200 research outputs found
Testing the black hole "no-hair" hypothesis
Black holes in General Relativity are very simple objects. This property,
that goes under the name of "no-hair," has been refined in the last few decades
and admits several versions. The simplicity of black holes makes them ideal
testbeds of fundamental physics and of General Relativity itself. Here we
discuss the no-hair property of black holes, how it can be measured in the
electromagnetic or gravitational window, and what it can possibly tell us about
our universe.Comment: Commissioned by Classical and Quantum Gravit
A hybrid approach to black hole perturbations from extended matter sources
We present a new method for the calculation of black hole perturbations
induced by extended sources in which the solution of the nonlinear
hydrodynamics equations is coupled to a perturbative method based on
Regge-Wheeler/Zerilli and Bardeen-Press-Teukolsky equations when these are
solved in the frequency domain. In contrast to alternative methods in the time
domain which may be unstable for rotating black-hole spacetimes, this approach
is expected to be stable as long as an accurate evolution of the matter sources
is possible. Hence, it could be used under generic conditions and also with
sources coming from three-dimensional numerical relativity codes. As an
application of this method we compute the gravitational radiation from an
oscillating high-density torus orbiting around a Schwarzschild black hole and
show that our method is remarkably accurate, capturing both the basic
quadrupolar emission of the torus and the excited emission of the black hole.Comment: 12 pages, 4 figures. Phys. Rev. D, in pres
Scalar, Electromagnetic and Gravitational Perturbations of Kerr-Newman Black Holes in the Slow-Rotation Limit
In Einstein-Maxwell theory, according to classic uniqueness theorems, the
most general stationary black-hole solution is the axisymmetric Kerr-Newman
metric, which is defined by three parameters: mass, spin and electric charge.
The radial and angular dependence of gravitational and electromagnetic
perturbations in the Kerr-Newman geometry do not seem to be separable. In this
paper we circumvent this problem by studying scalar, electromagnetic and
gravitational perturbations of Kerr-Newman black holes in the slow-rotation
limit. We extend (and provide details of) the analysis presented in a recent
Letter [arXiv:1304.1160]. Working at linear order in the spin, we present the
first detailed derivation of the axial and polar perturbation equations in the
gravito-electromagnetic case, and we compute the corresponding quasinormal
modes for any value of the electric charge. Our study is the first
self-consistent stability analysis of the Kerr-Newman metric, and in principle
it can be extended to any order in the small rotation parameter. We find
numerical evidence that the axial and polar sectors are isospectral at first
order in the spin, and speculate on the possible implications of this result.Comment: 15 pages, 3 figures. Mathematica notebook with derivation of the
axial and polar equations available at
http://blackholes.ist.utl.pt/?page=Files and at
http://www.phy.olemiss.edu/~berti/qnms.htm
Tidal Love numbers of a slowly spinning neutron star
By extending our recent framework to describe the tidal deformations of a
spinning compact object, we compute for the first time the tidal Love numbers
of a spinning neutron star to linear order in the angular momentum. The spin of
the object introduces couplings between electric and magnetic distortions and
new classes of spin-induced ("rotational") tidal Love numbers emerge. We focus
on stationary tidal fields, which induce axisymmetric perturbations. We present
the perturbation equations for both electric-led and magnetic-led rotational
Love numbers for generic multipoles and explicitly solve them for various
tabulated equations of state and for a tidal field with an electric (even
parity) and magnetic (odd parity) component with . For a binary
system close to the merger, various components of the tidal field become
relevant. In this case we find that an octupolar magnetic tidal field can
significantly modify the mass quadrupole moment of a neutron star. Preliminary
estimates, assuming a spin parameter , show modifications
relative to the static case, at an orbital distance of five
stellar radii. Furthermore, the rotational Love numbers as functions of the
moment of inertia are much more sensitive to the equation of state than in the
static case, where approximate universal relations at the percent level exist.
