401 research outputs found
Multi-Point Propagators in Cosmological Gravitational Instability
We introduce the concept of multi-point propagators between linear cosmic
fields and their nonlinear counterparts in the context of cosmological
perturbation theory. Such functions express how a non-linearly evolved Fourier
mode depends on the full ensemble of modes in the initial density field. We
identify and resum the dominant diagrams in the large- limit, showing
explicitly that multi-point propagators decay into the nonlinear regime at the
same rate as the two-point propagator. These analytic results generalize the
large- limit behavior of the two-point propagator to arbitrary order. We
measure the three-point propagator as a function of triangle shape in numerical
simulations and confirm the results of our high- resummation. We show that
any point spectrum can be reconstructed from multi-point propagators, which
leads to a physical connection between nonlinear corrections to the power
spectrum at small scales and higher-order correlations at large scales. As a
first application of these results, we calculate the reduced bispectrum at
one-loop in renormalized perturbation theory and show that we can predict the
decrease in its dependence on triangle shape at redshift zero, when standard
perturbation theory is least successful.Comment: 21 pages, 14 figures. Minor changes to match published version (Fig
11 changed, added reference
2-Point Moments in Cosmological Large Scale Structure: I. Theory and Comparison with Simulations
We present new perturbation theory (PT) predictions in the Spherical Collapse
(SC) model for the 2-point moments of the large-scale distribution of dark
matter density in the universe. We assume that these fluctuations grow under
gravity from small Gaussian initial conditions. These predictions are compared
with numerical simulations and with previous PT results to assess their domain
of validity. We find that the SC model provides in practice a more accurate
description of 2-point moments than previous tree-level PT calculations. The
agreement with simulations is excellent for a wide range of scales (5-50 Mpc/h)
and fluctuations amplitudes (0.02-2 variance). When normalized to unit variance
these results are independent of the cosmological parameters and of the initial
amplitude of fluctuations. The 2-point moments provide a convenient tool to
study the statistical properties of gravitational clustering for fairly
non-linear scales and complicated survey geometries, such as those probing the
clustering of the Ly-alpha forest. In this context, the perturbative SC
predictions presented here, provide a simple and novel way to test the
gravitational instability paradigm.Comment: 10 LaTeX pages, 9 figs, submitted to MNRA
MATLAB and Practical Applications on Climate Variability Studies
In every three-hour session of the tutorial, students will be introduced to practical applications for the study of the
climate system. Those applications will be based on Matlab. For those students that are not familiar
on using the Matlab, in every three-hour sessions there will be an introduction to the working
environment, dealing with matrices, useful functions, logical conditions, saving and loading, data
management, functions & scripts, loops and vectorizaton, etc.Main objective of the course (5 sessions) is the transfer of know-how in practical applications and
management of statistical tools commonly used to explore meteorological time series, using MATLAB, focusing on
applications to study issues related with the climate variability and climate change.Download esercizi.zip from the following alternative location, in order to create your "tutorial work environment".CLARIS EU Project (A Europe-South America
Network for Climate Change Assessment and Impact Studies; GOCE-CT-2003-01454).Unpublishedope
The Matter Bispectrum in N-body Simulations with non-Gaussian Initial Conditions
We present measurements of the dark matter bispectrum in N-body simulations
with non-Gaussian initial conditions of the local kind for a large variety of
triangular configurations and compare them with predictions from Eulerian
Perturbation Theory up to one-loop corrections. We find that the effects of
primordial non-Gaussianity at large scales, when compared to Perturbation
Theory, are well described by the initial component of the matter bispectrum,
linearly extrapolated at the redshift of interest. In addition, we find that,
for f_NL=100, the nonlinear corrections due to non-Gaussian initial conditions
are of the order of ~3, 4% for generic triangles up to ~20% for squeezed
configurations, at any redshift. We show that the predictions of Perturbation
Theory at tree-level fail to describe the simulation results at redshift z=0
already at scales corresponding to k ~ 0.02 - 0.08 h/Mpc, depending on the
triangle, while one-loop corrections can significantly extend their validity to
smaller scales. At higher redshift, one-loop Perturbation Theory provides
indeed quite accurate predictions, particularly with respect to the relative
correction due to primordial non-Gaussianity.Comment: 17 pages, 7 figures. Revised to match journal version with updated
references. Accepted for publication in MNRAS
