3,432 research outputs found
The Einstein-Friedrich-nonlinear scalar field system and the stability of scalar field Cosmologies
A frame representation is used to derive a first order quasi-linear symmetric
hyperbolic system for a scalar field minimally coupled to gravity. This
procedure is inspired by similar evolution equations introduced by Friedrich to
study the Einstein-Euler system. The resulting evolution system is used to show
that small nonlinear perturbations of expanding
Friedman-Lema\^itre-Robertson-Walker backgrounds, with scalar field potentials
satisfying certain future asymptotic conditions, decay exponentially to zero,
in synchronous time.Comment: Version 4: Matches final published versio
Canonical Realizations of Doubly Special Relativity
Doubly Special Relativity is usually formulated in momentum space, providing
the explicit nonlinear action of the Lorentz transformations that incorporates
the deformation of boosts. Various proposals have appeared in the literature
for the associated realization in position space. While some are based on
noncommutative geometries, others respect the compatibility of the spacetime
coordinates. Among the latter, there exist several proposals that invoke in
different ways the completion of the Lorentz transformations into canonical
ones in phase space. In this paper, the relationship between all these
canonical proposals is clarified, showing that in fact they are equivalent. The
generalized uncertainty principles emerging from these canonical realizations
are also discussed in detail, studying the possibility of reaching regimes
where the behavior of suitable position and momentum variables is classical,
and explaining how one can reconstruct a canonical realization of doubly
special relativity starting just from a basic set of commutators. In addition,
the extension to general relativity is considered, investigating the kind of
gravity's rainbow that arises from this canonical realization and comparing it
with the gravity's rainbow formalism put forward by Magueijo and Smolin, which
was obtained from a commutative but noncanonical realization in position space.Comment: 18 pages, accepted for publication in International Journal of Modern
Physics
First order perturbations of the Einstein-Straus and Oppenheimer-Snyder models
We derive the linearly perturbed matching conditions between a Schwarzschild
spacetime region with stationary and axially symmetric perturbations and a FLRW
spacetime with arbitrary perturbations. The matching hypersurface is also
perturbed arbitrarily and, in all cases, the perturbations are decomposed into
scalars using the Hodge operator on the sphere. This allows us to write down
the matching conditions in a compact way. In particular, we find that the
existence of a perturbed (rotating, stationary and vacuum) Schwarzschild cavity
in a perturbed FLRW universe forces the cosmological perturbations to satisfy
constraints that link rotational and gravitational wave perturbations. We also
prove that if the perturbation on the FLRW side vanishes identically, then the
vacuole must be perturbatively static and hence Schwarzschild. By the dual
nature of the problem, the first result translates into links between
rotational and gravitational wave perturbations on a perturbed
Oppenheimer-Snyder model, where the perturbed FLRW dust collapses in a
perturbed Schwarzschild environment which rotates in equilibrium. The second
result implies in particular that no region described by FLRW can be a source
of the Kerr metric.Comment: LaTeX; 29 page
Prescriptions in Loop Quantum Cosmology: A comparative analysis
Various prescriptions proposed in the literature to attain the polymeric
quantization of a homogeneous and isotropic flat spacetime coupled to a
massless scalar field are carefully analyzed in order to discuss their
differences. A detailed numerical analysis confirms that, for states which are
not deep in the quantum realm, the expectation values and dispersions of some
natural observables of interest in cosmology are qualitatively the same for all
the considered prescriptions. On the contrary, the amplitude of the wave
functions of those states differs considerably at the bounce epoch for these
prescriptions. This difference cannot be absorbed by a change of
representation. Finally, the prescriptions with simpler superselection sectors
are clearly more efficient from the numerical point of view.Comment: 18 pages, 6 figures, RevTex4-1 + BibTe
From stellar to planetary composition: Galactic chemical evolution of Mg/Si mineralogical ratio
The main goal of this work is to study element ratios that are important for
the formation of planets of different masses. We study potential correlations
between the existence of planetary companions and the relative elemental
abundances of their host stars. We use a large sample of FGK-type dwarf stars
for which precise Mg, Si, and Fe abundances have been derived using HARPS
high-resolution and high-quality data. A first analysis of the data suggests
that low-mass planet host stars show higher [Mg/Si] ratios, while giant planet
hosts present [Mg/Si] that is lower than field stars. However, we found that
the [Mg/Si] ratio significantly depends on metallicity through Galactic
