22,046 research outputs found
Bouncing solutions from generalized EoS
We present an exact analytical bouncing solution for a closed universe filled
with only one exotic fluid with negative pressure, obeying a Generalized
Equations of State (GEoS) of the form , where
, and are constants. In our solution and
and is kept as a free parameter. For particular values of
the initial conditions, we obtain that our solution obeys Null Energy Condition
(NEC), which allows us to reinterpret the matter source as that of a real
scalar field, , with a positive kinetic energy and a potential .
We compute numerically the scalar field as a function of time as well as its
potential , and find an analytical function for the potential that
fits very accurately with the numerical results obtained. The shape of this
potential can be well described by a Gaussian-type of function, and hence,
there is no spontaneous symmetry minimum of . We further show that the
bouncing scenario is structurally stable under small variations of the
parameter , such that a family of bouncing solutions can be find
numerically, in a small vicinity of the value .Comment: 12 pages, 12 figure
The effects of intrinsic noise on the behaviour of bistable cell regulatory systems under quasi-steady state conditions
We analyse the effect of intrinsic fluctuations on the properties of bistable
stochastic systems with time scale separation operating under1 quasi-steady
state conditions. We first formulate a stochastic generalisation of the
quasi-steady state approximation based on the semi-classical approximation of
the partial differential equation for the generating function associated with
the Chemical Master Equation. Such approximation proceeds by optimising an
action functional whose associated set of Euler-Lagrange (Hamilton) equations
provide the most likely fluctuation path. We show that, under appropriate
conditions granting time scale separation, the Hamiltonian can be re-scaled so
that the set of Hamilton equations splits up into slow and fast variables,
whereby the quasi-steady state approximation can be applied. We analyse two
particular examples of systems whose mean-field limit has been shown to exhibit
bi-stability: an enzyme-catalysed system of two mutually-inhibitory proteins
and a gene regulatory circuit with self-activation. Our theory establishes that
the number of molecules of the conserved species are order parameters whose
variation regulates bistable behaviour in the associated systems beyond the
predictions of the mean-field theory. This prediction is fully confirmed by
direct numerical simulations using the stochastic simulation algorithm. This
result allows us to propose strategies whereby, by varying the number of
molecules of the three conserved chemical species, cell properties associated
to bistable behaviour (phenotype, cell-cycle status, etc.) can be controlled.Comment: 33 pages, 9 figures, accepted for publication in the Journal of
Chemical Physic
Accelerating universes driven by bulk particles
We consider our universe as a 3d domain wall embedded in a 5d dimensional
Minkowski space-time. We address the problem of inflation and late time
acceleration driven by bulk particles colliding with the 3d domain wall. The
expansion of our universe is mainly related to these bulk particles. Since our
universe tends to be permeated by a large number of isolated structures, as
temperature diminishes with the expansion, we model our universe with a 3d
domain wall with increasing internal structures. These structures could be
unstable 2d domain walls evolving to fermi-balls which are candidates to cold
dark matter. The momentum transfer of bulk particles colliding with the 3d
domain wall is related to the reflection coefficient. We show a nontrivial
dependence of the reflection coefficient with the number of internal dark
matter structures inside the 3d domain wall. As the population of such
structures increases the velocity of the domain wall expansion also increases.
The expansion is exponential at early times and polynomial at late times. We
connect this picture with string/M-theory by considering BPS 3d domain walls
with structures which can appear through the bosonic sector of a
five-dimensional supergravity theory.Comment: To appear in Phys. Rev. D, 16 pages, 3 eps figures, minor changes and
references adde
Orbital magnetism in axially deformed sodium clusters: From scissors mode to dia-para magnetic anisotropy
Low-energy orbital magnetic dipole excitations, known as scissors mode (SM),
are studied in alkali metal clusters. Subsequent dynamic and static effects are
explored. The treatment is based on a self-consistent microscopic approach
using the jellium approximation for the ionic background and the Kohn-Sham mean
field for the electrons. The microscopic origin of SM and its main features
(structure of the mode in light and medium clusters, separation into low- and
high-energy plasmons, coupling high-energy M1 scissors and E2 quadrupole
plasmons, contributions of shape isomers, etc) are discussed. The scissors M1
strength acquires large values with increasing cluster size. The mode is
responsible for the van Vleck paramagnetism of spin-saturated clusters. Quantum
shell effects induce a fragile interplay between Langevin diamagnetism and van
Vleck paramagnetism and lead to a remarkable dia-para anisotropy in magnetic
susceptibility of particular light clusters. Finally, several routes for
observing the SM experimentally are discussed.Comment: 21 pages, 7 figure
The effect of transverse magnetic correlations on a coupled order parameter: shifted transition temperatures and thermal hysteresis
We use a Green's function method with Random Phase Approximation to show how
magnetic correlations may affect electric polarization in multiferroic
materials with magnetic-exchange-type magnetoelectric coupling. We use a model
spin 1/2 ferromagnetic ferroelectric system but our results are expected to
apply to multiferroic materials with more complex magnetic structures. In
particular, we find that transverse magnetic correlations result in a change in
the free energy of the ferroelectric solutions leading to the possibility for
thermal hysteresis of the electric polarization above the magnetic Curie
temperature. Although we are motivated by multiferroic materials, this problem
represents a more general calculation of the effect of fluctuations on coupled
order parameters
InfluĂȘncia do uso nas caracterĂsticas fĂsico quĂmicas de um latossolo amarelo, textura muito argilosa, Manaus, AM.
O objetivo deste estudo foi: verificar alteraçÔes em algumas propriedades fĂsico-quĂmicas do solos provocadas pelo uso; a que profundidade ocorre e verificar o efeito da cobertura do solo com kudzu. As ĂĄreas em que foram feitas as amostragens se localizam no Campo Experimental da EMBRAPA-CPAA, Km 29 da AM-010, no municĂpio de Manaus, AM
Anxiolytic effect of Mozart music over short and long photoperiods as part of environmental enrichment in captive Rattus norvegicus (Rodentia: Muridae)
Music is known to be able to elicit emotional changes, including anxiolytic effects on humans and animals. Photoperiod has also been reported to play an important role in the modulation of anxiety. In the present study, we examined whether the effect of music on anxiety is influenced by day length, comparing, short day (SD; 8:16 h light/dark) and long day (LD; 16:8 h light/dark) with controls (CD; 12:12 h light/dark). After 8 weeks of photoperiod treatment, rats were randomly assigned to 2 groups: silence and music. In the music group, rats were exposed to music 24 h before behavioral tests to quantify anxiety level. Exposure to Mozart music reduced anxiety in rats in the CD group. These effects of music were abolished by LD. Independently of music, rats exposed to SD exhibited higher levels of anxiety-like behavior than rats exposed to CD, in elevated plus-maze and open-field tests. The present findings suggest that the anxiolytic effects of Mozart music are photoperiod-dependent
High frequency limit of the Transport Cross Section and boundedness of the Total Cross Section in scattering by an obstacle with impedance boundary conditions
The scalar scattering of the plane wave by a strictly convex obstacle with
impedance boundary conditions is considered. The uniform boundedness of the
Total Cross Section for all values of frequencies is proved. The high frequency
limit of the Transport Cross Section is founded and presented as a classical
functional of the variational theory
- âŠ