186 research outputs found
Relativistic dynamical friction in a collisional fluid
The dynamical friction force experienced by a body moving at relativistic
speed in a gaseous medium is examined. This force, which arises due to the
gravitational interaction of the body with its own gravitationally-induced
wake, is calculated for straight-line and circular motion, generalizing
previous results by several authors. Possible applications to the study of
extreme mass-ratio inspirals around strongly-accreting supermassive black holes
are suggested.Comment: 9 pages and 1 figure. Accepted for publication in MNRAS. Replaced to
include minor changes made in proo
Perturbed Kerr Black Holes can probe deviations from General Relativity
Although the Kerr solution is common to many gravity theories, its
perturbations are different in different theories. Hence, perturbed Kerr black
holes can probe deviations from General Relativity.Comment: minor changes to match version published in Phys. Rev. Let
Testing the strong equivalence principle with gravitational-wave observations of binary black holes
The recent LIGO detection of gravitational waves from black-hole binaries offers the exciting possibility of testing gravitational theories in the previously inaccessible strong-field, highly relativistic regime. While the LIGO detections are so far consistent with the predictions of General Relativity, future gravitational-wave observations will allow us to explore this regime to unprecedented accuracy. One of the generic predictions of theories of gravity that extend General Relativity is the violation of the strong equivalence principle, i.e. strongly gravitating bodies such as neutron stars and black holes follow trajectories that depend on their nature and composition. This has deep consequences for gravitational-wave emission, which takes place with additional degrees of freedom besides the tensor polarizations of General Relativity. I will briefly review the formalism needed to describe these extra emission channels, and show that binary black-hole observations probe a set of gravitational theories that are largely disjoint from those that are testable with binary pulsars or neutron stars. \ua9 Copyright owned by the author(s)
Exploring gravity theories with gravitational waves and compact objects
This thesis is devoted to the study of tests of General Relativitywhich could be performed using astrophysical observations of stars or compact objects. The thesis consists of two parts. In the first one, I have investigated how the future gravitational wave observations by the space-based detector LISA will permit mapping the spacetime of the supermassive black holes which are thought to reside in galactic centres. In particular, I have analysed the dynamics of a stellar black hole orbiting around a supermassive black hole and have investigated under which conditions the gravitational wave signal emitted by such a system can allow one to detect the presence of an accretion torus around the supermassive black hole. I have also studied the motion of a stellar black hole in the very strong field region of a nearly extreme supermassive black hole: contrary to our expectations and to suggestions present in the literature, we have found that although the motion presents peculiar characteristics, the emitted gravitational waves do not retain an observable imprint of the almost maximal rotation of the supermassive black hole. Also, I considered black hole binaries with arbitrary masses and spins. Although the coalescence of such systems can be studied only with numerical simulations, I have derived a compact analytic formula for the spin of the final remnant. This formula is in agreement with all the numerical simulations available to date
Test bodies and naked singularities: is the self-force the cosmic censor?
