110 research outputs found
Non-Gaussianities in the local curvature of the 5-year WMAP data
Using the 5 year WMAP data, we re-investigate claims of non-Gaussianities and
asymmetries detected in local curvature statistics of the 1 year WMAP data. In
Hansen et al 2004, it was found that the northern ecliptic hemisphere was
non-Gaussian at the ~1% level testing the densities of hill-, lake and saddle
points based on the second derivatives of the CMB temperature map. The 5 year
WMAP data has a much lower noise level and better control of systematics. Using
these, we find that the anomalies are still present at a consistent level. Also
the direction of maximum non-Gaussianity remains. Due to limited availability
of computer resources, Hansen et al. 2004 were unable to calculate the full
covariance matrix for the chi^2 test used. Here we apply the full covariance
matrix instead of the diagonal approximation and find that the
non-Gaussianities disappear and there is no preferred non-Gaussian direction.
We compare with simulations of weak lensing to see if this may cause the
observed non-Gaussianity when using diagonal covariance matrix. We conclude
that weak lensing does not produce non-Gaussianity in the local curvature
statistics at the scales investigated in this paper. The cause of the
non-Gaussian detection in the case of a diagonal matrix remains unclear.Comment: 7 pages, 6 figures, included test on weak lensing simulation
Bayesian analysis of an anisotropic universe model: systematics and polarization
We revisit the anisotropic universe model previously developed by Ackerman,
Carroll and Wise (ACW), and generalize both the theoretical and computational
framework to include polarization and various forms of systematic effects. We
apply our new tools to simulated WMAP data in order to understand the potential
impact of asymmetric beams, noise mis-estimation and potential Zodiacal light
emission. We find that neither has any significant impact on the results. We
next show that the previously reported ACW signal is also present in the 1-year
WMAP temperature sky map presented by Liu & Li, where data cuts are more
aggressive. Finally, we reanalyze the 5-year WMAP data taking into account a
previously neglected (-i)^{l-l'}-term in the signal covariance matrix. We still
find a strong detection of a preferred direction in the temperature map.
Including multipoles up to l=400, the anisotropy amplitude for the W-band is
found to be g = 0.29 +- 0.031, nonzero at 9 sigma. However, the corresponding
preferred direction is also shifted very close to the ecliptic poles at (l,b)=
(96,30), in agreement with the analysis of Hanson & Lewis, indicating that the
signal is aligned along the plane of the solar system. This strongly suggests
that the signal is not of cosmological origin, but most likely is a product of
an unknown systematic effect. Determining the nature of the systematic effect
is of vital importance, as it might affect other cosmological conclusions from
the WMAP experiment. Finally, we provide a forecast for the Planck experiment
including polarization.Comment: 9 pages, 8 figure
Detection of transplanckian effects in the cosmic microwave background
Quantum gravity effects are expected to modify the primordial density
fluctuations produced during inflation and leave their imprint on the cosmic
microwave background observed today. We present a new analysis discussing
whether these effects are detectable, considering both currently available data
and simulated results from an optimal CMB experiment. We find that the WMAP
(Wilkinson Microwave Anisotropy Probe) data show no evidence for the particular
signature considered in this work but give an upper bound on the parameters of
the model. However, a hypothetical experiment shows that with proper data, the
trans-Planckian effects should be detectable through alternate sampling
methods. This fuzzy conclusion is a result of the nature of the oscillations,
since they give rise to a likelihood hypersurface riddled with local maxima. A
simple Bayesian analysis shows no significant evidence for the simulated data
to prefer a trans-Planckian model. Conventional Markov chain Monte Carlo (MCMC)
methods are not suitable for exploring this complicated landscape, but
alternative methods are required to solve the problem. This, however, requires
extremely high-precision data.Comment: 9 pages, 22 figure
Inconsistency of the MLE for the joint distribution of interval censored survival times and continuous marks
This paper considers the nonparametric maximum likelihood estimator (MLE) for
the joint distribution function of an interval censored survival time and a
continuous mark variable. We provide a new explicit formula for the MLE in this
