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Olivier Doré

Publications and source records attributed to Olivier Doré.

122 records · Page 7Linked to original sources
arXiv2009

The Non-Linear Fisher Information content of cosmic shear surveys

We quantify the Fisher information content of the cosmic shear survey two-point function as a function of noise and resolution. The two point information of dark matter saturates at the trans-linear scale. We investigate the impact of non-linear non-Gaussianity on the information content for lensing, which probes the same dark matter. To do so we heavily utilize N-body simulations in order to probe accurately the non-linear regime. While we find that even in a perfect survey, there is no clear saturation scale, we observe that non-linear growth induced non-Gaussianity could lead to a factor of ~4 reduction for the common Dark Energy figure of merit. This effect is however mitigated by realistic levels of shot noise and we find that for future surveys, the effect is closer to a factor of 1.5. To do so, we develop a new scheme to compute the relevant covariant matrix. It leads us to claim an unbiased estimator with an order of magnitude improvement in accuracy with only twice more simulations than previously used. Finally, we evaluate the error on the errors using bootstrap methods.

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arXiv2009

Dark Energy from Large-Scale Structure Lensing Information

Wide area Large-Scale Structure (LSS) surveys are planning to map a substantial fraction of the visible universe to quantify dark energy through Baryon Acoustic Oscillations (BAO). At increasing redshift, for example that probed by proposed 21-cm intensity mapping surveys, gravitational lensing potentially limits the fidelity (Hui et al., 2007) because it distorts the apparent matter distribution. In this paper we show that these distortions can be reconstructed, and actually used to map the distribution of intervening dark matter. The lensing information for sources at z=1-3 allows accurate reconstruction of the gravitational potential on large scales, l <~ 100, which is well matched for Integrated Sachs-Wolfe (ISW) effect measurements of dark energy and its sound speed, and a strong constraint for modified gravity models of dark energy. We built an optimal quadratic lensing estimator for non-Gaussian sources, which is necessary for LSS. The phenomenon of "information saturation" (Rimes & Hamilton, 2005) saturates reconstruction at mildly non-linear scales, where the linear source power spectrum Delta^2 ~ 0.2-0.5. We find that steeper power spectra, saturate more quickly. We compute the effective number densities of independent lensing sources for LSS lensing, and find that they increase rapidly with redshifts. For LSS/21-cm sources at z ~ 2-4, the lensing reconstruction is limited by cosmic variance at l<~ 100.

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arXiv2008

CMBPol Mission Concept Study: Gravitational Lensing

Gravitational lensing of the cosmic microwave background by large-scale structure in the late universe is both a source of cosmological information and a potential contaminant of primordial gravity waves. Because lensing imprints growth of structure in the late universe on the CMB, measurements of CMB lensing will constrain parameters to which the CMB would not otherwise be sensitive, such as neutrino mass. If the instrumental noise is sufficiently small (<~ 5 uK-arcmin), the gravitational lensing contribution to the large-scale B-mode will be the limiting source of contamination when constraining a stochastic background of gravity waves in the early universe, one of the most exciting prospects for future CMB polarization experiments. High-sensitivity measurements of small-scale B-modes can reduce this contamination through a lens reconstruction technique that separates the lensing and primordial contributions to the B-mode on large scales. A fundamental design decision for a future CMB polarization experiment such as CMBpol is whether to have coarse angular resolution so that only the large-scale B-mode (and the large-scale E-mode from reionization) is measured, or high resolution to additionally measure CMB lensing. The purpose of this white paper is to evaluate the science case for CMB lensing in polarization: constraints on cosmological parameters, increased sensitivity to the gravity wave B-mode via lens reconstruction, expected level of contamination from non-CMB foregrounds, and required control of beam systematics.

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arXiv2008

The imprints of primordial non-gaussianities on large-scale structure: scale dependent bias and abundance of virialized objects

We study the effect of primordial nongaussianity on large-scale structure, focusing upon the most massive virialized objects. Using analytic arguments and N-body simulations, we calculate the mass function and clustering of dark matter halos across a range of redshifts and levels of nongaussianity. We propose a simple fitting function for the mass function valid across the entire range of our simulations. We find pronounced effects of nongaussianity on the clustering of dark matter halos, leading to strongly scale-dependent bias. This suggests that the large-scale clustering of rare objects may provide a sensitive probe of primordial nongaussianity. We very roughly estimate that upcoming surveys can constrain nongaussianity at the level |fNL| <~ 10, competitive with forecasted constraints from the microwave background.

