8,001 research outputs found
2+1 Einstein Gravity as a Deformed Chern-Simons Theory
The usual description of 2+1 dimensional Einstein gravity as a Chern-Simons
(CS) theory is extended to a one parameter family of descriptions of 2+1
Einstein gravity. This is done by replacing the Poincare' gauge group symmetry
by a q-deformed Poincare' gauge group symmetry, with the former recovered when
q-> 1. As a result, we obtain a one parameter family of Hamiltonian
formulations for 2+1 gravity. Although formulated in terms of noncommuting
dreibeins and spin-connection fields, our expression for the action and our
field equations, appropriately ordered, are identical in form to the ordinary
ones. Moreover, starting with a properly defined metric tensor, the usual
metric theory can be built; the Christoffel symbols and space-time curvature
having the usual expressions in terms of the metric tensor, and being
represented by c-numbers. In this article, we also couple the theory to
particle sources, and find that these sources carry exotic angular momentum.
Finally, problems related to the introduction of a cosmological constant are
discussed.Comment: Latex file, 26 pages, no figure
Gravitational-wave astrophysics with effective-spin measurements: asymmetries and selection biases
Gravitational waves emitted by coalescing compact objects carry information
about the spin of the individual bodies. However, with present detectors only
the mass-weighted combination of the components of the spin along the orbital
angular momentum can be measured accurately. This quantity, the effective spin
, is conserved up to at least the second post-Newtonian
order. The measured distribution of values from a
population of detected binaries, and in particular whether this distribution is
symmetric about zero, encodes valuable information about the underlying
compact-binary formation channels. In this paper we focus on two important
complications of using the effective spin to study astrophysical population
properties: (i) an astrophysical distribution for values
which is symmetric does not necessarily lead to a symmetric distribution for
the detected effective spin values, leading to a \emph{selection bias}; and
(ii) the posterior distribution of for individual events
is \emph{asymmetric} and it cannot usually be treated as a Gaussian. We find
that the posterior distributions for systematically show
fatter tails toward larger positive values, unless the total mass is large or
the mass ratio is smaller than . Finally we show that
uncertainties in the measurement of are systematically
larger when the true value is negative than when it is positive. All these
factors can bias astrophysical inference about the population when we have more
than events and should be taken into account when using
gravitational-wave measurements to characterize astrophysical populations.Comment: An online generator for synthetic posteriors
can be found at: http://superstring.mit.edu/welcome.html Comments are welcom
LISA Pathfinder
LISA Pathfinder (formerly known as SMART-2) is an ESA mission designed to pave the way for the joint ESA/NASA Laser Interferometer Space Antenna (LISA) mission by testing in-flight the critical technologies required for space-borne gravitational wave detection; it will put two test masses in a near-perfect gravitational free fall, and control and measure their motion with an unprecedented accuracy. This is achieved through technology comprising inertial sensors, high-precision laser metrology, drag-free control and an ultra-precise micro-Newton propulsion system. The LISA Pathfinder mission is now in Phase C/D—the Implementation Phase, and is due to be launched in 2010, with results on the performance of the system being available within 6 months thereafter
Measuring the delay time distribution of binary neutron stars. II. Using the redshift distribution from third-generation gravitational wave detectors network
We investigate the ability of current and third-generation gravitational wave
(GW) detectors to determine the delay time distribution (DTD) of binary neutron
stars (BNS) through a direct measurement of the BNS merger rate as a function
of redshift. We assume that the DTD follows a power law distribution with a
slope and a minimum merger time , and also allow the
overall BNS formation efficiency per unit stellar mass to vary. By convolving
the DTD and mass efficiency with the cosmic star formation history, and then
with the GW detector capabilities, we explore two relevant regimes. First, for
the current generation of GW detectors, which are only sensitive to the local
universe, but can lead to precise redshift determinations via the
identification of electromagnetic counterparts and host galaxies, we show that
the DTD parameters are strongly degenerate with the unknown mass efficiency and
therefore cannot be determined uniquely. Second, for third-generation detectors
such as Einstein Telescope (ET) and Cosmic Explorer (CE), which will detect BNS
mergers at cosmological distances, but with a redshift uncertainty inherent to
GW-only detections (), we show that the DTD and mass
efficiency can be well-constrained to better than 10\% with a year of
observations. This long-term approach to determining the DTD through a direct
mapping of the BNS merger redshift distribution will be supplemented by more
near term studies of the DTD through the properties of BNS merger host galaxies
at (Safarzadeh & Berger 2019).Comment: 10 pages, Accepted to ApJ Letter
The c-terminal extension of a hybrid immunoglobulin A/G heavy chain is responsible for its Golgi-mediated sorting to the vacuole
We have assessed the ability of the plant secretory pathway to handle the expression of complex heterologous proteins by investigating the fate of a hybrid immunoglobulin A/G in tobacco cells. Although plant cells can express large amounts of the antibody, a relevant proportion is normally lost to vacuolar sorting and degradation. Here we show that the synthesis of high amounts of IgA/G does not impose stress on the plant secretory pathway. Plant cells can assemble antibody chains with high efficiency and vacuolar transport occurs only after the assembled immunoglobulins have traveled through the Golgi complex. We prove that vacuolar delivery of IgA/G depends on the presence of a cryptic sorting signal in the tailpiece of the IgA/G heavy chain. We also show that unassembled light chains are efficiently secreted as monomers by the plant secretory pathway
