3,138 research outputs found
Accuracy and effectualness of closed-form, frequency-domain waveforms for non-spinning black hole binaries
The coalescences of binary black hole (BBH) systems, here taken to be
non-spinning, are among the most promising sources for gravitational wave (GW)
ground-based detectors, such as LIGO and Virgo. To detect the GW signals
emitted by BBHs, and measure the parameters of the source, one needs to have in
hand a bank of GW templates that are both effectual (for detection), and
accurate (for measurement). We study the effectualness and the accuracy of the
two types of parametrized banks of templates that are directly defined in the
frequency-domain by means of closed-form expressions, namely 'post-Newtonian'
(PN) and 'phenomenological' models. In absence of knowledge of the exact
waveforms, our study assumes as fiducial, target waveforms the ones generated
by the most accurate version of the effective one body (EOB) formalism. We find
that, for initial GW detectors the use, at each point of parameter space, of
the best closed-form template (among PN and phenomenological models) leads to
an effectualness >97% over the entire mass range and >99% in an important
fraction of parameter space; however, when considering advanced detectors, both
of the closed-form frequency-domain models fail to be effectual enough in
significant domains of the two-dimensional [total mass and mass ratio]
parameter space. Moreover, we find that, both for initial and advanced
detectors, the two closed-form frequency-domain models fail to satisfy the
minimal required accuracy standard in a very large domain of the
two-dimensional parameter space. In addition, a side result of our study is the
determination, as a function of the mass ratio, of the maximum frequency at
which a frequency-domain PN waveform can be 'joined' onto a NR-calibrated EOB
waveform without undue loss of accuracy.Comment: 29 pages, 8 figures, 1 table. Accepted for publication in Phys. Rev.
Inflation without Slow Roll
We draw attention to the possibility that inflation (i.e. accelerated
expansion) might continue after the end of slow roll, during a period of fast
oscillations of the inflaton field \phi . This phenomenon takes place when a
mild non-convexity inequality is satisfied by the potential V(\phi). The
presence of such a period of \phi-oscillation-driven inflation can
substantially modify reheating scenarios.
In some models the effect of these fast oscillations might be imprinted on
the primordial perturbation spectrum at cosmological scales.Comment: 9 pages, Revtex, psfig, 1 figure, minor modifications, references
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Possible surface plasmon polariton excitation under femtosecond laser irradiation of silicon
The mechanisms of ripple formation on silicon surface by femtosecond laser
pulses are investigated. We demonstrate the transient evolution of the density
of the excited free-carriers. As a result, the experimental conditions required
for the excitation of surface plasmon polaritons are revealed. The periods of
the resulting structures are then investigated as a function of laser
parameters, such as the angle of incidence, laser fluence, and polarization.
The obtained dependencies provide a way of better control over the properties
of the periodic structures induced by femtosecond laser on the surface of a
semiconductor material.Comment: 11 pages, 8 figures, accepted for publication in Journal of Applied
Physic
Quantifying intermittent transport in cell cytoplasm
Active cellular transport is a fundamental mechanism for protein and vesicle
delivery, cell cycle and molecular degradation. Viruses can hijack the
transport system and use it to reach the nucleus. Most transport processes
consist of intermittent dynamics, where the motion of a particle, such as a
virus, alternates between pure Brownian and directed movement along
microtubules. In this communication, we estimate the mean time for particle to
attach to a microtubule network. This computation leads to a coarse grained
equation of the intermittent motion in radial and cylindrical geometries.
Finally, by using the degradation activity inside the cytoplasm, we obtain
refined asymptotic estimations for the probability and the mean time a virus
reaches a small nuclear pore.Comment: 4 pages, 5 figures accepted as rapid communication in Phys. Rev.
Photon rockets and gravitational radiation
The absence of gravitational radiation in Kinnersley's ``photon rocket''
solution of Einstein's equations is clarified by studying the mathematically
well-defined problem of point-like photon rockets in Minkowski space (i.e.