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arXiv2008

Probing the Largest Cosmological Scales with the CMB-Velocity Correlation

Cross-correlation between the CMB and large-scale structure is a powerful probe of dark-energy and gravity on the largest physical scales. We introduce a novel estimator, the CMB-velocity correlation, that has most of his power on large scales and that, at low redshift, delivers up to factor of two higher signal-to-noise ratio than the recently detected CMB-dark matter density correlation expected from the Integrated Sachs-Wolfe effect. We propose to use a combination of peculiar velocities measured from supernovae type Ia and kinetic Sunyaev-Zeldovich cluster surveys to reveal this signal and forecast dark-energy constraints that can be achieved with future surveys. We stress that low redshift peculiar velocity measurements should be exploited with complementary deeper large-scale structure surveys for precision cosmology.

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arXiv2007

The Signature of Patchy Reionization in the Polarization Anisotropy of the CMB

The inhomogeneous ionization state of the universe when the first sources of ionizing radiation appeared should lead to anisotropies in the polarization of the cosmic microwave background. We use cosmological simulations of the process by which the first sources ionized the intergalactic medium to study the induced polarization anisotropies. We find that the polarization anisotropies have rms of order ~0.01 μK, and local peak values of ~0.1 μK, smaller than those due to gravitational lensing on small scales. The polarization direction is highly coherent over degree scales. This directional coherence is not expected from either primary anisotropy or gravitational lensing effects, making the largest signals due to inhomogeneous ionization relatively easy to isolate, should experiments achieve the necessary very low noise levels.

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arXiv2006

The Cosmic Reionization History as Revealed by the CMB Doppler--21-cm Correlation

We show that the epoch(s) of reionization when the ionization fraction of the universe is about half can be determined by correlating Cosmic Microwave Background (CMB) temperature maps with 21-cm line maps at degree scales ($l\sim 100$). During reionization peculiar motion of free electrons induces the Doppler anisotropy of the CMB, while density fluctuations of neutral hydrogen induce the 21-cm line anisotropy. In our simplified model of inhomogeneous reionization, a positive correlation arises as the universe reionizes whereas a negative correlation arises as it recombines; thus, the sign of the correlation provides information on the reionization history which cannot be obtained by presently. The signal comes mainly from large scales (k~0.01 Mpc^-1) where linear perturbation theory is valid and complexity due to patchy reionization is averaged out. Since the Doppler signal comes from ionized regions and the 21-cm comes from neutral ones, the correlation has a well-defined peak(s) in redshift when the ionization fraction of the universe is about half. Furthermore, the cross-correlation is much less sensitive to systematic errors, especially foreground emission, than the auto-correlation of 21-cm lines: this is analogous to the temperature-polarization correlation of the CMB being more immune to systematic errors than the polarization-polarization. Therefore, we argue that the Doppler-21cm correlation provides a robust measurement of the 21-cm anisotropy, which can also be used as a diagnostic tool for detected signals in the 21-cm data -- detection of the cross-correlation provides the strongest confirmation that the signal is of cosmological origin. We show that the Square Kilometer Array can easily measure the predicted correlation signal for 1~year of survey observation.

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arXiv2004

The CMB Quadrupole in a Polarized Light

The low quadrupole of the cosmic microwave background (CMB), measured by COBE and confirmed by WMAP, has generated much discussion recently. We point out that the well-known correlation between temperature and polarization anisotropies of the CMB further constrains the low multipole anisotropy data. This correlation originates from the fact that the low-multipole polarization signal is sourced by the CMB quadrupole as seen by free electrons during the relatively recent cosmic history. Consequently, the large-angle temperature anisotropy data make restrictive predictions for the large-angle polarization anisotropy, which depend primarily on the optical depth for electron scattering after cosmological recombination, tau. We show that if current cosmological models for the generation of large angle anisotropy are correct and the COBE/WMAP data are not significantly contaminated by non-CMB signals, then the observed C_te amplitude on the largest scales is discrepant at the 99.8% level with the observed C_tt for the concordance LCDM model with tau=0.10. Using tau=0.17, the preferred WMAP model-independent value, the discrepancy is at the level of 98.5%.

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arXiv2003

Beyond the Damping Tail: Cross-Correlating the Kinetic Sunyaev-Zel'dovich Effect with Cosmic Shear

Secondary anisotropies of the CMB have the potential to reveal intricate details about the history of our universe between the present and recombination epochs. However, because the CMB we observe is the projected sum of a multitude of effects, the interpretation of small scale anisotropies by future high resolution experiments will be marred by uncertainty and speculation without the handles provided by other observations. In this paper we show that cross correlating the CMB with an overlapping weak lensing survey will isolate the elusive kinetic Sunyaev-Zel'dovich Effect from secondary anisotropies generated at higher redshifts. We show that if upcoming high angular resolution CMB experiments, like PLANCK/ACT/SPT, cover the same area of sky as current and future weak lensing surveys, like CFTHLS/SNAP/LSST, the cross correlation of cosmic shear with the kSZ effect will be detected with high signal to noise ratio, increasing the potential science accessible to both sets of surveys. For example, if ACT and a CFHTLS like survey were to overlap this cross-correlation would be detected with a total signal to noise ratio greater than 220, reaching 1.8 per individual multipole around l \sim 5000. Furthermore, this cross-correlation probes the three point coupling between the underlying dark matter and the "momentum" of the ionized baryons in the densest regions of the universe at intermediate redshifts. Similar to the tSZ power spectrum, its strength is extremely sensitive to the power spectrum normalization parameter, σ_8, scaling roughly as σ_8^7. It provides an effective mechanism to isolate any component of anisotropy due to patchy reionization and rule out primordial small scale anisotropy.

