690 research outputs found
Pure state estimation and the characterization of entanglement
A connection between the state estimation problem and the separability
problem is noticed and exploited to find efficient numerical algorithms to
solve the first one. Based on these ideas, we also derive a systematic method
to obtain upper bounds on the maximum local fidelity when the states are
distributed among several distant parties.Comment: Closer to published versio
Perturbations in Hybrid Loop Quantum Cosmology: Continuum Limit in Fourier Space
We analyze the passage to a continuum limit of the mode spectrum of
primordial perturbations around flat cosmological spacetimes in hybrid Loop
Quantum Cosmology, showing that this limit can be reached even if one starts by
considering a finite fiducial cell as spatial slice. We focus our attention on
regimes in which the background cosmology follows the effective dynamics of
Loop Quantum Cosmology, although we comment on extensions of our arguments
beyond this regime, as well as to some formalisms other than the hybrid
approach. Whereas the perturbed system can be described in an invariant way
under changes of the fiducial volume using the standard variables of the
improved prescription for Loop Quantum Cosmology, we show that the desired
continuum limit can be established by means of scaling transformations of the
physical volume when this volume grows unboundedly. These transformations lead
to a model with a continuum of modes and independent of any scale of reference
for the physical volume. For the sake of comparison, we also consider an
alternative road to the continuum in Fourier space that has been employed in
geometrodynamics and is based on the use of scaling transformations of the
fiducial volume, together with variables that are independent of them.Comment: 13 page
Modified FRW cosmologies arising from states of the hybrid quantum Gowdy model
We construct approximate solutions of the hybrid quantum Gowdy cosmology with
three-torus topology, linear polarization, and local rotational symmetry, in
the presence of a massless scalar field. More specifically, we determine some
families of states for which the complicated inhomogeneous and anisotropic
Hamiltonian constraint operator of the Gowdy model is approximated by a much
simpler one. Our quantum states follow the dynamics governed by this simpler
constraint, while being at the same time also approximate solutions of the full
Gowdy model. This is so thanks to the quantum correlations that the considered
states present between the isotropic and anisotropic sectors of the model.
Remarkably, this simpler constraint can be regarded as that of a flat
Friedmann-Robertson-Walker universe filled with different kinds of perfect
fluids and geometrically corrected by homogeneous and isotropic curvature-like
terms. Therefore, our quantum states, which are intrinsically inhomogeneous,
admit approximate homogeneous and isotropic effective descriptions similar to
those considered in modified theories of gravity.Comment: Version accepted for publication in PR
Fermions in Hybrid Loop Quantum Cosmology
This work pioneers the quantization of primordial fermion perturbations in
hybrid Loop Quantum Cosmology (LQC). We consider a Dirac field coupled to a
spatially flat, homogeneous, and isotropic cosmology, sourced by a scalar
inflaton, and treat the Dirac field as a perturbation. We describe the
inhomogeneities of this field in terms of creation and annihilation variables,
chosen to admit a unitary evolution if the Dirac fermion were treated as a test
field. Considering instead the full system, we truncate its action at quadratic
perturbative order and construct a canonical formulation. In particular this
implies that, in the global Hamiltonian constraint of the model, the
contribution of the homogeneous sector is corrected with a quadratic
perturbative term. We then adopt the hybrid LQC approach to quantize the full
model, combining the loop representation of the homogeneous geometry with the
Fock quantization of the inhomogeneities. We assume a Born-Oppenheimer ansatz
for physical states and show how to obtain a Schr\"odinger equation for the
quantum evolution of the perturbations, where the role of time is played by the
homogeneous inflaton. We prove that the resulting quantum evolution of the
Dirac field is indeed unitary, despite the fact that the underlying homogeneous
geometry has been quantized as well. Remarkably, in such evolution, the fermion
field couples to an infinite sequence of quantum moments of the homogeneous
geometry. Moreover, the evolved Fock vacuum of our fermion perturbations is
shown to be an exact solution of the Schr\"odinger equation. Finally, we
discuss in detail the quantum backreaction that the fermion field introduces in
the global Hamiltonian constraint. For completeness, our quantum study includes
since the beginning (gauge-invariant) scalar and tensor perturbations, that
were studied in previous works.Comment: 29 pages. It matches published versio
VOSA: Virtual Observatory SED Analyzer. An application to the Collinder 69 open cluster
The physical properties of almost any kind of astronomical object can be
derived by fitting synthetic spectra or photometry extracted from theoretical
models to observational data.
We want to develop an automatic procedure to perform this kind of fittings to
a relatively large sample of members of a stellar association and apply this
methodology to the case of Collinder 69.
We combine the multiwavelength data of our sources and follow a work-flow to
derive the physical parameters of the sources. The key step of the work-flow is
performed by a new VO-tool, VOSA. All the steps in this process are done in a
VO environment.
