437 research outputs found
Photon emission in a constant magnetic field in 2+1 dimensional space-time
We calculate by the proper-time method the amplitude of the two-photon
emission by a charged fermion in a constant magnetic field in (2+1)-dimensional
space-time. The relevant dynamics reduces to that of a supesymmetric
quantum-mechanical system with one bosonic and one fermionic degrees of
freedom.Comment: 18 pages. v2: references added, some significant changes in the
introductio
Solving advection equations by applying the crank-nicolson scheme combined with the richardson extrapolation
Advection equations appear often in large-scale mathematical models arising in many fields of science and engineering. The Crank-Nicolson scheme can successfully be used in the numerical treatment of such equations. The accuracy of the numerical solution can sometimes be increased substantially by applying the Richardson Extrapolation. Two theorems related to the accuracy of the calculations will be formulated and proved in this paper. The usefulness of the combination consisting of the Crank-Nicolson scheme and the Richardson Extrapolation will be illustrated by numerical examples. Copyright Zahari Zlatev et al
Application of Richardson extrapolation for multi-dimensional advection equations
A Crank-Nicolson type scheme, which is of order two with respect to all independent variables, is used in the numerical solution of multi-dimensional advection equations. Normally, the order of accuracy of any numerical scheme can be increased by one when the well-known Richardson Extrapolation is used. It is proved that in this particular case the order of accuracy of the combined numerical method, the method consisting of the Crank-Nicolson scheme and the Richardson Extrapolation, is not three but four. (C) 2014 Elsevier Ltd. All rights reserved
A New Cosmological Model of Quintessence and Dark Matter
We propose a new class of quintessence models in which late times
oscillations of a scalar field give rise to an effective equation of state
which can be negative and hence drive the observed acceleration of the
universe. Our ansatz provides a unified picture of quintessence and a new form
of dark matter we call "Frustrated Cold Dark Matter" (FCDM). FCDM inhibits
gravitational clustering on small scales and could provide a natural resolution
to the core density problem for disc galaxy halos. Since the quintessence field
rolls towards a small value, constraints on slow-roll quintessence models are
safely circumvented in our model.Comment: Revised. Important new results added in response to referees comment
Natural Quintessence with Gauge Coupling Unification
We show that a positive accelerating universe can be obtained simply by the
dynamics of a non-abelian gauge group. It is the condensates of the chiral
fields that obtain a negative power potential, below the condensation scale,
and allow for a quintessence interpretation of these fields. The only free
parameters in this model are and and the number of dynamically
gauge singlet bilinear fields generated below the condensation scale. We
show that it is possible to have unification of all coupling constants,
including the standard and non standard model couplings, while having an
acceptable phenomenology of as the cosmological constant. This is done
without any fine tuning of the initial conditions. The problem of coincidence
(why the universe has only recently started an accelerating period) is not
solved but it is put at the same level as what the particle content of the
standard model is.Comment: minor changes(discussion on field normalization included), reference
added, accepted in Phy.Rev.Lett., 5 pages,LateX,2 Figure
Gravitational field of vacuumless defects
It has been recently shown that topological defects can arise in symmetry
breaking models where the scalar field potential has no minima and is
a monotonically decreasing function of . Here we study the
gravitational fields produced by such vacuumless defects in the cases of both
global and gauge symmetry breaking. We find that a global monopole has a
strongly repulsive gravitational field, and its spacetime has an event horizon
similar to that in de Sitter space. A gauge monopole spacetime is essentially
that of a magnetically charged black hole. The gravitational field of a global
string is repulsive and that of a gauge string is attractive at small distances
and repulsive at large distances. Both gauge and global string spacetimes have
singularities at a finite distance from the string core.Comment: 19 pages, REVTeX, 6 Postscript figure
Cosmological models from quintessence
A generalized quintessence model is presented which corresponds to a richer
vacuum structure that, besides a time-dependent, slowly varying scalar field,
contains a varying cosmological term. From first principles we determine a
number of scalar-field potentials that satisfy the constraints imposed by the
field equations and conservations laws, both in the conventional and
generalized quintessence models. Besides inverse-power law solutions, these
potentials are given in terms of hyperbolic functions or the twelve Jacobian
elliptic functions, and are all related to the luminosity distance by means of
an integral equation. Integration of this equation for the different solutions
leads to a large family of cosmological models characterized by luminosity
distance-redshift relations. Out of such models, only four appear to be able to
predict a required accelerating universe conforming to observations on
supernova Ia, at large or moderate redshifts.Comment: 9 pages, RevTex, to appear in Phys. Rev.
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