127 research outputs found
The Planck mission
These lecture from the 100th Les Houches summer school on "Post-planck
cosmology" of July 2013 discuss some aspects of the Planck mission, whose prime
objective was a very accurate measurement of the temperature anisotropies of
the Cosmic Microwave Background (CMB). We announced our findings a few months
ago, on March 21, 2013. I describe some of the relevant steps we took to
obtain these results, sketching the measurement process, how we processed the
data to obtain full sky maps at 9 different frequencies, and how we extracted
the CMB temperature anisotropies map and angular power spectrum. I conclude by
describing some of the main cosmological implications of the statistical
characteristics of the CMB we found. Of course, this is a very much shortened
and somewhat biased view of the \Planck\ 2013 results, written with the hope
that it may lead some of the students to consult the original papers.Comment: 53 p.-34 fig; for spacetime consideration, the file here is not
paying justice to the actual thing; a closer approximation of it can be found
at
https://www.researchgate.net/profile/Francois_Bouchet/publication/262004262_The_Planck_Mission/file/e0b495363b042e81dd.pd
Kurtosis in Large-Scale Structure as a Constraint on Non-Gaussian Initial Conditions
We calculate the kurtosis of a large-scale density field which has undergone
weakly non-linear gravitational evolution from arbitrary non-Gaussian initial
conditions. It is well known that the weakly evolved {\twelveit skewness} is
equal to its initial value plus the term induced by gravity, which scales with
the rms density fluctuation in precisely the same way as for Gaussian initial
conditions. As in the case of skewness, the evolved {\twelveit kurtosis} is
equal to its initial value plus the contribution induced by gravity. The
scaling of this induced contribution, however, turns out to be qualitatively
different for Gaussian versus non-Gaussian initial conditions. Therefore,
measurements of the kurtosis can serve as a powerful discriminating test
between the hypotheses of Gaussian and non-Gaussian nature of primordial
density fluctuations.Comment: uuencoded compressed tar file including postscript text (17 pages)
and 2 postscript figures, submitted to MNRA
The COBRAS/SAMBA CMB Project
COBRAS/SAMBA is a second generation satelitte dedicated to mapping at high
resolution and sensitivity the anisotropies of the Cosmic Microwave Background
(CMB). This mission is in the assessment study phase (A) at ESA, with a
decision expected mid 1996, for a launch around 2003.Comment: PostScript, 4 pages, 4 figures in text, to appear in the Proceedings
of the 1995 Moriond Meeting on ``Clustering in the Universe'
All sky CMB map from cosmic strings integrated Sachs-Wolfe effect
By actively distorting the Cosmic Microwave Background (CMB) over our past
light cone, cosmic strings are unavoidable sources of non-Gaussianity.
Developing optimal estimators able to disambiguate a string signal from the
primordial type of non-Gaussianity requires calibration over synthetic full sky
CMB maps, which till now had been numerically unachievable at the resolution of
modern experiments. In this paper, we provide the first high resolution full
sky CMB map of the temperature anisotropies induced by a network of cosmic
strings since the recombination. The map has about 200 million sub-arcminute
pixels in the healpix format which is the standard in use for CMB analyses
(Nside=4096). This premiere required about 800,000 cpu hours; it has been
generated by using a massively parallel ray tracing method piercing through a
thousands of state of art Nambu-Goto cosmic string numerical simulations which
pave the comoving volume between the observer and the last scattering surface.
We explicitly show how this map corrects previous results derived in the flat
sky approximation, while remaining completely compatible at the smallest
scales.Comment: 8 pages, 4 figures, uses RevTeX. References added, matches published
versio
Stochastic gravitational waves from long cosmic strings
We compute the expected strain power spectrum and energy density parameter of
the stochastic gravitational wave background (SGWB) created by a network of
long cosmic strings evolving during the whole cosmic history. As opposed to
other studies, the contribution of cosmic string loops is discarded and our
result provides a robust lower bound of the expected signal that is applicable
to most string models. Our approach uses Nambu-Goto numerical simulations,
running during the radiation, transition and matter eras, in which we compute
the two-point unequal-time anisotropic stress correlators. These ones act as
source terms in the linearised equations of motion for the tensor modes, that
we solve using an exact Green's function integrator. Today, we find that the
rescaled strain power spectrum peaks on
Hubble scales and exhibits, at large wavenumbers, high frequency oscillations
around a plateau of amplitude . Most of the high frequency power is
generated by the long strings present in the matter era, the radiation era
contribution being smaller.Comment: 28 pages, 9 figures, uses jcappub. References and discussions added.
Matches published versio
CMB Polarization can constrain cosmology better than CMB temperature
We demonstrate that for a cosmic variance limited experiment, CMB E
polarization alone places stronger constraints on cosmological parameters than
CMB temperature. For example, we show that EE can constrain parameters better
than TT by up to a factor 2.8 when a multipole range of l=30-2500 is
considered. We expose the physical effects at play behind this remarkable
result and study how it depends on the multipole range included in the
analysis. In most relevant cases, TE or EE surpass the TT based cosmological
constraints. This result is important as the small scale astrophysical
foregrounds are expected to have a much reduced impact on polarization, thus
opening the possibility of building cleaner and more stringent constraints of
the LCDM model. This is relevant specially for proposed future CMB satellite
missions, such as CORE or PRISM, that are designed to be cosmic variance
limited in polarization till very large multipoles. We perform the same
analysis for a Planck-like experiment, and conclude that even in this case TE
alone should determine the constraint on better than TT by 15%,
while determining , and with comparable accuracy.
Finally, we explore a few classical extensions of the LCDM model and show again
that CMB polarization alone provides more stringent constraints than CMB
temperature in case of a cosmic variance limited experiment.Comment: 14 pages, 16 figure
Small-Angle CMB Temperature Anisotropies Induced by Cosmic Strings
We use Nambu-Goto numerical simulations to compute the cosmic microwave
background (CMB) temperature anisotropies induced at arcminute angular scales
by a network of cosmic strings in a Friedmann-Lemaitre-Robertson-Walker (FLRW)
expanding universe. We generate 84 statistically independent maps on a 7.2
degree field of view, which we use to derive basic statistical estimators such
as the one-point distribution and two-point correlation functions. At high
multipoles, the mean angular power spectrum of string-induced CMB temperature
anisotropies can be described by a power law slowly decaying as \ell^{-p}, with
p=0.889 (+0.001,-0.090) (including only systematic errors). Such a behavior
suggests that a nonvanishing string contribution to the overall CMB
anisotropies may become the dominant source of fluctuations at small angular
scales. We therefore discuss how well the temperature gradient magnitude
operator can trace strings in the context of a typical arcminute
diffraction-limited experiment. Including both the thermal and nonlinear
kinetic Sunyaev-Zel'dovich effects, the Ostriker-Vishniac effect, and the
currently favored adiabatic primary anisotropies, we find that, on such a map,
strings should be ``eye visible,'' with at least of order ten distinctive
string features observable on a 7.2 degree gradient map, for tensions U down to
GU \simeq 2 x 10^{-7} (in Planck units). This suggests that, with upcoming
experiments such as the Atacama Cosmology Telescope (ACT), optimal
non-Gaussian, string-devoted statistical estimators applied to small-angle CMB
temperature or gradient maps may put stringent constraints on a possible cosmic
string contribution to the CMB anisotropies.Comment: 17 pages, 9 figures. v2: matches published version, minor
clarifications added, typo in Eq. (8) fixed, results unchange
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