7,372 research outputs found
Cosmological perturbations in f(T) gravity
We investigate the cosmological perturbations in f(T) gravity. Examining the
pure gravitational perturbations in the scalar sector using a diagonal
vierbien, we extract the corresponding dispersion relation, which provides a
constraint on the f(T) ansatzes that lead to a theory free of instabilities.
Additionally, upon inclusion of the matter perturbations, we derive the fully
perturbed equations of motion, and we study the growth of matter overdensities.
We show that f(T) gravity with f(T) constant coincides with General Relativity,
both at the background as well as at the first-order perturbation level.
Applying our formalism to the power-law model we find that on large subhorizon
scales (O(100 Mpc) or larger), the evolution of matter overdensity will differ
from LCDM cosmology. Finally, examining the linear perturbations of the vector
and tensor sectors, we find that (for the standard choice of vierbein) f(T)
gravity is free of massive gravitons.Comment: 11 pages, 4 figures. Analysis of the vector and tensor sectors adde
Grain Fragmentation and Frictional Melting During Initial Experimental Deformation and Implications for Seismic Slip at Shallow Depths
During seismic slip, the elastic strain energy released by the wall rocks drives grain fragmentation and flash heating in the slipping zone, resulting in formation of (nano)powders and melt droplets, which lower the fault resistance. With progressive seismic slip, the frictional melt covers the slip surface and behaves as a lubricant reducing the coseismic fault strength. However, the processes associated to the transition from grain fragmentation to bulk frictional melting remain poorly understood. Here we discuss in situ microanalytical investigations performed on experimentally produced solidified frictional melts from the transition regime between grain fragmentation and frictional melting. The experiments were performed on granitic gneiss at seismic slip rates (1.3 and 5 m/s), normal stresses ranging from 3 to 30 MPa. At normal stresses <12 MPa, the apparent friction coefficient \u3bcapp (shear stress versus normal stress) evolves in a complex manner with slip: \u3bcapp decreases because of flash weakening, increases up to a peak value \u3bcp1 ~0.6\u20131.0, slightly decreases and increases again to a second peak value \u3bcp2 ~0.44\u20130.83, and eventually decreases with displacement to a steady-state value \u3bcss ~0.3\u20130.45. In situ synchrotron observations of the solidified frictional melt show abundance of ultrafine quartz grains before \u3bcp2 and enrichment in SiO2 at \u3bcp2. Because partial melting occurs on the ultrafine quartz grains and, as a consequence, it suggested that the second re-strengthening (\u3bcp2) is induced by the higher viscosity of the melt due to its enrichment in Si from melting of the ultrafine quartz grains derived from grain fragmentation
GALEX Detection of Shock Breakout in Type II-P Supernova PS1-13arp: Implications for the Progenitor Star Wind
We present the GALEX detection of a UV burst at the time of explosion of an
optically normal Type II-P supernova (PS1-13arp) from the Pan-STARRS1 survey at
z=0.1665. The temperature and luminosity of the UV burst match the theoretical
predictions for shock breakout in a red supergiant, but with a duration a
factor of ~50 longer than expected. We compare the light curve of
PS1-13arp to previous GALEX detections of Type IIP SNe, and find clear
distinctions that indicate that the UV emission is powered by shock breakout,
and not by the subsequent cooling envelope emission previously detected in
these systems. We interpret the ~ 1 d duration of the UV signal with a shock
breakout in the wind of a red supergiant with a pre-explosion mass-loss rate of
~ 10^-3 Msun yr^-1. This mass-loss rate is enough to prolong the duration of
the shock breakout signal, but not enough to produce an excess in the optical
plateau light curve or narrow emission lines powered by circumstellar
interaction. This detection of non-standard, potentially episodic high
mass-loss in a RSG SN progenitor has favorable consequences for the prospects
of future wide-field UV surveys to detect shock breakout directly in these
systems, and provide a sensitive probe of the pre-explosion conditions of SN
progenitors.Comment: Accepted for Publication in Ap
Poincar\'e gauge theory with even and odd parity dynamic connection modes: isotropic Bianchi cosmological models
The Poincar\'e gauge theory of gravity has a metric compatible connection
with independent dynamics that is reflected in the torsion and curvature. The
theory allows two good propagating spin-0 modes. Dynamical investigations using
a simple expanding cosmological model found that the oscillation of the 0
mode could account for an accelerating expansion similar to that presently
observed. The model has been extended to include a mode and more
recently cross parity couplings. We investigate the dynamics of this model in a
situation which is simple, non-trivial, and yet may give physically interesting
results that might be observable. We consider homogeneous cosmologies, more
specifically, isotropic Bianchi class A models. We find an effective Lagrangian
for our dynamical system, a system of first order equations, and present some
