2,012 research outputs found
CMB anisotropies in the presence of a stochastic magnetic field
Primordial magnetic fields present since before the epoch of matter-radiation
equality have an effect on the anisotropies of the cosmic microwave background.
The CMB anisotropies due to scalar perturbations are calculated in the gauge
invariant formalism for magnetized adiabatic initial conditions. Furthermore
the linear matter power spectrum is calculated. Numerical solutions are
complemented by a qualitative analysis.Comment: 26 pages, 21 figures; sections 2 and 4 expanded; matches version
published in PR
CMB power spectrum parameter degeneracies in the era of precision cosmology
Cosmological parameter constraints from the CMB power spectra alone suffer
several well-known degeneracies. These degeneracies can be broken by numerical
artefacts and also a variety of physical effects that become quantitatively
important with high-accuracy data e.g. from the Planck satellite. We study
degeneracies in models with flat and non-flat spatial sections, non-trivial
dark energy and massive neutrinos, and investigate the importance of various
physical degeneracy-breaking effects. We test the CAMB power spectrum code for
numerical accuracy, and demonstrate that the numerical calculations are
accurate enough for degeneracies to be broken mainly by true physical effects
(the integrated Sachs-Wolfe effect, CMB lensing and geometrical and other
effects through recombination) rather than numerical artefacts. We quantify the
impact of CMB lensing on the power spectra, which inevitably provides
degeneracy-breaking information even without using information in the
non-Gaussianity. Finally we check the numerical accuracy of sample-based
parameter constraints using CAMB and CosmoMC. In an appendix we document recent
changes to CAMB's numerical treatment of massive neutrino perturbations, which
are tested along with other recent improvements by our degeneracy exploration
results.Comment: 27 pages, 28 figures. Latest CAMB version available from
http://camb.info/. Reduced number of figures, plot legend corrected and minor
edits to match published versio
Morphing the CMB: a technique for interpolating power spectra
The confrontation of the Cosmic Microwave Background (CMB) theoretical
angular power spectrum with available data often requires the calculation of
large numbers of power spectra. The standard practice is to use a fast code to
compute the CMB power spectra over some large parameter space, in order to
estimate likelihoods and constrain these parameters. But as the dimensionality
of the space under study increases, then even with relatively fast anisotropy
codes, the computation can become prohibitive. This paper describes the
employment of a "morphing" strategy to interpolate new power spectra based on
previously calculated ones. We simply present the basic idea here, and
illustrate with a few examples; optimization of interpolation schemes will
depend on the specific application. In addition to facilitating the exploration
of large parameter spaces, this morphing technique may be helpful for Fisher
matrix calculations involving derivatives.Comment: 18 pages, including 6 figures, uses elsart.cls, accepted for
publication in New Astronomy, changes to match published versio
Statistical isotropy violation in WMAP CMB maps resulting from non-circular beams
Statistical isotropy (SI) of Cosmic Microwave Background (CMB) fluctuations
is a key observational test to validate the cosmological principle underlying
the standard model of cosmology. While a detection of SI violation would have
immense cosmological ramification, it is important to recognise their possible
origin in systematic effects of observations. WMAP seven year (WMAP-7) release
claimed significant deviation from SI in the bipolar spherical harmonic
(BipoSH) coefficients and . Here we present the
first explicit reproduction of the measurements reported in WMAP-7, confirming
that beam systematics alone can completely account for the measured SI
violation. The possibility of such a systematic origin was alluded to in WMAP-7
paper itself and other authors but not as explicitly so as to account for it
accurately. We simulate CMB maps using the actual WMAP non-circular beams and
scanning strategy. Our estimated BipoSH spectra from these maps match the
WMAP-7 results very well. It is also evident that only a very careful and
adequately detailed modelling, as carried out here, can conclusively establish
that the entire signal arises from non-circular beam effect. This is important
since cosmic SI violation signals are expected to be subtle and dismissing a
large SI violation signal as observational artefact based on simplistic
plausibility arguments run the serious risk of "throwing the baby out with the
bathwater".Comment: 4 pages, 3 figures. Published version. Includes major revision in the
text and one important figure. No change in the result
Cosmic Microwave Background temperature and polarization anisotropies from the large-N limit of global defects
We determine the full C_l spectra and correlation functions of the
temperature and polarization anisotropies in the CMB, generated by a source
modeled by the large N limit of spontaneously broken global O(N)-theories. We
point out a problem in the standard approach of treating the radiation-matter
transition by interpolating the eigenvectors of the unequal-time correlators of
the source energy-momentum tensor. This affects the CMB predictions from all
type of cosmic defects. We propose a method to overcome this difficulty, and
find that in the large-N global model that we study, differences in the final
CMB power spectra amplitudes reach up to 25%, when compared to implementations
of the eigenvector interpolation technique. We discuss as well how to optimally
search for the contribution in the CMB from active sources such as cosmic
defects, in experiments like Planck, COrE and PRISM.Comment: 16+4 pages, 13 figures (Version 2: minor changes to match published
version in PRD
High precision simulations of weak lensing effect on Cosmic Microwave Background polarization
We study accuracy, robustness and self-consistency of pixel-domain
simulations of the gravitational lensing effect on the primordial CMB
anisotropies due to the large-scale structure of the Universe. In particular,
we investigate dependence of the results precision on some crucial parameters
of such techniques and propose a semi-analytic framework to determine their
values so the required precision is a priori assured and the numerical workload
simultaneously optimized. Our focus is on the B-mode signal but we discuss also
other CMB observables, such as total intensity, T, and E-mode polarization,
emphasizing differences and similarities between all these cases. Our
semi-analytic considerations are backed up by extensive numerical results.
Those are obtained using a code, nicknamed lenS2HAT -- for Lensing using
Scalable Spherical Harmonic Transforms (S2HAT) -- which we have developed in
the course of this work. The code implements a version of the pixel-domain
approach of Lewis (2005) and permits performing the simulations at very high
resolutions and data volumes, thanks to its efficient parallelization provided
by the S2HAT library -- a parallel library for a calculation of the spherical
harmonic transforms. The code is made publicly available.Comment: 20 pages, 14 figures, submitted to A&A, matches version accepted for
publication in A&
A Line of Sight Approach to Cosmic Microwave Background Anisotropies
We present a new method for calculating linear cosmic microwave background
(CMB) anisotropy spectra based on integration over sources along the photon
past light cone. In this approach the temperature anisotropy is written as a
time integral over the product of a geometrical term and a source term. The
geometrical term is given by radial eigenfunctions which do not depend on the
particular cosmological model. The source term can be expressed in terms of
photon, baryon and metric perturbations, all of which can be calculated using a
small number of differential equations. This split clearly separates between
the dynamical and geometrical effects on the CMB anisotropies. More
importantly, it allows to significantly reduce the computational time compared
to standard methods. This is achieved because the source term, which depends on
the model and is generally the most time consuming part of calculation, is a
slowly varying function of wavelength and needs to be evaluated only in a small
number of points. The geometrical term, which oscillates much more rapidly than
the source term, does not depend on the particular model and can be precomputed
in advance. Standard methods that do not separate the two terms and require a
much higher number of evaluations. The new method leads to about two orders of
magnitude reduction in CPU time when compared to standard methods and typically
requires a few minutes on a workstation for a single model. The method should
be especially useful for accurate determinations of cosmological parameters
from CMB anisotropy and polarization measurements that will become possible
with the next generation of experiments. A programm implementing this method
can be obtained from the authors.Comment: 20 pages, 5 figures. Fortran code available from the author
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