570 research outputs found
Coherent control of population transfer between communicating defects
Population transfer between two identical, communicating defects in a
one-dimensional tight-binding lattice can be systematically controlled by
external time-periodic forcing. Employing a force with slowly changing
amplitude, the time it takes to transfer a particle from one defect to the
other can be altered over several orders of magnitude. An analytical expression
is derived which shows how the forcing effectively changes the energy splitting
between the defect states, and numerical model calculations illustrate the
possibility of coherent control of the transfer.Comment: 7 pages, 6 figures, to appear in Phys. Rev.
Astrophysical constraints on primordial black holes in Brans-Dicke theory
We consider cosmological evolution in Brans-Dicke theory with a population of
primordial black holes. Hawking radiation from the primordial black holes
impacts various astrophysical processes during the evolution of the Universe.
The accretion of radiation by the black holes in the radiation dominated era
may be effective in imparting them a longer lifetime. We present a detailed
study of how this affects various standard astrophysical constraints coming
from the evaporation of primordial black holes. We analyze constraints from the
present density of the Universe, the present photon spectrum, the distortion of
the cosmic microwave background spectrum and also from processes affecting
light element abundances after nucleosynthesis. We find that the constraints on
the initial primordial black hole mass fractions are tightened with increased
accretion efficiency.Comment: 15 page
Origin of Correlations between Central Black Holes Masses and Galactic Bulge Velocity Dispersions
We argue that the observed correlations between central black holes masses
M_{BH} and galactic bulge velocity dispersions \sigma_e in the form
M_{BH}\propto\sigma_e^4 may witness on the pregalactic origin of massive black
holes. Primordial black holes would be the centers for growing protogalaxies
which experienced multiple mergers with ordinary galaxies. This process is
accompanied by the merging of black holes in the galactic nuclei.Comment: 6 pages, 1 figure, submitted to Astron. and Astrophys. Transaction
Non-equilibrium symmetry restoration beyond one loop
We calculate the strength of symmetry restoration effects in highly
non-equilibrium states which can arise, for example, during preheating after
inflation. We show that in certain parameter range the one-loop results are
unstable, requiring summation of multiloop diagrams. We solve this problem for
the model in the large -limit and show that the symmetry restoration
may be less effective than what predicted by the one-loop estimate.Comment: Latex, 12 pages, 2 postscript figure
Simple Model of Propagating Flame Pulsations
A simple model which exhibits dynamical flame properties in 1D is presented.
It is investigated analytically and numerically. The results are applicable to
problems of flame propagation in supernovae Ia.Comment: 10 pages, 8 figures, revised version accepted by MNRA
Parity-Violating Electron Scattering and Neucleon Structure
The measurement of parity violation in the helicity dependence of
electron-nucleon scattering provides unique information about the basic quark
structure of the nucleons. In this review, the general formalism of
parity-violating electron scattering is presented, with emphasis on elastic
electron-nucleon scattering. The physics issues addressed by such experiments
is discussed, and the major goals of the presently envisioned experimental
program are identified. %General aspects of the experimental technique are
reviewed and A summary of results from a recent series of experiments is
presented and the future prospects of this program are also discussed.Comment: 45 pages, 9 figure
Nonlinear cellular instabilities of planar premixed flames: numerical simulations of the Reactive Navier-Stokes equations
Two-dimensional compressible Reactive Navier-Stokes numerical simulations of intrinsic planar, premixed flame instabilities are performed. The initial growth of a sinusoidally perturbed planar flame is first compared with the predictions of a recent exact linear stability analysis, and it is shown the analysis provides a necessary but not sufficient test problem for validating numerical schemes intended for flame simulations. The long-time nonlinear evolution up to the final nonlinear stationary cellular flame is then examined for numerical domains of increasing width. It is shown that for routinely computationally affordable domain widths, the evolution and final state is, in general, entirely dependent on the width of the domain and choice of numerical boundary conditions. It is also shown that the linear analysis has no relevance to the final nonlinear cell size. When both hydrodynamic and thermal-diffusive effects are important, the evolution consists of a number of symmetry breaking cell splitting and re-merging processes which results in a stationary state of a single very asymmetric cell in the domain, a flame shape which is not predicted by weakly nonlinear evolution equations. Resolution studies are performed and it is found that lower numerical resolutions, typical of those used in previous works, do not give even the qualitatively correct solution in wide domains. We also show that the long-time evolution, including whether or not a stationary state is ever achieved, depends on the choice of the numerical boundary conditions at the inflow and outflow boundaries, and on the numerical domain length and flame Mach number for the types of boundary conditions used in some previous works
Absolute calibration of Analog Detectors using Stimulated Parametric Down Conversion
Spontaneous parametric down conversion has been largely exploited as a tool
for absolute calibration of photon counting detectors, photomultiplier tubes or
avalanche photodiodes working in Geiger regime. In this work we investigate the
extension of this technique from very low photon flux of photon counting regime
to the absolute calibration of analog photodetectors at higher photon flux.
Moving toward higher photon rate, i.e. at high gain regime, with the
spontaneous parametric down conversion shows intrinsic limitations of the
method, while the stimulated parametric down conversion process, where a seed
beam properly injected into the crystal in order to increase the photon
generation rate in the conjugate arm, allows us to work around this problem. A
preliminary uncertainty budget is discussed
Dressed matter waves
We suggest to view ultracold atoms in a time-periodically shifted optical
lattice as a "dressed matter wave", analogous to a dressed atom in an
electromagnetic field. A possible effect lending support to this concept is a
transition of ultracold bosonic atoms from a superfluid to a Mott-insulating
state in response to appropriate "dressing" achieved through time-periodic
lattice modulation. In order to observe this effect in a laboratory experiment,
one has to identify conditions allowing for effectively adiabatic motion of a
many-body Floquet state.Comment: 9 pages, 4 figures, to be published in: J. Phys.: Conference Serie
Spherically symmetric space-time with the regular de Sitter center
The requirements are formulated which lead to the existence of the class of
globally regular solutions to the minimally coupled GR equations which are
asymptotically de Sitter at the center. The brief review of the resulting
geometry is presented. The source term, invariant under radial boots, is
classified as spherically symmetric vacuum with variable density and pressure,
associated with an r-dependent cosmological term, whose asymptotic in the
origin, dictated by the weak energy condition, is the Einstein cosmological
term. For this class of metrics the ADM mass is related to both de Sitter
vacuum trapped in the origin and to breaking of space-time symmetry. In the
case of the flat asymptotic, space-time symmetry changes smoothly from the de
Sitter group at the center to the Lorentz group at infinity. Dependently on
mass, de Sitter-Schwarzschild geometry describes a vacuum nonsingular black
hole, or G-lump - a vacuum selfgravitating particlelike structure without
horizons. In the case of de Sitter asymptotic at infinity, geometry is
asymptotically de Sitter at both origin and infinity and describes, dependently
on parameters and choice of coordinates, a vacuum nonsingular cosmological
black hole, selfgravitating particlelike structure at the de Sitter background
and regular cosmological models with smoothly evolving vacuum energy density.Comment: Latex, 10 figures, extended version of the plenary talk at V
Friedmann Intern. Conf. on Gravitation and Cosmology, Brazil 2002, to appear
in Int.J.Mod.Phys.
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