Aims. We present an exploration of weak lensing by large-scale structure in the linear regime, using the third-year (T0003) CFHTLS Wide data release. Our results place tight constraints on the scaling of the amplitude of the matter power spectrum σ 8 with the matter density Ω m . Methods. Spanning 57 square degrees to i AB = 24.5 over three independent fields, the unprecedented contiguous area of this survey permits high signal-to-noise measurements of two-point shear statistics from 1 arcmin to 4 degrees. Understanding systematic errors in our analysis is vital in interpreting the results. We therefore demonstrate the percent-level accuracy of our method using STEP simulations, an E/B-mode decomposition of the data, and the star-galaxy cross correlation function. We also present a thorough analysis of the galaxy redshift distribution using redshift data from the CFHTLS T0003 Deep fields that probe the same spatial regions as the Wide fields. Results. We find σ 8 (Ω m /0.25) 0.64 = 0.785 ± 0.043 using the aperture-mass statistic for the full range of angular scales for an assumed flat cosmology, in excellent agreement with WMAP3 constraints. The largest physical scale probed by our analysis is 85 Mpc, assuming a mean redshift of lenses of 0.5 and a ΛCDM cosmology. This allows for the first time to constrain cosmology using only cosmic shear measurements in the linear regime. Using only angular scales θ > 85 arcmin, we find σ 8 (Ω m /0.25) 0.53 lin = 0.837 ± 0.084, which agree with the results from our full analysis. Combining our results with data from WMAP3, we find Ω m = 0.248 ± 0.019 and σ 8 = 0.771 ± 0.029.

We present the first cosmic shear measurements obtained from the T0001 release of the Canada-France-Hawaii Telescope Legacy Survey. The data set covers three uncorrelated patches (D1, D3 and D4) of one square degree each, observed in u * , g , r , i and z bands, to a depth of i = 25.5. The deep, multi-colour observations in these fields allow for several data-quality controls. The lensing signal is detected in both r and i bands and shows similar amplitude and slope in both filters. B-modes are found to be statistically zero at all scales. Using multi-colour information, we derived a photometric redshift for each galaxy and use this to separate the background source sample into low-z and high-z subsamples. A stronger shear signal is detected from the high-z subsample than from the low-z subsample, as expected from weak lensing tomography. While further work is needed to model the effects of errors in the photometric redshifts, this result suggests that it will be possible to obtain constraints on the growth of dark matter fluctuations with lensing wide field surveys. The combined Deep and Wide surveys give σ 8 = 0.89 ± 0.06 assuming the Peacock & Dodds non-linear scheme (P&D), and σ 8 = 0.86 ± 0.05 for the halo model and Ω m = 0.3. We assumed a Cold Dark Matter model with flat geometry and have marginalized over the systematics, the Hubble constant and redshift uncertainties. Using data from the Deep survey, the 1σ upper bound for w 0 , the constant equation of state parameter is w 0 < −0.8.

Based on observations obtained at the Very Large Telescope UT1 (ANTU) which is operated by the European Southern Observatory (program 63.O-0039A).

We consider the effects of the elastic Thomson scattering between cosmic background radiation (CBR) and primordial molecules, like H2, H2(+), HeH(+), HD, HD(+), LiH, LiH(+): primary CBR anisotropies may be erased or attenuated for angular scales is less than or equal to 10 deg up to 50 deg and frequencies nu is less than or equal to 50 GHz, if LiH primordial abundance exceed 10-10(H). Conversely, CBR anisotropy data may be used to put stringent upper limits on the abundance of primordial elements. Already available data on CBR anisotropies at various frequencies impose primordial LiH abundance to be not larger than 2 x 10-9(H). Secondary anisotropies are expected at arcsecond-arcminute angular scales, due to the anisotropic scattering by moving molecular clouds: they are stronger in the millimetric region and close to be detected in IRAM 200 GHz observations

Application of the aperture mass (M ap -) statistics provides a weak lensing method for the detection of cluster-sized dark matter halos. We present a new aperture filter function and maximise the effectiveness of the M ap -statistics to detect cluster-sized halos using analytical models. We then use weak lensing mock catalogues generated from ray-tracing through N-body simulations, to analyse the effect of image treatment on the expected number density of halos. Using the M ap -statistics, the aperture radius is typically several arcminutes, hence the aperture often lies partly outside a data field, consequently the signal-to-noise ratio of a halo detection decreases. We study these border effects analytically and by using mock catalogues. We find that the expected number density of halos decreases by a factor of two if the size of a field is comparable to the diameter of the aperture used. We finally report on the results of a weak lensing cluster search applying the M ap -statistics to 50 randomly selected fields which were observed with FORS1 at the VLT. Altogether the 50 VLT fields cover an area of 0.64 square degrees. The I-band images were taken under excellent seeing conditions (average seeing ≈ 0. 6) which results in a high number density of galaxies used for the weak lensing analysis (n ≈ 26 arcmin −2 ). In five of the VLT fields, we detect a significant M ap -signal which coincides with an overdensity of the light distribution. These detections are thus excellent candidates for shear-selected clusters.