1,955 research outputs found
Gravitational Wave Background from Neutrino-Driven Gamma-Ray Bursts
We discuss the gravitational wave background (GWB) from a cosmological
population of gamma-ray bursts (GRBs). Among various emission mechanisms for
the gravitational waves (GWs), we pay a particular attention to the vast
anisotropic neutrino emissions from the accretion disk around the black hole
formed after the so-called failed supernova explosions. The produced GWs by
such mechanism are known as burst with memory, which could dominate over the
low-frequency regime below \sim 10Hz. To estimate their amplitudes, we derive
general analytic formulae for gravitational waveform from the axisymmetric
jets. Based on the formulae, we first quantify the spectrum of GWs from a
single GRB. Then, summing up its cosmological population, we find that the
resultant value of the density parameter becomes roughly \Omega_{GW} \approx
10^{-20} over the wide-band of the low-frequency region, f\sim 10^{-4}-10^1Hz.
The amplitude of GWB is sufficiently smaller than the primordial GWBs
originated from an inflationary epoch and far below the detection limit.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Study of gravitational radiation from cosmic domain walls
In this paper, following the previous study, we evaluate the spectrum of
gravitational wave background generated by domain walls which are produced if
some discrete symmetry is spontaneously broken in the early universe. We apply
two different methods to calculate the gravitational wave spectrum: One is to
calculate the gravitational wave spectrum directly from numerical simulations,
and another is to calculate it indirectly by estimating the unequal time
anisotropic stress power spectrum of the scalar field. Both analysises indicate
that the slope of the spectrum changes at two characteristic frequencies
corresponding to the Hubble radius at the decay of domain walls and the width
of domain walls, and that the spectrum between these two characteristic
frequencies becomes flat or slightly red tilted. The second method enables us
to evaluate the GW spectrum semi-analytically for the frequencies which can not
be resolved in the finite box lattice simulations, but relies on the
assumptions for the unequal time correlations of the source.Comment: 17 pages, 9 figures; revised version of the manuscript, accepted for
publication in JCA
A Closure Theory for Non-linear Evolution of Cosmological Power Spectra
We apply a non-linear statistical method in turbulence to the cosmological
perturbation theory and derive a closed set of evolution equations for matter
power spectra. The resultant closure equations consistently recover the
one-loop results of standard perturbation theory and beyond that, it is still
capable of treating the non-linear evolution of matter power spectra. We find
the exact integral expressions for the solutions of closure equations. These
analytic expressions coincide with the renormalized one-loop results presented
by Crocce & Scoccimarro (2006,2007). By constructing the non-linear propagator,
we analytically evaluate the non-linear matter power spectra based on the
first-order Born approximation of the integral expressions and compare it with
those of the renormalized perturbation theory.Comment: 22 pages, 4 figures, accepted for publication in Ap
Los Huarpes, el agua y el ambiente
Estamos realizando un proyecto de investigación del cual una de las problemáticas
planteadas es el agua y el ambiente de los pueblos originarios. El objetivo de este trabajo es
describir la situación de este pueblo en las áreas no irrigadas de Lavalle, su acceso al agua
y a un ambiente sano. Las ciencias que nos brindan sus aportes para profundizar en la
comprensión de esta problemática son la historia y geografÃa. Este acceso es un derecho
consagrado a partir de la reforma de la Constitución Nacional de 1994. Nuestra hipótesis es
que no se está cumpliendo con este derecho.Fil: Hiramatsu, T..
Universidad Nacional de Cuyo. Facultad de Ciencias AgrariasFil: GarcÃa Cardoni, M.O..
Universidad Nacional de Cuyo. Facultad de Ciencias Agraria
Non-linear Evolution of Matter Power Spectrum in Modified Theory of Gravity
We present a formalism to calculate the non-linear matter power spectrum in
modified gravity models that explain the late-time acceleration of the Universe
without dark energy. Any successful modified gravity models should contain a
mechanism to recover General Relativity (GR) on small scales in order to avoid
the stringent constrains on deviations from GR at solar system scales. Based on
our formalism, the quasi non-linear power spectrum in the
Dvali-Gabadadze-Porratti (DGP) braneworld models and gravity models are
derived by taking into account the mechanism to recover GR properly. We also
extrapolate our predictions to fully non-linear scales using the Parametrized
Post Friedmann (PPF) framework. In gravity models, the predicted
non-linear power spectrum is shown to reproduce N-body results. We find that
the mechanism to recover GR suppresses the difference between the modified
gravity models and dark energy models with the same expansion history, but the
difference remains large at weakly non-linear regime in these models. Our
formalism is applicable to a wide variety of modified gravity models and it is
ready to use once consistent models for modified gravity are developed.Comment: 25 pages, 8 figures, comparison to N-body simulations in DGP added,
published in PR
Separation of Low Reynolds Number Flows Around a Corner
It was shown experimentally that low Reynolds number flows around a sharp corner do not separate at the corner, and separation points move downstream from the corner with decrease in Reynolds number. For the flow in a channel with a backward-facing step, the experimental results showed a good agreement with those of numerical calculation for the position of separation points and the length of standing vortices
Evolution of String-Wall Networks and Axionic Domain Wall Problem
We study the cosmological evolution of domain walls bounded by strings which
arise naturally in axion models. If we introduce a bias in the potential, walls
become metastable and finally disappear. We perform two dimensional lattice
simulations of domain wall networks and estimate the decay rate of domain
walls. By using the numerical results, we give a constraint for the bias
parameter and the Peccei-Quinn scale. We also discuss the possibility to probe
axion models by direct detection of gravitational waves produced by domain
walls.Comment: 19 pages, 7 figures; revised version of the manuscript, accepted for
publication in JCA
Scalar perturbations in braneworld cosmology
We study the behaviour of scalar perturbations in the radiation-dominated era
of Randall-Sundrum braneworld cosmology by numerically solving the coupled bulk
and brane master wave equations. We find that density perturbations with
wavelengths less than a critical value (set by the bulk curvature length) are
amplified during horizon re-entry. This means that the radiation era matter
power spectrum will be at least an order of magnitude larger than the
predictions of general relativity (GR) on small scales. Conversely, we
explicitly confirm from simulations that the spectrum is identical to GR on
large scales. Although this magnification is not relevant for the cosmic
microwave background or measurements of large scale structure, it will have
some bearing on the formation of primordial black holes in Randall-Sundrum
models.Comment: 17 pages, 7 figure
Advantageous grain boundaries in iron pnictide superconductors
High critical temperature superconductors have zero power consumption and
could be used to produce ideal electric power lines. The principal obstacle in
fabricating superconducting wires and tapes is grain boundaries-the
misalignment of crystalline orientations at grain boundaries, which is
unavoidable for polycrystals, largely deteriorates critical current density.
Here, we report that High critical temperature iron pnictide superconductors
have advantages over cuprates with respect to these grain boundary issues. The
transport properties through well-defined bicrystal grain boundary junctions
with various misorientation angles (thetaGB) were systematically investigated
for cobalt-doped BaFe2As2 (BaFe2As2:Co) epitaxial films fabricated on bicrystal
substrates. The critical current density through bicrystal grain boundary
(JcBGB) remained high (> 1 MA/cm2) and nearly constant up to a critical angle
thetac of ~9o, which is substantially larger than the thetac of ~5o for YBCO.
Even at thetaGB > thetac, the decay of JcBGB was much smaller than that of
YBCO.Comment: to appear in Nature Communication
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