127 research outputs found
Primordial Non-Gaussianity in the Cosmic Microwave Background
In the last few decades, advances in observational cosmology have given us a
standard model of cosmology. We know the content of the universe to within a
few percent. With more ambitious experiments on the way, we hope to move beyond
the knowledge of what the universe is made of, to why the universe is the way
it is. In this review paper we focus on primordial non-Gaussianity as a probe
of the physics of the dynamics of the universe at the very earliest moments. We
discuss 1) theoretical predictions from inflationary models and their
observational consequences in the cosmic microwave background (CMB)
anisotropies; 2) CMB--based estimators for constraining primordial
non-Gaussianity with an emphasis on bispectrum templates; 3) current
constraints on non-Gaussianity and what we can hope to achieve in the near
future; and 4) non-primordial sources of non-Gaussianities in the CMB such as
bispectrum due to second order effects, three way cross-correlation between
primary-lensing-secondary CMB, and possible instrumental effects.Comment: 27 pages, 8 figures; Invited Review for the Journal "Advances in
Astronomy"; references adde
Probing Primordial Magnetism with Off-Diagonal Correlators of CMB Polarization
Primordial magnetic fields (PMF) can create polarization -modes in the
cosmic microwave background (CMB) through Faraday rotation (FR), leading to
non-trivial 2-point and 4-point correlators of the CMB temperature and
polarization. We discuss the detectability of primordial magnetic fields using
different correlators and evaluate their relative merits. We have fully
accounted for the contamination by weak lensing, which contributes to the
variance, but whose contribution to the 4-point correlations is orthogonal to
that of FR. We show that a Planck-like experiment can detect scale-invariant
PMF of nG strength using the FR diagnostic at 90GHz, while realistic future
experiments at the same frequency can detect 10^{-10} G. Utilizing multiple
frequencies will improve on these prospects, making FR of CMB a powerful probe
of scale-invariant PMF.Comment: 11 pages, 4 figures; unit typos fixed in fig 1 and
Primordial B-mode Diagnostics and Self Calibrating the CMB Polarization
Distortions in the primordial cosmic microwave background (CMB) along the
line-of-sight can be modeled and described using 11 fields. These distortion
fields correspond to various cosmological signals such as weak gravitational
lensing of the CMB by large-scale structure, screening from patchy
reionization, rotation of the plane of polarization due to magnetic fields or
parity violating physics. Various instrumental systematics such as gain
fluctuations, pixel rotation, differential gain, pointing, differential
ellipticity are also described by the same distortion model. All these
distortions produce B-mode that contaminate the primordial tensor B-modes
signal. In this paper we show that apart from generating B-modes, each
distortion uniquely couples different modes (\bfl_1\ne \bfl_2) of the CMB
anisotropies, generating correlations which for the primordial CMB are
zero. We describe and implement unbiased minimum variance quadratic estimators
which using the off diagonal correlations in the CMB can extract the map of
distortions. We perform Monte-Carlo simulations to characterize the estimators
and illustrate the level of distortions that can be detected with current and
future experiments. The estimators can be used to look for cosmological
signals, or to check for any residual systematics in the data. As a specific
example of primordial tensor B-mode diagnostics we compare the level of minimum
detectable distortions using our method with maximum allowed distortion level
for the B-modes detection. We show that for any experiment, the distortions
will be detected at high significance using correlations before they would show
up as spurious B-modes in the power spectrum.Comment: 14 pages, 4 figure
Constraining a spatially dependent rotation of the Cosmic Microwave Background Polarization
Following Kamionkowski (2008), a quadratic estimator of the rotation of the
plane of polarization of the CMB is constructed. This statistic can estimate a
spatially varying rotation angle. We use this estimator to quantify the
prospects of detecting such a rotation field with forthcoming experiments. For
PLANCK and CMBPol we find that the estimator containing the product of the E
and B components of the polarization field is the most sensitive. The variance
