21,339 research outputs found
Scale dependence of cosmological backreaction
Due to the non-commutation of spatial averaging and temporal evolution,
inhomogeneities and anisotropies (cosmic structures) influence the evolution of
the averaged Universe via the cosmological backreaction mechanism. We study the
backreaction effect as a function of averaging scale in a perturbative approach
up to higher orders. We calculate the hierarchy of the critical scales, at
which 10% effects show up from averaging at different orders. The dominant
contribution comes from the averaged spatial curvature, observable up to scales
of 200 Mpc. The cosmic variance of the local Hubble rate is 10% (5%) for
spherical regions of radius 40 (60) Mpc. We compare our result to the one from
Newtonian cosmology and Hubble Space Telescope Key Project data.Comment: 6 pages, 2 figures; v3: substantial modifications, new figure
Quantization and holomorphic anomaly
We study wave functions of B-model on a Calabi-Yau threefold in various
polarizations.Comment: 15 page
Polarizations and Nullcone of Representations of Reductive Groups
The paper starts with the following simple observation. Let V be a representation of a reductive group G, and let f_1,f_2,...,f_n be homogeneous invariant functions. Then the polarizations of f_1,f_2,...,f_n define the nullcone of k 0} h(t) x = 0 for all x in L. This is then applied to many examples. A surprising result is about the group SL(2,C) where almost all representations V have the property that all linear subspaces of the nullcone are annihilated. Again, this has interesting applications to the invariants on several copies. Another result concerns the n-qubits which appear in quantum computing. This is the representation of a product of n copies of on the n-fold tensor product C^2 otimes C^2 otimes ... otimes C^2. Here we show just the opposite, namely that the polarizations never define the nullcone of several copies if n <= 3. (An earlier version of this paper, distributed in 2002, was split into two parts; the first part with the title ``On the nullcone of representations of reductive groups'' is published in Pacific J. Math. {bf 224} (2006), 119--140.
Evidence of sympathetic cooling of Na+ ions by a Na MOT in a hybrid trap
A hybrid ion-neutral trap provides an ideal system to study collisional
dynamics between ions and neutrals. This system provides a general cooling
method that can be applied to optically inaccessible species and can also
potentially cool internal degrees of freedom. The long range polarization
potentials () between ions and neutrals result in large
scattering cross sections at cold temperatures, making the hybrid trap a
favorable system for efficient sympathetic cooling of ions by collisions with
neutral atoms. We present experimental evidence of sympathetic cooling in a
hybrid trap of \ce{Na+} ions, which are closed shell and therefore do not have
a laser induced atomic transition, by equal mass cold Na atoms in a
magneto-optical trap (MOT).Comment: 7 figure
Tests of Bayesian Model Selection Techniques for Gravitational Wave Astronomy
The analysis of gravitational wave data involves many model selection
problems. The most important example is the detection problem of selecting
between the data being consistent with instrument noise alone, or instrument
noise and a gravitational wave signal. The analysis of data from ground based
gravitational wave detectors is mostly conducted using classical statistics,
and methods such as the Neyman-Pearson criteria are used for model selection.
Future space based detectors, such as the \emph{Laser Interferometer Space
Antenna} (LISA), are expected to produced rich data streams containing the
signals from many millions of sources. Determining the number of sources that
are resolvable, and the most appropriate description of each source poses a
challenging model selection problem that may best be addressed in a Bayesian
framework. An important class of LISA sources are the millions of low-mass
binary systems within our own galaxy, tens of thousands of which will be
detectable. Not only are the number of sources unknown, but so are the number
of parameters required to model the waveforms. For example, a significant
subset of the resolvable galactic binaries will exhibit orbital frequency
evolution, while a smaller number will have measurable eccentricity. In the
Bayesian approach to model selection one needs to compute the Bayes factor
between competing models. Here we explore various methods for computing Bayes
factors in the context of determining which galactic binaries have measurable
frequency evolution. The methods explored include a Reverse Jump Markov Chain
Monte Carlo (RJMCMC) algorithm, Savage-Dickie density ratios, the Schwarz-Bayes
Information Criterion (BIC), and the Laplace approximation to the model
evidence. We find good agreement between all of the approaches.Comment: 11 pages, 6 figure
Classical and quantum regimes of the superfluid turbulence
We argue that turbulence in superfluids is governed by two dimensionless
parameters. One of them is the intrinsic parameter q which characterizes the
friction forces acting on a vortex moving with respect to the heat bath, with
1/q playing the same role as the Reynolds number Re=UR/\nu in classical
hydrodynamics. It marks the transition between the "laminar" and turbulent
regimes of vortex dynamics. The developed turbulence described by Kolmogorov
cascade occurs when Re >> 1 in classical hydrodynamics, and q << 1 in the
superfluid hydrodynamics. Another parameter of the superfluid turbulence is the
superfluid Reynolds number Re_s=UR/\kappa, which contains the circulation
quantum \kappa characterizing quantized vorticity in superfluids. This
parameter may regulate the crossover or transition between two classes of
superfluid turbulence: (i) the classical regime of Kolmogorov cascade where
vortices are locally polarized and the quantization of vorticity is not
important; and (ii) the quantum Vinen turbulence whose properties are
determined by the quantization of vorticity. The phase diagram of the dynamical
vortex states is suggested.Comment: 12 pages, 1 figure, version accepted in JETP Letter
Weak Scale Superstrings
Recent developments in string duality suggest that the string scale may not
be irrevocably tied to the Planck scale. Two explicit but unrealistic examples
are described where the ratio of the string scale to the Planck scale is
arbitrarily small. Solutions which are more realistic may exist in the
intermediate coupling or ``truly strong coupling'' region of the heterotic
string. Weak scale superstrings have dramatic experimental consequences for
both collider physics and cosmology.Comment: harvmac, 14 pages. References added, 3 typos fixed, Comments added at
beginning of section 4 emphasizing flaws of the toy example
Comet Machholz (C/2004 Q2): morphological structures in the inner coma and rotation parameters
Extensive observations of comet C/2004 Q2 (Machholz) were carried out between
August 2004 and May 2005. The images obtained were used to investigate the
comet's inner coma features at resolutions between 350 and 1500 km/pixel. A
photometric analysis of the dust outflowing from the comet's nucleus and the
study of the motion of the morphological structures in the inner coma indicated
that the rotation period of the nucleus was most likely around 0.74 days. A
thorough investigation of the inner coma morphology allowed us to observe two
main active sources on the comet's nucleus, at a latitude of +85{\deg} \pm
5{\deg} and +45{\deg} \pm 5{\deg}, respectively. Further sources have been
observed, but their activity ran out quite rapidly over time; the most relevant
was at latcom. = 25{\deg} \pm 5{\deg}. Graphic simulations of the geometrical
conditions of observation of the inner coma were compared with the images and
used to determine a pole orientation at RA=95{\deg} \pm 5{\deg}, Dec=+35{\deg}
\pm 5{\deg}. The comet's spin axis was lying nearly on the plane of the sky
during the first decade of December 2004.Comment: 29 pages, 8 figures, 3 table
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