problem. We use this formula and the mark specific cumulative hazard function
of Huang and Louis (1998) to obtain the almost sure limit of the MLE. This
result leads to necessary and sufficient conditions for consistency of the MLE
which imply that the MLE is inconsistent in general. We show that the
inconsistency can be repaired by discretizing the marks. Our theoretical
results are supported by simulations.Comment: 27 pages, 4 figure
The effect of asymmetric beams in the Wilkinson Microwave Anisotropy Probe experiment
We generate simulations of the CMB temperature field as observed by the WMAP
satellite, taking into account the detailed shape of the asymmetric beams and
scanning strategy of the experiment, and use these to re-estimate the WMAP beam
transfer functions. This method avoids the need of artificially symmetrizing
the beams, as done in the baseline WMAP approach, and instead measures the
total convolution effect by direct simulation. We find only small differences
with respect to the nominal transfer functions, typically less than 1%
everywhere, and less than 0.5% at l<400. The net effect on the CMB power
spectrum is less than 0.6%. The effect on all considered cosmological
parameters are negligible. For instance, we find that the spectral index of
scalar perturbations after taking into account the beam asymmetries is n_s =
0.964 +- 0.014, corresponding to a negative shift of -0.1 sigma compared to the
previously released WMAP results. Our CMB sky simulations are made publicly
available, and can be used for general studies of asymmetric beam effects in
the WMAP data.Comment: 14 pages, 7 figures, published versio
The Effect of Asymmetric Beams in the Wilkinson Microwave Anisotropy Probe Experiment
We generate simulations of the cosmic microwave background (CMB) temperature field as observed by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, taking into account the detailed shape of the asymmetric beams and scanning strategy of the experiment, and use these to re-estimate the WMAP beam transfer functions. This method avoids the need of artificially symmetrizing the beams, as done in the baseline WMAP approach, and instead measures the total convolution effect by direct simulation. We find only small differences with respect to the nominal transfer functions, typically less than 1% everywhere, and less than 0.5% at ℓ < 400. The net effect on the CMB power spectrum is less than 0.6%. The effect on all considered cosmological parameters is negligible. For instance, we find that the spectral index of scalar perturbations after taking into account the beam asymmetries is n_s = 0.964 ± 0.014, corresponding to a negative shift of –0.1σ compared to the previously released WMAP results. Our CMB sky simulations are made publicly available and can be used for general studies of asymmetric beam effects in the WMAP data
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
Frequentist comparison of CMB local extrema statistics in the five-year WMAP data with two anisotropic cosmological models
We present local extrema studies of two models that introduce a preferred
direction into the observed cosmic microwave background (CMB) temperature
field. In particular, we make a frequentist comparison of the one- and
two-point statistics for the dipole modulation and ACW models with data from
the five-year Wilkinson Microwave Anisotropy Probe (WMAP). This analysis is
motivated by previously revealed anomalies in the WMAP data, and particularly
the difference in the statistical nature of the temperature anisotropies when
analysed in hemispherical partitions.
The analysis of the one-point statistics indicates that the previously
determined hemispherical variance difficulties can be apparently overcome by a
dipole modulation field, but new inconsistencies arise if the mean and the
l-dependence of the statistics are considered. The two-point correlation
functions of the local extrema, the temperature pair product and the
point-point spatial pair-count, demonstrate that the impact of such a
modulation is to over-`asymmetrise' the temperature field on smaller scales
than the wave-length of the dipole or quadrupole, and this is disfavored by the
observed data.The results from the ACW model predictions, however, are
consistent with the standard isotropic hypothesis. The two-point analysis
confirms that the impact of this type of violation of isotropy on the
temperature extrema statistics is relatively weak.
From this work, we conclude that a model with more spatial structure than the
dipole modulated or rotational-invariance breaking models are required to fully
explain the observed large-scale anomalies in the WMAP data.Comment: 10 pages, 4 figures, 3 tables, accepted for publication in MNRA
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