For a neutron-star binary approaching the merger, we estimate that the
approximate universality of the induced mass quadrupole moment deteriorates
from in the static case to roughly when . Our
results suggest that spin-tidal couplings can introduce important corrections
to the gravitational waveforms of spinning neutron-star binaries approaching
the merger.Comment: v1: 16+11 pages, 6 appendices, 11 figures. v2: improved estimates of
the tidal-spin corrections to the quadrupole moment of spinning neutron-star
binaries approaching the merger. v3: version published in PR
Hawking emission of gravitons in higher dimensions: non-rotating black holes
We compute the absorption cross section and the total power carried by
gravitons in the evaporation process of a higher-dimensional non-rotating black
hole. These results are applied to a model of extra dimensions with standard
model fields propagating on a brane. The emission of gravitons in the bulk is
highly enhanced as the spacetime dimensionality increases. The implications for
the detection of black holes in particle colliders and ultrahigh-energy cosmic
ray air showers are briefly discussed.Comment: 16 pages, no figures, revtex4. v3: Misprints in Tables and
four-dimensional power for fermions correcte
Constraining the equation of state of nuclear matter with gravitational wave observations: Tidal deformability and tidal disruption
We study how to extract information on the neutron star equation of state
from the gravitational wave signal emitted during the coalescence of a binary
system composed of two neutron stars or a neutron star and a black hole. We use
post-Newtonian templates which include the tidal deformability parameter and,
when tidal disruption occurs before merger, a frequency cut-off. Assuming that
this signal is detected by Advanced LIGO/Virgo or ET, we evaluate the
uncertainties on these parameters using different data analysis strategies
based on the Fisher matrix approach, and on recently obtained analytical fits
of the relevant quantities. We find that the tidal deformability is more
effective than the stellar compactness to discriminate among different possible
equations of state.Comment: 13 pages, 4 figures, 4 tables. Minor changes to match the version
appearing on Phys. Rev.
Threshold anomalies in Horava-Lifshitz-type theories
Recently the study of threshold kinematic requirements for
particle-production processes has played a very significant role in the
phenomenology of theories with departures from Poincare' symmetry. We here
specialize these threshold studies to the case of a class of violations of
Poincare' symmetry which has been much discussed in the literature on
Horava-Lifshitz scenarios. These involve modifications of the energy-momentum
("dispersion") relation that may be different for different types of particles,
but always involve even powers of energy-momentum in the correction terms. We
establish the requirements for compatibility with the observed cosmic-ray
spectrum, which is sensitive to the photopion-production threshold. We find
that the implications for the electron-positron pair-production threshold are
rather intriguing, in light of some recent studies of TeV emissions by Blazars.
Our findings should also provide motivation for examining the fate of the law
of energy-momentum conservation in Horava-Lifshitz-type theories.Comment: LaTex, 6 page
Non-linear relativistic perturbation theory with two parameters
An underlying fundamental assumption in relativistic perturbation theory is
the existence of a parametric family of spacetimes that can be Taylor expanded
around a background. Since the choice of the latter is crucial, sometimes it is
convenient to have a perturbative formalism based on two (or more) parameters.
A good example is the study of rotating stars, where generic perturbations are
constructed on top of an axisymmetric configuration built by using the slow
rotation approximation. Here, we discuss the gauge dependence of non-linear
perturbations depending on two parameters and how to derive explicit higher
order gauge transformation rules.Comment: 5 pages, LaTeX2e. Contribution to the Spanish Relativity Meeting (ERE
2002), Mao, Menorca, Spain, 22-24.September.200
The return of the membrane paradigm? Black holes and strings in the water tap
Several general arguments indicate that the event horizon behaves as a
stretched membrane. We propose using this relation to understand gravity and
dynamics of black objects in higher dimensions. We provide evidence that (i)
the gravitational Gregory-Laflamme instability has a classical counterpart in
the Rayleigh-Plateau instability of fluids. Each known feature of the
gravitational instability can be accounted for in the fluid model. These
features include threshold mode, dispersion relation, time evolution and
critical dimension of certain phase transitions. Thus, we argue that black
strings break in much the same way as water from a faucet breaks up into small
droplets. (ii) General rotating black holes can also be understood with this
analogy. In particular, instability and bifurcation diagrams for black objects
can easily be inferred. This correspondence can and should be used as a guiding
tool to understand and explore physics of gravity in higher dimensions.Comment: This essay received an honorable mention in the Gravity Research
Foundation Essay Competition, 2007. v2: Published versio
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