Patrick Valageas

Cosmology with weak lensing surveys

Weak gravitational lensing is responsible for the shearing and magnification of the images of high-redshift sources due to the presence of intervening mass. Since the lensing effects arise from deflections of the light rays due to fluctuations of the gravitational potential, they can be directly related to the underlying density field of the large-scale structures. Weak gravitational surveys are complementary to both galaxy surveys and cosmic microwave background observations as they probe unbiased nonlinear matter power spectra at medium redshift. Ongoing CMBR experiments such as WMAP and a future Planck satellite mission will measure the standard cosmological parameters with unprecedented accuracy. The focus of attention will then shift to understanding the nature of dark matter and vacuum energy: several recent studies suggest that lensing is the best method for constraining the dark energy equation of state. During the next 5 year period, ongoing and future weak lensing surveys such as the Joint Dark Energy Mission (JDEM; e.g. SNAP) or the Large-aperture Synoptic Survey Telescope will play a major role in advancing our understanding of the universe in this direction. In this review article, we describe various aspects of probing the matter power spectrum and the bi-spectrum and other related statistics with weak lensing surveys. This can be used to probe the background dynamics of the universe as well as the nature of dark matter and dark energy.

The XXL Survey I. Scientific motivations - XMM-Newton observing plan - Follow-up observations and simulation programme

Context. The quest for the cosmological parameters that describe our universe continues to motivate the scientific community to undertake very large survey initiatives across the electromagnetic spectrum. Over the past two decades, the Chandra and XMM-Newton observatories have supported numerous studies of X-ray-selected clusters of galaxies, active galactic nuclei (AGNs), and the X-ray background. The present paper is the first in a series reporting results of the XXL-XMM survey; it comes at a time when the Planck mission results are being finalised. Aims. We present the XXL Survey, the largest XMM programme totaling some 6.9 Ms to date and involving an international consortium of roughly 100 members. The XXL Survey covers two extragalactic areas of 25 deg(2) each at a point-source sensitivity of similar to 5 x 10(-15) erg s(-1) cm(-2) in the [0.5-2] keV band (completeness limit). The surveys main goals are to provide constraints on the dark energy equation of state from the space-time distribution of clusters of galaxies and to serve as a pathfinder for future, wide-area X-ray missions. We review science objectives, including cluster studies, AGN evolution, and large-scale structure, that are being conducted with the support of approximately 30 follow-up programmes. Methods. We describe the 542 XMM observations along with the associated multi-lambda and numerical simulation programmes. We give a detailed account of the X-ray processing steps and describe innovative tools being developed for the cosmological analysis. Results. The paper provides a thorough evaluation of the X-ray data, including quality controls, photon statistics, exposure and background maps, and sky coverage. Source catalogue construction and multi-lambda associations are briefly described. This material will be the basis for the calculation of the cluster and AGN selection functions, critical elements of the cosmological and science analyses. Conclusions. The XXL multi-lambda data set will have a unique lasting legacy value for cosmological and extragalactic studies and will serve as a calibration resource for future dark energy studies with clusters and other X-ray selected sources. With the present article, we release the XMM XXL photon and smoothed images along with the corresponding exposure maps.

Combining perturbation theories with halo models

Aims. We investigate the building of unified models that can predict the matter-density power spectrum and the two-point correlation function from very large to small scales, being consistent with perturbation theory at low k and with halo models at high k. Methods. We use a Lagrangian framework to re-interpret the halo model and to decompose the power spectrum into “2-halo” and “1-halo” contributions, related to “perturbative” and “non-perturbative” terms. We describe a simple implementation of this model and present a detailed comparison with numerical simulations, from k ∼ 0.02 up to 100 h Mpc −1 , and from x ∼ 0.02 up to 150 h −1 Mpc. Results. We show that the 1-halo contribution contains a counterterm that ensures a k 2 tail at low k and this contribution is important not to spoil the predictions on the scales probed by baryon acoustic oscillations, k ∼ 0.02 to 0.3 h Mpc −1 . On the other hand, we show that standard perturbation theory is inadequate for the 2-halo contribution, because higher order terms grow too fast at high k, so that resummation schemes must be used. Moreover, we explain why such a model, which is based on the combination of perturbation theories and halo models, remains consistent with standard perturbation theory up to the order of the resummation scheme. We describe a simple implementation, based on a 1-loop “direct steepest-descent” resummation for the 2-halo contribution that allows fast numerical computations, and we check that we obtain a good match to simulations at low and high k. We also study the dependence of such predictions on the details of the underlying model, such as the choice of the perturbative resummation scheme or the properties of halo profiles. Our simple implementation already fares better than both standard 1-loop perturbation theory on large scales and simple fits to the power spectrum at high k, with a typical accuracy of 1% on large scales and 10% on small scales. We obtain similar results for the two-point correlation function. However, there remains room for improvement on the transition scale between the 2-halo and 1-halo contributions, which may be the most difficult regime to describe.