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arXiv2003

Are Chaplygin gases serious contenders to the dark energy throne?

We study the implications on both background and perturbation evolution of introducing a Chaplygin gas component in the universe's ingredients. We perform likelihood analyses using wide-ranging, SN1a, CMB and large scale structure observations to assess whether such a component could be a genuine alternative to a cosmological constant. We find that current data favors an adiabatic Chaplygin gas with behavior akin to a cosmological constant.

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arXiv2003

Probing dark energy perturbations: the dark energy equation of state and speed of sound as measured by WMAP

We review the implications of having a non-trivial matter component in the universe and the potential for detecting such a component through the matter power spectrum and ISW effect. We adopt a phenomenological approach and consider the mysterious dark energy to be a cosmic fluid. It is thus fully characterized, up to linear order, by its equation of state and its speed of sound. Whereas the equation of state has been widely studied in the literature, less interest has been devoted to the speed of sound. Its observational consequences come predominantly from very large scale modes of dark matter perturbations (k < 0.01 h.Mpc^{-1}). Since these modes have hardly been probed so far by large scale galaxy surveys, we investigate whether joint constraints that can be placed on those two quantities using the recent CMB fluctuations measurements by WMAP as well as the recently measured CMB large scale structure cross-correlation. We find only a tentative 1$sigma$ detection of the speed of sound, from CMB alone, c_{s}^{2}<0.04 at this low significance level. Furthermore, the current uncertainties in bias in the matter power spectrum preclude any constraints being placed using the cross correlation of CMB with the NVSS radio survey. We believe however that improvements in bias through improved survey scales and depths in the near future will improve hopes of detecting the speed of sound.

astro-ph↗
arXiv2003

Skeleton as a probe of the cosmic web: the 2D case

We discuss the skeleton as a probe of the filamentary structures of a 2D random field. It can be defined for a smooth field as the ensemble of pairs of field lines departing from saddle points, initially aligned with the major axis of local curvature and connecting them to local maxima. This definition is thus non local and makes analytical predictions difficult, so we propose a local approximation: the local skeleton is given by the set of points where the gradient is aligned with the local curvature major axis and where the second component of the local curvature is negative. We perform a statistical analysis of the length of the total local skeleton, chosen for simplicity as the set of all points of space where the gradient is either parallel or orthogonal to the main curvature axis. In all our numerical experiments, which include Gaussian and various non Gaussian realizations such as χ^2 fields and Zel'dovich maps, the differential length is found within a normalization factor to be very close to the probability distribution function of the smoothed field. This is in fact explicitly demonstrated in the Gaussian case. This result might be discouraging for using the skeleton as a probe of non Gausiannity, but our analyses assume that the total length of the skeleton is a free, adjustable parameter. This total length could in fact be used to constrain cosmological models, in CMB maps but also in 3D galaxy catalogs, where it estimates the total length of filaments in the Universe. Making the link with other works, we also show how the skeleton can be used to study the dynamics of large scale structure.

astro-ph↗
arXiv2002

Gravitational Potential Reconstruction from Peculiar Velocity and Weak Lensing Measurements

We present an analytic method for rapidly forecasting the accuracy of gravitational potential reconstruction possible from measurement of radial peculiar velocities of every galaxy cluster with M > M_th in solid angle θ^2 and over redshift range z_min < z < z_max. These radial velocities can be determined from measurement of the kinetic and thermal Sunyaev-Zeldovich effects. For a shallow survey with 0.2 < z < 0.4, coincident with the SDSS photometric survey, one mode of the gravitational potential (on length scales > 60 Mpc) can be reconstructed for every ~8 cluster velocity determinations. Deeper surveys require measurement of more clusters per S/N > 1 mode. Accuracy is limited by the ``undersampling noise'' due to our non-observation of the large fraction of mass that is not in galaxy clusters. Determining the gravitational potential will allow for detailed study of the relationship between galaxies and their surrounding large-scale density fields over a wide range of redshifts, and test the gravitational instability paradigm on very large scales. Observation of weak lensing by large-scale structure provides complementary information since lensing is sensitive to the tangential modes that do not affect the velocity.

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