2,663 research outputs found
The nonlinear redshift-space power spectrum of galaxies
We study the power spectrum of galaxies in redshift space, with third order
perturbation theory to include corrections that are absent in linear theory. We
assume a local bias for the galaxies: i.e. the galaxy density is sampled from
some local function of the underlying mass distribution. We find that the
effect of the nonlinear bias in real space is to introduce two new features:
first, there is a contribution to the power which is constant with wavenumber,
whose nature we reveal as essentially a shot-noise term. In principle this
contribution can mask the primordial power spectrum, and could limit the
accuracy with which the latter might be measured on very large scales.
Secondly, the effect of second- and third-order bias is to modify the effective
bias (defined as the square root of the ratio of galaxy power spectrum to
matter power spectrum). The effective bias is almost scale-independent over a
wide range of scales. These general conclusions also hold in redshift space. In
addition, we have investigated the distortion of the power spectrum by peculiar
velocities, which may be used to constrain the density of the Universe. We look
at the quadrupole-to-monopole ratio, and find that higher-order terms can mimic
linear theory bias, but the bias implied is neither the linear bias, nor the
effective bias referred to above. We test the theory with biased N-body
simulations, and find excellent agreement in both real and redshift space,
providing the local biasing is applied on a scale whose fractional r.m.s.
density fluctuations are .Comment: 13 pages, 7 figures. Accepted by MNRA
Reducing sample variance: halo biasing, non-linearity and stochasticity
Comparing clustering of differently biased tracers of the dark matter
distribution offers the opportunity to reduce the cosmic variance error in the
measurement of certain cosmological parameters. We develop a formalism that
includes bias non-linearities and stochasticity. Our formalism is general
enough that can be used to optimise survey design and tracers selection and
optimally split (or combine) tracers to minimise the error on the
cosmologically interesting quantities. Our approach generalises the one
presented by McDonald & Seljak (2009) of circumventing sample variance in the
measurement of . We analyse how the bias, the noise,
the non-linearity and stochasticity affect the measurements of and explore
in which signal-to-noise regime it is significantly advantageous to split a
galaxy sample in two differently-biased tracers. We use N-body simulations to
find realistic values for the parameters describing the bias properties of dark
matter haloes of different masses and their number density.
We find that, even if dark matter haloes could be used as tracers and
selected in an idealised way, for realistic haloes, the sample variance limit
can be reduced only by up to a factor .
This would still correspond to the gain from a three times larger survey volume
if the two tracers were not to be split. Before any practical application one
should bear in mind that these findings apply to dark matter haloes as tracers,
while realistic surveys would select galaxies: the galaxy-host halo relation is
likely to introduce extra stochasticity, which may reduce the gain further.Comment: 21 pages, 13 figures. Published version in MNRA
Multi-variate joint PDF for non-Gaussianities: exact formulation and generic approximations
We provide an exact expression for the multi-variate joint probability
distribution function of non-Gaussian fields primordially arising from local
transformations of a Gaussian field. This kind of non-Gaussianity is generated
in many models of inflation. We apply our expression to the non- Gaussianity
estimation from Cosmic Microwave Background maps and the halo mass function
where we obtain analytical expressions. We also provide analytic approximations
and their range of validity. For the Cosmic Microwave Background we give a fast
way to compute the PDF which is valid up to 7{\sigma} for fNL values (both true
and sampled) not ruled out by current observations, which consists of
expressing the PDF as a combination of bispectrum and trispectrum of the
temperature maps. The resulting expression is valid for any kind of
non-Gaussianity and is not limited to the local type. The above results may
serve as the basis for a fully Bayesian analysis of the non-Gaussianity
parameter.Comment: Matches accepted verion to JCAP; conclusions unchaged, extra
references adde
Exploring data and model poisoning attacks to deep learning-based NLP systems
Natural Language Processing (NLP) is being recently explored also to its application in supporting malicious activities and objects detection. Furthermore, NLP and Deep Learning have become targets of malicious attacks too. Very recent researches evidenced that adversarial attacks are able to affect also NLP tasks, in addition to the more popular adversarial attacks on deep learning systems for image processing tasks. More precisely, while small perturbations applied to the data set adopted for training typical NLP tasks (e.g., Part-of-Speech Tagging, Named Entity Recognition, etc..) could be easily recognized, models poisoning, performed by the means of altered data models, typically provided in the transfer learning phase to a deep neural networks (e.g., poisoning attacks by word embeddings), are harder to be detected. In this work, we preliminary explore the effectiveness of a poisoned word embeddings attack aimed at a deep neural network trained to accomplish a Named Entity Recognition (NER) task. By adopting the NER case study, we aimed to analyze the severity of such a kind of attack to accuracy in recognizing the right classes for the given entities. Finally, this study represents a preliminary step to assess the impact and the vulnerabilities of some NLP systems we adopt in our research activities, and further investigating some potential mitigation strategies, in order to make these systems more resilient to data and models poisoning attacks
Exploring the impact of data poisoning attacks on machine learning model reliability
Recent years have seen the widespread adoption of Artificial Intelligence techniques in several domains, including healthcare, justice, assisted driving and Natural Language Processing (NLP) based applications (e.g., the Fake News detection). Those mentioned are just a few examples of some domains that are particularly critical and sensitive to the reliability of the adopted machine learning systems. Therefore, several Artificial Intelligence approaches were adopted as support to realize easy and reliable solutions aimed at improving the early diagnosis, personalized treatment, remote patient monitoring and better decision-making with a consequent reduction of healthcare costs. Recent studies have shown that these techniques are venerable to attacks by adversaries at phases of artificial intelligence. Poisoned data set are the most common attack to the reliability of Artificial Intelligence approaches. Noise, for example, can have a significant impact on the overall performance of a machine learning model. This study discusses the strength of impact of noise on classification algorithms. In detail, the reliability of several machine learning techniques to distinguish correctly pathological and healthy voices by analysing poisoning data was evaluated. Voice samples selected by available database, widely used in research sector, the Saarbruecken Voice Database, were processed and analysed to evaluate the resilience and classification accuracy of these techniques. All analyses are evaluated in terms of accuracy, specificity, sensitivity, F1-score and ROC area
Simulação da Produção de Cupuaçu (Theobroma grandiflorum) e Pupunha (Bactris gasipaes) em Sistemas Agroflorestais Utilizando Modelos de Dinâmica de Sistemas.
bitstream/item/64869/1/cot32-2009-safs-marcelo.pd
Análise financeira de sistemas produtivos integrados.
bitstream/item/123060/1/Doc.-274-ArcoVerde.pd
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