37,976 research outputs found
Forecasts for Low Spin Black Hole Spectroscopy in Horndeski Gravity
We investigate the prospect of using black hole spectroscopy to constrain the
parameters of Horndeski gravity through observations of gravitational waves
from perturbed black holes. We study the gravitational waves emitted during
ringdown from black holes without hair in Horndeski gravity, demonstrating the
qualitative differences between such emission in General Relativity and
Horndeski theory. In particular, Quasi-Normal Mode frequencies associated with
the scalar field spectrum can appear in the emitted gravitational radiation.
Analytic expressions for error estimates for both the black hole and Horndeski
parameters are calculated using a Fisher Matrix approach, with constraints on
the `effective mass' of the Horndeski scalar field of order eV or tighter being shown to be achievable in some scenarios.
Estimates for the minimum signal-noise-ratio required to observe such a signal
are also presented.Comment: Updated to match published versio
Ultra-Light Scalar Fields and the Growth of Structure in the Universe
Ultra-light scalar fields, with masses of between m=10^{-33} eV and
m=10^{-22} eV, can affect the growth of structure in the Universe. We identify
the different regimes in the evolution of ultra-light scalar fields, how they
affect the expansion rate of the universe and how they affect the growth rate
of cosmological perturbations. We find a number of interesting effects, discuss
how they might arise in realistic scenarios of the early universe and comment
on how they might be observed.Comment: 12 pages, 11 figure
Magnetic braking in young late-type stars: the effect of polar spots
The concentration of magnetic flux near the poles of rapidly rotating cool
stars has been recently proposed as an alternative mechanism to dynamo
saturation in order to explain the saturation of angular momentum loss. In this
work we study the effect of magnetic surface flux distribution on the coronal
field topology and angular momentum loss rate. We investigate if magnetic flux
concentration towards the pole is a reasonable alternative to dynamo
saturation. We construct a 1D wind model and also apply a 2-D self-similar
analytical model, to evaluate how the surface field distribution affects the
angular momentum loss of the rotating star. From the 1D model we find that, in
a magnetically dominated low corona, the concentrated polar surface field
rapidly expands to regions of low magnetic pressure resulting in a coronal
field with small latitudinal variation. We also find that the angular momentum
loss rate due to a uniform field or a concentrated field with equal total
magnetic flux is very similar. From the 2D wind model we show that there are
several relevant factors to take into account when studying the angular
momentum loss from a star. In particular, we show that the inclusion of force
balance across the field in a wind model is fundamental if realistic
conclusions are to be drawn from the effect of non-uniform surface field
distribution on magnetic braking. This model predicts that a magnetic field
concentrated at high latitudes leads to larger Alfven radii and larger braking
rates than a smoother field distribution. From the results obtained, we argue
that the magnetic surface field distribution towards the pole does not directly
limit the braking efficiency of the wind.Comment: 11 pages, 10 figures, accepted in A&
Are Magnetic Wind-Driving Disks Inherently Unstable?
There have been claims in the literature that accretion disks in which a
centrifugally driven wind is the dominant mode of angular momentum transport
are inherently unstable. This issue is considered here by applying an
equilibrium-curve analysis to the wind-driving, ambipolar diffusion-dominated,
magnetic disk model of Wardle & Konigl (1993). The equilibrium solution curves
for this class of models typically exhibit two distinct branches. It is argued
that only one of these branches represents unstable equilibria and that a real
disk/wind system likely corresponds to a stable solution.Comment: 5 pages, 2 figures, to be published in ApJ, vol. 617 (2004 Dec 20).
Uses emulateapj.cl
The 4 Year COBE DMR data is non-Gaussian
I review our recent claim that there is evidence of non-Gaussianity in the 4
Year COBE DMR data. I describe the statistic we apply, the result we obtain and
make a detailed list of the systematics we have analysed. I finish with a
qualitative understanding of what it might be and its implications.Comment: Proceedings of Rome 3K conference, 5 pages, 3 figure
A covariant approach to parameterised cosmological perturbations
We present a covariant formulation for constructing general quadratic actions
for cosmological perturbations, invariant under a given set of gauge symmetries
for a given field content. This approach allows us to analyse scalar, vector
and tensor perturbations at the same time in a straightforward manner. We apply
the procedure to diffeomorphism invariant single-tensor, scalar-tensor and
vector-tensor theories and show explicitly the full covariant form of the
quadratic actions in such cases, in addition to the actions determining the
evolution of vector and tensor perturbations. We also discuss the role of the
symmetry of the background in identifying the set of cosmologically relevant
free parameters describing these classes of theories, including calculating the
relevant free parameters for an axisymmetric Bianchi-I vacuum universe.Comment: Updated to match published versio
Information profiles for DNA pattern discovery
Finite-context modeling is a powerful tool for compressing and hence for
representing DNA sequences. We describe an algorithm to detect genomic
regularities, within a blind discovery strategy. The algorithm uses information
profiles built using suitable combinations of finite-context models. We used
the genome of the fission yeast Schizosaccharomyces pombe strain 972 h- for
illustration, unveilling locations of low information content, which are
usually associated with DNA regions of potential biological interest.Comment: Full version of DCC 2014 paper "Information profiles for DNA pattern
discovery
A multi-flow model for microquasars
We present a new picture for the central regions of Black Hole X-ray
Binaries. In our view, these central regions have a multi-flow configuration
which consists in (1) an outer standard accretion disc down to a transition
radius r_J, (2) an inner magnetized accretion disc below r_J driving (3) a non
relativistic self-collimated electron-proton jet surrounding, when adequate
conditions for pair creation are met, (4) a ultra relativistic
electron-positron beam. This accretion-ejection paradigm provides a simple
explanation to the canonical spectral states, from radio to X/gamma-rays, by
varying the transition radius r_J and disc accretion rate independently. Large
values of r_J and low accretion rate correspond to Quiescent and Hard states.
These states are characterized by the presence of a steady electron-proton MHD
jet emitted by the disc below r_J. The hard X-ray component is expect to form
at the jet basis. When r_J becomes smaller than the marginally stable orbit
r_i, the whole disc resembles a standard accretion disc with no jet,
characteristic of the Soft state. Intermediate states correspond to situations
where r_J ~ r_i. At large accretion rate, an unsteady pair cascade process is
triggered within the jet axis, giving birth to flares and ejection of
relativistic pair blobs. This would correspond to the luminous intermediate
state, with its associated superluminal motions.Comment: 12 pages, 3 figures. Proceedings of ``High Energies in the
Highlands'', Fort-William, 27 June-1 July 200
Modifying gravity with the Aether: an alternative to Dark Matter
There is evidence that Newton and Einstein's theories of gravity cannot
explain the dynamics of a universe made up solely of baryons and radiation. To
be able to understand the properties of galaxies, clusters of galaxies and the
universe on the whole it has become commonplace to invoke the presence of dark
matter. An alternative approach is to modify the gravitational field equations
to accommodate observations. We propose a new class of gravitational theories
in which we add a new degree of freedom, the Aether, in the form of a vector
field that is coupled covariantly, but non-minimally, with the space-time
metric. We explore the Newtonian and non-Newtonian limits, discuss the
conditions for these theories to be consistent and explore their effect on
cosmology.Comment: Updated version: Notation improved - TG
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