19,878 research outputs found
Chemical Potential and the Nature of the Dark Energy: The case of phantom
The influence of a possible non zero chemical potential on the nature
of dark energy is investigated by assuming that the dark energy is a
relativistic perfect simple fluid obeying the equation of state (EoS),
(). The entropy condition, ,
implies that the possible values of are heavily dependent on the
magnitude, as well as on the sign of the chemical potential. For , the
-parameter must be greater than -1 (vacuum is forbidden) while for not only the vacuum but even a phantomlike behavior () is
allowed. In any case, the ratio between the chemical potential and temperature
remains constant, that is, . Assuming that the dark energy
constituents have either a bosonic or fermionic nature, the general form of the
spectrum is also proposed. For bosons is always negative and the extended
Wien's law allows only a dark component with which includes
vacuum and the phantomlike cases. The same happens in the fermionic branch for
are permmited only if . The thermodynamics and statistical arguments constrain the
EoS parameter to be , a result surprisingly close to the maximal
value required to accelerate a FRW type universe dominated by matter and dark
energy ().Comment: 7 pages, 5 figure
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An analysis of media reporting on the closure of freestanding midwifery units in England
PROBLEM: Despite clinical guidelines and policy promoting choice of place of birth, 14 Freestanding Midwifery Units were closed between 2008 and 2015, closures justified by low use and financial constraints.
BACKGROUND: The Birthplace in England Programme found that freestanding midwifery units provided the most cost-effective birthplace for women at low risk of complications. Women planning birth in a freestanding unit were less likely to experience interventions than those planning obstetric unit birth, with no difference in outcomes for babies.
METHODS: This paper uses an interpretative technique developed for policy analysis to explore the representation of these closures in 191 news articles, to explore the public climate in which they occurred.
FINDINGS AND DISCUSSION: The articles focussed on underuse by women and financial constraints on services. Despite the inclusion of service user voices, the power of framing was held by service managers and commissioners. The analysis exposed how neoliberalist and austerity policies has privileged representation of individual consumer choice and market-driven provision as drivers of changes in health services. This normative framing makes the reasons given for closure as hard to refute and cultural norms persist that birth is safest in an obstetric setting, despite evidence to the contrary.
CONCLUSION: The rise of neoliberalism and austerity in contemporary Britain has influenced the reform of maternity services, in particular the closure of midwifery units. Justifications given for closure silence other narratives, predominantly from service users, that attempt to present women's choice in terms of rights and a social model of care
Clustering, Angular Size and Dark Energy
The influence of dark matter inhomogeneities on the angular size-redshift
test is investigated for a large class of flat cosmological models driven by
dark energy plus a cold dark matter component (XCDM model). The results are
presented in two steps. First, the mass inhomogeneities are modeled by a
generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is
characterized by a smoothness parameter and a power index ,
and, second, we provide a statistical analysis to angular size data for a large
sample of milliarcsecond compact radio sources. As a general result, we have
found that the parameter is totally unconstrained by this sample of
angular diameter data.Comment: 9 pages, 7 figures, accepted in Physical Review
Magnetic phases evolution in the LaMn1-xFexO3+y system
We have investigated the crystal structure and magnetic properties for
polycrystalline samples of LaMn1-xFexO3+y, in the whole range x=0.0 to x=1.0,
prepared by solid state reaction in air. All samples show the ORT-2
orthorhombic structure that suppresses the Jahn-Teller distortion, thus
favoring a ferromagnetic (FM) superexchange (SE) interaction between
Mn^{3+}-O-Mn^{3+}. For x=0.0 the oxygen excess (y ~ 0.09) produces vacancies in
the La and Mn sites and generates a fraction around 18% of Mn^{4+} ions and 82%
of the usual Mn^{3+} ions, with possible double exchange interaction between
them. The Fe doping in this system is known to produce only stable Fe^{3+}
ions. We find an evolution from a fairly strong FM phase with a Curie
temperature T_{C} ~ 160 K, for x=0.0, to an antiferromagnetic (AFM) phase with
T_{N} = 790 K, for x=1.0, accompanied by clear signatures of a cluster-glass
behavior. For intermediate Fe contents a mixed-phase state occurs, with a
gradual decrease (increase) of the FM (AFM) phase, accompanied by a systematic
transition broadening for 0.2 < x < 0.7. A model based on the expected exchange
interaction among the various magnetic-ion types, accounts very well for the
saturation-magnetization dependence on Fe doping.Comment: 27 pages, 9 figure
Large magnetic anisotropy in Ferrihydrite nanoparticles synthesized from reverse micelles
