2,453 research outputs found
Improving the modelling of redshift-space distortions: I. A bivariate Gaussian description for the galaxy pairwise velocity distributions
As a step towards a more accurate modelling of redshift-space distortions in
galaxy surveys, we develop a general description of the probability
distribution function of galaxy pairwise velocities within the framework of the
so-called streaming model. For a given galaxy separation , such
function can be described as a superposition of virtually infinite local
distributions. We characterize these in terms of their moments and then
consider the specific case in which they are Gaussian functions, each with its
own mean and dispersion . Based on physical considerations, we
make the further crucial assumption that these two parameters are in turn
distributed according to a bivariate Gaussian, with its own mean and covariance
matrix. Tests using numerical simulations explicitly show that with this
compact description one can correctly model redshift-space distorsions on all
scales, fully capturing the overall linear and nonlinear dynamics of the galaxy
flow at different separations. In particular, we naturally obtain
Gaussian/exponential, skewed/unskewed distribution functions, depending on
separation as observed in simulations and data. Also, the recently proposed
single-Gaussian description of redshift-space distortions is included in this
model as a limiting case, when the bivariate Gaussian is collapsed to a
two-dimensional Dirac delta function. We also show how this description
naturally allows for the Taylor expansion of around
, which leads to the Kaiser linear formula when truncated to second
order, expliciting its connection with the moments of the velocity distribution
functions. More work is needed, but these results indicate a very promising
path to make definitive progress in our program to improve RSD estimators.Comment: 11 pages, 3 figures, 2 table
A dyadic model on a tree
We study an infinite system of non-linear differential equations coupled in a
tree-like structure. This system was previously introduced in the literature
and it is the model from which the dyadic shell model of turbulence was
derived. It mimics 3d Euler and Navier-Stokes equations in a rough
approximation of a wavelet decomposition. We prove existence of finite energy
solutions, anomalous dissipation in the inviscid unforced case, existence and
uniqueness of stationary solutions (either conservative or not) in the forced
case
Stochastic Navier-Stokes Equations and Related Models
Regularization by noise for certain classes of fluid dynamic equations, a
theme dear to Giuseppe Da Prato (see G. Da Prato and A. Debussche, Ergodicity
for the 3D stochastic Navier-Stokes equations, J. Math. Pures Appl., 2003), is
reviewed focusing on 3D Navier-Stokes equations and dyadic models of
turbulence
The impact of white noise on a supercritical bifurcation in the Swift-Hohenberg equation
We consider the impact of additive Gaussian white noise on a supercritical
pitchfork bifurcation in an unbounded domain. As an example we focus on the
stochastic Swift-Hohenberg equation with polynomial nonlinearity. Here we
identify the order where small noise first impacts the bifurcation. Using an
approximation via modulation equations, we provide a tool to analyse how the
noise influences the dynamics close to a change of stability.Comment: To appear on Physica D: Nonlinear Phenomen
Group-galaxy correlations in redshift space as a probe of the growth of structure
We investigate the use of the cross-correlation between galaxies and galaxy
groups to measure redshift-space distortions (RSD) and thus probe the growth
rate of cosmological structure. This is compared to the classical approach
based on using galaxy auto-correlation. We make use of realistic simulated
galaxy catalogues that have been constructed by populating simulated dark
matter haloes with galaxies through halo occupation prescriptions. We adapt the
classical RSD dispersion model to the case of the group-galaxy
cross-correlation function and estimate the RSD parameter by fitting
both the full anisotropic correlation function and its multipole
moments. In addition, we define a modified version of the latter statistics by
truncating the multipole moments to exclude strongly non-linear distortions at
small transverse scales. We fit these three observable quantities in our set of
simulated galaxy catalogues and estimate statistical and systematic errors on
for the case of galaxy-galaxy, group-group, and group-galaxy
correlation functions. When ignoring off-diagonal elements of the covariance
matrix in the fitting, the truncated multipole moments of the group-galaxy
cross-correlation function provide the most accurate estimate, with systematic
errors below 3% when fitting transverse scales larger than . Including
the full data covariance enlarges statistical errors but keep unchanged the
level of systematic error. Although statistical errors are generally larger for
groups, the use of group-galaxy cross-correlation can potentially allow the
reduction of systematics while using simple linear or dispersion models.Comment: 18 pages, 16 figure
D-STREAMON: from middlebox to distributed NFV framework for network monitoring
Many reasons make NFV an attractive paradigm for IT security: lowers costs,
agile operations and better isolation as well as fast security updates,
improved incident responses and better level of automation. On the other side,
the network threats tend to be increasingly complex and distributed, implying
huge traffic scale to be monitored and increasingly strict mitigation delay
requirements. Considering the current trend of the net- working and the
requirements to counteract to the evolution of cyber-threats, it is expected
that also network monitoring will move towards NFV based solutions. In this
paper, we present D- StreaMon an NFV-capable distributed framework for network
monitoring realized to face the above described challenges. It relies on the
StreaMon platform, a solution for network monitoring originally designed for
traditional middleboxes. An evolution path which migrates StreaMon from
middleboxes to Virtual Network Functions (VNFs) has been realized.Comment: Short paper at IEEE LANMAN 2017. arXiv admin note: text overlap with
arXiv:1608.0137
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