The phase-diagram of cosmological baryons

We investigate the behaviour of cosmological baryons at low redshifts

Large-N expansions applied to gravitational clustering

z \la 5

Secondary non-Gaussianity and cross-correlation analysis

after reionization, through analytic means. In particular, we study the density-temperature phase-diagram that describes the history of the gas. We show how the location of the matter in this

K -mouflage cosmology: The background evolution

(\rho,T)

Propagators in Lagrangian space

diagram expresses the various constraints implied by usual hierarchical scenarios. This yields robust model-independent results that agree with numerical simulations. The IGM is seen to be formed via two phases: a “cool” photo-ionized component and a “warm” component governed by shock-heating. We also briefly describe how the remainder of the matter is distributed over galaxies, groups and clusters. We recover the fraction of matter and the spatial clustering computed by numerical simulations. We also check that the soft X-ray background due to the “warm” IGM component is consistent with observations. We find in the present universe a baryon fraction of 7% in hot gas, 24% in the warm IGM, 38% in the cool IGM, 9% within star-like objects and, as a still un-observed component, 22% of dark baryons associated with collapsed structures, with a relative uncertainty no larger than 30% on these numbers.

Angular averaged consistency relations of large-scale structures

We develop a path-integral formalism to study the formation of large-scale structures in the universe. Starting from the equations of motion of hydrodynamics (single-stream approximation) we derive the action which describes the statistical properties of the density and velocity fields for Gaussian initial conditions. Then, we present large- N expansions (associated with a generalization to N fields or with a semi-classical expansion) of the path-integral defined by this action. This provides a systematic expansion for two-point functions such as the response function and the usual two-point correlation. We present the results of two such expansions (and related variants) at one-loop order for a SCDM and a Λ CDM cosmology. We find that the response function exhibits fast oscillations in the non-linear regime with an amplitude which either follows the linear prediction (for the direct steepest-descent scheme) or decays (for the 2PI effective action scheme). On the other hand, the correlation function agrees with the standard one-loop result in the quasi-linear regime and remains well-behaved in the highly non-linear regime. This suggests that these large- N expansions could provide a good framework to study the dynamics of gravitational clustering in the non-linear regime. Moreover, the use of various expansion schemes allows one to estimate their range of validity without the need of N -body simulations and could provide a better accuracy in the weakly non-linear regime.

Using the Zeldovich dynamics to test expansion schemes

We develop optimised estimators of two sorts of power spectra for fields defined on the sky, in the pres- ence of partial sky coverage. The first is the cross-power spe ctrum of two fields on the sky; the second is the skew spectrum of three fields. The cross-power spectru m of the Cosmic Microwave Background (CMB) sky with tracers of large-scale-structure is useful a s it provides valuable information on cosmo- logical parameters. Numerous recent studies have proved the usefulness of cross-correlating CMB sky with external data sets, which probes the Integrated Sachs Wolfe Effect (ISW) at large angular scales and the Sunyaev Zeldovich (SZ) effect from hot gas in clusters at small angular scales. The skew spec- trum, recently introduced by Munshi & Heavens (2009), is an optimised statistic which can be tuned to study a particular form of non-gaussianity, such as may aris e in the early Universe, but which retains information on the nature of non-gaussianity. In this paper we develop the mathematical formalism for the skew spectrum of 3 different fields. When applied to th e CMB, this allows us to explore the contamination of the skew spectrum by secondary sources of CMB fluctuations. Considering the three- point function, the study of the bispectrum provides valuable information regarding cross-correlation of secondaries with lensing of CMB with much higher significanc e compared to just the study involving CMB sky alone. After developing the analytical model we use them to study specific cases of cosmo- logical interest which include cross-correlating CMB with various large scale tracers to probe ISW and SZ effects for cross spectral analysis. Next we use the forma lism to study the signal-to-noise ratio for detection of the weak lensing of the CMB by cross-correlating it with different tracers as well as point sources for CMB experiments such as Planck.