114 research outputs found
Wavelets and WMAP non-Gaussianity
We study the statistical properties of the 1st year WMAP data on different
scales using the spherical mexican hat wavelet transform. Consistent with the
results of Vielva et al. (2003) we find a deviation from Gaussianity in the
form of kurtosis of wavelet coefficients on scales in the southern
Galactic hemisphere. This paper extends the work of Vielva et al. as follows.
We find that the non-Gaussian signal shows up more strongly in the form of a
larger than expected number of cold pixels and also in the form of scale-scale
correlations amongst wavelet coefficients. We establish the robustness of the
non-Gaussian signal under more wide-ranging assumptions regarding the Galactic
mask applied to the data and the noise statistics. This signal is unlikely to
be due to the usual quadratic term parametrized by the non-linearity parameter
. We use the skewness of the spherical mexican hat wavelet coefficients
to constrain with the 1st year WMAP data. Our results constrain
to be at 68% confidence, and less than 280 at 99%
confidence.Comment: 22 pages, 10 figures, ApJ accepte
General CMB bispectrum analysis using wavelets and separable modes
In this paper we combine partial-wave (`modal') methods with a wavelet analysis of the CMB bispectrum. Our implementation exploits the advantages of both approaches to produce robust, reliable and efficient estimators which can constrain the amplitude of arbitrary primordial bispectra. This will be particularly important for upcoming surveys such as \emph{Planck}. A key advantage is the computational efficiency of calculating the inverse covariance matrix in wavelet space, producing an error bar which is close to optimal. We verify the efficacy and robustness of the method by applying it to WMAP7 data, finding \fnllocal=38.4 \pm 23.6 and \fnlequil=-119.2 \pm 123.6
On what scale should inflationary observables be constrained?
We examine the choice of scale at which constraints on inflationary
observables are presented. We describe an implementation of the hierarchy of
inflationary consistency equations which ensures that they remain enforced on
different scales, and then seek to optimize the scale for presentation of
constraints on marginalized inflationary parameters from WMAP3 data. For models
with spectral index running, we find a strong variation of the constraints
through the range of observational scales available, and optimize by finding
the scale which decorrelates constraints on the spectral index n_S and the
running. This scale is k=0.017 Mpc^{-1}, and gives a reduction by a factor of
more than four in the allowed parameter area in the n_S-r plane (r being the
tensor-to-scalar ratio) relative to k=0.002 Mpc^{-1}. These optimized
constraints are similar to those obtained in the no-running case. We also
extend the analysis to a larger compilation of data, finding essentially the
same conclusions.Comment: 7 pages RevTeX4 with 9 figures included. v2: References added, new
section added analyzing additional datasets alongside WMAP3. v3: Minor
corrections to match version accepted by PR
Model selection in cosmology
Model selection aims to determine which theoretical models are most plausible given some data, without necessarily considering preferred values of model parameters. A common model selection question is to ask when new data require introduction of an additional parameter, describing a newly discovered physical effect. We review model selection statistics, then focus on the Bayesian evidence, which implements Bayesian analysis at the level of models rather than parameters. We describe our CosmoNest code, the first computationally efficient implementation of Bayesian model selection in a cosmological context. We apply it to recent WMAP satellite data, examining the need for a perturbation spectral index differing from the scaleinvariant (Harrison–Zel'dovich) case
Looking for Cosmological Alfven Waves in WMAP Data
A primordial cosmological magnetic field induces and supports vorticity or
Alfven waves, which in turn generate cosmic microwave background (CMB)
anisotropies. A homogeneous primordial magnetic field with fixed direction
induces correlations between the and multipole
coefficients of the CMB temperature anisotropy field. We discuss the
constraints that can be placed on the strength of such a primordial magnetic
field using CMB anisotropy data from the WMAP experiment. We place 3
upper limits on the strength of the magnetic field of nG for vector
perturbation spectral index and nG for .Comment: 14 pages, 3 figures, minor changes, references added, ApJ, in pres
Observational Bounds on Modified Gravity Models
Modified gravity provides a possible explanation for the currently observed
cosmic accelaration. In this paper, we study general classes of modified
gravity models. The Einstein-Hilbert action is modified by using general
functions of the Ricci and the Gauss-Bonnet scalars, both in the metric and in
the Palatini formalisms. We do not use an explicit form for the functions, but
