60 research outputs found
Black-hole spectroscopy: quasinormal modes, ringdown stability and the pseudospectrum
Black-hole spectroscopy is a powerful tool to probe the Kerr nature of
astrophysical compact objects and their environment. The observation of
multiple ringdown modes in gravitational waveforms could soon lead to
high-precision gravitational-wave spectroscopy, thus it is critical to
understand if the quasinormal mode spectrum itself is affected by astrophysical
environments, quantum corrections, and other generic modifications. In this
chapter, we will review the black-hole spectroscopy program and its challenges
regarding quasinormal mode detection, the overtone status and the recent
evidence that supports the existence of nonlinearities in the spectrum of black
holes. We will then discuss a newly introduced non-modal tool in black-hole
physics, namely the pseudospectrum; a mathematical notion that can shed light
on the spectral stability of quasinormal modes, and discuss its novel
applications in black holes and exotic horizonless compact objects. We will
show that quasinormal modes generically suffer from spectral instabilities,
explore how such phenomena can further affect black-hole spectroscopy, and
discuss potential ringdown imprints and waveform stability issues in current
and future gravitational-wave detectors.Comment: 35 pages, 24 figures, Topical review presented on the 11th Aegean
Summer School, Syros, Greece, References adde
Διεπιφάνειες τοπολογικού μονωτή με κύματα πυκνότητας φορτίου, σπιν και υπεραγώγιμες καταστάσεις τάξης
Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Μικροσυστήματα και Νανοδιατάξεις
Slowly-rotating compact objects: the nonintegrability of Hartle-Thorne particle geodesics
X-ray astronomy provides information regarding the electromagnetic emission
of active galactic nuclei and X-ray binaries. These events provide details
regarding the astrophysical environment of black holes and stars, and help us
understand gamma-ray bursts. They produce estimates for the maximum mass of
neutron stars and eventually will contribute to the discovery of their equation
of state. Thus, it is crucial to study these configurations to increase the
yield of X-ray astronomy when combined with multimessenger gravitational-wave
astrophysics and black hole shadows. Unfortunately, an exact solution of the
field equations does not exist for neutron stars. Nevertheless, there exist a
variety of approximate compact objects that may characterize massive or neutron
stars. The most studied approximation is the Hartle-Thorne metric that
represents slowly-rotating compact objects, like massive stars, white dwarfs
and neutron stars. Recent investigations of photon orbits and shadows of such
metric revealed that it exhibits chaos close to resonances. Here, we thoroughly
investigate particle orbits around the Hartle-Thorne spacetime. We perform an
exhaustive analysis of bound motion, by varying all parameters involved in the
system. We demonstrate that chaotic regions, known as Birkhoff islands, form
around resonances, where the ratio of the radial and polar frequency of
geodesics, known as the rotation number, is shared throughout the island. This
leads to the formation of plateaus in rotation curves during the most prominent
resonance, which designate nonintegrability. We measure their width and
show how each parameter affects it. The nonintegrability of Hartle-Thorne
metric may affect quasiperiodic oscillations of low-mass X-ray binaries, when
chaos is taken into account, and improve estimates of mass, angular momentum
and multipole moments of astrophysical compact objects.Comment: 12 pages, 5 figure
Quasinormal mode (in)stability and strong cosmic censorship
Recent studies have shown that quasinormal modes suffer from spectral
instabilities, a frailty of black holes that leads to disproportional migration
of their spectra in the complex plane when black-hole effective potentials are
modified by minuscule perturbations. Similar results have been found with the
mathematical notion of the pseudospectrum which was recently introduced in
gravitational physics. Environmental effects, such as the addition of a thin
accretion disk or a matter shell, lead to a secondary bump that appears in the
effective potential of black hole perturbations. Regardless of the
environment's small contribution to the effective potential, its presence can
completely destabilize the fundamental quasinormal mode and may potentially
affect black hole spectroscopy. Here, we perform a comprehensive analysis of
such phenomenon for Schwarzschild, Reissner-Nordstr\"om, Schwarzschild-de
Sitter, and Reissner-Nordstr\"om-de Sitter black holes by considering the
potential for a test scalar field with the addition of a tiny bump sufficiently
away from the photon sphere. We find a qualitatively similar destabilization
pattern for photon sphere, complex, scalar quasinormal modes in all cases, and
a surprising spectral stability for dominant scalar, purely imaginary, de
Sitter and near-extremal modes that belong to different families of the
spectrum. For Reissner-Nordstr\"om-de Sitter black holes, we re-evaluate the
validity of the strong cosmic censorship and find that the addition of a
realistic bump in the effective potential cannot prevent its violation due to a
combination of the spectral stability of dominant de Sitter and near-extremal
modes for small cosmological constants and an ineffective migration of the
photon sphere modes that dominate the late-time ringdown signal for
sufficiently large cosmological constants.Comment: 16 pages, 12 figure
Geodesics and gravitational waves in chaotic extreme-mass-ratio inspirals: the curious case of Zipoy-Voorhees black-hole mimickers
Due to the growing capacity of gravitational-wave astronomy and black-hole
imaging, we will soon be able to emphatically decide if astrophysical objects
lurking in galactic centers are black holes. Sgr A*, one of the most prolific
astronomical radio sources in our galaxy, is the focal point for tests of
general relativity. Current mass and spin constraints predict that the central
object of the Milky Way is supermassive and slowly rotating, thus can be
conservatively modeled as a Schwarzschild black hole. The well-established
presence of accretion disks and astrophysical environments around supermassive
objects can deform their geometry and complicate their observational scientific
yield. Here, we study extreme-mass-ratio binaries comprised of a minuscule
secondary object inspiraling onto a supermassive Zipoy-Voorhees compact object;
the simplest exact solution of general relativity that describes a static,
spheroidal deformation of Schwarzschild spacetime. We examine geodesics of
prolate and oblate deformations for generic orbits and reevaluate the
non-integrability of Zipoy-Voorhees spacetime through the existence of resonant
islands in the orbital phase space. By including radiation loss with
post-Newtonian techniques, we evolve stellar-mass secondary objects around a
supermassive Zipoy-Voorhees primary and find clear imprints of
non-integrability in these systems. The peculiar structure of the primary,
allows for, not only typical single crossings of transient resonant islands,
that are well-known for non-Kerr objects, but also inspirals that traverse
through several islands, in a brief period of time, that lead to multiple
glitches in the gravitational-wave frequency evolution of the binary. The
detectability of glitches with future spaceborne detectors can, therefore,
narrow down the parameter space of exotic solutions that, otherwise, can cast
identical shadows with black holes.Comment: 16 pages, 7 figures, minor revision, accepted for publication in
General Relativity and Gravitation, abstract minimally trimmed due to arxiv
limitation
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