10,674 research outputs found
Separation of foregrounds from cosmic microwave background observations with the MAP satellite
Simulated observations of a 10\dg \times 10\dg field by the Microwave
Anisotropy Probe (MAP) are analysed in order to separate cosmic microwave
background (CMB) emission from foreground contaminants and instrumental noise
and thereby determine how accurately the CMB emission can be recovered. The
simulations include emission from the CMB, the kinetic and thermal
Sunyaev-Zel'dovich (SZ) effects from galaxy clusters, as well as Galactic dust,
free-free and synchrotron. We find that, even in the presence of these
contaminating foregrounds, the CMB map is reconstructed with an rms accuracy of
about 20 K per 12.6 arcmin pixel, which represents a substantial
improvement as compared to the individual temperature sensitivities of the raw
data channels. We also find, for the single 10\dg \times 10\dg field, that
the CMB power spectrum is accurately recovered for \ell \la 600.Comment: 7 pages, 7 figures, MNRAS submitte
An Optimal Skorokhod Embedding for Diffusions
Given a Brownian motion and a general target law (not necessarily
centered or even integrable) we show how to construct an embedding of in
. This embedding is an extension of an embedding due to Perkins, and is
optimal in the sense that it simultaneously minimises the distribution of the
maximum and maximises the distribution of the minimum among all embeddings of
. The embedding is then applied to regular diffusions, and used to
characterise the target laws for which a -embedding may be found.Comment: 22 pages, 4 figure
Do wavelets really detect non-Gaussianity in the 4-year COBE data?
We investigate the detection of non-Gaussianity in the 4-year COBE data
reported by Pando, Valls-Gabaud & Fang (1998), using a technique based on the
discrete wavelet transform. Their analysis was performed on the two DMR faces
centred on the North and South Galactic poles respectively, using the
Daubechies 4 wavelet basis. We show that these results depend critically on the
orientation of the data, and so should be treated with caution. For two
distinct orientations of the data, we calculate unbiased estimates of the
skewness, kurtosis and scale-scale correlation of the corresponding wavelet
coefficients in all of the available scale domains of the transform. We obtain
several detections of non-Gaussianity in the DMR-DSMB map at greater than the
99 per cent confidence level, but most of these occur on pixel-pixel scales and
are therefore not cosmological in origin. Indeed, after removing all multipoles
beyond from the COBE maps, only one robust detection remains.
Moreover, using Monte-Carlo simulations, we find that the probability of
obtaining such a detection by chance is 0.59. We repeat the analysis for the
53+90 GHz coadded COBE map. In this case, after removing
multipoles, two non-Gaussian detections at the 99 per cent level remain.
Nevertheless, again using Monte-Carlo simulations, we find that the probability
of obtaining two such detections by chance is 0.28. Thus, we conclude the
wavelet technique does {\em not} yield strong evidence for non-Gaussianity of
cosmological origin in the 4-year COBE data.Comment: 7 pages, 5 figures. Revised version including discussion of
orientation sensitivity of the wavelet decomposition. MNRAS submitte
Markov chain Monte Carlo analysis of Bianchi VII_h models
We have extended the analysis of Jaffe et al. to a complete Markov chain
Monte Carlo (MCMC) study of the Bianchi type models including a
dark energy density, using 1-year and 3-year Wilkinson Microwave Anisotropy
Probe (WMAP) cosmic microwave background (CMB) data. Since we perform the
analysis in a Bayesian framework our entire inference is contained in the
multidimensional posterior distribution from which we can extract marginalised
parameter constraints and the comparative Bayesian evidence. Treating the
left-handed Bianchi CMB anisotropy as a template centred upon the `cold-spot'
in the southern hemisphere, the parameter estimates derived for the total
energy density, `tightness' and vorticity from 3-year data are found to be:
, , with orientation ). This template is preferred by a factor of roughly
unity in log-evidence over a concordance cosmology alone. A Bianchi type
template is supported by the data only if its position on the sky is heavily
restricted. The low total energy density of the preferred template, implies a
geometry that is incompatible with cosmologies inferred from recent CMB
observations. Jaffe et al. found that extending the Bianchi model to include a
term in creates a degeneracy in the plane. We explore this region fully by MCMC and find that the
degenerate likelihood contours do not intersect areas of parameter space that 1
or 3 year WMAP data would prefer at any significance above . Thus we
can confirm that a physical Bianchi model is not responsible for
this signature.Comment: 8 pages, 10 figures, significant update to include more accurate
results and conclusions to match version accepted by MNRA
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