1,613 research outputs found
Pseudo-Dipole Signal Removal from WMAP Data
It is discovered in our previous work that different observational
systematics, e.g., errors of antenna pointing directions, asynchronous between
the attitude and science data, can generate pseudo-dipole signal in full-sky
maps of the cosmic microwave background (CMB) anisotropy published by The
Wilkinson Microwave Anisotropy Probe (WMAP) team. Now the antenna sidelobe
response to the Doppler signal is found to be able to produce similar effect as
well. In this work, independent to the sources, we uniformly model the
pseudo-dipole signal and remove it from published WMAP7 CMB maps by model
fitting. The result demonstrates that most of the released WMAP CMB quadrupole
is artificial.Comment: V3: using WMAP7 dat
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
Hot pixel contamination in the CMB correlation function?
Recently, it was suggested that the map-making procedure, which is applied to
the time-ordered CMB data by the WMAP team, might be flawed by hot pixels. This
could lead to a bias in the pixels having an angular distance of about 141
degrees from hot pixels due to the differential measuring process of the
satellite WMAP. Here, the bias is confirmed, and the temperature two-point
correlation function C(theta) is reevaluated by excluding the affected pixels.
It is shown that the most significant effect occurs in C(theta) at the largest
angles near theta = 180 degrees. Furthermore, the corrected correlation
function C(theta) is applied to the cubic topology of the Universe, and it is
found that such a multi-connected universe matches the temperature correlation
better than the LCDM concordance model, provided the cubic length scale is
close to L=4 measured in units of the Hubble length
Statistical Isotropy violation of the CMB brightness fluctuations
Certain anomalies at large angular scales in the cosmic microwave background
measured by WMAP have been suggested as possible evidence of breakdown of
statistical isotropy(SI). Most CMB photons free-stream to the present from the
surface of last scattering. It is thus reasonable to expect statistical
isotropy violation in the CMB photon distribution observed now to have
originated from SI violation in the baryon-photon fluid at last scattering, in
addition to anisotropy of the primordial power spectrum studied earlier in
literature.
We consider the generalized anisotropic brightness distribution fluctuations,
(at conformal time ) in contrast to the
SI case where it is simply a function of and . The brightness fluctuations expanded in Bipolar Spherical Harmonic
(BipoSH) series, can then be written as where terms encode deviations from statistical isotropy. We
study the evolution of from
non-zero terms at last
scattering. Similar to the SI case, power at small spherical harmonic (SH)
multipoles of at the last
scattering, is transferred to at
larger SH multipoles. The structural similarity is more apparent in the
asymptotic expression for large values of the final SH multipoles. This
formalism allows an elegant identification of any SI violation observed today
to a possible origin in the SI violation present in the baryon-photon fluid
(eg., due to the presence of significant magnetic field).Comment: 14 pages, 4 figures, added illustrative example of SI violation in
baryon-photon fluid, matches version accepted for publication in Phys. Rev.
Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Angular Power Spectra
We present the temperature and polarization angular power spectra of the
cosmic microwave background (CMB) derived from the first 5 years of WMAP data.
The 5-year temperature (TT) spectrum is cosmic variance limited up to multipole
l=530, and individual l-modes have S/N>1 for l<920. The best fitting
six-parameter LambdaCDM model has a reduced chi^2 for l=33-1000 of
chi^2/nu=1.06, with a probability to exceed of 9.3%. There is now significantly
improved data near the third peak which leads to improved cosmological
constraints. The temperature-polarization correlation (TE) is seen with high
significance. After accounting for foreground emission, the low-l reionization
feature in the EE power spectrum is preferred by \Delta\chi^2=19.6 for optical
depth tau=0.089 by the EE data alone, and is now largely cosmic variance
limited for l=2-6. There is no evidence for cosmic signal in the BB, TB, or EB
spectra after accounting for foreground emission. We find that, when averaged
over l=2-6, l(l+1)C^{BB}_l/2\pi < 0.15 uK^2 (95% CL).Comment: 29 pages, 13 figures, accepted by ApJ
Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Are There Cosmic Microwave Background Anomalies?
