arXiv subjects

François R. Bouchet

Publications and source records attributed to François R. Bouchet.

24 records · Page 2Linked to original sources

arXiv2014

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$^{st}$, 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.

astro-ph.CO↗

arXiv2008

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.

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arXiv1999

Foregrounds and CMB Experiments: I. Semi-analytical estimates of contamination

As Cosmic Microwave Background (CMB) measurements are becoming more ambitious, the issue of foreground contamination is becoming more pressing. This is especially true at the level of sensitivity, angular resolution and for the sky coverage of the planned space experiments MAP and PLANCK. We present in this paper an indicator of the accuracy of the separation of the CMB anisotropies from those induced by foregrounds. Of course, the outcome will depend on the spectral and spatial characteristics of the sources of anisotropies. We thus start by summarising present knowledge on the spectral and spatial properties of Galactic foregrounds, point sources, and clusters of galaxies. This information comes in support of a modelling of the microwave sky including the relevant components. The accuracy indicator we introduce is based on a generalisation of the Wiener filtering method to multi-frequency, multi-resolution data. While the development and use of this indicator was prompted by the preparation of the scientific case for the \plancks satellite, it has broader application since it allows assessing the effective capabilities of an instrumental set-up once foregrounds are fully accounted for, with a view to enabling comparisons between different experimental arrangements. The real sky might well be different from the one assumed here, and the analysis method might not be in the end Wiener filtering, but this work still allow meaningful {\em comparative} studies. As a matter of examples, we compare the CMB reconstruction errors for the \maps and \plancks space missions, as well as the robustness of the \plancks outcome to possible failures of specific spectral channels or global variations of the detectors noise level across spectral channels.

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arXiv1995

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.

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arXiv1995

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.

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arXiv1995

Lyman Alpha Forest Clustering Using Cell Counts Statistics

We present a novel technique for calculating \Lya forest correlations based on cell counts. It is applied to the line lists from 7 QSOs observed at high resolution ($Δv<25\kms$). Two spectra (Q0055$-$259 and Q0014$+$813) appear to be sufficiently free of biases to obtain meaningful estimates. We find for both positive correlations, with a maximum of $0.5-1$, on the scale of $0.5-3 h^{-1}$ Mpc (comoving), or $100-600\kms$, which is consistent with the primordial power spectrum inferred from optically-selected galaxy surveys. Strong evidence for anti-correlation on the scale of $3-6 h^{-1}$ Mpc is also found. If this first detection of anti-correlations is physical, additional factors to gravity may play a role in determining the clustering of the \Lya forest on comoving scales $> 3h^{-1}\Mpc$.}

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