Six-line ferrihydrite(FH) nanoparticles have been synthesized in the core of
reverse micelles, used as nanoreactors to obtain average particle sizes
2 to 4 nm. The blocking temperatures extracted from
magnetization data increased from to 20 K for increasing particle
size. Low-temperature \MOS measurements allowed to observe the onset of
differentiated contributions from particle core and surface as the particle
size increases. The magnetic properties measured in the liquid state of the
original emulsion showed that the \FH phase is not present in the liquid
precursor, but precipitates in the micelle cores after the free water is
freeze-dried. Systematic susceptibility \chi_{ac}(\emph{f},T) measurements
showed the dependence of the effective magnetic anisotropy energies
with particle volume, and yielded an effective anisotropy value of kJ/m.Comment: 8 pages, 10 figures. Nanotechnology, v17 (Nov. 2006) In pres
Accelerating Cold Dark Matter Cosmology ()
A new kind of accelerating flat model with no dark energy that is fully
dominated by cold dark matter (CDM) is investigated. The number of CDM
particles is not conserved and the present accelerating stage is a consequence
of the negative pressure describing the irreversible process of gravitational
particle creation. A related work involving accelerating CDM cosmology has been
discussed before the SNe observations [Lima, Abramo & Germano, Phys. Rev. D53,
4287 (1996)]. However, in order to have a transition from a decelerating to an
accelerating regime at low redshifts, the matter creation rate proposed here
includes a constant term of the order of the Hubble parameter. In this case,
does not need to be small in order to solve the age problem and the
transition happens even if the matter creation is negligible during the
radiation and part of the matter dominated phase. Therefore, instead of the
vacuum dominance at redshifts of the order of a few, the present accelerating
stage in this sort of Einstein-de Sitter CDM cosmology is a consequence of the
gravitational particle creation process. As an extra bonus, in the present
scenario does not exist the coincidence problem that plagues models with
dominance of dark energy. The model is able to harmonize a CDM picture with the
present age of the universe, the latest measurements of the Hubble parameter
and the Supernovae observations.Comment: 9 pages, 6 figures, typos corrected, references added, discussion in
Appendix B extende
Physical approximations for the nonlinear evolution of perturbations in dark energy scenarios
The abundance and distribution of collapsed objects such as galaxy clusters
will become an important tool to investigate the nature of dark energy and dark
matter. Number counts of very massive objects are sensitive not only to the
equation of state of dark energy, which parametrizes the smooth component of
its pressure, but also to the sound speed of dark energy as well, which
determines the amount of pressure in inhomogeneous and collapsed structures.
Since the evolution of these structures must be followed well into the
nonlinear regime, and a fully relativistic framework for this regime does not
exist yet, we compare two approximate schemes: the widely used spherical
collapse model, and the pseudo-Newtonian approach. We show that both
approximation schemes convey identical equations for the density contrast, when
the pressure perturbation of dark energy is parametrized in terms of an
effective sound speed. We also make a comparison of these approximate
approaches to general relativity in the linearized regime, which lends some
support to the approximations.Comment: 15 pages, 2 figure
Beyond Mean-Field Low-Lying Excitations of Dipolar Bose Gases
We theoretically investigate various beyond mean-field effects on Bose gases
at zero temperature featuring the anisotropic and long-range dipole-dipole
interaction in addition to the isotropic and short-range contact interaction.
Within the realm of the Bogoliubov-de Gennes theory, we consider static
properties and low-lying excitations of both homogeneous and harmonically
trapped dipolar bosonic gases. For the homogeneous system, the condensate
depletion, the ground-state energy, the equation of state, and the speed of
sound are discussed in detail. Making use of the local density approximation,
we extend these results in order to study the properties of a dipolar Bose gas
in a harmonic trap and in the regime of large particle numbers. After deriving
the equations of motion for the general case of a triaxial trap, we analyze the
influence of quantum fluctuations on important properties of the gas, such as
the equilibrium configuration and the low-lying excitations in the case of a
cylinder-symmetric trap. In addition to the monopole and quadrupole oscillation
modes, we also discuss the radial quadrupole mode. We find that the latter
acquires a quantum correction exclusively due to the dipole-dipole interaction.
As a result, we identify the radial quadrupole as a reasonably accessible
source for the signature of dipolar many-body effects and stress the enhancing
character that dipolar interactions have for quantum fluctuations in the other
oscillation modes.Comment: Version published in PR
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