1,421 research outputs found
Partially massless gravitons do not destroy general relativity black holes
Recent nonlinear completions of Fierz-Pauli theory for a massive spin-2 field
include nonlinear massive gravity and bimetric theories. The spectrum of
black-hole solutions in these theories is rich, and comprises the same vacuum
solutions of Einstein's gravity enlarged to include a cosmological constant. It
was recently shown that Schwarzschild (de Sitter) black holes in these theories
are generically unstable against spherical perturbations. Here we show that a
notable exception is partially massless gravity, where the mass of the graviton
is fixed in terms of the cosmological constant by \mu^2=2\Lambda/3 and a new
gauge invariance emerges. We find that general-relativity black holes are
stable in this limit. Remarkably, the spectrum of massive gravitational
perturbations is isospectral.Comment: 5 pages, 1 figure; v2: matched to published version; v3: references
adde
Black hole superradiant instability from ultralight spin-2 fields
Ultralight bosonic fields are compelling dark-matter candidates and arise in
a variety of beyond-Standard-Model scenarios. These fields can tap energy and
angular momentum from spinning black holes through superradiant instabilities,
during which a macroscopic bosonic condensate develops around the black hole.
Striking features of this phenomenon include gaps in the spin-mass distribution
of astrophysical black holes and a continuous gravitational-wave~(GW) signal
emitted by the condensate. So far these processes have been studied in great
detail for scalar fields and, more recently, for vector fields. Here we take an
important step forward in the black-hole superradiance program by computing,
analytically, the instability time scale, the direct GW emission, and the
stochastic background, in the case of massive tensor (i.e., spin-) fields.
Our analysis is valid for any black hole spin and for small boson masses. The
instability of massive spin- fields shares some properties with the scalar
and vector cases, but its phenomenology is much richer, for example there exist
multiple modes with comparable instability time scales, and the dominant GW
signal is hexadecapolar rather than quadrupolar. Electromagnetic and GW
observations of spinning black holes in the mass range
can constrain the mass of a putative spin- field in the range . For , the space mission LISA could detect the continuous
GW signal for sources at redshift , or even larger.Comment: 13 pages, 4 Figures; v3: minor changes to match published version in
Physical Review Letter
Electromagnetic emission from axionic clouds and the quenching of superradiant instabilities
The nature of dark matter is one of the longest-standing puzzles in science.
Axions or axion-like particles are a key possibility, and arise in mechanisms
to solve the strong CP problem but also in low-energy limits of string theory.
Extensive experimental and observational efforts are actively looking for
`axionic' imprints. Independently on their nature, their abundance, and on
their contribution to the dark matter problem, axions form dense clouds around
spinning black holes, grown by superradiant mechanisms. It was recently
suggested that once couplings to photons are considered, an exponential
(quantum) stimulated emission of photons ensues at large enough axion number.
Here we solve numerically the classical problem in different setups. We show
that laser-like emission from clouds exists at the classical level, and we
provide the first quantitative description of the problem.Comment: 6 pages, RevTex4. v2: Overall improvement. Accepted for publication
in Physical Review Letter
Massive spin-2 fields on black hole spacetimes: Instability of the Schwarzschild and Kerr solutions and bounds on the graviton mass
Massive bosonic fields of arbitrary spin are predicted by general extensions
of the Standard Model. It has been recently shown that there exists a family of
bimetric theories of gravity - including massive gravity - which are free of
Boulware-Deser ghosts at the nonlinear level. This opens up the possibility to
describe consistently the dynamics of massive spin-2 particles in a
gravitational field. Within this context, we develop the study of massive
spin-2 fluctuations - including massive gravitons - around Schwarzschild and
slowly-rotating Kerr black holes. Our work has two important outcomes. First,
we show that the Schwarzschild geometry is linearly unstable for small tensor
masses, against a spherically symmetric mode. Second, we provide solid evidence
that the Kerr geometry is also generically unstable, both against the spherical
mode and against long-lived superradiant modes. In the absence of nonlinear
effects, the observation of spinning black holes bounds the graviton mass to be
smaller than 5x10^{-23} eV.Comment: 24 pages, 8 figures. v2: references added and discussion extended.