typical dynamical evolution.Comment: 8 pages, 1 figures, submitted to IARD 2010 Conference Proceedings in
{\em Journal of Physics: Conference Series}, eds. L. Horwitz and M. Land
(2011
The effect of typhoon on particulate organic carbon flux in the southern East China Sea
Severe tropical storms play an important role in triggering phytoplankton blooms, but the extent to which such storms influence biogenic carbon flux from the euphotic zone is unclear. In 2008, typhoon Fengwong provided a unique opportunity to study the in situ biological responses including phytoplankton blooms and particulate organic carbon fluxes associated with a severe storm in the southern East China Sea (SECS). After passage of the typhoon, the sea surface temperature (SST) in the SECS was markedly cooler (&sim;25 to 26 °C) than before typhoon passage (&sim;28 to 29 °C). The POC flux 5 days after passage of the typhoon was 265 &plusmn; 14 mg C m<sup>−2</sup> d<sup>−1</sup>, which was &sim;1.7-fold that (140–180 mg C m<sup>−2</sup> d<sup>−1</sup>) recorded during a period (June–August, 2007) when no typhoons occurred. A somewhat smaller but nevertheless significant increase in POC flux (224–225 mg C m<sup>−2</sup> d<sup>−1</sup>) was detected following typhoon Sinlaku which occurred approximately 1 month after typhoon Fengwong, indicating that typhoon events can increase biogenic carbon flux efficiency in the SECS. Remarkably, phytoplankton uptake accounted for only about 5% of the nitrate injected into the euphotic zone by typhoon Fengwong. It is likely that phytoplankton population growth was constrained by a combination of light limitation and grazing pressure. Modeled estimates of new/export production were remarkably consistent with the average of new and export production following typhoon Fengwong. The same model suggested that during non-typhoon conditions approximately half of the export of organic carbon occurs via convective mixing of dissolved organic carbon, a conclusion consistent with earlier work at comparable latitudes in the open ocean
Nanomechanical-resonator-assisted induced transparency in a Cooper-pair-box system
We propose a scheme to demonstrate the electromagnetically induced
transparency (EIT) in a system of a superconducting Cooper-pair box coupled to
a nanomechanical resonator. In this scheme, the nanomechanical resonator plays
an important role to contribute additional auxiliary energy levels to the
Cooper-pair box so that the EIT phenomenon could be realized in such a system.
We call it here resonator-assisted induced transparency (RAIT). This RAIT
technique provides a detection scheme in a real experiment to measure physical
properties, such as the vibration frequency and the decay rate, of the coupled
nanomechanical resonator.Comment: To appear in New Journal of Physics: Special Issue "Mechanical
Systems at the Quantum Limit
Probing the Rho Spectral Function in Hot and Dense Nuclear Matter by Dileptons
We present a dynamical study of and production in
proton-nucleus and nucleus-nucleus collisions at CERN-SPS energies on the basis
of the covariant transport approach HSD employing a momentum-dependent
-meson spectral function that includes the pion modifications in the
nuclear medium as well as the polarization of the -meson due to resonant
scattering. We find that the experimental data from the CERES and
HELIOS-3 Collaborations can be described equally well as within the dropping
-mass scenario. Whereas corresponding dilepton -spectra are found to
be very similar, the inclusive dilepton yield in the invariant mass range GeV should allow to disentangle the two scenarios
experimentally.Comment: 13 pages RevTeX slightly revised, 6 eps-figure
The Stony Brook / SMARTS Atlas of mostly Southern Novae
We introduce the Stony Brook / SMARTS Atlas of (mostly) Southern Novae. This
atlas contains both spectra and photometry obtained since 2003. The data
archived in this atlas will facilitate systematic studies of the nova
phenomenon and correlative studies with other comprehensive data sets. It will
also enable detailed investigations of individual objects. In making the data
public we hope to engender more interest on the part of the community in the
physics of novae. The atlas is on-line at
\url{http://www.astro.sunysb.edu/fwalter/SMARTS/NovaAtlas/} .Comment: 11 figures; 5 table
Versatile transporter apparatus for experiments with optically trapped Bose-Einstein condensates
We describe a versatile and simple scheme for producing magnetically and
optically-trapped Rb-87 Bose-Einstein condensates, based on a moving-coil
transporter apparatus. The apparatus features a TOP trap that incorporates the
movable quadrupole coils used for magneto-optical trapping and long-distance
magnetic transport of atomic clouds. As a stand-alone device, this trap allows
for the stable production of condensates containing up to one million atoms. In
combination with an optical dipole trap, the TOP trap acts as a funnel for
efficient loading, after which the quadrupole coils can be retracted, thereby
maximizing optical access. The robustness of this scheme is illustrated by
realizing the superfluid-to-Mott insulator transition in a three-dimensional
optical lattice
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