1,324 research outputs found
Contact Term, its Holographic Description in QCD and Dark Energy
In this work we study the well known contact term, which is the key element
in resolving the so-called problem in QCD. We study this term using
the dual Holographic Description. We argue that in the dual picture the contact
term is saturated by the D2 branes which can be interpreted as the tunnelling
events in Minkowski space-time. We quote a number of direct lattice results
supporting this identification. We also argue that the contact term receives a
Casimir -like correction \sim (\Lqcd R)^{-1} rather than naively expected
\exp(-\Lqcd R) when the Minkowski space-time is replaced by
a large but finite manifold with a size . Such a behaviour is consistent
with other QFT-based computations when power like corrections are due to
nontrivial properties of topological sectors of the theory. In holographic
description such a behaviour is due to massless Ramond-Ramond (RR) field living
in the bulk of multidimensional space when power like corrections is a natural
outcome of massless RR field. In many respects the phenomenon is similar to the
Aharonov -Casher effect when the "modular electric field" can penetrate into a
superconductor where the electric field is exponentially screened. The role of
"modular operator" from Aharonov -Casher effect is played by large gauge
transformation operator in 4d QCD, resulting the transparency of the
system to topologically nontrivial pure gauge configurations. We discuss some
profound consequences of our findings. In particular, we speculate that a slow
variation of the contact term in expanding universe might be the main source of
the observed Dark Energy.Comment: Final version to appear in Phys. Rev. D. Comments added on
interpretation of the "topological Casimir effect" from 5d viewpoint where it
can be thought as conventional Casimir effec
Burgers turbulence with pressure
The randomly driven Burgers equation with pressure is considered as a 1D
model of strong turbulence of compressible fluid. It is shown that infinitely
small pressure provides a finite effect on the velocity and density statistics
and this case therefore is qualitatively different from turbulence without
pressure. We establish the corresponding operator product expansion and predict
the intermittent velocity- difference and mass-difference PDFs. We then apply
the developed methods to the statistics of a passive scalar advected by the
Burgers field.Comment: 4 pages, revte
Periodically Aligned Liquid Crystal: Potential application for projection displays
A nematic liquid crystal (NLC) layer with the anisotropy axis modulated at a
fixed rate q in the transverse direction is considered. If the layer locally
constitutes a half-wave plate, then the thin-screen approximation predicts 100%
-efficient diffraction of normal incident wave. The possibility of implementing
such a layer via anchoring at both surfaces of a cell with thickness L is
studied as a function of parameter qL and threshold values of this parameter
are found for a variety of cases. Distortions of the structure of director in
comparison with the preferable ideal profile are found via numerical modeling.
Freedericksz transition is studied for this configuration. Coupled-mode theory
is applied to light propagation through such cell allowing to account for
walk-off effects and effects of nematic distortion. In summary, this cell is
suggested as a means for projection display; high efficiency is predicted.Comment: 25 pages, 6 figures, 1 tabl
Making information flow explicit in HiStar
HiStar is a new operating system designed to minimize the amount of code that must be trusted. HiStar provides strict information flow control, which allows users to specify precise data security policies without unduly limiting the structure of applications. HiStar's security features make it possible to implement a Unix-like environment with acceptable performance almost entirely in an untrusted user-level library. The system has no notion of superuser and no fully trusted code other than the kernel. HiStar's features permit several novel applications, including privacy-preserving, untrusted virus scanners and a dynamic Web server with only a few thousand lines of trusted code.National Science Foundation (U.S.) (Cybertrust Award CNS-0716806)National Science Foundation (U.S.) (Cybertrust/DARPA Grant CNS-0430425
Nonlinear turbulent magnetic diffusion and effective drift velocity of large-scale magnetic field in a two-dimensional magnetohydrodynamic turbulence
We study a nonlinear quenching of turbulent magnetic diffusion and effective
drift velocity of large-scale magnetic field in a developed two-dimensional MHD
turbulence at large magnetic Reynolds numbers. We show that transport of the
mean-square magnetic potential strongly changes quenching of turbulent magnetic
diffusion. In particularly, the catastrophic quenching of turbulent magnetic
diffusion does not occur for the large-scale magnetic fields when a divergence of the flux of the mean-square magnetic
potential is not zero, where is the equipartition mean magnetic
field determined by the turbulent kinetic energy and Rm is the magnetic
Reynolds number. In this case the quenching of turbulent magnetic diffusion is
independent of magnetic Reynolds number. The situation is similar to
three-dimensional MHD turbulence at large magnetic Reynolds numbers whereby the
catastrophic quenching of the alpha effect does not occur when a divergence of
the flux of the small-scale magnetic helicity is not zero.Comment: 8 pages, Physical Review E, in pres
Vacuum energy and spectral function sum rules