Constraints on Galaxy Bias, Matter Density, and Primordial Non--Gausianity from the PSCz Galaxy Redshift Survey
We compute the bispectrum for the \IRAS PSCz catalog and find that the galaxy
distribution displays the characteristic signature of gravity. Assuming
Gaussian initial conditions, we obtain galaxy biasing parameters
and , with no sign of
scale-dependent bias for h/Mpc. These results impose stringent
constraints on non-Gaussian initial conditions. For dimensional scaling models
with statistics, we find N>49, which implies a constraint on
primordial skewness .Comment: 4 pages, 3 embedded figures, uses revtex style file, minor changes to
reflect published versio
Generation of Vorticity and Velocity Dispersion by Orbit Crossing
We study the generation of vorticity and velocity dispersion by orbit
crossing using cosmological numerical simulations, and calculate the
backreaction of these effects on the evolution of large-scale density and
velocity divergence power spectra. We use Delaunay tessellations to define the
velocity field, showing that the power spectra of velocity divergence and
vorticity measured in this way are unbiased and have better noise properties
than for standard interpolation methods that deal with mass weighted
velocities. We show that high resolution simulations are required to recover
the correct large-scale vorticity power spectrum, while poor resolution can
spuriously amplify its amplitude by more than one order of magnitude. We
measure the scalar and vector modes of the stress tensor induced by orbit
crossing using an adaptive technique, showing that its vector modes lead, when
input into the vorticity evolution equation, to the same vorticity power
spectrum obtained from the Delaunay method. We incorporate orbit crossing
corrections to the evolution of large scale density and velocity fields in
perturbation theory by using the measured stress tensor modes. We find that at
large scales (k~0.1 h/Mpc) vector modes have very little effect in the density
power spectrum, while scalar modes (velocity dispersion) can induce percent
level corrections at z=0, particularly in the velocity divergence power
spectrum. In addition, we show that the velocity power spectrum is smaller than
predicted by linear theory until well into the nonlinear regime, with little
contribution from virial velocities.Comment: 27 pages, 14 figures. v2: reorganization of the material, new
appendix. Accepted by PR
Modelling large-scale halo bias using the bispectrum
We study the relation between the halo and matter density fields -- commonly
termed bias -- in the LCDM framework. In particular, we examine the local model
of biasing at quadratic order in the matter density. This model is
characterized by parameters b_1 and b_2. Using an ensemble of N-body
simulations, we apply several statistical methods to estimate the parameters.
We measure halo and matter fluctuations smoothed on various scales and find
that the parameters vary with smoothing scale. We argue that, for real-space
measurements, owing to the mixing of wavemodes, no scale can be found for which
the parameters are independent of smoothing. However, this is not the case in
Fourier space. We measure halo power spectra and construct estimates for an
effective large-scale bias. We measure the configuration dependence of the halo
bispectra B_hhh and reduced bispectra Q_hhh for very large-scale k-space
triangles. From this we constrain b_1 and b_2. Using the lowest-order
perturbation theory, we find that for B_hhh the best-fit parameters are in
reasonable agreement with one another as the triangle scale is varied, but that
the fits become poor as smaller scales are included. The same is true for
Q_hhh. The best-fit parameters depend on the discreteness correction. This led
us to consider halo-mass cross-bispectra. The results from these statistics
support our earlier findings. We develop a test to explore the importance of
missing higher-order terms in the models. We prove that low-order expansions
are not able to correctly model the data, even on scales k_1~0.04 h/Mpc. If
robust inferences are to be drawn from galaxy surveys, then accurate models for
the full nonlinear matter bispectrum and trispectrum will be essential.Comment: 23 pages, 7 figures; accepted for publication in MNRA
Observed shift towards earlier spring discharge in the main Alpine rivers
In this study, we analyse the observed long-term discharge time-series of the Rhine, the Danube, the Rhone and the Po rivers. These rivers are characterised by different seasonal cycles reflecting the diverse climates and morphologies of the Alpine basins. However, despite the intensive and varied water management adopted in the four basins, we found common features in the trend and low-frequency variability of the spring discharge timings. All the discharge time-series display a tendency towards earlier spring peaks of more than two weeks per century. These results can be explained in terms of snowmelt, total precipitation (i.e. the sum of snowfall and rainfall) and rainfall variability. The relative importance of these factors might be different in each basin. However, we show that the change of seasonality of total precipitation plays a major role in the earlier spring runoff over most of the Alps
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