4,267 research outputs found
Some remarks on the observational constraints on the self-interacting scalar field model for dark energy
The dark energy component of the cosmic budget is represented by a
self-interacting scalar field. The violation of the null energy condition is
allowed. Hence, such component can also represent a phantom fluid. The model is
tested using supernova type Ia and matter power spectrum data. The supernova
test leads to preferred values for configurations representing the phantom
fluid. The matter power spectrum constraints for the dark energy equation of
state parameter are highly degenerated. In both cases, values for the equation
of state parameter corresponding to the phantom fluid are highly admitted if no
particular prior is used.Comment: Latex file, 12 pages, 12 figures in eps forma
Scalar models for the unification of the dark sector
We review the difficulties of the generalized Chaplygin gas model to fit
observational data, due to the tension between background and perturbative
tests. We argue that such issues may be circumvented by means of a
self-interacting scalar field representation of the model. However, this
proposal seems to be successful only if the self-interacting scalar field has a
non-canonical form. The latter can be implemented in Rastall's theory of
gravity.Comment: Latex file, 8 pages, 3 figures in eps format. To appear in the
proceedings of the CosmoSul conference, held in Rio de Janeiro, Brazil, 01-05
august of 201
Cosmology and stellar equilibrium using Newtonian hydrodynamics with general relativistic pressure
We revisit the analysis made by Hwang and Noh [JCAP 1310 (2013)] aiming the
construction of a Newtonian set of equations incorporating pressure effects
typical of the General Relativity theory. We explicitly derive the Hwang-Noh
equations, comparing them with similar computations found in the literature.
Then, we investigate the cosmological expansion, linear cosmological
perturbations theory and stellar equilibrium by using the new set of
equations and comparing the results with those coming from the usual Newtonian
theory, from the Neo-Newtonian theory and from the General Relativity theory.
We show that the predictions for the background evolution of the Universe are
deeply changed with respect to the General Relativity theory: the acceleration
of the Universe is achieved with positive pressure. On the other hand, the
behaviour of small cosmological perturbations reproduces the one found in the
relativistic context, even if only at small scales. We argue that this last
result may open new possibilities for numerical simulations for structure
formation in the Universe. Finally, the properties of neutron stars are
qualitatively reproduced by Hwang-Noh equations, but the upper mass limit is at
least one order of magnitude higher than the one obtained in General
Relativity.Comment: 15 pages, 4 figures. Section 2 greatly extended with a post-Newtonian
analysis. Final results strengthe
Bouncing solutions in Rastall's theory with a barotropic fluid
Rastall's theory is a modification of Einstein's theory of gravity where the
covariant divergence of the stress-energy tensor is no more vanishing, but
proportional to the gradient of the Ricci scalar. The motivation of this theory
is to investigate a possible non-minimal coupling of the matter fields to
geometry which, being proportional to the curvature scalar, may represent an
effective description of quantum gravity effects. Non-conservation of the
stress-energy tensor, via Bianchi identities, implies new field equations which
have been recently used in a cosmological context, leading to some interesting
results. In this paper we adopt Rastall's theory to reproduce some features of
the effective Friedmann's equation emerging from loop quantum cosmology. We
determine a class of bouncing cosmological solutions and comment about the
possibility of employing these models as effective descriptions of the full
quantum theory.Comment: Latex file, 14 pages, 1 figure in eps format. Typos corrected, one
reference added. Published versio
Magnetically-induced electric polarization in an organo-metallic magnet
The coupling between magnetic order and ferroelectricity has been under
intense investigation in a wide range of transition-metal oxides. The strongest
coupling is obtained in so-called magnetically-induced multiferroics where
ferroelectricity arises directly from magnetic order that breaks inversion
symmetry. However, it has been difficult to find non-oxide based materials in
which these effects occur. Here we present a study of copper dimethyl sulfoxide
dichloride (CDC), an organo-metallic quantum magnet containing Cu
spins, in which electric polarization arises from non-collinear magnetic order.
We show that the electric polarization can be switched in a stunning hysteretic
fashion. Because the magnetic order in CDC is mediated by large organic
molecules, our study shows that magnetoelectric interactions can exist in this
important class of materials, opening the road to designing magnetoelectrics
and multiferroics using large molecules as building blocks. Further, we
demonstrate that CDC undergoes a magnetoelectric quantum phase transition where
both ferroelectric and magnetic order emerge simultaneously as a function of
magnetic field at very low temperatures
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