We reformulate the problem of the cancellation of the ultraviolet
divergencies of the vacuum energy, particularly important at the cosmological
level, in terms of a saturation of spectral function sum rules which leads to a
set of conditions on the spectrum of the fundamental theory. We specialize the
approach to both Minkowski and de Sitter space-times and investigate some
examples.Comment: 11 pages, revtex4, no figures, version to be published on PR
Dynamics of Logamediate Inflation
A computation of the inflationary observables n_{s} and r is made for
`logamediate' inflation where the cosmological scale factor expands as , and is compared to their predicted values in the
intermediate inflationary theory, where . Both versions prove
to be consistent with observational measurements of the cosmic background
radiation. It is shown that the dynamics of a single inflaton field can be
mimicked by a system of several fields in an analogous manner to that created
by the joint evolution of the fields in assisted power-law inflation.Comment: 7 pages, 5 figures. Extended introductio
Protein and DNA sequence determinants of thermophilic adaptation
Prokaryotes living at extreme environmental temperatures exhibit pronounced
signatures in the amino acid composition of their proteins and nucleotide
compositions of their genomes reflective of adaptation to their thermal
environments. However, despite significant efforts, the definitive answer of
what are the genomic and proteomic compositional determinants of Optimal Growth
Temperature of prokaryotic organisms remained elusive. Here the authors
performed a comprehensive analysis of amino acid and nucleotide compositional
signatures of thermophylic adaptation by exhaustively evaluating all
combinations of amino acids and nucleotides as possible determinants of Optimal
Growth Temperature for all prokaryotic organisms with fully sequences genomes..
The authors discovered that total concentration of seven amino acids in
proteomes, IVYWREL, serves as a universal proteomic predictor of Optimal Growth
Temperature in prokaryotes. Resolving the old-standing controversy the authors
determined that the variation in nucleotide composition (increase of purine
load, or A+G content with temperature) is largely a consequence of thermal
adaptation of proteins. However, the frequency with which A and G nucleotides
appear as nearest neighbors in genome sequences is strongly and independently
correlated with Optimal Growth Temperature. as a result of codon bias in
corresponding genomes. Together these results provide a complete picture of
proteomic and genomic determinants of thermophilic adaptation.Comment: in press PLoS Computational Biology; revised versio
Formation of the First Supermassive Black Holes
We consider the physical conditions under which supermassive black holes
could have formed inside the first galaxies. Our SPH simulations indicate that
metal-free galaxies with a virial temperature ~10^4 K and with suppressed H2
formation (due to an intergalactic UV background) tend to form a binary black
hole system which contains a substantial fraction (>10%) of the total baryonic
mass of the host galaxy. Fragmentation into stars is suppressed without
substantial H2 cooling. Our simulations follow the condensation of ~5x10^6
M_sun around the two centers of the binary down to a scale of < 0.1pc. Low-spin
galaxies form a single black hole instead. These early black holes lead to
quasar activity before the epoch of reionization. Primordial black hole
binaries lead to the emission of gravitational radiation at redshifts z>10 that
would be detectable by LISA.Comment: 11 pages, 9 figures, revised version, ApJ in press (October 10, 2003
Creation of the CMB spectrum: precise analytic solutions for the blackbody photosphere
The blackbody spectrum of CMB was created in the blackbody photosphere at
redshifts z>2x10^6. At these early times, the Universe was dense and hot enough
that complete thermal equilibrium between baryonic matter (electrons and ions)
and photons could be established. Any perturbation away from the blackbody
spectrum was suppressed exponentially. New physics, for example annihilation
and decay of dark matter, can add energy and photons to CMB at redshifts z>10^5
and result in a Bose-Einstein spectrum with a non-zero chemical potential
(). Precise evolution of the CMB spectrum around the critical redshift of
z~2x10^6 is required in order to calculate the -type spectral distortion
and constrain the underlying new physics. Although numerical calculation of
important processes involved (double Compton process, comptonization and
bremsstrahlung) is not difficult, analytic solutions are much faster and easier
to calculate and provide valuable physical insights. We provide precise (better
than 1%) analytic solutions for the decay of , created at an earlier
epoch, including all three processes, double Compton, Compton scattering on
thermal electrons and bremsstrahlung in the limit of small distortions. This is
a significant improvement over the existing solutions with accuracy ~10% or
worse. We also give a census of important sources of energy injection into CMB
in standard cosmology. In particular, calculations of distortions from
electron-positron annihilation and primordial nucleosynthesis illustrate in a
dramatic way the strength of the equilibrium restoring processes in the early
Universe. Finally, we point out the triple degeneracy in standard cosmology,
i.e., the and distortions from adiabatic cooling of baryons and
electrons, Silk damping and annihilation of thermally produced WIMP dark matter
are of similar order of magnitude (~ 10^{-8}-10^{-10})
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