61,278 research outputs found
Ricci dark energy in Chern-Simons modified gravity
In this work, we have considered the Ricci dark energy model, where the
energy density of the universe is proportional to the Ricci scalar curvature,
in the dynamic Chern-Simons modified gravity. We show that in this context the
evolution of the scale factor is similar to that displayed by the modified
Chaplygin gas.Comment: 7 pages; to appear in EPJ
Asteroseismology and Magnetic Cycles
Small cyclic variations in the frequencies of acoustic modes are expected to
be a common phenomenon in solar-like pulsators, as a result of stellar magnetic
activity cycles. The frequency variations observed throughout the solar and
stellar cycles contain information about structural changes that take place
inside the stars as well as about variations in magnetic field structure and
intensity. The task of inferring and disentangling that information is,
however, not a trivial one. In the sun and solar-like pulsators, the direct
effect of the magnetic field on the oscillations might be significantly
important in regions of strong magnetic field (such as solar- / stellar-spots),
where the Lorentz force can be comparable to the gas-pressure gradient. Our aim
is to determine the sun- / stellar-spots effect on the oscillation frequencies
and attempt to understand if this effect contributes strongly to the frequency
changes observed along the magnetic cycle. The total contribution of the spots
to the frequency shifts results from a combination of direct and indirect
effects of the magnetic field on the oscillations. In this first work we
considered only the indirect effect associated with changes in the
stratification within the starspot. Based on the solution of the wave equation
and the variational principle we estimated the impact of these stratification
changes on the oscillation frequencies of global modes in the sun and found
that the induced frequency shifts are about two orders of magnitude smaller
than the frequency shifts observed over the solar cycle.Comment: 4 pages, 6 figures, ESF Conference: The Modern Era of Helio- and
Asteroseismology, to be published on 3 December 2012 at Astronomische
Nachrichten 333, No. 10, 1032-103
Wave Equations for Classical Two-Component Proca Fields in Curved Spacetimes with Torsionless Affinities
The world formulation of the full theory of classical Proca fields in
generally relativistic spacetimes is concisely reviewed and the entire set of
pertinent field equations is transcribed in a straightforward way into the
framework of one of the Infeld-van der Waerden formalisms. Some well-known
calculational techniques are then utilized for deriving the wave equations that
control the propagation of the fields allowed for. It appears that no
interaction couplings between such fields and electromagnetic curvatures are
carried by the wave equations at issue. What results is, in effect, that the
only interactions which ultimately occur in the theoretical context under
consideration involve strictly Proca fields and wave functions for gravitons.Comment: Many improvements on the paper have still been made. In particular,
its title has been modified so as to conform further to one of its main aim
Non-equilibrium tube length fluctuations of entangled polymers
We investigate the nonequilibrium tube length fluctuations during the
relaxation of an initially stretched, entangled polymer chain. The
time-dependent variance of the tube length follows in the early-time
regime a simple universal power law originating in the
diffusive motion of the polymer segments. The amplitude is calculated
analytically both from standard reptation theory and from an exactly solvable
lattice gas model for reptation and its dependence on the initial and
equilibrium tube length respectively is discussed. The non-universality
suggests the measurement of the fluctuations (e.g. using flourescence
microscopy) as a test for reptation models.Comment: 12 pages, 2 figures. Minor typos correcte
The two-site Bose--Hubbard model
The two-site Bose--Hubbard model is a simple model used to study Josephson
tunneling between two Bose--Einstein condensates. In this work we give an
overview of some mathematical aspects of this model. Using a classical
analysis, we study the equations of motion and the level curves of the
Hamiltonian. Then, the quantum dynamics of the model is investigated using
direct diagonalisation of the Hamiltonian. In both of these analyses, the
existence of a threshold coupling between a delocalised and a self-trapped
phase is evident, in qualitative agreement with experiments. We end with a
discussion of the exact solvability of the model via the algebraic Bethe
ansatz.Comment: 10 pages, 5 figures, submitted for publication in Annales Henri
Poincar
On FRW Model in Conformal Teleparallel Gravity
In this paper we use the conformal teleparallel gravity to study an isotropic
and homogeneous Universe which is settled by the FRW metric. We solve the field
equations and we obtain the behavior of some cosmological parameters such as
scale factor, deceleration parameter and the energy density of the perfect
fluid which is the matter field of our model. The field equations, that we
called modified Friedmann equations, allow us to define a dark fluid, with dark
energy density and dark pressure, responsible for the acceleration in the
Universe.Comment: Accepted in EPJ
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