5,759 research outputs found
On tests of general relativity with binary radio pulsars
The timing of radio pulsars in binary systems provides a superb testing
ground of general relativity. Here we propose a Bayesian approach to carry out
these tests, and a relevant efficient numerical implementation, that has
several conceptual and practical advantages with respect to traditional methods
based on least-square-fits that have been used so far: (i) it accounts for the
actual structure of the likelihood function - and it is not predicated on the
Laplace approximation which is implicitly built in least-square fits that can
potentially bias the inference - (ii) it provides the ratio of the evidences of
any two models under consideration as the statistical quantity to compare
different theories, and (iii) it allows us to put joint constraints from the
monitoring of multiple systems, that can be expressed in terms of ratio of
evidences or probability intervals of global (thus not system-dependent)
parameters of the theory, if any exists. Our proposed approach optimally
exploits the progress in timing of radio pulsars and the increase in the number
of observed systems. We demonstrate the power of this framework using simulated
data sets that are representative of current observations.Comment: Accepted for publication on MNRAS Letter
Scattering parabolic solutions for the spatial N-centre problem
For the -centre problem in the three dimensional space, where ,
and , we prove the existence of entire parabolic trajectories
having prescribed asymptotic directions. The proof relies on a variational
argument of min-max type. Morse index estimates and regularization techniques
are used in order to rule out the possible occurrence of collisions
Asymptotic and chaotic solutions of a singularly perturbed Nagumo-type equation
We deal with the singularly perturbed Nagumo-type equation where is a real parameter and is a piecewise constant function satisfying for all . We prove the existence of chaotic, homoclinic and
heteroclinic solutions, when is small enough. We use a dynamical
systems approach, based on the Stretching Along Paths method and on the
Conley-Wazewski's method
No tension between assembly models of supermassive black hole binaries and pulsar observations
Pulsar timing arrays (PTAs) are presently the only means to search for the
gravitational wave stochastic background from supermassive black hole binary
populations, considered to be within the grasp of current or near future
observations. However, the stringent upperlimit set by the Parkes PTA (Shannon
et al. 2013, 2015) has been interpreted as excluding at confidence the
current paradigm of binary assembly through galaxy mergers and hardening via
stellar interactions, suggesting evolution is accelerated (by stars and/or gas)
or stalled. Using Bayesian hierarchical modelling, we consider implications of
this upperlimit for a comprehensive range of astrophysical scenarios, without
invoking stalling nor more exotic physical processes. We find they are fully
consistent with the upperlimit, but (weak) bounds on population parameters can
be inferred. Bayes factors between models vary between --
and Kullback-Leibler divergences between characteristic amplitude prior and
posterior lie between -- . Considering prior astrophysical
information on galaxy merger rates, recent upwards revisions of the black
hole-galaxy bulge mass relation (Kormendy & Ho 2013) are disfavoured at
against lighter models (e.g. Shankar et al. 2016). We also show, if
no detection is achieved once sensitivity improves by an order of magnitude,
the most optimistic scenario is disfavoured at .Comment: 20 pages, 7 figure
Efficient computation of the gravitational wave spectrum emitted by eccentric massive black hole binaries in stellar environments
We present a fast and versatile method to calculate the characteristic
spectrum of the gravitational wave background (GWB) emitted by a
population of eccentric massive black hole binaries (MBHBs). We fit the
spectrum of a reference MBHB with a simple analytic function and show that the
spectrum of any other MBHB can be derived from this reference spectrum via
simple scalings of mass, redshift and frequency. We then apply our calculation
to a realistic population of MBHBs evolving via 3-body scattering of stars in
galactic nuclei. We demonstrate that our analytic prescription satisfactorily
describes the signal in the frequency band relevant to pulsar timing array
(PTA) observations. Finally we model the high frequency steepening of the GWB
to provide a complete description of the features characterizing the spectrum.
