372 research outputs found
Primordial Non-Gaussianity and the Statistics of Weak Lensing and other Projected Density Fields
Estimators for weak lensing observables such as shear and convergence
generally have non-linear corrections, which, in principle, make weak lensing
power spectra sensitive to primordial non-Gaussianity. In this paper, we
quantitatively evaluate these contributions for weak lensing auto- and
cross-correlation power spectra, and show that they are strongly suppressed by
projection effects. This is a consequence of the central limit theorem, which
suppresses departures from Gaussianity when the projection reaches over several
correlation lengths of the density field, L_P~55 [Mpc/h]. Furthermore, the
typical scales that contribute to projected bispectra are generally smaller
than those that contribute to projected power spectra. Both of these effects
are not specific to lensing, and thus affect the statistics of non-linear
tracers (e.g., peaks) of any projected density field. Thus, the clustering of
biased tracers of the three-dimensional density field is generically more
sensitive to non-Gaussianity than observables constructed from projected
density fields.Comment: 10 pages, 6 figure
DEMNUni: Massive neutrinos and the bispectrum of large scale structures
The main effect of massive neutrinos on the large-scale structure consists in
a few percent suppression of matter perturbations on all scales below their
free-streaming scale. Such effect is of particular importance as it allows to
constraint the value of the sum of neutrino masses from measurements of the
galaxy power spectrum. In this work, we present the first measurements of the
next higher-order correlation function, the bispectrum, from N-body simulations
that include massive neutrinos as particles. This is the simplest statistics
characterising the non-Gaussian properties of the matter and dark matter halos
distributions. We investigate, in the first place, the suppression due to
massive neutrinos on the matter bispectrum, comparing our measurements with the
simplest perturbation theory predictions, finding the approximation of
neutrinos contributing at quadratic order in perturbation theory to provide a
good fit to the measurements in the simulations. On the other hand, as
expected, a linear approximation for neutrino perturbations would lead to
O() errors on the total matter bispectrum at large scales. We then
attempt an extension of previous results on the universality of linear halo
bias in neutrino cosmologies, to non-linear and non-local corrections finding
consistent results with the power spectrum analysis.Comment: 22 pages, 11 figure
Nonlinear effects of dark energy clustering beyond the acoustic scales
We extend the resummation method of Anselmi & Pietroni (2012) to compute the
total density power spectrum in models of quintessence characterized by a
vanishing speed of sound. For standard CDM cosmologies, this
resummation scheme allows predictions with an accuracy at the few percent level
beyond the range of scales where acoustic oscillations are present, therefore
comparable to other, common numerical tools. In addition, our theoretical
approach indicates an approximate but valuable and simple relation between the
power spectra for standard quintessence models and models where scalar field
perturbations appear at all scales. This, in turn, provides an educated guess
for the prediction of nonlinear growth in models with generic speed of sound,
particularly valuable since no numerical results are yet available.Comment: 28 pages, 12 figure
Extragalactic gamma-ray signal from Dark Matter annihilation: a power spectrum based computation
We revisit the computation of the extragalactic gamma-ray signal from
cosmological dark matter annihilations. The prediction of this signal is
notoriously model dependent, due to different descriptions of the clumpiness of
the dark matter distribution at small scales, responsible for an enhancement
with respect to the smoothly distributed case. We show how a direct computation
of this "flux multiplier" in terms of the nonlinear power spectrum offers a
conceptually simpler approach and may ease some problems, such as the
extrapolation issue. In fact very simple analytical recipes to construct the
power spectrum yield results similar to the popular Halo Model expectations,
with a straightforward alternative estimate of errors. For this specific
application, one also obviates to the need of identifying (often
literature-dependent) concepts entering the Halo Model, to compare different
simulations.Comment: 6 pages, 2 figures; minor changes, additional references, matches
published version; Mon. Not. R. Astron. Soc. Letters, Feb. 7 (2012
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
Accurate fitting functions for peculiar velocity spectra in standard and massive-neutrino cosmologies
We estimate the velocity field in a large set of -body simulations
including massive neutrino particles, and measure the auto-power spectrum of
the velocity divergence field as well as the cross-power spectrum between the
cold dark matter density and the velocity divergence. We perform these
measurements at four different redshifts and within four different cosmological
scenarios, covering a wide range in neutrino masses. We find that the nonlinear
correction to the velocity power spectra largely depend on the degree of
nonlinear evolution with no specific dependence on the value of neutrino mass.
We provide a fitting formula, based on the value of the r.m.s. of the matter
fluctuations in spheres of Mpc, describing the nonlinear corrections
with 3\% accuracy on scales below Mpc.Comment: 8 pages, 5 figures, accepted by A&A, typos corrected in equation 1
Extragalactic gamma-ray signal from dark matter annihilation: an appraisal
We re-evaluate the extragalactic gamma-ray flux prediction from dark matter
annihilation in the approach of integrating over the nonlinear matter power
spectrum, extrapolated to the free-streaming scale. We provide an estimate of
the uncertainty based entirely on available N-body simulation results and
minimal theoretical assumptions. We illustrate how an improvement in the
simulation resolution, exemplified by the comparison between the Millennium and
Millennium II simulations, affects our estimate of the flux uncertainty and we
provide a "best guess" value for the flux multiplier, based on the assumption
of stable clustering for the dark matter perturbations described as a
collision-less fluid. We achieve results comparable to traditional Halo Model
calculations, but with a much simpler procedure and a more general approach, as
it relies only on one, directly measurable quantity. In addition we discuss the
extension of our calculation to include baryonic effects as modeled in
hydrodynamical cosmological simulations and other possible sources of
uncertainty that would in turn affect indirect dark matter signals. Upper limit
on the integrated power spectrum from supernovae lensing magnification are also
derived and compared with theoretical expectations.Comment: 20 pages, 9 figures, 1 table. Updated to match the published version.
New material and figures added, conclusions unchange
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