(Abridged) A simple six-parameter LCDM model provides a successful fit to
WMAP data, both when the data are analyzed alone and in combination with other
cosmological data. Even so, it is appropriate to search for any hints of
deviations from the now standard model of cosmology, which includes inflation,
dark energy, dark matter, baryons, and neutrinos. The cosmological community
has subjected the WMAP data to extensive and varied analyses. While there is
widespread agreement as to the overall success of the six-parameter LCDM model,
various "anomalies" have been reported relative to that model. In this paper we
examine potential anomalies and present analyses and assessments of their
significance. In most cases we find that claimed anomalies depend on posterior
selection of some aspect or subset of the data. Compared with sky simulations
based on the best fit model, one can select for low probability features of the
WMAP data. Low probability features are expected, but it is not usually
straightforward to determine whether any particular low probability feature is
the result of the a posteriori selection or of non-standard cosmology. We
examine in detail the properties of the power spectrum with respect to the LCDM
model. We examine several potential or previously claimed anomalies in the sky
maps and power spectra, including cold spots, low quadrupole power,
quadropole-octupole alignment, hemispherical or dipole power asymmetry, and
quadrupole power asymmetry. We conclude that there is no compelling evidence
for deviations from the LCDM model, which is generally an acceptable
statistical fit to WMAP and other cosmological data.Comment: 19 pages, 17 figures, also available with higher-res figures on
http://lambda.gsfc.nasa.gov; accepted by ApJS; (v2) text as accepte
The Spectrum of the CMB Anisotropy from the Combined COBE FIRAS and DMR Observations
We analyze the Cosmic Microwave Background (CMB) anisotropy data from the
independent COBE FIRAS and DMR observations. We extract the frequency spectrum
of the FIRAS signal that has the spatial distribution seen by DMR and show that
it is consistent with CMB temperature fluctuations in the radiation well into
the Wien region of the spectrum. Conversely, we form a map of the Planckian
component of the sky temperature from FIRAS and show that it correlates with
the DMR anisotropy map. The rms fluctuations at angular scales of 7 degrees are
48 \pm 14 uK for the FIRAS data compared with 35 \pm 2 uK for the DMR data and
31 \pm 6 uK for the correlated combination (1 sigma uncertainties). The
consistency of these data, from very different instruments with very different
observing strategies, provide compelling support for the interpretation that
the signal seen by DMR is, in fact, temperature anisotropy of cosmological
origin. The data also limit rms fluctuations in the Compton y parameter,
observable via the Sunyaev- Zel'dovich effect, to Delta_y < 3 x 10^{-6} (95%
CL) on 7 degree angular scales.Comment: 15 pages, Latex (AASv4 macro) including 3 Postscript figures, to
appear in ApJ, vol. 486, Sept 10, 199
Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Sky Maps, Systematic Errors, and Basic Results
(Abridged) New full sky temperature and polarization maps based on seven
years of data from WMAP are presented. The new results are consistent with
previous results, but have improved due to reduced noise from the additional
integration time, improved knowledge of the instrument performance, and
improved data analysis procedures. The improvements are described in detail.
The seven year data set is well fit by a minimal six-parameter flat Lambda-CDM
model. The parameters for this model, using the WMAP data in conjunction with
baryon acoustic oscillation data from the Sloan Digital Sky Survey and priors
on H_0 from Hubble Space Telescope observations, are: Omega_bh^2 = 0.02260
+-0.00053, Omega_ch^2 = 0.1123 +-0.0035, Omega_Lambda = 0.728 +0.015 -0.016,
n_s = 0.963 +-0.012, tau = 0.087 +-0.014 and sigma_8 = 0.809 +-0.024 (68 % CL
uncertainties). The temperature power spectrum signal-to-noise ratio per
multipole is greater that unity for multipoles < 919, allowing a robust
measurement of the third acoustic peak. This measurement results in improved
constraints on the matter density, Omega_mh^2 = 0.1334 +0.0056 -0.0055, and the
epoch of matter- radiation equality, z_eq = 3196 +134 -133, using WMAP data
alone. The new WMAP data, when combined with smaller angular scale microwave
background anisotropy data, results in a 3 sigma detection of the abundance of
primordial Helium, Y_He = 0.326 +-0.075.The power-law index of the primordial
power spectrum is now determined to be n_s = 0.963 +-0.012, excluding the
Harrison-Zel'dovich-Peebles spectrum by >3 sigma. These new WMAP measurements
provide important tests of Big Bang cosmology.Comment: 42 pages, 9 figures, Submitted to Astrophysical Journal Supplement
Serie
Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Bayesian Estimation of CMB Polarization Maps
We describe a sampling method to estimate the polarized CMB signal from
observed maps of the sky. We use a Metropolis-within-Gibbs algorithm to
estimate the polarized CMB map, containing Q and U Stokes parameters at each
pixel, and its covariance matrix. These can be used as inputs for cosmological
analyses. The polarized sky signal is parameterized as the sum of three
components: CMB, synchrotron emission, and thermal dust emission. The polarized
Galactic components are modeled with spatially varying power law spectral
indices for the synchrotron, and a fixed power law for the dust, and their
component maps are estimated as by-products. We apply the method to simulated
low resolution maps with pixels of side 7.2 degrees, using diagonal and full
noise realizations drawn from the WMAP noise matrices. The CMB maps are
recovered with goodness of fit consistent with errors. Computing the likelihood
of the E-mode power in the maps as a function of optical depth to reionization,
tau, for fixed temperature anisotropy power, we recover tau=0.091+-0.019 for a
simulation with input tau=0.1, and mean tau=0.098 averaged over 10 simulations.
A `null' simulation with no polarized CMB signal has maximum likelihood
consistent with tau=0. The method is applied to the five-year WMAP data, using
the K, Ka, Q and V channels. We find tau=0.090+-0.019, compared to
tau=0.086+-0.016 from the template-cleaned maps used in the primary WMAP
analysis. The synchrotron spectral index, beta, averaged over high
signal-to-noise pixels with standard deviation sigma(beta)<0.25, but excluding
~6% of the sky masked in the Galactic plane, is -3.03+-0.04. This estimate does
not vary significantly with Galactic latitude, although includes an informative
prior.Comment: 11 pages, 9 figures, matches version accepted by Ap
Gravitational Lensing Signature of Long Cosmic Strings
The gravitational lensing by long, wiggly cosmic strings is shown to produce
a large number of lensed images of a background source. In addition to pairs of
images on either side of the string, a number of small images outline the
string due to small-scale structure on the string. This image pattern could
provide a highly distinctive signature of cosmic strings. Since the optical
depth for multiple imaging of distant quasar sources by long strings may be
comparable to that by galaxies, these image patterns should be clearly
observable in the next generation of redshift surveys such as the Sloan Digital
Sky Survey.Comment: 4 pages, revtex with 3 postscript figures include
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