wave propagation is affected by the galactic potential in a nontrivial way, and may be characterized with future observatories.</p
Destabilizing the fundamental mode of black holes: the elephant and the flea
No full textRecent work applying the notion of pseudospectrum to gravitational physics showed that the quasinormal mode spectrum of black holes is unstable, with the possible exception of the longest-lived (fundamental) mode. The fundamental mode dominates the expected signal in gravitational wave astronomy, and there is no reason why it should have privileged status. We compute the quasinormal mode spectrum of two model problems where the Schwarzschild potential is perturbed by a small "bump"consisting of either a Pöschl-Teller potential or a Gaussian, and we show that the fundamental mode is destabilized under generic perturbations. We present phase diagrams and study a simple double-barrier toy problem to clarify the conditions under which the spectral instability occurs.</p
Gravitational-wave imprints of compact and galactic-scale environments in extreme-mass-ratio binaries
No full textCircumambient and galactic-scale environments are intermittently present
around black holes that reside in active galactic nuclei. As supermassive black
holes impart energy on their host galaxy, so the galactic environment affects
the dynamics of solar-mass objects around black holes and the gravitational
waves emitted from non-vacuum asymmetric binaries. Only recently an exact
general-relativistic solution has been found that describes a Schwarzschild
black hole immersed in a dark matter halo of the Hernquist type. We perform an
extensive analysis of generic geodesics delving in such non-vacuum spacetimes
and compare our results with those obtained in Schwarzschild, as well as
calculate their gravitational-wave emission. Our findings indicate that the
radial and polar oscillation frequency ratios descend deeper into the strong
gravity region as the compactness of the halo increases. This translates to a
redshift of non-vacuum geodesics and their resulting waveforms with respect to
the vacuum ones. We calculate the overlap between waveforms resulting from
Schwarzschild and non-vacuum geometries and find that it decreases as the halo
compactness grows, meaning that dark matter environments should be
distinguishable by space-borne detectors. For compact environments, we find
that the apsidal precession is strongly affected due to the gravitational pull
of dark matter; the orbit's axis can rotate in the opposite direction as that
of the orbital motion, leading to a retrograde precession drift that depends on
the halo mass, as opposed to the typical prograde precession transpiring in
galactic-scale environments. Gravitational waves in retrograde-to-prograde
alterations demonstrate transient frequency phenomena around critical
non-precessing turning points, thus they may serve as `smoking guns' for the
presence of compact dark matter environments around supermassive black holes.Comment: 19 pages, 10 figures, revisions regarding detectability and addition
of new figures and sections, abstract reduced to fit arxiv limits, accepted
for publication in PR
Gravitational-wave glitches in chaotic extreme-mass-ratio inspirals
No full textThe Kerr geometry admits the Carter symmetry, which ensures that the geodesic equations are integrable. It is shown that gravitational waveforms associated with extreme-mass-ratio inspirals involving a nonintegrable compact object display "glitch" phenomena, where the frequencies of gravitational waves increase abruptly, when the orbit crosses certain spacetime regions known as Birkhoff islands. The presence or absence of these features in data from upcoming space-borne detectors will therefore allow not only for tests of general relativity but also of fundamental spacetime symmetries
Testing spacetime symmetry through gravitational waves from extreme-mass-ratio inspirals
No full textOne of the primary aims of upcoming spaceborne gravitational wave detectors is to measure radiation in the mHz range from extreme-mass-ratio inspirals. Such a detection would place strong constraints on hypothetical departures from a Kerr description for astrophysically stable black holes. The Kerr geometry, which is unique in general relativity, admits a higher-order symmetry in the form of a Carter constant, which implies that the equations of motion describing test particle motion in a Kerr background are Liouville-integrable. In this article, we investigate whether the Carter symmetry itself is discernible from a generic deformation of the Kerr metric in the gravitational waveforms for such inspirals. We build on previous studies by constructing a new metric which respects current observational constraints, describes a black hole, and contains two non-Kerr parameters, one of which controls the presence or absence of the Carter symmetry, thereby controlling the existence of chaotic orbits, and another which serves as a generic deformation parameter. We find that these two parameters introduce fundamentally distinct features into the orbital dynamics, and evince themselves in the gravitational waveforms through a significant dephasing. Although only explored in the quadrupole approximation, this, together with a Fisher metric analysis, suggests that gravitational wave data analysis may be able to test, in addition to the governing theory of gravity, the underlying symmetries of spacetime
Strong cosmic censorship in charged black-hole spacetimes: Still subtle
No full textIt was recently shown that strong cosmic censorship may be violated in highly charged black-hole spacetimes living in a universe with a positive cosmological constant. Several follow-up works have since suggested that such a result, while conceptually interesting, cannot be upheld in practice. We focus here on the claim that the presence of charged massive scalars suffices to save strong cosmic censorship. To the contrary, we show that there still exists a finite region in parameter space where strong cosmic censorship is expected to be violated
Quasinormal Modes and Strong Cosmic Censorship
No full textThe fate of Cauchy horizons, such as those found inside charged black holes, is intrinsically connected to the decay of small perturbations exterior to the event horizon. As such, the validity of the strong cosmic censorship (SCC) conjecture is tied to how effectively the exterior damps fluctuations. Here, we study massless scalar fields in the exterior of Reissner-Nordstrm-de Sitter black holes. Their decay rates are governed by quasinormal modes of the black hole. We identify three families of modes in these spacetimes: one directly linked to the photon sphere, well described by standard WKB-type tools; another family whose existence and time scale is closely related to the de Sitter horizon; finally, a third family which dominates for near-extremally charged black holes and which is also present in asymptotically flat spacetimes. The last two families of modes seem to have gone unnoticed in the literature. We give a detailed description of linear scalar perturbations of such black holes, and conjecture that SCC is violated in the near extremal regime
Διεπιφάνειες τοπολογικού μονωτή με κύματα πυκνότητας φορτίου, σπιν και υπεραγώγιμες καταστάσεις τάξης
No full textΕθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Μικροσυστήματα και Νανοδιατάξεις