We present this new tool, and provide physical parameters such as T, gravity, luminosity, etc. for 170 candidate members to Collinder
69, and an upper-limit for the age of this stellar association.
This kind of studies of star forming regions, clusters, etc. produces a huge
amount of data, very tedious to analyse using the traditional methodology.
Thus, they are excellent examples where to apply the VO capabilities.Comment: Accepted for publication in A&
Modeling effective FRW cosmologies with perfect fluids from states of the hybrid quantum Gowdy model
We employ recently developed approximation methods in the hybrid quantization
of the Gowdy model with linear polarization and a massless scalar field
to obtain physically interesting solutions of this inhomogeneous cosmology.
More specifically, we propose approximate solutions of the quantum Gowdy model
constructed in such a way that, for the Hamiltonian constraint, they
effectively behave as those corresponding to a flat homogeneous and isotropic
universe filled with a perfect fluid, even though these quantum states are far
from being homogeneous and isotropic. We analyze how one can get different
perfect fluid effective behaviors, including the cases of dust, radiation, and
cosmological constant.Comment: Version accepted for publication in PR
Time-dependent mass of cosmological perturbations in the hybrid and dressed metric approaches to loop quantum cosmology
Loop quantum cosmology has recently been applied in order to extend the
analysis of primordial perturbations to the Planck era and discuss the possible
effects of quantum geometry on the cosmic microwave background. Two approaches
to loop quantum cosmology with admissible ultraviolet behavior leading to
predictions that are compatible with observations are the so-called hybrid and
dressed metric approaches. In spite of their similarities and relations, we
show in this work that the effective equations that they provide for the
evolution of the tensor and scalar perturbations are somewhat different. When
backreaction is neglected, the discrepancy appears only in the time- dependent
mass term of the corresponding field equations. We explain the origin of this
difference, arising from the distinct quantization procedures. Besides, given
the privileged role that the big bounce plays in loop quantum cosmology, e.g.
as a natural instant of time to set initial conditions for the perturbations,
we also analyze the positivity of the time-dependent mass when this bounce
occurs. We prove that the mass of the tensor perturbations is positive in the
hybrid approach when the kinetic contribution to the energy density of the
inflaton dominates over its potential, as well as for a considerably large
sector of backgrounds around that situation, while this mass is always
nonpositive in the dressed metric approach. Similar results are demonstrated
for the scalar perturbations in a sector of background solutions that includes
the kinetically dominated ones; namely, the mass then is positive for the
hybrid approach, whereas it typically becomes negative in the dressed metric
case. More precisely, this last statement is strictly valid when the potential
is quadratic for values of the inflaton mass that are phenomenologically
favored.Comment: 16 pages, 3 figures. Version to be published in PR
The young, wide and very low mass visual binary LOri167
We look for wide, faint companions around members of the 5 Myr Lambda Orionis
open cluster. We used optical, near-infrared, and Spitzer/IRAC photometry. We
report the discovery of a very wide very low mass visual binary, LOri167,
formed by a brown dwarf and a planetary-mass candidate located at 5 arcsec,
which seems to belong to the cluster. We derive Teff of 2125 and 1750 K. If
they are members, comparisons with theoretical models indicate masses of 17
(20-15) Mjup and 8 (13-7) Mjup, with a projected separation of 2000 AU. Such a
binary system would be difficult to explain in most models, particularly those
where substellar objects form in the disks surrounding higher mass stars.Comment: Astronomy & Astrophysics Letters, in pres
The Vacuum State of Primordial Fluctuations in Hybrid Loop Quantum Cosmology
We investigate the role played by the vacuum of the primordial fluctuations
in hybrid Loop Quantum Cosmology. We consider scenarios where the inflaton
potential is a mass term and the unperturbed quantum geometry is governed by
the effective dynamics of Loop Quantum Cosmology. In this situation, the
phenomenologically interesting solutions have a preinflationary regime where
the kinetic energy of the inflaton dominates over the potential. For these kind
of solutions, we show that the primordial power spectra depend strongly on the
choice of vacuum. We study in detail the case of adiabatic states of low order
and the non-oscillating vacuum introduced by Mart\'in de Blas and Olmedo, all
imposed at the bounce. The adiabatic spectra are typically suppressed at large
scales, and display rapid oscillations with an increase of power at
intermediate scales. In the non-oscillating vacuum, there is power suppression
for large scales, but the rapid oscillations are absent. We argue that the
oscillations are due to the imposition of initial adiabatic conditions in the
region of kinetic dominance, and that they would also be present in General
Relativity. Finally, we discuss the sensitivity of our results to changes of
the initial time and other data of the model.Comment: 29 pages, 13 figure
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