4,523 research outputs found
Parametrizing growth in dark energy and modified gravity models
It is well-known that an extremely accurate parametrization of the growth
function of matter density perturbations in CDM cosmology, with errors
below , is given by with . In this work, we show that a simple modification of this
expression also provides a good description of growth in modified gravity
theories. We consider the model-independent approach to modified gravity in
terms of an effective Newton constant written as and show
that provides fits to the numerical
solutions with similar accuracy to that of CDM. In the
time-independent case with , simple analytic expressions for
and are presented. In the time-dependent (but
scale-independent) case , we show that has the same time
dependence as . As an example, explicit formalae are provided in the
DGP model. In the general case, for theories with , we obtain a
perturbative expansion for around the General Relativity case
which, for theories, reaches an accuracy below . Finally,
as an example we apply the obtained fitting functions in order to forecast the
precision with which future galaxy surveys will be able to measure the
parameter.Comment: 12 pages, 12 figures. New section on applications to forecasts for
galaxy surveys and new references included. Matches version published in PR
Representative galaxy age-metallicity relationships
The ongoing surveys of galaxies and those for the next generation of
telescopes will demand the execution of high-CPU consuming machine codes for
recovering detailed star formation histories (SFHs) and hence age-metallicity
relationships (AMRs). We present here an expeditive method which provides
quick-look AMRs on the basis of representative ages and metallicities obtained
from colour-magnitude diagram (CMD) analyses. We have tested its perfomance by
generating synthetic CMDs for a wide variety of galaxy SFHs. The representative
AMRs turn out to be reliable down to a magnitude limit with a photometric
completeness factor higher than 85 per cent, and trace the chemical
evolution history for any stellar population (represented by a mean age and an
intrinsic age spread) with a total mass within ~ 40 per cent of the more
massive stellar population in the galaxy.Comment: 12 pages, 11 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Predicción de la frescura del aceite de oliva virgen extra durante el almacenamiento mediante espectroscopía de fluorescencia
Virgin olive oil quality relates to flavor and unique health benefits. Some of these properties are
at the most desirable level when the oil is just extracted, since it is not a product that improves with age. On the
contrary, the concentrations of many compounds change during its shelf-life. These changes reveal the aging of
the oil but do not necessarily mean decay in sensory properties, so in some cases an aged oil from healthy olives
may be better qualified than a fresh one from olives affected by fermentation. The aim of this work is to analyze
different methodologies proposed for assessing the quality of virgin olive oil with implications in freshness and
aging of the oil, and to highlight the possibilities of rapid spectrofluorimetric techniques for assessing oil freshness by checking the evolution of pigments during storage. The observed change in the selected spectral features
and mathematical modelling over time was compared with the accepted model for predicting the amount of
pyropheophytin a, which is based on isokinetic studies. The best regression was obtained for 655 nm (adjustedR2
= 0.91) wavelength, which matches the distinctive band of pigments. The two mathematical models described
in this study highlight the usefulness of pigments in the prediction of the shelf-life of extra virgin olive oil.La calidad del aceite de oliva virgen está relacionada con su flavor y sus beneficios
únicos para la salud. Algunas de estas propiedades se encuentran en el nivel más deseable cuando el aceite está
recién extraído, ya que no es un producto que mejore con el tiempo. Por el contrario, las concentraciones de
muchos compuestos cambian a lo largo de la vida útil. Estos cambios revelan el envejecimiento del aceite, pero
no implican necesariamente la alteración de las propiedades sensoriales, por lo que en algunos casos un aceite
envejecido procedente de aceitunas sanas puede presentar mejor calidad que uno fresco procedente de aceitunas
afectadas por procesos de fermentación. El objetivo de este trabajo es estudiar diferentes metodologías propuestas para evaluar la calidad del aceite de oliva virgen con implicaciones en la frescura y el envejecimiento del
aceite, destacando las posibilidades de las rápidas técnicas espectrofluorométricas para evaluar la frescura del
aceite verificando la evolución de los pigmentos durante el almacenamiento. El cambio observado en las características espectrales seleccionadas y su modelado matemático a lo largo del tiempo se comparó con el modelo
aceptado para predecir la cantidad de pirofeofitina a, que se basa en estudios isocinéticos. Los dos modelos
matemáticos descritos en este estudio pusieron de manifiesto la utilidad de los pigmentos en la predicción de la
vida útil del aceite de oliva virgen extra. La mejor regresión se obtuvo para 655 nm (R2
-ajustado = 0,91), longitud de onda que coincide con la banda distintiva de pigmentos.Secretaría de Estado de Investigación, Desarrollo e Innovación de España-AGL2015-69320-
The age-metallicity relationship in the Fornax spheroidal dwarf galaxy
We produce a comprehensive field star age-metallicity relationship (AMR) from
the earliest epoch until ~ 1 Gyr ago for three fields in the Fornax dSph galaxy
by using VI photometric data obtained with FORS1 at the VLT. We find that the
innermost one does not contains dominant very old stars (age > 12 Gyr), whereas
the relatively outer field does not account for representative star field
populations younger than ~ 3 Gyr. When focusing on the most prominent stellar
populations, we find that the derived AMRs are engraved by the evidence of a
outside-in star formation process. The studied fields show bimodal metallicity
distributions peaked at [Fe/H] = (-0.95 +- 0.15) dex and (-1.15 or -1.25 +-
0.05) dex, respectively, but only during the first half of the entire galaxy
lifetime. Furthermore, the more metal-rich population appears to be more
numerous in the outer fields, while in the innermost Fornax field the
contribution of both metallicity populations seems to be similar. We also find
that the metallicity spread ~ 6 Gyr ago is remarkable large, while the
intrinsic metallicity dispersion at ~ 1-2 Gyr results smaller than that for the
relatively older generations of stars. We interpret these outcomes as a result
of a possible merger of two galaxies that would have triggered a star formation
bursting process that peaked between ~ 6 and 9 Gyr ago, depending on the
position of the field in the galaxy.Comment: 7 pages, 5 figures, MNRAS, in pres
Detection of advanced persistent threat using machine-learning correlation analysis
As one of the most serious types of cyber attack, Advanced Persistent Threats (APT) have caused major concerns on a global scale. APT refers to a persistent, multi-stage attack with the intention to compromise the system and gain information from the targeted system, which has the potential to cause significant damage and substantial financial loss. The accurate detection and prediction of APT is an ongoing challenge. This work proposes a novel machine learning-based system entitled MLAPT, which can accurately and rapidly detect and predict APT attacks in a systematic way. The MLAPT runs through three main phases: (1) Threat detection, in which eight methods have been developed to detect different techniques used during the various APT steps. The implementation and validation of these methods with real traffic is a significant contribution to the current body of research; (2) Alert correlation, in which a correlation framework is designed to link the outputs of the detection methods, aims to identify alerts that could be related and belong to a single APT scenario; and (3) Attack prediction, in which a machine learning-based prediction module is proposed based on the correlation framework output, to be used by the network security team to determine the probability of the early alerts to develop a complete APT attack. MLAPT is experimentally evaluated and the presented sy
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