267 research outputs found
Gravitational particle production in braneworld cosmology
Gravitational particle production in time variable metric of an expanding
universe is efficient only when the Hubble parameter is not too small in
comparison with the particle mass. In standard cosmology, the huge value of the
Planck mass makes the mechanism phenomenologically irrelevant. On the
other hand, in braneworld cosmology the expansion rate of the early universe
can be much faster and many weakly interacting particles can be abundantly
created. Cosmological implications are discussed.Comment: 4 pages, 1 figure, v3 with new definition of and minor text
modification
Zeta-function approach to Casimir energy with singular potentials
In the framework of zeta-function approach the Casimir energy for three
simple model system: single delta potential, step function potential and three
delta potentials is analyzed. It is shown that the energy contains
contributions which are peculiar to the potentials. It is suggested to
renormalize the energy using the condition that the energy of infinitely
separated potentials is zero which corresponds to subtraction all terms of
asymptotic expansion of zeta-function. The energy obtained in this way obeys
all physically reasonable conditions. It is finite in the Dirichlet limit and
it may be attractive or repulsive depending on the strength of potential. The
effective action is calculated and it is shown that the surface contribution
appears. The renormalization of the effective action is discussed.Comment: 17 pages, 2 figures, added reference, address correcte
Thermodynamics via Creation from Nothing: Limiting the Cosmological Constant Landscape
The creation of a quantum Universe is described by a {\em density matrix}
which yields an ensemble of universes with the cosmological constant limited to
a bounded range . The
domain is ruled out by a cosmological bootstrap
requirement (the self-consistent back reaction of hot matter). The upper cutoff
results from the quantum effects of vacuum energy and the conformal anomaly
mediated by a special ghost-avoidance renormalization. The cutoff establishes a new quantum scale -- the accumulation point of an infinite
sequence of garland-type instantons. The dependence of the cosmological
constant range on particle phenomenology suggests a possible dynamical
selection mechanism for the landscape of string vacua.Comment: RevTex, 4 pages, 4 figure
Field of homogeneous Plane in Quantum Electrodynamics
We study quantum electrodynamics coupled to the matter field on singular
background, which we call defect. For defect on the infinite plane we
calculated the fermion propagator and mean electromagnetic field. We show that
at large distances from the defect plane, the electromagnetic field is constant
what is in agreement with the classical results. The quantum corrections
determining the field near the plane are calculated in the leading order of
perturbation theory.Comment: 16 page
Strain-Compensated AlInGaAs-GaAsP Superlattices for Highly-Polarized Electron Emission
Spin-polarized electron emission from the first superlattice photocathodes
developed with strain compensation is investigated. An opposite strain in the
quantum well and barrier layers is complished using an InAlGaAs/GaAsP
superlattice structure. The measured values of maximum polarization and quantum
yield for the structure with a 0.18 um-thick working layer are close to the
best results reported for any strained superlattice photocathode structure,
demonstrating the high potential of strain compensation for future photocathode
applications. An analysis of the photoemission spectra is used to estimate the
parameters responsible for the polarization losses.Comment: 10 pages, 2 figure
Svortices and the fundamental modes of the "snake instability": Possibility of observation in the gaseous Bose-Einstein Condensate
The connection between quantized vortices and dark solitons in a long and
thin, waveguide-like trap geometry is explored in the framework of the
non-linear Schr\"odinger equation. Variation of the transverse confinement
leads from the quasi-1D regime where solitons are stable to 2D (or 3D)
confinement where soliton stripes are subject to a transverse modulational
instability known as the ``snake instability''. We present numerical evidence
of a regime of intermediate confinement where solitons decay into single,
deformed vortices with solitonic properties, also called svortices, rather than
vortex pairs as associated with the ``snake'' metaphor. Further relaxing the
transverse confinement leads to production of 2 and then 3 vortices, which
correlates perfectly with a Bogoliubov-de Gennes stability analysis. The decay
of a stationary dark soliton (or, planar node) into a single svortex is
predicted to be experimentally observable in a 3D harmonically confined dilute
gas Bose-Einstein condensate.Comment: 4 pages, 4 figure
Casimir interaction of two plates inside a cylinder
The new exact formulas for the attractive Casimir force acting on each of the
two identical perfectly conducting plates moving freely inside an infinite
perfectly conducting cylinder with the same cross section are derived at zero
and finite temperatures by making use of the zeta function technique. The long
and short distance behaviour of the plates' free energy is investigated.Comment: 14 pages, 1 figure, latex2
Axion braneworld cosmology
We study axion cosmology in a 5D Universe, in the case of flat and warped
extra dimension. The comparison between theoretical predictions and
observations constrains the 5D axion decay constant and the 5D Planck mass,
which has to be taken into account in building 5D axion models. The framework
developed in this paper can be readily applied to other bulk fields in brane
universes.Comment: uses revtex
Watching dark solitons decay into vortex rings in a Bose-Einstein condensate
We have created spatial dark solitons in two-component Bose-Einstein
condensates in which the soliton exists in one of the condensate components and
the soliton nodal plane is filled with the second component. The filled
solitons are stable for hundreds of milliseconds. The filling can be
selectively removed, making the soliton more susceptible to dynamical
instabilities. For a condensate in a spherically symmetric potential, these
instabilities cause the dark soliton to decay into stable vortex rings. We have
imaged the resulting vortex rings.Comment: 4 pages, 4 figure
Renormalization of the nonequilibrium dynamics of fermions in a flat FRW universe
We derive the renormalized equations of motion and the renormalized
energy-momentum tensor for fermions coupled to a spatially homogeneous scalar
field (inflaton) in a flat FRW geometry. The fermion back reaction to the
metric and to the inflaton field is formulated in one-loop approximation.
Having determined the infinite counter terms in an scheme we
formulate the finite terms in a form suitable for numerical computation. We
comment on the trace anomaly which is inferred from the standard analysis. We
also address the problem of initial singularities and determine the Bogoliubov
transformation by which they are removed.Comment: 26 pages, LaTe
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