a general form with a valid Taylor expansion up to second order about redshift
zero in the Riemann-scalars. The coefficients of this expansion are then
reconstructed via the cosmic expansion history measured using current
cosmological observations. These are the quantities of interest for theoretical
considerations relating to ghosts and instabilities. We find that current data
provide interesting constraints on the coefficients. The next-generation dark
energy surveys should shrink the allowed parameter space for modifed gravity
models quite dramatically.Comment: 23 pages, 5 figures, uses RevTe
A Comparative Study of Dark Energy Constraints from Current Observational Data
We examine how dark energy constraints from current observational data depend
on the analysis methods used: the analysis of Type Ia supernovae (SNe Ia), and
that of galaxy clustering data. We generalize the flux-averaging analysis
method of SNe Ia to allow correlated errors of SNe Ia, in order to reduce the
systematic bias due to weak lensing of SNe Ia. We find that flux-averaging
leads to larger errors on dark energy and cosmological parameters if only SN Ia
data are used. When SN Ia data (the latest compilation by the SNLS team) are
combined with WMAP 7 year results (in terms of our Gaussian fits to the
probability distributions of the CMB shift parameters), the latest Hubble
constant (H_0) measurement using the Hubble Space Telescope (HST), and gamma
ray burst (GRB) data, flux-averaging of SNe Ia increases the concordance with
other data, and leads to significantly tighter constraints on the dark energy
density at z=1, and the cosmic curvature \Omega_k. The galaxy clustering
measurements of H(z=0.35)r_s(z_d) and r_s(z_d)/D_A(z=0.35) (where H(z) is the
Hubble parameter, D_A(z) is the angular diameter distance, and r_s(z_d) is the
sound horizon at the drag epoch) by Chuang & Wang (2011) are consistent with SN
Ia data, given the same pirors (CMB+H_0+GRB), and lead to significantly
improved dark energy constraints when combined. Current data are fully
consistent with a cosmological constant and a flat universe.Comment: 11 pages, 9 figures. Slightly revised version, to appear in PRD.
Supernova flux-averaging code available at
http://www.nhn.ou.edu/~wang/SNcode
Model-Independent Constraints on Dark Energy Density from Flux-averaging Analysis of Type Ia Supernova Data
We reconstruct the dark energy density as a free function from
current type Ia supernova (SN Ia) data (Tonry et al. 2003; Barris et al. 2003;
Knop et al. 2003), together with the Cosmic Microwave Background (CMB) shift
parameter from CMB data (WMAP, CBI, and ACBAR), and the large scale structure
(LSS) growth factor from 2dF galaxy survey data. We parametrize as
a continuous function, given by interpolating its amplitudes at equally spaced
values in the redshift range covered by SN Ia data, and a constant at
larger (where is only weakly constrained by CMB data). We
assume a flat universe, and use the Markov Chain Monte Carlo (MCMC) technique
in our analysis. We find that the dark energy density is constant
for 0 \la z \la 0.5 and increases with redshift for 0.5 \la z \la 1 at
68.3% confidence level, but is consistent with a constant at 95% confidence
level. For comparison, we also give constraints on a constant equation of state
for the dark energy.
Flux-averaging of SN Ia data is required to yield cosmological parameter
constraints that are free of the bias induced by weak gravitational lensing
\citep{Wang00b}. We set up a consistent framework for flux-averaging analysis
of SN Ia data, based on \cite{Wang00b}. We find that flux-averaging of SN Ia
data leads to slightly lower and smaller time-variation in
. This suggests that a significant increase in the number of SNe Ia
from deep SN surveys on a dedicated telescope \citep{Wang00a} is needed to
place a robust constraint on the time-dependence of the dark energy density.Comment: Slightly revised in presentation, ApJ accepted. One color figure
shows rho_X(z) reconstructed from dat
Detecting and distinguishing topological defects in future data from the CMBPol satellite
The proposed CMBPol mission will be able to detect the imprint of topological defects on the CMB provided the contribution is sufficiently strong. We quantify the detection threshold for cosmic strings and for textures, and analyze the satellite's ability to distinguish between these different types of defects. We also assess the level of danger of misidentification of a defect signature as from the wrong defect type or as an effect of primordial gravitational waves. A 0.002 fractional contribution of cosmic strings to the CMB temperature spectrum at multipole ten, and similarly a 0.001 fractional contribution of textures, can be detected and correctly identified at the 3 level. We also confirm that a tensor contribution of r=0.0018 can be detected at over 3, in agreement with the CMBPol mission concept study. These results are supported by a model selection analysis
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