591 research outputs found
Measuring primordial non-gaussianity without cosmic variance
Non-gaussianity in the initial conditions of the universe is one of the most
powerful mechanisms to discriminate among the competing theories of the early
universe. Measurements using bispectrum of cosmic microwave background
anisotropies are limited by the cosmic variance, i.e. available number of
modes. Recent work has emphasized the possibility to probe non-gaussianity of
local type using the scale dependence of large scale bias from highly biased
tracers of large scale structure. However, this power spectrum method is also
limited by cosmic variance, finite number of structures on the largest scales,
and by the partial degeneracy with other cosmological parameters that can mimic
the same effect. Here we propose an alternative method that solves both of
these problems. It is based on the idea that on large scales halos are biased,
but not stochastic, tracers of dark matter: by correlating a highly biased
tracer of large scale structure against an unbiased tracer one eliminates the
cosmic variance error, which can lead to a high signal to noise even from the
structures comparable to the size of the survey. The square of error
improvement on non-gaussianity parameter f_nl relative to the power spectrum
method scales as Pn/2, where P and n is the power spectrum and the number
density of the biased tracer, respectively. For an ideal survey out to z=2 the
error reduction can be as large as a factor of seven, which should guarantee a
detection of non-gaussianity from an all sky survey of this type. The
improvements could be even larger if high density tracers that are sensitive to
non-gaussianity can be identified and measured over a large volume.Comment: 7 page
Program for refan JT8D engine design, fabrication and test, phase 2
The objective of the JT8D refan program was to design, fabricate, and test certifiable modifications of the JT8D engine which would reduce noise generated by JT8D powered aircraft. This was to be accomplished without affecting reliability and maintainability, at minimum retrofit cost, and with no performance penalty. The mechanical design, engine performance and stability characteristics at sea-level and altitude, and the engine noise characteristics of the test engines are documented. Results confirmed the structural integrity of the JT8D-109. Engine operation was stable throughout the airplane flight envelope. Fuel consumption of the test engines was higher than that required to meet the goal of no airplane performance penalty, but the causes were identified and corrected during a normal pre-certification engine development program. Compared to the baseline JT8D-109 engine, the acoustically treated JT8D-109 engine showed noise reductions of 6 PNdB at takeoff and 11 PNdB at a typical approach power setting
Does the galaxy correlation length increase with the sample depth?
We have analyzed the behavior of the correlation length, , as a function
of the sample depth by extracting from the CfA2 redshift survey volume--limited
samples out to increasing distances. For a fractal distribution, the value of
would increase with the volume occupied by the sample. We find no linear
increase for the CfA2 samples of the sort that would be expected if the
Universe preserved its small scale fractal character out to the distances
considered (60--100\hmpc). The results instead show a roughly constant value
for as a function of the size of the sample, with small fluctuations due
to local inhomogeneities and luminosity segregation. Thus the fractal picture
can safely be discarded.Comment: Accepted for publication in ApJ
The power spectrum of systematics in cosmic shear tomography and the bias on cosmological parameters
Cosmic shear tomography has emerged as one of the most promising tools to
both investigate the nature of dark energy and discriminate between General
Relativity and modified gravity theories. In order to successfully achieve
these goals, systematics in shear measurements have to be taken into account;
their impact on the weak lensing power spectrum has to be carefully
investigated in order to estimate the bias induced on the inferred cosmological
parameters. To this end, we develop here an efficient tool to compute the power
spectrum of systematics by propagating, in a realistic way, shear measurement,
source properties and survey setup uncertainties. Starting from analytical
results for unweighted moments and general assumptions on the relation between
measured and actual shear, we derive analytical expressions for the
multiplicative and additive bias, showing how these terms depend not only on
the shape measurement errors, but also on the properties of the source galaxies
(namely, size, magnitude and spectral energy distribution). We are then able to
compute the amplitude of the systematics power spectrum and its scaling with
redshift, while we propose a multigaussian expansion to model in a
non-parametric way its angular scale dependence. Our method allows to
self-consistently propagate the systematics uncertainties to the finally
observed shear power spectrum, thus allowing us to quantify the departures from
the actual spectrum. We show that even a modest level of systematics can induce
non-negligible deviations, thus leading to a significant bias on the recovered
cosmological parameters.Comment: 19 pages, 5 tables, 4 figure
Constraining Modified Gravity with Euclid
Future proposed satellite missions as Euclid can offer the opportunity to
test general relativity on cosmic scales through mapping of the galaxy weak
lensing signal. In this paper we forecast the ability of these experiments to
constrain modified gravity scenarios as those predicted by scalar-tensor and
theories. We found that Euclid will improve constraints expected from
the PLANCK satellite on these modified gravity models by two orders of
magnitude. We discuss parameter degeneracies and the possible biases introduced
by modified gravity
Cosmic homogeneity demonstrated with luminous red galaxies
We test the homogeneity of the Universe at with the Luminous Red
Galaxy (LRG) spectroscopic sample of the Sloan Digital Sky Survey. First, the
mean number of LRGs within completely surveyed LRG-centered spheres of
comoving radius is shown to be proportional to at radii greater than
. The test has the virtue that it does not rely
on the assumption that the LRG sample has a finite mean density; its results
show, however, that there \emph{is} such a mean density. Secondly, the survey
sky area is divided into 10 disjoint solid angular regions and the fractional
rms density variations of the LRG sample in the redshift range
among these () regions is found to be 7
percent of the mean density. This variance is consistent with typical biased
\lcdm models and puts very strong constraints on the quality of SDSS
photometric calibration.Comment: submitted to Ap
Endoscopy during the Covid-19 outbreak : experience and recommendations from a single center in a high-incidence scenario
A dramatic SARS-Cov-2 outbreak is hitting Italy hard. To face the new scenario all the hospitals have been re-organised in order to reduce all the outpatient services and to devote almost all their personnel and resources to the management of Covid-19 patients. As a matter of fact, all the services have undergone a deep re-organization guided by: the necessity to reduce exams, to create an environment that helps reduce the virus spread, and to preserve the medical personnel from infection. In these days a re-organization of the endoscopic unit, sited in a high-incidence area, has been adopted, with changes to logistics, work organization and patients selection. With the present manuscript, we want to support gastroenterologists and endoscopists in the organization of a \u201cnew\u201d endoscopy unit that responds to the \u201cnew\u201d scenario, while remaining fully aware that resources, availability and local circumstances may extremely vary from unit to unit
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