25 research outputs found
Kerr black holes with synchronised Proca hair: lensing, shadows and EHT constraints
We investigate the gravitational lensing by spinning Proca stars and the shadows
and lensing by Kerr black holes (BHs) with synchronised Proca hair, discussing both theoretical
aspects and observational constraints from the Event Horizon Telescope (EHT) M87* and Sgr
A* data. On the theoretical side, this family of BHs interpolates between Kerr-like solutions —
exhibiting a similar optical appearance to that of Kerr BHs — to very non-Kerr like solutions,
exhibiting exotic features such as cuspy shadows, egg-like shadows and ghost shadows. We
interpret these features in terms of the structure of the fundamental photon orbits, for which
different branches exist, containing both stable and unstable orbits, with some of the latter not
being shadow related. On the observational side, we show that current EHT constraints are
compatible with all such BHs that could form from the growth of the superradiant instability
of Kerr BHs. Unexpectedly, given the (roughly) 10% error bars in the EHT data — and in
contrast to their scalar cousin model —, some of the BHs with up to 40% of their energy in
their Proca hair are compatible with the current data. We estimate the necessary resolution
of future observations to better constrain this model.publishe
Constraining black holes with light boson hair and boson stars using epicyclic frequencies and quasiperiodic oscillations
Light bosonic fields are ubiquitous in extensions of the Standard Model. Even when minimally coupled to gravity, these fields might evade the assumptions of the black-hole no-hair theorems and give rise to spinning black holes which can be drastically different from the Kerr metric. Furthermore, they allow for self-gravitating compact solitons, known as (scalar or Proca) boson stars. The quasiperiodic oscillations (QPOs) observed in the x-ray flux emitted by accreting compact objects carry information about the strong-field region, thus providing a powerful tool to constrain deviations from Kerr’s geometry and to search for exotic compact objects. By using the relativistic precession model as a proxy to interpret the QPOs in terms of geodesic frequencies, we investigate how the QPO frequencies could be used to test the no-hair theorem and the existence of light bosonic fields near accreting compact objects. We show that a detection of two QPO triplets with current sensitivity can already constrain these models and that the future eXTP mission or a LOFT-like mission can set very stringent constraints on black holes with bosonic hair and on (scalar or Proca) boson stars. The peculiar geodesic structure of compact scalar/Proca boson stars implies that these objects can easily be ruled out as alternative models for x-ray source GRO J1655-40
Spinning deformations of the D1-D5 system and a geometric resolution of Closed Timelike Curves
The SO(4) isometry of the extreme Reissner-Nordstrom black hole of N=1, D=5
supergravity can be partly broken, without breaking any supersymmetry, in two
different ways. The ``right'' solution is a rotating black hole (BMPV); the
``left'' is interpreted as a black hole in a Godel universe. In ten dimensions,
both spacetimes are described by deformations of the D1-D5-pp-wave system with
the property that the non-trivial Closed Timelike Curves of the five
dimensional manifold are absent in the universal covering space of the ten
dimensional manifold. In the decoupling limit, the BMPV deformation is
normalizable. It corresponds to the vev of an IR relevant operator of dimension
\Delta=1. The Godel deformation is sub-leading in \alpha' unless we take an
infinite vorticity limit; in such case it is a non-normalizable perturbation.
It corresponds to the insertion of a vector operator of dimension \Delta=5.
Thus we conclude that from the dual (1+1)-CFT viewpoint the SO(4) R-symmetry is
broken `spontaneously' in the BMPV case and explicitly in the Godel case.Comment: 20 pages, no figures, LaTeX; v2: Minor improvements, to appear in
Nucl. Phys.
Hagedorn transition and chronology protection in string theory
We conjecture chronology is protected in string theory due to the
condensation of light winding strings near closed null curves. This
condensation triggers a Hagedorn phase transition, whose end-point target space
geometry should be chronological. Contrary to conventional arguments,
chronology is protected by an infrared effect. We support this conjecture by
studying strings in the O-plane orbifold, where we show that some winding
string states are unstable and condense in the non-causal region of spacetime.
The one-loop string partition function has infrared divergences associated to
the condensation of these states.Comment: 40 pages, 11 figures. Expanded discussion on evolution of on-shell
modes and added appendi
On a class of 4D Kahler bases and AdS_5 supersymmetric Black Holes
We construct a class of toric Kahler manifolds, M_4, of real dimension four,
a subset of which corresponds to the Kahler bases of all known 5D
asymptotically AdS_5 supersymmetric black-holes. In a certain limit, these
Kahler spaces take the form of cones over Sasaki spaces, which, in turn, are
fibrations over toric manifolds of real dimension two. The metric on M_4 is
completely determined by a single function H(x), which is the conformal factor
of the two dimensional space. We study the solutions of minimal five
dimensional gauged supergravity having this class of Kahler spaces as base and
show that in order to generate a five dimensional solution H(x) must obey a
simple sixth order differential equation. We discuss the solutions in detail,
which include all known asymptotically AdS_5 black holes as well as other
spacetimes with non-compact horizons. Moreover we find an infinite number of
supersymmetric deformations of these spacetimes with less spatial isometries
than the base space. These deformations vanish at the horizon, but become
relevant asymptotically.Comment: 34 pages, 3 figures. v2: formula (8.35) and other minor typos
corrected; references added; accepted for publication in JHE
HKT Geometry and de Sitter Supergravity
Solutions of five dimensional minimal de Sitter supergravity admitting
Killing spinors are considered. It is shown that the "timelike'' solutions are
determined in terms of a four dimensional hyper-Kahler torsion (HKT) manifold.