massive particles emitting null fluid anisotro\-pically and accelerating
because of the recoil). We explicitly compute the (uniquely defined) {\it
linearized} retarded gravitational waves emitted by such objects, which are the
coherent superposition of the gravitational waves generated by the motion of
the massive point-like rocket and of those generated by the energy-momentum
distribution of the photon fluid. In the special case (corresponding to
Kinnersley's solution) where the anisotropy of the photon emission is purely
dipolar we find that the gravitational wave amplitude generated by the
energy-momentum of the photons exactly cancels the usual gravitational
wave amplitude generated by the accelerated motion of the rocket. More general
photon anisotropies would, however, generate genuine gravitational radiation at
infinity. Our explicit calculations show the compatibility between the
non-radiative character of Kinnersley's solution and the currently used
gravitational wave generation formalisms based on post-Minkowskian perturbation
theory.Comment: 21 pages, LATEX, submitted to Class. Quant. Gra
Quantum effects in gravitational wave signals from cuspy superstrings
We study the gravitational emission, in Superstring Theory, from fundamental
strings exhibiting cusps. The classical computation of the gravitational
radiation signal from cuspy strings features strong bursts in the special null
directions associated to the cusps. We perform a quantum computation of the
gravitational radiation signal from a cuspy string, as measured in a
gravitational wave detector using matched filtering and located in the special
null direction associated to the cusp. We study the quantum statistics
(expectation value and variance) of the measured filtered signal and find that
it is very sharply peaked around the classical prediction. Ultimately, this
result follows from the fact that the detector is a low-pass filter which is
blind to the violent high-frequency quantum fluctuations of both the string
worldsheet, and the incoming gravitational field.Comment: 16 pages, no figur
Gravitational Recoil during Binary Black Hole Coalescence using the Effective One Body Approach
Using the Effective One Body approach, that includes nonperturbative resummed
estimates for the damping and conservative parts of the compact binary
dynamics, we compute the recoil during the late inspiral and the subsequent
plunge of non-spinning black holes of comparable masses moving in
quasi-circular orbits. Further, using a prescription that smoothly connects the
plunge phase to a perturbed single black hole, we obtain an estimate for the
total recoil associated with the binary black hole coalescence. We show that
the crucial physical feature which determines the magnitude of the terminal
recoil is the presence of a ``burst'' of linear momentum flux emitted slightly
before coalescence. When using the most natural expression for the linear
momentum flux during the plunge, together with a Taylor-expanded
correction factor, we find that the maximum value of the terminal recoil is
km/s and occurs for a mass ratio . We comment,
however, on the fact that the above `best bet estimate' is subject to strong
uncertainties because the location and amplitude of the crucial peak of linear
momentum flux happens at a moment during the plunge where most of the
simplifying analytical assumptions underlying the Effective One Body approach
are no longer justified. Changing the analytical way of estimating the linear
momentum flux, we find maximum recoils that range between 49 and 172 km/s.
(Abridged)Comment: 46 pages, new figures and discussions, to appear in PR
Light deflection by gravitational waves from localized sources
We study the deflection of light (and the redshift, or integrated time delay)
caused by the time-dependent gravitational field generated by a localized
material source lying close to the line of sight. Our calculation explicitly
takes into account the full, near-zone, plus intermediate-zone, plus wave-zone,
retarded gravitational field. Contrary to several recent claims in the
literature, we find that the deflections due to both the wave-zone 1/r
gravitational wave and the intermediate-zone 1/r^2 retarded fields vanish
exactly. The leading total time-dependent deflection caused by a localized
material source, such as a binary system, is proven to be given by the
quasi-static, near-zone quadrupolar piece of the gravitational field, and
therefore to fall off as the inverse cube of the impact parameter.Comment: 12 pages, REVTeX 3.0, no figur
The Shaken Baby Syndrome: A Clinical, Pathological, and Biomechanical Study
Because a history of shaking is often lacking in the so-called “shaken baby syndrome,” diagnosis is usually based on a constellation of clinical and radiographic findings. Forty-eight cases of infants and young children with this diagnosis seen between 1978 and 1985 at the Children\u27s Hospital of Philadelphia were reviewed. All patients had a presenting history thought to be suspicious for child abuse, and either retinal hemorrhages with subdural or subarachnoid hemorrhages or a computerized tomography scan showing subdural or subarachnoid hemorrhages with interhemispheric blood. The physical examination and presence of associated trauma were analyzed; autopsy findings for the 13 fatalities were reviewed. All fatal cases had signs of blunt impact to the head, although in more than half of them these findings were noted only at autopsy. All deaths were associated with uncontrollably increased intracranial pressure.
Models of 1-month-old infants with various neck and skull parameters were instrumented with accelerometers and shaken and impacted against padded or unpadded surfaces. Angular accelerations for shakes were smaller than those for impacts by a factor of 50. All shakes fell below injury thresholds established for subhuman primates scaled for the same brain mass, while impacts spanned concussion, subdural hematoma, and diffuse axonal injury ranges. It was concluded that severe head injuries commonly diagnosed as shaking injuries require impact to occur and that shaking alone in an otherwise normal baby is unlikely to cause the shaken baby syndrome
Inelastic collisions in molecular nitrogen at low temperature (2<T<50 K)
Theory and experiment are combined in a novel approach aimed at establishing a set of two-body state-to-state rates for elementary processes ij->lm in low temperature N2:N2 collisions involving the rotational states i, j, l, m. First, a set of 148 collision cross sections is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent potential energy surface for N2–N2. Then, the corresponding rates for the range of 2<T<50 K are derived from the cross sections. The link between theory and experiment, aimed at assessing the calculated rates, is a master equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume 2E-3 mm3 of N2 traveling along the axis of a supersonic jet. The calculated collisional rates are assessed experimentally in the range of 4<T<35 K by means of the master equation, and then are scaled by averaging over a large set of experimental data. The scaled rates account accurately for the evolution of the rotational populations measured in a wide range of conditions. Accuracy of 10%
is estimated for the main scaled rates.This work has been supported by the Spanish Ministerio de Educación y Ciencia, research Project Nos. FIS2004-02576, HF2004-232, ESP2004-21060-E, and ASTROCAM network. J.P.F. is indebted to the CSIC for an I3P grant.Peer reviewe
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