5,406 research outputs found
A Markov Chain Monte Carlo Algorithm for analysis of low signal-to-noise CMB data
We present a new Monte Carlo Markov Chain algorithm for CMB analysis in the
low signal-to-noise regime. This method builds on and complements the
previously described CMB Gibbs sampler, and effectively solves the low
signal-to-noise inefficiency problem of the direct Gibbs sampler. The new
algorithm is a simple Metropolis-Hastings sampler with a general proposal rule
for the power spectrum, C_l, followed by a particular deterministic rescaling
operation of the sky signal. The acceptance probability for this joint move
depends on the sky map only through the difference of chi-squared between the
original and proposed sky sample, which is close to unity in the low
signal-to-noise regime. The algorithm is completed by alternating this move
with a standard Gibbs move. Together, these two proposals constitute a
computationally efficient algorithm for mapping out the full joint CMB
posterior, both in the high and low signal-to-noise regimes.Comment: Submitted to Ap
The two-and three-point correlation functions of the polarized five-year WMAP sky maps
We present the two- and three-point real space correlation functions of the
five-year WMAP sky maps, and compare the observed functions to simulated LCDM
concordance model ensembles. In agreement with previously published results, we
find that the temperature correlation functions are consistent with
expectations. However, the pure polarization correlation functions are
acceptable only for the 33GHz band map; the 41, 61, and 94 GHz band correlation
functions all exhibit significant large-scale excess structures. Further, these
excess structures very closely match the correlation functions of the two
(synchrotron and dust) foreground templates used to correct the WMAP data for
galactic contamination, with a cross-correlation statistically significant at
the 2sigma-3sigma confidence level. The correlation is slightly stronger with
respect to the thermal dust template than with the synchrotron template.Comment: 10 pages, 5 figures, published in ApJ. v2: New title, minor changes
to appendix, and fixed some typos. v3: Matches version published in Ap
Ecosystem services of biodiversity in organic grasslands
The use of multi-species mixtures in herbage production can add value in terms of improved conditions for pollinating insects, better resource utilization, carbon sequestration, yield stability, animal health and product quality. This is the hypothesis of a new project - EcoServe - where the goal is to design grasslands, which increase both the nature value and provide an economically sustainable food production
CMB likelihood approximation by a Gaussianized Blackwell-Rao estimator
We introduce a new CMB temperature likelihood approximation called the
Gaussianized Blackwell-Rao (GBR) estimator. This estimator is derived by
transforming the observed marginal power spectrum distributions obtained by the
CMB Gibbs sampler into standard univariate Gaussians, and then approximate
their joint transformed distribution by a multivariate Gaussian. The method is
exact for full-sky coverage and uniform noise, and an excellent approximation
for sky cuts and scanning patterns relevant for modern satellite experiments
such as WMAP and Planck. A single evaluation of this estimator between l=2 and
200 takes ~0.2 CPU milliseconds, while for comparison, a single pixel space
likelihood evaluation between l=2 and 30 for a map with ~2500 pixels requires
~20 seconds. We apply this tool to the 5-year WMAP temperature data, and
re-estimate the angular temperature power spectrum, , and likelihood,
L(C_l), for l<=200, and derive new cosmological parameters for the standard
six-parameter LambdaCDM model. Our spectrum is in excellent agreement with the
official WMAP spectrum, but we find slight differences in the derived
cosmological parameters. Most importantly, the spectral index of scalar
perturbations is n_s=0.973 +/- 0.014, 1.9 sigma away from unity and 0.6 sigma
higher than the official WMAP result, n_s = 0.965 +/- 0.014. This suggests that
an exact likelihood treatment is required to higher l's than previously
believed, reinforcing and extending our conclusions from the 3-year WMAP
analysis. In that case, we found that the sub-optimal likelihood approximation
adopted between l=12 and 30 by the WMAP team biased n_s low by 0.4 sigma, while
here we find that the same approximation between l=30 and 200 introduces a bias
of 0.6 sigma in n_s.Comment: 10 pages, 7 figures, submitted to Ap
Species competition in multispecies grass swards
There is growing interest in establishing highly biodiverse grasslands that are also capable of maintaining high yields. In order to design successful multispecies mixtures it is necessary to know the competitiveness of individual species and how different management regimes affect this. Some species can survive in highly productive pastures,
while others need nursing in special mixtures if they are to make a significant contribution to the forage.
