3,678 research outputs found
Information entropy and dark energy evolution
The information entropy is here investigated in the context of early and late
cosmology under the hypothesis that distinct phases of universe evolution are
entangled between them. The approach is based on the \emph{entangled state
ansatz}, representing a coarse-grained definition of primordial \emph{dark
temperature} associated to an \emph{effective entangled energy density}. The
dark temperature definition comes from assuming either Von Neumann or linear
entropy as sources of cosmological thermodynamics. We interpret the involved
information entropies by means of probabilities of forming structures during
cosmic evolution. Following this recipe, we propose that quantum entropy is
simply associated to the thermodynamical entropy and we investigate the
consequences of our approach using the adiabatic sound speed. As byproducts, we
analyze two phases of universe evolution: the late and early stages. To do so,
we first recover that dark energy reduces to a pure cosmological constant, as
zero-order entanglement contribution, and second that inflation is
well-described by means of an effective potential. In both cases, we infer
numerical limits which are compatible with current observations.Comment: 12 pages, 1 figur
Cosmic acceleration from a single fluid description
We here propose a new class of barotropic factor for matter, motivated by
properties of isotropic deformations of crystalline solids. Our approach is
dubbed Anton-Schmidt's equation of state and provides a non-vanishing, albeit
small, pressure term for matter. The corresponding pressure is thus
proportional to the logarithm of universe's volume, i.e. to the density itself
since . In the context of solid state physics, we
demonstrate that by only invoking standard matter with such a property, we are
able to frame the universe speed up in a suitable way, without invoking a dark
energy term by hand. Our model extends a recent class of dark energy paradigms
named \emph{logotropic} dark fluids and depends upon two free parameters,
namely and . Within the Debye approximation, we find that and
are related to the Gr\"uneisen parameter and the bulk modulus of crystals. We
thus show the main differences between our model and the logotropic scenario,
and we highlight the most relevant properties of our new equation of state on
the background cosmology. Discussions on both kinematics and dynamics of our
new model have been presented. We demonstrate that the CDM model is
inside our approach, as limiting case. Comparisons with CPL parametrization
have been also reported in the text. Finally, a Monte Carlo analysis on the
most recent low-redshift cosmological data allowed us to place constraints on
and . In particular, we found and .Comment: 13 pages, 7 figure
Kinematic model-independent reconstruction of Palatini cosmology
A kinematic treatment to trace out the form of cosmology, within the
Palatini formalism, is discussed by only postulating the universe homogeneity
and isotropy. To figure this out we build model-independent approximations of
the luminosity distance through rational expansions. These approximants extend
the Taylor convergence radii computed for usual cosmographic series. We thus
consider both Pad\'e and the rational Chebyshev polynomials. They can be used
to accurately describe the universe late-time expansion history, providing
further information on the thermal properties of all effective cosmic fluids
entering the energy momentum tensor of Palatini's gravity. To perform our
numerical analysis, we relate the Palatini's Ricci scalar with the Hubble
parameter and thus we write down a single differential equation in terms of
the redshift . Therefore, to bound , we make use of the most recent
outcomes over the cosmographic parameters obtained from combined data surveys.
In particular our clue is to select two scenarios, i.e. Pad\'e and
Chebyshev approximations, since they well approximate the luminosity
distance at the lowest possible order. We find that best analytical matches to
the numerical solutions lead to with free parameters given by the
set for Pad\'e approximation,
whereas with for rational Chebyshev approximation. Finally, our results are
compared with the CDM predictions and with previous studies in the
literature. Slight departures from General Relativity are also discussed.Comment: 10 pages, 6 figures. Accepted for publication in Gen. Rel. Gra
Impacts of fragmented accretion streams onto Classical T Tauri Stars: UV and X-ray emission lines
Context. The accretion process in Classical T Tauri Stars (CTTSs) can be
studied through the analysis of some UV and X-ray emission lines which trace
hot gas flows and act as diagnostics of the post-shock downfalling plasma. In
the UV band, where higher spectral resolution is available, these lines are
characterized by rather complex profiles whose origin is still not clear.
Aims. We investigate the origin of UV and X-ray emission at impact regions of
density structured (fragmented) accretion streams.We study if and how the
stream fragmentation and the resulting structure of the post-shock region
determine the observed profiles of UV and X-ray emission lines.
Methods. We model the impact of an accretion stream consisting of a series of
dense blobs onto the chromosphere of a CTTS through 2D MHD simulations. We
explore different levels of stream fragmentation and accretion rates. From the
model results, we synthesize C IV (1550 {\AA}) and OVIII (18.97 {\AA}) line
profiles.
Results. The impacts of accreting blobs onto the stellar chromosphere produce
reverse shocks propagating through the blobs and shocked upflows. These
upflows, in turn, hit and shock the subsequent downfalling fragments. As a
result, several plasma components differing for the downfalling velocity,
density, and temperature are present altoghether. The profiles of C IV doublet
are characterized by two main components: one narrow and redshifted to speed
50 km s and the other broader and consisting of subcomponents
with redshift to speed in the range 200 400 km s. The profiles
of OVIII lines appear more symmetric than C IV and are redshifted to speed
150 km s.
Conclusions. Our model predicts profiles of C IV line remarkably similar to
those observed and explains their origin in a natural way as due to stream
fragmentation.Comment: 11 pages, 10 figure
Learning Relatedness Measures for Entity Linking
Entity Linking is the task of detecting, in text documents, relevant mentions to entities of a given knowledge base. To this end, entity-linking algorithms use several signals and features extracted from the input text or from the knowl- edge base. The most important of such features is entity relatedness. Indeed, we argue that these algorithms benefit from maximizing the relatedness among the relevant enti- ties selected for annotation, since this minimizes errors in disambiguating entity-linking.
