5,687 research outputs found
Testing the Isotropy of the Universe with Type Ia Supernovae
We analyze the magnitude-redshift data of type Ia supernovae included in the
Union and Union2 compilations in the framework of an anisotropic Bianchi type I
cosmological model and in the presence of a dark energy fluid with anisotropic
equation of state. We find that the amount of deviation from isotropy of the
equation of state of dark energy, the skewness \delta, and the present level of
anisotropy of the large-scale geometry of the Universe, the actual shear
\Sigma_0, are constrained in the ranges -0.16 < \delta < 0.12 and -0.012 <
\Sigma_0 < 0.012 (1\sigma C.L.) by Union2 data. Supernova data are then
compatible with a standard isotropic universe (\delta = \Sigma_0 = 0), but a
large level of anisotropy, both in the geometry of the Universe and in the
equation of state of dark energy, is allowed.Comment: 12 pages, 7 figures, 2 tables. Union2 analysis added. New references
added. To appear in Phys. Rev.
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
Constraints on the anisotropy of dark energy
If the equation of state of dark energy is anisotropic there will be
additional quadrupole anisotropy in the cosmic microwave background induced by
the time dependent anisotropic stress quantified in terms of .
Assuming that the entire amplitude of the observed quadrupole is due to this
anisotropy, we conservatively impose a limit of for any value of assuming that . This is
considerably tighter than that which comes from SNe. Stronger limits, upto a
factor of 10, are possible for specific values of and .
Since we assume this component is uncorrelated with the stochastic component
from inflation, we find that both the expectation value and the sample variance
are increased. There no improvement in the likelihood of an anomalously low
quadrupole as suggested by previous work on an elliptical universe
The Lazarus project: A pragmatic approach to binary black hole evolutions
We present a detailed description of techniques developed to combine 3D
numerical simulations and, subsequently, a single black hole close-limit
approximation. This method has made it possible to compute the first complete
waveforms covering the post-orbital dynamics of a binary black hole system with
the numerical simulation covering the essential non-linear interaction before
the close limit becomes applicable for the late time dynamics. To determine
when close-limit perturbation theory is applicable we apply a combination of
invariant a priori estimates and a posteriori consistency checks of the
robustness of our results against exchange of linear and non-linear treatments
near the interface. Once the numerically modeled binary system reaches a regime
that can be treated as perturbations of the Kerr spacetime, we must
approximately relate the numerical coordinates to the perturbative background
coordinates. We also perform a rotation of a numerically defined tetrad to
asymptotically reproduce the tetrad required in the perturbative treatment. We
can then produce numerical Cauchy data for the close-limit evolution in the
form of the Weyl scalar and its time derivative
with both objects being first order coordinate and tetrad invariant. The
Teukolsky equation in Boyer-Lindquist coordinates is adopted to further
continue the evolution. To illustrate the application of these techniques we
evolve a single Kerr hole and compute the spurious radiation as a measure of
the error of the whole procedure. We also briefly discuss the extension of the
project to make use of improved full numerical evolutions and outline the
approach to a full understanding of astrophysical black hole binary systems
which we can now pursue.Comment: New typos found in the version appeared in PRD. (Mostly found and
collected by Bernard Kelly
A perturbative solution for gravitational waves in quadratic gravity
We find a gravitational wave solution to the linearized version of quadratic
gravity by adding successive perturbations to the Einstein's linearized field
equations. We show that only the Ricci squared quadratic invariant contributes
to give a different solution of those found in Einstein's general relativity.
The perturbative solution is written as a power series in the
parameter, the coefficient of the Ricci squared term in the quadratic
gravitational action. We also show that, for monochromatic waves of a given
angular frequency , the perturbative solution can be summed out to give
an exact solution to linearized version of quadratic gravity, for
.