This is investigated in the ECOSERVE project and has also been studied in the earlier ORGGRASS project
The contribution of grass and clover root turnover to N leaching
Sources of inorganic and organic N leaching from grass-clover mixtures at field sites in Denmark, Germany and Iceland were investigated. Grass or clover was labelled with 15N-urea four times (autumn 2007, spring, summer and autumn 2008) prior to the leaching season in autumn and winter 2008. Soil water was sampled at 30 cm depth and analyzed for 15N-enrichment of dissolved inorganic N (DIN) and dissolved organic N (DON). Most 15N was recovered in DON for both labelled grass and clover at all sites. At the Danish site, grass and clover contributed more to the DON pool than the DIN whereas the opposite was observed at the German and Icelandic sites. The results show that both clover and grass contribute directly to N leaching from the root zone in mixtures, and that clover contribution is higher than grass. Furthermore, the present study indicates that roots active in the growth season prior to the drainage period contribute more to N leaching than roots active in the growth season the previous year, which is consistent with estimates of root longevity at the three sites
Mathematical modelling of the pathogenesis of multiple myeloma-induced bone disease
Multiple myeloma (MM) is the second most common haematological malignancy and results in destructive bone lesions. The interaction between MM cells and the bone microenvironment plays an important role in the development of the tumour cells and MM-induced bone disease and forms a 'vicious cycle' of tumour development and bone destruction, intensified by suppression of osteoblast activity and promotion of osteoclast activity. In this paper, a mathematical model is proposed to simulate how the interaction between MM cells and the bone microenvironment facilitates the development of the tumour cells and the resultant bone destruction. It includes both the roles of inhibited osteoblast activity and stimulated osteoclast activity. The model is able to mimic the temporal variation of bone cell concentrations and resultant bone volume after the invasion and then removal of the tumour cells and explains why MM-induced bone lesions rarely heal even after the complete removal of MM cells. The behaviour of the model compares well with published experimental data. The model serves as a first step to understand the development of MM-induced bone disease and could be applied further to evaluate the current therapies against MM-induced bone disease and even suggests new potential therapeutic targets
Cosmological Parameters from CMB Maps without Likelihood Approximation
We propose an efficient Bayesian MCMC algorithm for estimating cosmological
parameters from CMB data without use of likelihood approximations. It builds on
a previously developed Gibbs sampling framework that allows for exploration of
the joint CMB sky signal and power spectrum posterior, P(s,Cl|d), and addresses
a long-standing problem of efficient parameter estimation simultaneously in
high and low signal-to-noise regimes. To achieve this, our new algorithm
introduces a joint Markov Chain move in which both the signal map and power
spectrum are synchronously modified, by rescaling the map according to the
proposed power spectrum before evaluating the Metropolis-Hastings accept
probability. Such a move was already introduced by Jewell et al. (2009), who
used it to explore low signal-to-noise posteriors. However, they also found
that the same algorithm is inefficient in the high signal-to-noise regime,
since a brute-force rescaling operation does not account for phase information.
This problem is mitigated in the new algorithm by subtracting the Wiener filter
mean field from the proposed map prior to rescaling, leaving high
signal-to-noise information invariant in the joint step, and effectively only
rescaling the low signal-to-noise component. To explore the full posterior, the
new joint move is then interleaved with a standard conditional Gibbs sky map
move. We apply our new algorithm to simplified simulations for which we can
evaluate the exact posterior to study both its accuracy and performance, and
find good agreement with the exact posterior; marginal means agree to less than
0.006 sigma, and standard deviations to better than 3%. The Markov Chain
correlation length is of the same order of magnitude as those obtained by other
standard samplers in the field.Comment: 9 pages, 3 figures, Published in Ap
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