Jacobson and Sotiriou showed that rotating black holes could be spun-up past
the extremal limit by the capture of non-spinning test bodies, if one neglects
radiative and self-force effects. This would represent a violation of the
Cosmic Censorship Conjecture in four-dimensional, asymptotically flat
spacetimes. We show that for some of the trajectories giving rise to naked
singularities, radiative effects can be neglected. However, for these orbits
the conservative self-force is important, and seems to have the right sign to
prevent the formation of naked singularities.Comment: 4 pages, 1 table. Phys. Rev. Lett. in press. Substantially improved
version, showing that the conservative self-force's sign is the right one to
prevent the formation of naked singularities for all orbit
Black Holes in General Relativity and Beyond
The recent detections of gravitational waves from binary systems of black holes are in remarkable agreement with the predictions of General Relativity. In this pedagogical mini-review, I go through the physics of the different phases of the evolution of black hole binary systems, providing a qualitative physical interpretation of each one of them. I also briefly describe how these phases would be modified if gravitation were described by a theory extending or deforming General Relativity, or if the binary components turned out to be more exotic compact objects than black holes
The importance of precession in modelling the direction of the final spin from a black-hole merger
The prediction of the spin of the black hole resulting from the merger of a
generic black-hole binary system is of great importance to study the
cosmological evolution of supermassive black holes. Several attempts have been
recently made to model the spin via simple expressions exploiting the results
of numerical-relativity simulations. Here, I first review the derivation of a
formula, proposed in Barausse & Rezzolla, Apj 704 L40, which accurately
predicts the final spin magnitude and direction when applied to binaries with
separations of hundred or thousands of gravitational radii. This makes my
formula particularly suitable for cosmological merger-trees and N-body
simulations, which provide the spins and angular momentum of the two black
holes when their separation is of thousands of gravitational radii. More
importantly, I investigate the physical reason behind the good agreement
between my formula and numerical relativity simulations, and nail it down to
the fact that my formula takes into account the post-Newtonian precession of
the spins and angular momentum in a consistent manner.Comment: 6 pages, 2 figures. Panel added to fig 2, discussion extended to
comply with referee's comments. Version accepted for publication as
proceeding of the 8th Amaldi International Conference on Gravitational Waves,
NYC, 21-26 June 200
Black holes in Einstein-aether and Horava-Lifshitz gravity
We study spherical black-hole solutions in Einstein-aether theory, a
Lorentz-violating gravitational theory consisting of General Relativity with a
dynamical unit timelike vector (the "aether") that defines a preferred timelike
direction. These are also solutions to the infrared limit of Horava-Lifshitz
gravity. We explore parameter values of the two theories where all presently
known experimental constraints are satisfied, and find that spherical
black-hole solutions of the type expected to form by gravitational collapse
exist for all those parameters. Outside the metric horizon, the deviations away
from the Schwarzschild metric are typically no more than a few percent for most
of the explored parameter regions, which makes them difficult to observe with
electromagnetic probes, but in principle within reach of future
gravitational-wave detectors. Remarkably, we find that the solutions possess a
universal horizon, not far inside the metric horizon, that traps waves of any
speed relative to the aether. A notion of black hole thus persists in these
theories, even in the presence of arbitrarily high propagation speeds.Comment: 18 pages, 12 figures; v2: typos corrected, matches published versio
The complete non-spinning effective-one-body metric at linear order in the mass ratio
Using the main result of a companion paper, in which the binding energy of a
circular-orbit non-spinning compact binary system is computed at leading-order
beyond the test-particle approximation, the exact expression of the
effective-one-body (EOB) metric component g^eff_tt is obtained through first
order in the mass ratio. Combining these results with the recent gravitational
self-force calculation of the periastron advance for circular orbits in the
Schwarzschild geometry, the EOB metric component g^eff_rr is also determined at
linear order in the mass ratio. These results assume that the mapping between
the real and effective Hamiltonians at the second and third post-Newtonian (PN)
orders holds at all PN orders. Our findings also confirm the advantage of
resumming the PN dynamics around the test-particle limit if the goal is to
obtain a flexible model that can smoothly connect the test-mass and equal-mass
limits.Comment: 11 pages, 2 figures; appendix generalized to include the logarithmic
contributions in the post-Newtonian Hamiltonian. Results unchanged. Matches
version to be published in Phys. Rev.
Polytropic spheres in Palatini f(R) gravity
We examine static spherically symmetric polytropic spheres in Palatini f(R)
gravity and show that no regular solutions to the field equations exist for
physically relevant cases such as a monatomic isentropic gas or a degenerate
electron gas, thus casting doubt on the validity of Palatini f(R) gravity as an
alternative to General Relativity.Comment: Talk given by EB at the 30th Spanish Relativity Meeting, 10 - 14
September 2007, Tenerife (Spain). Based on arXiv:gr-qc/0703132 and
arXiv:0712.1141 [gr-qc
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