This result may lead to implications to the predictions for gravitational
wave backgrounds of cosmological origin.Comment: 9 pages, to appear in CQ
The close limit from a null point of view: the advanced solution
We present a characteristic algorithm for computing the perturbation of a
Schwarzschild spacetime by means of solving the Teukolsky equation. We
implement the algorithm as a characteristic evolution code and apply it to
compute the advanced solution to a black hole collision in the close
approximation. The code successfully tracks the initial burst and quasinormal
decay of a black hole perturbation through 10 orders of magnitude and tracks
the final power law decay through an additional 6 orders of magnitude.
Determination of the advanced solution, in which ingoing radiation is absorbed
by the black hole but no outgoing radiation is emitted, is the first stage of a
two stage approach to determining the retarded solution, which provides the
close approximation waveform with the physically appropriate boundary condition
of no ingoing radiation.Comment: Revised version, published in Phys. Rev. D, 34 pages, 13 figures,
RevTe
Cosmic Parallax in Ellipsoidal Universe
The detection of a time variation of the angle between two distant sources
would reveal an anisotropic expansion of the Universe. We study this effect of
"cosmic parallax" within the "ellipsoidal universe" model, namely a particular
homogeneous anisotropic cosmological model of Bianchi type I, whose attractive
feature is the potentiality to account for the observed lack of power of the
large-scale cosmic microwave background anisotropy. The preferred direction in
the sky, singled out by the axis of symmetry inherent to planar symmetry of
ellipsoidal universe, could in principle be constrained by future cosmic
parallax data. However, that will be a real possibility if and when the
experimental accuracy will be enhanced at least by two orders of magnitude.Comment: 9 pages, 2 figures, 1 table. Revised version to match published
version. References adde
Cosmology of a Scalar Field Coupled to Matter and an Isotropy-Violating Maxwell Field
Motivated by the couplings of the dilaton in four-dimensional effective
actions, we investigate the cosmological consequences of a scalar field coupled
both to matter and a Maxwell-type vector field. The vector field has a
background isotropy-violating component. New anisotropic scaling solutions
which can be responsible for the matter and dark energy dominated epochs are
identified and explored. For a large parameter region the universe expands
almost isotropically. Using that the CMB quadrupole is extremely sensitive to
shear, we constrain the ratio of the matter coupling to the vector coupling to
be less than 10^(-5). Moreover, we identify a large parameter region,
corresponding to a strong vector coupling regime, yielding exciting and viable
cosmologies close to the LCDM limit.Comment: Refs. added, some clarifications. Published in JHEP10(2012)06
Anisotropic dark energy and ellipsoidal universe
A cosmological model with anisotropic dark energy is analyzed. The amount of
deviation from isotropy of the equation of state of dark energy, the skewness
\delta, generates an anisotropization of the large-scale geometry of the
Universe, quantifiable by means of the actual shear \Sigma_0. Requiring that
the level of cosmic anisotropization at the time of decoupling is such to solve
the "quadrupole problem" of cosmic microwave background radiation, we find that
|\delta| \sim 10^{-4} and |\Sigma_0| \sim 10^{-5}, compatible with existing
limits derived from the magnitude-redshift data on type Ia supernovae.Comment: 10 pages, 3 figures. Revised version to match published version.
References adde
Reconstruction of Black Hole Metric Perturbations from Weyl Curvature
Perturbation theory of rotating black holes is usually described in terms of
Weyl scalars and , which each satisfy Teukolsky's complex
master wave equation and respectively represent outgoing and ingoing radiation.
On the other hand metric perturbations of a Kerr hole can be described in terms
of (Hertz-like) potentials in outgoing or ingoing {\it radiation
gauges}. In this paper we relate these potentials to what one actually computes
in perturbation theory, i.e and . We explicitly construct
these relations in the nonrotating limit, preparatory to devising a
corresponding approach for building up the perturbed spacetime of a rotating
black hole. We discuss the application of our procedure to second order
perturbation theory and to the study of radiation reaction effects for a
particle orbiting a massive black hole.Comment: 6 Pages, Revtex
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