The definition of an e↵ective relatedness function is thus a crucial point in any entity-linking algorithm. In this paper we address the problem of learning high-quality entity relatedness functions. First, we formalize the problem of learning entity relatedness as a learning-to-rank problem. We propose a methodology to create reference datasets on the basis of manually annotated data. Finally, we show that our machine-learned entity relatedness function performs better than other relatedness functions previously proposed, and, more importantly, improves the overall performance of dif- ferent state-of-the-art entity-linking algorithms
Extended Gravity Cosmography
Cosmography can be considered as a sort of a model-independent approach to
tackle the dark energy/modified gravity problem. In this review, the success
and the shortcomings of the CDM model, based on General Relativity and
standard model of particles, are discussed in view of the most recent
observational constraints. The motivations for considering extensions and
modifications of General Relativity are taken into account, with particular
attention to and theories of gravity where dynamics is
represented by curvature or torsion field respectively. The features of
models are explored in metric and Palatini formalisms. We discuss the
connection between gravity and scalar-tensor theories highlighting the
role of conformal transformations in the Einstein and Jordan frames.
Cosmological dynamics of models is investigated through the
corresponding viability criteria. Afterwards, the equivalent formulation of
General Relativity (Teleparallel Equivalent General Relativity) in terms of
torsion and its extension to gravity is considered. Finally, the
cosmographic method is adopted to break the degeneracy among dark energy
models. A novel approach, built upon rational Pad\'e and Chebyshev polynomials,
is proposed to overcome limits of standard cosmography based on Taylor
expansion. The approach provides accurate model-independent approximations of
the Hubble flow. Numerical analyses, based on Monte Carlo Markov Chain
integration of cosmic data, are presented to bound coefficients of the
cosmographic series. These techniques are thus applied to reconstruct
and functions and to frame the late-time expansion history of the
universe with no \emph{a priori} assumptions on its equation of state. A
comparison between the CDM cosmological model with and
models is reported.Comment: 82 pages, 35 figures. Accepted for publication in IJMP
Cosmographic analysis with Chebyshev polynomials
The limits of standard cosmography are here revised addressing the problem of
error propagation during statistical analyses. To do so, we propose the use of
Chebyshev polynomials to parameterize cosmic distances. In particular, we
demonstrate that building up rational Chebyshev polynomials significantly
reduces error propagations with respect to standard Taylor series. This
technique provides unbiased estimations of the cosmographic parameters and
performs significatively better than previous numerical approximations. To
figure this out, we compare rational Chebyshev polynomials with Pad\'e series.
In addition, we theoretically evaluate the convergence radius of (1,1)
Chebyshev rational polynomial and we compare it with the convergence radii of
Taylor and Pad\'e approximations. We thus focus on regions in which convergence
of Chebyshev rational functions is better than standard approaches. With this
recipe, as high-redshift data are employed, rational Chebyshev polynomials
remain highly stable and enable one to derive highly accurate analytical
approximations of Hubble's rate in terms of the cosmographic series. Finally,
we check our theoretical predictions by setting bounds on cosmographic
parameters through Monte Carlo integration techniques, based on the
Metropolis-Hastings algorithm. We apply our technique to high-redshift cosmic
data, using the JLA supernovae sample and the most recent versions of Hubble
parameter and baryon acoustic oscillation measurements. We find that
cosmography with Taylor series fails to be predictive with the aforementioned
data sets, while turns out to be much more stable using the Chebyshev approach.Comment: 17 pages, 6 figures, 5 table
Accretion disk coronae of Intermediate Polar Cataclysmic Variables - 3D MagnetoHydro-Dynamic modeling and thermal X-ray emission
IPCVs contain a magnetic, rotating white dwarf surrounded by a magnetically
truncated accretion disk. To explain their strong flickering X-ray emission,
accretion has been successfully taken into account. Nevertheless, observations
suggest that accretion phenomena could not be the only process behind it. An
intense flaring activity occurring on the surface of the disk may generate a
corona, contribute to the thermal X-ray emission and influence the system
stability. Our purposes are: investigating the formation of an extended corona
above the accretion disk, due to an intense flaring activity occurring on the
disk surface; studying its effects on the disk and stellar magnetosphere;
assessing its contribution to the observed X-ray flux. We have developed a 3D
MHD model of a IPCV. The model takes into account gravity, disk viscosity,
thermal conduction, radiative losses and coronal flare heating. To perform a
parameter space exploration, several system conditions have been considered,
with different magnetic field intensity and disk density values. From the
results of the evolution of the model, we have synthesized the thermal X-ray
emission. The simulations show the formation of an extended corona, linking
disk and star. The flaring activity is capable of strongly influencing the disk
configuration and its stability, effectively deforming the magnetic field
lines. Hot plasma evaporation phenomena occur in the layer immediately above
the disk. The flaring activity gives rise to a thermal X-ray emission in both
the [0.1-2.0] keV and the [2.0-10] keV bands. An intense coronal activity
occurring on the disk surface of an IPCV can affect the structure of the disk
depending noticeably on the density of the disk and the magnetic field of the
central object. Moreover, the synthesis of the thermal X-ray fluxes shows that
this flaring activity may contribute to the observed thermal X-ray emission
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