chemical evolution. After removing the Galactic evolution trend only the
difference in the [Mg/Si] elemental ratio between low-mass planet hosts and
non-hosts was present in a significant way. These results suggests that
low-mass planets are more prevalent around stars with high [Mg/Si]. Our results
demonstrate the importance of Galactic chemical evolution and indicate that it
may play an important role in the planetary internal structure and composition.Comment: Accepted by A&A (Letter to the Editor
Publindex: Aweb service to automatically evaluate research publications according to customized criteria
We introduce Publindex, a system that retrieves, classifies, and returns research publications of a given researcher according to the criteria and in the format predefined by the user
Big Bounce and inhomogeneities
The dynamics of an inhomogeneous universe is studied with the methods of Loop
Quantum Cosmology as an example of the quantization of vacuum cosmological
spacetimes containing gravitational waves (Gowdy spacetimes). The analysis
performed at the effective level shows that: (i) The initial Big Bang
singularity is replaced (as in the case of homogeneous cosmological models) by
a Big Bounce, joining deterministically two large universes, (ii) the universe
size at the bounce is at least of the same order of magnitude as that of the
background homogeneous universe, (iii) for each gravitational wave mode, the
difference in amplitude at very early and very late times has a vanishing
statistical average when the bounce dynamics is strongly dominated by the
inhomogeneities, whereas this average is positive when the dynamics is in a
near-vacuum regime, so that statistically the inhomogeneities are amplified.Comment: RevTex4, 4 pages, 2 figure
Searching for solar siblings among the HARPS data
The search for the solar siblings has been particularly fruitful in the last
few years. Until now, there are four plausible candidates pointed out in the
literature: HIP21158, HIP87382, HIP47399, and HIP92831. In this study we
conduct a search for solar siblings among the HARPS high-resolution FGK dwarfs
sample, which includes precise chemical abundances and kinematics for 1111
stars. Using a new approach based on chemical abundance trends with the
condensation temperature, kinematics, and ages we found one (additional)
potential solar sibling candidate: HIP97507.Comment: 4 pages, 2 figures, 1 table. Accepted in A&
Inhomogeneous Loop Quantum Cosmology: Hybrid Quantization of the Gowdy Model
The Gowdy cosmologies provide a suitable arena to further develop Loop
Quantum Cosmology, allowing the presence of inhomogeneities. For the particular
case of Gowdy spacetimes with the spatial topology of a three-torus and a
content of linearly polarized gravitational waves, we detail a hybrid quantum
theory in which we combine a loop quantization of the degrees of freedom that
parametrize the subfamily of homogeneous solutions, which represent Bianchi I
spacetimes, and a Fock quantization of the inhomogeneities. Two different
theories are constructed and compared, corresponding to two different schemes
for the quantization of the Bianchi I model within the {\sl improved dynamics}
formalism of Loop Quantum Cosmology. One of these schemes has been recently put
forward by Ashtekar and Wilson-Ewing. We address several issues including the
quantum resolution of the cosmological singularity, the structure of the
superselection sectors in the quantum system, or the construction of the
Hilbert space of physical states.Comment: 16 pages, version accepted for publication in Physical Review
Hybrid Quantization: From Bianchi I to the Gowdy Model
The Gowdy cosmologies are vacuum solutions to the Einstein equations which
possess two space-like Killing vectors and whose spatial sections are compact.
We consider the simplest of these cosmological models: the case where the
spatial topology is that of a three-torus and the gravitational waves are
linearly polarized. The subset of homogeneous solutions to this Gowdy model are
vacuum Bianchi I spacetimes with a three-torus topology. We deepen the analysis
of the loop quantization of these Bianchi I universes adopting the improved
dynamics scheme put forward recently by Ashtekar and Wilson-Ewing. Then, we
revisit the hybrid quantization of the Gowdy cosmologies by combining
this loop quantum cosmology description with a Fock quantization of the
inhomogeneities over the homogeneous Bianchi I background. We show that, in
vacuo, the Hamiltonian constraint of both the Bianchi I and the Gowdy models
can be regarded as an evolution equation with respect to the volume of the
Bianchi I universe. This evolution variable turns out to be discrete, with a
strictly positive minimum. Furthermore, we argue that this evolution is
well-defined inasmuch as the associated initial value problem is well posed:
physical solutions are completely determined by the data on an initial section
of constant Bianchi I volume. This fact allows us to carry out to completion
the quantization of these two cosmological models.Comment: 20 pages, version accepted for publication in Physical Review
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