Flat band properties of twisted transition metal dichalcogenide homo- and heterobilayers of MoS2, MoSe2, WS2 and WSe2
Twisted bilayers of two-dimensional materials, such as twisted bilayer graphene, often feature flat electronic bands that enable the observation of electron correlation effects. In this work, we study the electronic structure of twisted transition metal dichalcogenide (TMD) homo- and heterobilayers that are obtained by combining MoS, WS, MoSe and WSe monolayers, and show how flat band properties depend on the chemical composition of the bilayer as well as its twist angle. We determine the relaxed atomic structure of the twisted bilayers using classical force fields and calculate the electronic band structure using a tight-binding model parametrized from first-principles density-functional theory. We find that the highest valence bands in these systems can derive either from -point or /-point states of the constituent monolayers. For homobilayers, the two highest valence bands are composed of monolayer -point states, exhibit a graphene-like dispersion and become flat as the twist angle is reduced. The situation is more complicated for heterobilayers where the ordering of -derived and /-derived states depends both on the material composition and also the twist angle. In all systems, qualitatively different band structures are obtained when atomic relaxations are neglected
Acceleration disturbances and requirements for ASTROD I
ASTRODynamical Space Test of Relativity using Optical Devices I (ASTROD I)
mainly aims at testing relativistic gravity and measuring the solar-system
parameters with high precision, by carrying out laser ranging between a
spacecraft in a solar orbit and ground stations. In order to achieve these
goals, the magnitude of the total acceleration disturbance of the proof mass
has to be less than 10−13 m s−2 Hz−1/2 at 0.1 m Hz. In this
paper, we give a preliminary overview of the sources and magnitude of
acceleration disturbances that could arise in the ASTROD I proof mass. Based on
the estimates of the acceleration disturbances and by assuming a simple
controlloop model, we infer requirements for ASTROD I. Our estimates show that
most of the requirements for ASTROD I can be relaxed in comparison with Laser
Interferometer Space Antenna (LISA).Comment: 19 pages, two figures, accepted for publication by Class. Quantum
Grav. (at press
Decomposition of 1,1-Dichloroethane and 1,1-Dichloroethene in an electron beam generated plasma reactor
An electron beam generated plasma reactor is used to decompose low concentrations (100–3000 ppm) of 1,1-dichloroethane and 1,1-dichloroethene in atmospheric pressure air streams. The energy requirements for 90% and 99% decomposition of each compound are reported as a function of inlet concentration. Dichloroethene decomposition is enhanced by a chlorine radical propagated chain reaction. The chain length of the dichloroethene reaction is estimated to increase with dichloroethene concentration from 10 at 100 ppm initial dichloroethene concentration to 30 at 3000 ppm. Both the dichloroethane and dichloroethene reactions seem to be inhibited by electron scavenging decomposition products. A simple analytic expression is proposed for fitting decomposition data where inhibition effects are important and simple first order kinetics are not observed
Preliminary investigations on sunburn in Chardonnay grapevine variety
The aim of this investigation was to determine if a temperature response curve can be used to describe sunburn in grape berries. Trials were carried out at the Viticulture and Enology Department (University of California, Davis) on cv Chardonnay (clone 29) grown under both field and greenhouse conditions.
Greenhouse plants were two years old, grown in 5 L pots, and watered daily with a modified Hoagland’s nutrient solution. The vines were pruned to two shoots with one or two clusters per shoot, and the shoots were vertically trained to approximately 1.5 m. Field-grown vines were clone 29 grafted onto 101-14 rootstock, planted at 2.5 x 3.7 m spacing, cane pruned, and VSP trained. Rows were north-south oriented.
In order to increase the temperature of the berry surface, solar radiation was concentrated using a normal reading lens with different magnifications degrees. Temperature was measured with a copper-constantan thermocouples attached to the berry surface. Experiments were performed just before harvest. Sunburn was caused by using different ranges of temperatures held constant for 2 or 5 minutes in the case of greenhouse plants and 5, 10, and 15 minutes in the case of field-grown plants. The effects of treatments were rated on visual basis by a panel of 3 people at one day intervals for three or four consecutive days after the treatments. On the last day, treated berries were harvested and analyzed for cell viability and membrane integrity using the fluorescein diacetate (FDA) technique.
In greenhouse grown vines, a temperature of 38-40 °C for 5 minutes was sufficient to cause visual symptoms of sunburn two days after the treatments, even if no cells were permanently damaged. In field-grown vines, 5 minutes at 40-43 °C caused 12.4% cell mortality and permanent surface deformation.
In conclusion, exposure of berries to a surface temperature of 40-43 °C appears to be effective in causing sunburn in greenhouse and field-grown plants. The radiation regime experienced by the cluster during the growing season may be important to determine the critical level of temperature causing sunburn
Bayesian parameter estimation in the second LISA Pathfinder Mock Data Challenge
A main scientific output of the LISA Pathfinder mission is to provide a noise
model that can be extended to the future gravitational wave observatory, LISA.
The success of the mission depends thus upon a deep understanding of the
instrument, especially the ability to correctly determine the parameters of the
underlying noise model. In this work we estimate the parameters of a simplified
model of the LISA Technology Package (LTP) instrument. We describe the LTP by
means of a closed-loop model that is used to generate the data, both injected
signals and noise. Then, parameters are estimated using a Bayesian framework
and it is shown that this method reaches the optimal attainable error, the
Cramer-Rao bound. We also address an important issue for the mission: how to
efficiently combine the results of different experiments to obtain a unique set
of parameters describing the instrument.Comment: 14 pages, 4 figures, submitted to PR
- …