93 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
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
Comment on `Tainted evidence: cosmological model selection versus fitting', by Eric V. Linder and Ramon Miquel (astro-ph/0702542v2)
In astro-ph/0702542v2, Linder and Miquel seek to criticize the use of
Bayesian model selection for data analysis and for survey forecasting and
design. Their discussion is based on three serious misunderstandings of the
conceptual underpinnings and application of model-level Bayesian inference,
which invalidate all their main conclusions. Their paper includes numerous
further inaccuracies, including an erroneous calculation of the Bayesian
Information Criterion. Here we seek to set the record straight.Comment: 6 pages RevTeX
Planck priors for dark energy surveys
Although cosmic microwave background (CMB) anisotropy data alone cannot
constrain simultaneously the spatial curvature and the equation of state of
dark energy, CMB data provide a valuable addition to other experimental
results. However computing a full CMB power spectrum with a Boltzmann code is
quite slow; for instance if we want to work with many dark energy and/or
modified gravity models, or would like to optimize experiments where many
different configurations need to be tested, it is possible to adopt a quicker
and more efficient approach.
In this paper we consider the compression of the projected Planck CMB data
into four parameters, R (scaled distance to last scattering surface), l_a
(angular scale of sound horizon at last scattering), Omega_b h^2 (baryon
density fraction) and n_s (powerlaw index of primordial matter power spectrum),
all of which can be computed quickly. We show that, although this compression
loses information compared to the full likelihood, such information loss
becomes negligible when more data is added. We also demonstrate that the method
can be used for scalar field dark energy independently of the parametrisation
of the equation of state, and discuss how this method should be used for other
kinds of dark energy models.Comment: 8 pages, 3 figures, 4 table
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