of this EB estimator, N(L) is roughly independent of the multipole L, and is
only weakly dependent on the instrumental beam. For FWHM of the beam size ~
5'-50', and instrument noise $\Delta_p ~ 5-50 uK-arcmin, the scaling of
variance N(L) can be fitted by a power law N(L)=3.3 x 10^{-7} \Delta^2_p
(FWHM)^{1.3} sq-deg. For small instrumental noise \Delta_p \leq 5 uK-arcmin,
the lensing B-modes become important, saturating the variance to ~10^{-6}
sq-deg even for an ideal experiment. Upcoming experiments like PLANCK will be
able to detect a power spectrum of the rotation angle, C^{\alpha \alpha}(L), as
small as 0.01 sq-deg, while futuristic experiment like CMBPol will be able to
detect rotation angle power spectrum as small as 2.5 x 10^{-5} sq-deg. We
discuss the implications of such constraints, both for the various physical
effects that can rotate the polarization as photons travel from the last
scattering surface as well as for constraints on instrumental systematics that
can also lead to a spurious rotation signal. Rotation of the CMB polarization
generates B-modes which will act as contamination for the primordial B-modes
detection. We discuss an application of our estimator to de-rotate the CMB to
increase the sensitivity for the primordial B-modes.Comment: 11 pages, 5 figure
Detection of primordial non-Gaussianity (fNL) in the WMAP 3-year data at above 99.5% confidence
We present evidence for the detection of primordial non-Gaussianity of the
local type (fNL), using the temperature information of the Cosmic Microwave
Background (CMB) from the WMAP 3-year data. We employ the bispectrum estimator
of non-Gaussianity described in (Yadav et al. 2007) which allows us to analyze
the entirety of the WMAP data without an arbitrary cut-off in angular scale.
Using the combined information from WMAP's two main science channels up to
lmax=750 and the conservative Kp0 foreground mask we find 27 < fNL < 147 at 95%
C.L., with a central value of fNL=87. This corresponds to a rejection of fNL=0
at more than 99.5% significance. We find that this detection is robust to
variations in lmax, frequency and masks, and that no known foreground,
instrument systematic, or secondary anisotropy explains our signal while
passing our suite of tests. We explore the impact of several analysis choices
on the stated significance and find 2.5 sigma for the most conservative view.
We conclude that the WMAP 3-year data disfavors canonical single field
slow-roll inflation.Comment: 4 pages, 2 figures, 1 tables, submitted to PRL, references added. New
version has several additional tests and systematic error estimates. Results
largely unchange
Higher-Order Gravitational Lensing Reconstruction using Feynman Diagrams
We develop a method for calculating the correlation structure of the Cosmic
Microwave Background (CMB) using Feynman diagrams, when the CMB has been
modified by gravitational lensing, Faraday rotation, patchy reionization, or
other distorting effects. This method is used to calculate the bias of the
Hu-Okamoto quadratic estimator in reconstructing the lensing power spectrum up
to O(\phi^4) in the lensing potential . We consider both the diagonal
noise TTTT, EBEB, etc. and, for the first time, the off-diagonal noise TTTE,
TBEB, etc. The previously noted large O(\phi^4) term in the second order noise
is identified to come from a particular class of diagrams. It can be
significantly reduced by a reorganization of the expansion. These
improved estimators have almost no bias for the off-diagonal case involving
only one component of the CMB, such as EEEB.Comment: 17 pages, 17 figure
Gravitational Lensing of the CMB: a Feynman Diagram Approach
We develop a Feynman diagram approach to calculating correlations of the
Cosmic Microwave Background (CMB) in the presence of distortions. As one
application, we focus on CMB distortions due to gravitational lensing by Large
Scale Structure (LSS). We study the Hu-Okamoto quadratic estimator for
extracting lensing from the CMB and derive the noise of the estimator up to
in the lensing potential . The previously noted
large term can be significantly reduced by a
reorganization of the expansion. Our approach makes it simple to obtain
expressions for quadratic estimators based on any CMB channel. We briefly
discuss other applications to cosmology of this diagrammatic approach, such as
distortions of the CMB due to patchy reionization, or due to Faraday rotation
from primordial axion fields.Comment: 5 pages, 8 figures, v2: journal versio
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