If the HKT manifold is conformally hyper-Kahler the most general solution can
be obtained from a sub-class of supersymmetric solutions of minimal N=2
ungauged supergravity, by means of a simple transformation. Examples include a
multi-BMPV de Sitter solution, describing multiple rotating black holes
co-moving with the expansion of the universe. If the HKT manifold is not
conformally hyper-Kahler, examples admitting a tri-holomorphic Killing vector
field are constructed in terms of certain solutions of three dimensional
Einstein-Weyl geometry.Comment: 26 pages, late
Stationary scalar clouds around a BTZ black hole
We establish the existence of stationary clouds of massive test scalar fields around BTZ black holes. These clouds are zero-modes of the superradiant instability and are possible when Robin boundary conditions (RBCs) are considered at the AdS boundary. These boundary conditions are the most general ones that ensure the AdS space is an isolated system, and include, as a particular case, the commonly considered Dirichlet or Neumann-type boundary conditions (DBCs or NBCs). We obtain an explicit, closed form, resonance condition, relating the RBCs that allow the existence of normalizable (and regular on and outside the horizon) clouds to the system's parameters. Such RBCs never include pure DBCs or NBCs. We illustrate the spatial distribution of these clouds, their energy and angular momentum density for some cases. Our results show that BTZ black holes with scalar hair can be constructed, as the non-linear realization of these clouds. (C) 2017 The Author(s). Published by Elsevier B.V
Asymptotically flat scalar, Dirac and Proca stars: Discrete vs. continuous families of solutions
The existence of localized, approximately stationary, lumps of the classical gravitational and electromagnetic field - geons - was conjectured more than half a century ago. If one insists on exactstationarity, topologically trivial configurations in electro-vacuum are ruled out by no-go theorems for solitons. But stationary, asymptotically flat geons found a realization in scalar-vacuum, where everywhere non-singular, localized field lumps exist, known as (scalar) boson stars. Similar geons have subsequently been found in Einstein-Dirac theory and, more recently, in Einstein-Proca theory. We identify the common conditions that allow these solutions, which may also exist for other spin fields. Moreover, we present a comparison of spherically symmetric geons for the spin 0, 1/2 and 1, emphasizingthe mathematical similarities and clarifying the physical differences, particularly between the bosonic and fermioniccases. We clarify that for the fermionic case, Pauli's exclusion principle prevents a continuous family of solutions for a fixed field mass; rather only a discrete set exists, in contrast with the bosonic case. (C) 2017 The Authors. Published by Elsevier B.V
Superradiance in the BTZ black hole with Robin boundary conditions
We show the existence of superradiant modes of massive scalar fields propagating in BTZ black holes when certain Robin boundary conditions, which never include the commonly considered Dirichlet boundary conditions, are imposed at spatial infinity. These superradiant modes are defined as those solutions whose energy flux across the horizon is towards the exterior region. Differently from rotating, asymptotically flat black holes, we obtain that notall modes which grow up exponentially in time are superradiant; for some of these, the growth is sourced by a bulk instability of AdS(3), triggered by the scalar field with Robin boundary conditions, rather than by energy extraction from the BTZ black hole. Thus, this setup provides an example wherein Bosonic modes with low frequency are pumping energy into, rather than extracting energy from, a rotating black hole. (C) 2018 The Authors. Published by Elsevier B.V.publishe
Flux-branes and the Dielectric Effect in String Theory
We consider the generalization to String and M-theory of the Melvin solution.
These are flux p-branes which have (p+1)-dimensional Poincare invariance and
are associated to an electric (p+1)-form field strength along their
worldvolume. When a stack of Dp-branes is placed along the worldvolume of a
flux (p+3)-brane it will expand to a spherical D(p+2)-brane due to the
dielectric effect. This provides a new setup to consider the gauge
theory/gravity duality. Compactifying M-theory on a circle we find the exact
gravity solution of the type IIA theory describing the dielectric expansion of
N D4-branes into a spherical bound state of D4-D6-branes, due to the presence
of a flux 7-brane. In the decoupling limit, the deformation of the dual field
theory associated with the presence of the flux brane is irrelevant in the UV.
We calculate the gravitational radius and energy of the dielectric brane which
give, respectively, a prediction for the VEV of scalars and vacuum energy of
the dual field theory. Consideration of a spherical D6-brane probe with n units
of D4-brane charge in the dielectric brane geometry suggests that the dual
theory arises as the Scherk-Schwarz reduction of the M5-branes (2,0) conformal
field theory. The probe potential has one minimum placed at the locus of the
bulk dielectric brane and another associated to an inner dielectric brane
shell.Comment: v2 Major Additions: dielectric radius in gravity solution matches
exactly stable and unstable points of Myers probe potential, as well as the
upper bound on D-brane charge. New section probing dielectric brane that
clarifies dual field theory discussion and is consistent with Scherk-Schwarz
reduction of (2,0) M5-brane theory. References added. 43 pages, 4 figure