v3: references added, matches published versio
Linear stability of nonbidiagonal black holes in massive gravity
We consider generic linear perturbations of a nonbidiagonal class of static
black-hole solutions in massive (bi)gravity. We show that the quasinormal
spectrum of these solutions coincides with that of a Schwarzschild black hole
in general relativity, thus proving that these solutions are mode stable. This
is in contrast to the case of bidiagonal black-hole solutions which are
affected by a radial instability. On the other hand, the full set of
perturbation equations is generically richer than that of a Schwarzschild black
hole in general relativity, and this affects the linear response of the black
hole to external perturbations. Finally, we argue that the generalization of
these solutions to the spinning case does not suffer from the superradiant
instability, despite the fact that the theory describes a massive graviton.Comment: 13 pages, 2 figures; v2: References added; v3: matches published
version; v4: acknowledgement adde
Superradiance -- the 2020 Edition
Superradiance is a radiation enhancement process that involves dissipative
systems. With a 60 year-old history, superradiance has played a prominent role
in optics, quantum mechanics and especially in relativity and astrophysics. In
General Relativity, black-hole superradiance is permitted by the ergoregion,
that allows for energy, charge and angular momentum extraction from the vacuum,
even at the classical level. Stability of the spacetime is enforced by the
event horizon, where negative energy-states are dumped. Black-hole
superradiance is intimately connected to the black-hole area theorem, Penrose
process, tidal forces, and even Hawking radiation, which can be interpreted as
a quantum version of black-hole superradiance. Various mechanisms (as diverse
as massive fields, magnetic fields, anti-de Sitter boundaries, nonlinear
interactions, etc...) can confine the amplified radiation and give rise to
strong instabilities. These "black-hole bombs" have applications in searches of
dark matter and of physics beyond the Standard Model, are associated to the
threshold of formation of new black hole solutions that evade the no-hair
theorems, can be studied in the laboratory by devising analog models of
gravity, and might even provide a holographic description of spontaneous
symmetry breaking and superfluidity through the gauge-gravity duality.
This work is meant to provide a unified picture of this multifaceted subject.
We focus on the recent developments in the field, and work out a number of
novel examples and applications, ranging from fundamental physics to
astrophysics.Comment: 279 pages. Second Edition of the "Lecture Notes in Physics" book by
Springer-Verlag. Overall improvement, typos and incorrect statements of
Edition 1 are now corrected; new sections were added, reflecting activity in
the field. Bounds on ultralight fields are summarized in Table 4, and updated
online regularly at https://centra.tecnico.ulisboa.pt/network/grit/ and
https://web.uniroma1.it/gmunu
Accretion of dark matter by stars
Searches for dark matter imprints are one of the most active areas of current
research. We focus here on light fields with mass , such as axions and
axion-like candidates. Using perturbative techniques and full-blown nonlinear
Numerical Relativity methods, we show that (i) dark matter can pile up in the
center of stars, leading to configurations and geometries oscillating with
frequency which is a multiple of f= /eV Hz. These
configurations are stable throughout most of the parameter space, and arise out
of credible mechanisms for dark-matter capture. Stars with bosonic cores may
also develop in other theories with effective mass couplings, such as
(massless) scalar-tensor theories. We also show that (ii) collapse of the host
star to a black hole is avoided by efficient gravitational cooling mechanisms.Comment: 5 pages, RevTeX 4. Published in Physical Review Letter
Follow-up signals from superradiant instabilities of black hole merger remnants
Superradiant instabilities can trigger the formation of bosonic clouds around
rotating black holes. If the bosonic field growth is sufficiently fast, these
clouds could form shortly after a binary black hole merger. Such clouds are
continuous sources of gravitational waves whose detection (or lack thereof) can
probe the existence of ultralight bosons (such as axion-like particles) and
their properties. Motivated by the binary black hole mergers seen by Advanced
LIGO so far, we investigate in detail the parameter space that can be probed
with continuous gravitational wave signals from ultralight scalar field clouds
around black hole merger remnants with particular focus on future ground-based
detectors (A+, Voyager and Cosmic Explorer). We also study the impact that the
confusion noise from a putative stochastic gravitational-wave background from
unresolved sources would have on such searches and we estimate, under different
astrophysical priors, the number of binary black-hole merger events that could
lead to an observable post-merger signal. Under our most optimistic
assumptions, Cosmic Explorer could detect dozens of post-merger signals.Comment: v1:10 pages, 8 figures, 2 tables; v2: matches published versio
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