For typical stellar distributions observed in massive galaxies, our calculation
shows that 3-body scattering alone is unlikely to affect the GWB in the PTA
band and a low frequency turnover in the spectrum is caused primarily by high
eccentricities.Comment: 12 pages, 9 figures, published in MNRA
Stellar binary black holes in the LISA band: a new class of standard sirens
The recent Advanced LIGO detections of coalescing black hole binaries (BHBs)
imply a large population of such systems emitting at milli-Hz frequencies,
accessible to the Laser Interferometer Space Antenna (LISA). We show that these
systems provide a new class of cosmological standard sirens. Direct LISA
luminosity distance -- measurements, combined with the inhomogeneous
redshift -- distribution of possible host galaxies provide an effective way
to populate the diagram at , thus allowing a precise local
measurement of the Hubble expansion rate. To be effective, the method requires
a sufficiently precise LISA distance determination and sky localization of a
sizeable number of BHBs, which is best achieved for a 6-link detector
configuration. We find that, for a BHB population consistent with current
fiducial LIGO rates, the Hubble constant can be determined at the
5% and 2% level (68% confidence) assuming two and five million Km
arm-length respectively.Comment: 9 pages 4 figures, to be submitted to MNRA
View Registration Using Interesting Segments of Planar Trajectories
We introduce a method for recovering the spatial and temporal alignment between two or more views of objects moving over a ground plane. Existing approaches either assume that the streams are globally synchronized, so that only solving the spatial alignment is needed, or that the temporal misalignment is small enough so that exhaustive search can be performed. In contrast, our approach can recover both the spatial and temporal alignment. We compute for each trajectory a number of interesting segments, and we use their description to form putative matches between trajectories. Each pair of corresponding interesting segments induces a temporal alignment, and defines an interval of common support across two views of an object that is used to recover the spatial alignment. Interesting segments and their descriptors are defined using algebraic projective invariants measured along the trajectories. Similarity between interesting segments is computed taking into account the statistics of such invariants. Candidate alignment parameters are verified checking the consistency, in terms of the symmetric transfer error, of all the putative pairs of corresponding interesting segments. Experiments are conducted with two different sets of data, one with two views of an outdoor scene featuring moving people and cars, and one with four views of a laboratory sequence featuring moving radio-controlled cars
Testing general relativity with compact coalescing binaries: comparing exact and predictive methods to compute the Bayes factor
The second generation of gravitational-wave detectors is scheduled to start
operations in 2015. Gravitational-wave signatures of compact binary
coalescences could be used to accurately test the strong-field dynamical
predictions of general relativity. Computationally expensive data analysis
pipelines, including TIGER, have been developed to carry out such tests. As a
means to cheaply assess whether a particular deviation from general relativity
can be detected, Cornish et al. and Vallisneri recently proposed an approximate
scheme to compute the Bayes factor between a general-relativity
gravitational-wave model and a model representing a class of alternative
theories of gravity parametrised by one additional parameter. This approximate
scheme is based on only two easy-to-compute quantities: the signal-to-noise
ratio of the signal and the fitting factor between the signal and the manifold
of possible waveforms within general relativity. In this work, we compare the
prediction from the approximate formula against an exact numerical calculation
of the Bayes factor using the lalinference library. We find that, using
frequency-domain waveforms, the approximate scheme predicts exact results with
good accuracy, providing the correct scaling with the signal-to-noise ratio at
a fitting factor value of 0.992 and the correct scaling with the fitting factor
at a signal-to-noise ratio of 20, down to a fitting factor of 0.9. We
extend the framework for the approximate calculation of the Bayes factor which
significantly increases its range of validity, at least to fitting factors of
0.7 or higher.Comment: 13 pages, 4 figures, accepted for publication in Classical and
Quantum Gravit
Producción agroexportadora e (in)seguridad alimentaria : el caso de la soja en Argentina
Durante los noventa, la producción agropecuaria argentina presentó importantes transformaciones, que sosteniéndose en un modelo tecnológico intensivo contribuyó a un perfil de país, que creyó encontrar en la creciente especialización productiva ventajas comerciales importantes. El paradigma de la eficiencia y la incorporación tecnológica de cultivares de soja transgénica adaptados a muchas de las ecoregiones argentinas facilitó el desplazamiento de variados sistemas, no sólo en la Región Pampeana sino en las zonas extraPampeanas, ricas en biodiversidad. Por otro lado, la disponibilidad y riqueza de alimentos para la población argentina (ricos y pobres) disminuyó drásticamente poniendo en riesgo la soberanía alimentaria del país, que al igual que muchas otras naciones sudamericanas facilitaron la transnacionalización de su agricultura y han perdido el rumbo. Es importante definir políticas agrícolo ambientales y la implementación de instrumentos de regulación, como las retenciones ambientales. A la lucha por los recursos, suelo, agua potable y biodiversidad, Argentina, Sud América y el mundo se envuelven hoy en un nuevo debate, la Batalla por la Proteína de mayor calidad entre el Sur y el Norte
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