296 research outputs found

### Wiggly tails: a gravitational wave signature of massive fields around black holes

Massive fields can exist in long-lived configurations around black holes. We
examine how the gravitational wave signal of a perturbed black hole is affected
by such `dirtiness' within linear theory. As a concrete example, we consider
the gravitational radiation emitted by the infall of a massive scalar field
into a Schwarzschild black hole. Whereas part of the scalar field is
absorbed/scattered by the black hole and triggers gravitational wave emission,
another part lingers in long-lived quasi-bound states. Solving numerically the
Teukolsky master equation for gravitational perturbations coupled to the
massive Klein-Gordon equation, we find a characteristic gravitational wave
signal, composed by a quasi-normal ringing followed by a late time tail. In
contrast to `clean' black holes, however, the late time tail contains small
amplitude wiggles with the frequency of the dominating quasi-bound state.
Additionally, an observer dependent beating pattern may also be seen. These
features were already observed in fully non-linear studies; our analysis shows
they are present at linear level, and, since it reduces to a 1+1 dimensional
numerical problem, allows for cleaner numerical data. Moreover, we discuss the
power law of the tail and that it only becomes universal sufficiently far away
from the `dirty' black hole. The wiggly tails, by constrast, are a generic
feature that may be used as a smoking gun for the presence of massive fields
around black holes, either as a linear cloud or as fully non-linear hair.Comment: 6 pages, 4 figure

### Non-linear Q-clouds around Kerr black holes

Q-balls are regular extended `objects' that exist for some non-gravitating,
self-interacting, scalar field theories with a global, continuous, internal
symmetry, on Minkowski spacetime. Here, analogous objects are also shown to
exist around rotating (Kerr) black holes, as non-linear bound states of a test
scalar field. We dub such configurations Q-clouds. We focus on a complex
massive scalar field with quartic plus hexic self-interactions. Without the
self-interactions, linear clouds have been shown to exist, in synchronous
rotation with the black hole horizon, along 1-dimensional subspaces - existence
lines - of the Kerr 2-dimensional parameter space. They are zero modes of the
superradiant instability. Non-linear Q-clouds, on the other hand, are also in
synchronous rotation with the black hole horizon; but they exist on a
2-dimensional subspace, delimited by a minimal horizon angular velocity and by
an appropriate existence line, wherein the non-linear terms become irrelevant
and the Q-cloud reduces to a linear cloud. Thus, Q-clouds provide an example of
scalar bound states around Kerr black holes which, generically, are not zero
modes of the superradiant instability. We describe some physical properties of
Q-clouds, whose backreaction leads to a new family of hairy black holes,
continuously connected to the Kerr family.Comment: 11 pages, 4 figure

### On the interaction between two Kerr black holes

The double-Kerr solution is generated using both a Backlund transformation
and the Belinskii-Zakharov inverse-scattering technique. We build a dictionary
between the parametrisations naturally obtained in the two methods and show
their equivalence. We then focus on the asymptotically flat double-Kerr system
obeying the axis condition which is Z_2^\phi invariant; for this system there
is an exact formula for the force between the two black holes, in terms of
their physical quantities and the coordinate distance. We then show that 1) the
angular velocity of the two black holes decreases from the usual Kerr value at
infinite distance to zero in the touching limit; 2) the extremal limit of the
two black holes is given by |J|=cM^2, where c depends on the distance and
varies from one to infinity as the distance decreases; 3) for sufficiently
large angular momentum the temperature of the black holes attains a maximum at
a certain finite coordinate distance. All of these results are interpreted in
terms of the dragging effects of the system.Comment: 19 pages, 4 figures. v2: changed statement about thermodynamical
equilibrium in section 3; minor changes; added references. v3: added
references to previous relevant work; removed one equation (see note added);
other minor corrections; final version to be published in JHE

### n-DBI gravity

n-DBI gravity is a gravitational theory introduced in arXiv:1109.1468
[hep-th], motivated by Dirac-Born-Infeld type conformal scalar theory and
designed to yield non-eternal inflation spontaneously. It contains a foliation
structure provided by an everywhere time-like vector field n, which couples to
the gravitational sector of the theory, but decouples in the small curvature
limit. We show that any solution of Einstein gravity with a particular
curvature property is a solution of n-DBI gravity. Amongst them is a class of
geometries isometric to Reissner-Nordstrom-(Anti) de Sitter black hole, which
is obtained within the spherically symmetric solutions of n-DBI gravity
minimally coupled to the Maxwell field. These solutions have, however, two
distinct features from their Einstein gravity counterparts: 1) the cosmological
constant appears as an integration constant and can be positive, negative or
vanishing, making it a variable quantity of the theory; 2) there is a
non-uniqueness of solutions with the same total mass, charge and effective
cosmological constant. Such inequivalent solutions cannot be mapped to each
other by a foliation preserving diffeomorphism. Physically they are
distinguished by the expansion and shear of the congruence tangent to n, which
define scalar invariants on each leave of the foliation.Comment: 13 page

### String Theory and Hybrid Inflation/Acceleration

We find a description of hybrid inflation in (3+1)-dimensions using brane
dynamics of Hanany-Witten type. P-term inflation/acceleration of the universe
with the hybrid potential has a slow-roll de Sitter stage and a waterfall stage
which leads towards an N=2 supersymmetric ground state. We identify the
slow-roll stage of inflation with a non-supersymmetric `Coulomb phase' with
Fayet-Iliopoulos term. This stage ends when the mass squared of one of the
scalars in the hypermultiplet becomes negative. At that moment the brane system
starts undergoing a phase transition via tachyon condensation to a fully
Higgsed supersymmetric vacuum which is the absolute ground state of P-term
inflation. A string theory/cosmology dictionary is provided, which leads to
constraints on parameters of the brane construction from cosmological
experiments. We display a splitting of mass levels reminiscent of the Zeeman
effect due to spontaneous supersymmetry breaking.Comment: 1+21 pages, 5 figures, LaTeX; one figure added; included computation
of supertrace of mass squared for the string theory and discussion of
relation to spontaneous breaking of supersymmetry; several typos corrected;
references adde

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