642 research outputs found
First On-Sky Demonstration of a Scintillation Correction technique using Tomographic Wavefront Sensing
Scintillation noise significantly limits high precision ground-based photometry of bright stars. In this paper we present the first ever on-sky demonstration of scintillation correction. The technique uses tomographic wavefront sensing to estimate the spatial-temporal intensity fluctuations induced by high altitude optical turbulence. With an estimate of the altitudes and relative strengths of the turbulent layers above the telescope, the wavefront sensor data from multiple guide stars can be combined to estimate the phase aberrations of the wavefront at each altitude through the use of a tomographic algorithm. This 3D model of the phase aberrations can then be used to estimate the intensity fluctuations across the telescope pupil via Fresnel propagation. The measured photometric data for a given target within the field of view can then be corrected for the effects of scintillation using this estimate in post-processing. A simple proof-of-concept experiment using a wavefront sensor and a stereo-SCIDAR turbulence profiler attached to the 2.5m Isaac Newton Telescope was performed for a range of exposure times using the Orion Trapezium cluster as the reference stars. The results from this on-sky demonstration as well as simulations estimating the expected performance for a full tomographic AO system with laser guide stars are presented. On-sky the scintillation index was reduced on average by a factor of 1.9, with a peak of 3.4. For a full tomographic system we expect to achieve a maximum reduction in the scintillation index by a factor of ∼25
Quarterly U.S. unemployment: cycles, seasons and asymmetries
This paper documents three stylized facts for the quarterly unemployment rate in the United States. Firstly, unemployment is asymmetric over the business cycle, i.e. it rises sharply in recessions and it falls slowly in expansions. Secondly, its seasonal fluctuations are not constant across the two business cycle stages in the sense that there is less seasonality in recession periods. Thirdly, the effect of shocks to the unemployment rate in expansions seem transitory, while this effect is permanent in recessions. Some implications of these stylized facts for empirical macroeconomics and seasonal adjustment are discussed
Trace-gas metabolic versatility of the facultative methanotroph Methylocella silvestris
The climate-active gas methane is generated both by biological processes and by thermogenic decomposition of fossil organic material, which forms methane and short-chain alkanes, principally ethane, propane and butane1, 2. In addition to natural sources, environments are exposed to anthropogenic inputs of all these gases from oil and gas extraction and distribution. The gases provide carbon and/or energy for a diverse range of microorganisms that can metabolize them in both anoxic3 and oxic zones. Aerobic methanotrophs, which can assimilate methane, have been considered to be entirely distinct from utilizers of short-chain alkanes, and studies of environments exposed to mixtures of methane and multi-carbon alkanes have assumed that disparate groups of microorganisms are responsible for the metabolism of these gases. Here we describe the mechanism by which a single bacterial strain, Methylocella silvestris, can use methane or propane as a carbon and energy source, documenting a methanotroph that can utilize a short-chain alkane as an alternative to methane. Furthermore, during growth on a mixture of these gases, efficient consumption of both gases occurred at the same time. Two soluble di-iron centre monooxygenase (SDIMO) gene clusters were identified and were found to be differentially expressed during bacterial growth on these gases, although both were required for efficient propane utilization. This report of a methanotroph expressing an additional SDIMO that seems to be uniquely involved in short-chain alkane metabolism suggests that such metabolic flexibility may be important in many environments where methane and short-chain alkanes co-occur
Causality constraints in AdS/CFT from conformal collider physics and Gauss-Bonnet gravity
We explore the relation between positivity of the energy constraints in
conformal field theories and causality in their dual gravity description. Our
discussion involves CFTs with different central charges whose description, in
the gravity side, requires the inclusion of quadratic curvature corrections. It
is enough, indeed, to consider the Gauss-Bonnet term. We find that both sides
of the AdS/CFT correspondence impose a restriction on the Gauss-Bonnet
coupling. In the case of 6d supersymmetric CFTs, we show the full matching of
these restrictions. We perform this computation in two ways. First by
considering a thermal setup in a black hole background. Second by scrutinizing
the scattering of gravitons with a shock wave in AdS. The different helicities
provide the corresponding lower and upper bounds. We generalize these results
to arbitrary higher dimensions and comment on some hints and puzzles they
prompt regarding the possible existence of higher dimensional CFTs and the
extent to which the AdS/CFT correspondence would be valid for them.Comment: 31 pages, 5 figures; v2: typos fixed, cosmetic amendments and
references adde
Lorentz violation, Gravity, Dissipation and Holography
We reconsider Lorentz Violation (LV) at the fundamental level. We show that
Lorentz Violation is intimately connected with gravity and that LV couplings in
QFT must always be fields in a gravitational sector. Diffeomorphism invariance
must be intact and the LV couplings transform as tensors under coordinate/frame
changes. Therefore searching for LV is one of the most sensitive ways of
looking for new physics, either new interactions or modifications of known
ones. Energy dissipation/Cerenkov radiation is shown to be a generic feature of
LV in QFT. A general computation is done in strongly coupled theories with
gravity duals. It is shown that in scale invariant regimes, the energy
dissipation rate depends non-triviallly on two characteristic exponents, the
Lifshitz exponent and the hyperscaling violation exponent.Comment: LateX, 51 pages, 9 figures. (v2) References and comments added.
Misprints correcte
Quantization of Midisuperspace Models
We give a comprehensive review of the quantization of midisuperspace models.
Though the main focus of the paper is on quantum aspects, we also provide an
introduction to several classical points related to the definition of these
models. We cover some important issues, in particular, the use of the principle
of symmetric criticality as a very useful tool to obtain the required
Hamiltonian formulations. Two main types of reductions are discussed: those
involving metrics with two Killing vector fields and spherically symmetric
models. We also review the more general models obtained by coupling matter
fields to these systems. Throughout the paper we give separate discussions for
standard quantizations using geometrodynamical variables and those relying on
loop quantum gravity inspired methods.Comment: To appear in Living Review in Relativit
Stochastic Gravity: Theory and Applications
Whereas semiclassical gravity is based on the semiclassical Einstein equation
with sources given by the expectation value of the stress-energy tensor of
quantum fields, stochastic semiclassical gravity is based on the
Einstein-Langevin equation, which has in addition sources due to the noise
kernel.In the first part, we describe the fundamentals of this new theory via
two approaches: the axiomatic and the functional. In the second part, we
describe three applications of stochastic gravity theory. First, we consider
metric perturbations in a Minkowski spacetime: we compute the two-point
correlation functions for the linearized Einstein tensor and for the metric
perturbations. Second, we discuss structure formation from the stochastic
gravity viewpoint. Third, we discuss the backreaction of Hawking radiation in
the gravitational background of a quasi-static black hole.Comment: 75 pages, no figures, submitted to Living Reviews in Relativit
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