86 research outputs found
Gravitational Waves in Gravity: Scalar Waves and the Chameleon Mechanism
We discuss the scalar mode of gravitational waves emerging in the context of
gravity by taking into account the chameleon mechanism. Assuming a toy
model with a specific matter distribution to reproduce the environment of
detection experiment by a ground-based gravitational wave observatory, we find
that chameleon mechanism remarkably suppresses the scalar wave in the
atmosphere of Earth, compared with the tensor modes of the gravitational waves.
We also discuss the possibility to detect and constrain scalar waves by the
current gravitational observatories and advocate a necessity of the future
space-based observations.Comment: 15 pages, 5 figures, accepted version in Physical Review
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
Axionic instabilities and new black hole solutions
The coupling between scalar and vector fields has a long and interesting
history. Axions are one key possibility to solve the strong CP problem and
axion-like particles could be one solution to the dark matter puzzle. Given the
nature of the coupling, and the universality of free fall, nontrivial important
effects are expected in regions where gravity is strong. Here, we show that i.
A background EM field induces an axionic instability in flat space, for large
enough electric fields. Conversely, a homogeneous harmonic axion field induces
an instability in the Maxwell sector. When carried over to curved spacetime,
this phenomena translates into generic instabilities of charged black holes
(BHs). ii. In the presence of charge, BH uniqueness results are lost. We find
solutions which are small deformations of the Kerr-Newman geometry and hairy
stationary solutions without angular momentum, which are `dragged' by the
axion. Axion fields must exist around spinning BHs if these are immersed in
external magnetic fields. The axion profile can be obtained perturbatively from
the electro-vacuum solution derived by Wald. iii. Ultralight axions trigger
superradiant instabilities of spinning BHs and form an axionic cloud in the
exterior geometry. The superradiant growth can be interrupted or suppressed
through axionic or scalar couplings to EM. These couplings lead to periodic
bursts of light, which occur throughout the history of energy extraction from
the BH. We provide numerical and simple analytical estimates for the rates of
these processes. iv. Finally, we discuss how plasma effects can affect the
evolution of superradiant instabilities.Comment: 28 pages, RevTeX4. v2: overall improvements, typos corrected; version
to appear in Physical Review
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