We use the 2dF QSO Redshift Survey, to investigate whether QSOs are detectably gravitationally lensed. Lensing could magnify and distort light from QSOs, influencing QSO numbers near galaxies, which trace structure in our Universe. Following Boyle, Fong & Shanks (1988), we find a 3σ anti-correlation between QSOs and galaxy groups of strength W (_gg)(< 10') = -0.049. We limit absorption by dust in groups to A(_B) < 0.04 mag. To explain the anti-correlation by dust would need Ab ≈ 0.2 mag. We demonstrate that if the dearth of QSOs around groups is due to statistical lensing, more mass would be required in groups than Ω(_m) = 0.3 models suggest. We use a mock catalogue to test how many of our "2D" galaxy groups, which are detected using angular information, are associated in redshift-space. We then utilise 2dF Galaxy Redshift Survey groups, which are selected to trace dark matter haloes, to test the hypothesis that there is more mass in groups than Ωr(_m) = 0.3 models suggest, finding we cannot discount a lensing mass of 2dFGRS groups that is consistent with ACDM. We find QSOs and galaxies are also anti-correlated at the 3σ level, with strength w(< 10’) = -0.007 and use stars as a control sample to rule out observational systematics as a cause. By measuring QSO colours as a function of QSO-galaxy separation, we argue that obscuration by dust in galaxies could explain at most 30-40 per cent of the anti correlation. We show that if the anti-correlation is due to lensing, galaxies would be anti-biased [b ~ 0.05) on small scales. We discuss two surveys carried out to count faint QSOs, which newly identify 160 QSOs. We calculate that the faint-end QSO number-counts have a slope of 0.29 ± 0.03. Finally, we use our faint QSO data, to estimate that ~ 85(75) per cent of g < 21.15 (≥ 21.15) candidates targeted by the 2dFSDSS survey will be QSOs.

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Cette thèse s’intéresse aux combinaisons d’observables cosmologiques provenant des mesures du fond diffus cosmologique et des relevés de galaxies, et est basée sur l’exploitation des données du satellite Planck et du Baryon Oscillation Spectroscopic Survey (BOSS) du Sloan Digital Sky Survey. On explore l’utilisation de corrélations croisées entre les jeux de données afin de mettre en évidence de nouveaux effets et d’améliorer les contraintes statistiques sur les paramètres cosmologiques. Dans un premier temps, on mesure pour la première fois une corrélation entre le lentillage gravitationnel du fond diffus cosmologique et le spectre de puissance des fluctuations de la forêt Lyman-α des quasars. Cet effet, d’origine purement non-linéaire, est interprété comme la réponse du spectre de puissance à des grandes échelles. Il montre comment les fluctuations dans la densité en hydrogène neutre dans le milieu intergalactique sont influencées par des fluctuations à grande échelle dans la densité de matière noire. Le signal mesuré est compatible avec l’approche théorique et des simulations menées par d’autres groupes. Dans un deuxième temps, on développe un formalisme permettant une analyse conjointe de la densité de galaxies et de quasars de BOSS avec le lentillage gravitationnel du fond diffus cosmologique. La prise en compte des corrélations croisées entre ces sondes permet de diminuer les barres d’erreurs de certains paramètres cosmologiques de 20%, ce qui équivaut à augmenter la surface couverte par les relevés de presque 50%. Cette analyse est complétée par la mesure des anisotropies de température du fond diffus cosmologique afin de contraindre tous les paramètres du modèle standard ΛCDM, ainsi que les biais des galaxies. Puis on étend le modèle afin d’explorer les contraintes sur l’équation d’état de l’énergie noire et la somme des masses des neutrinos / This thesis addresses the combinations of cosmological probes from the measurements of the cosmic microwave background (CMB) and galaxy redshift surveys, and exploits data from the Planck satellite and the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey. It explores how cross-correlations between different data sets can be used to detect new signals and improve contraints on cosmological parameters. First, we measure, for the first time, the cross-correlation between gravitational lensing of the CMB and the power spectrum of the Lyman-α forest in the spectra of quasars. This effect, which emerges from purely non-linear evolution, is interpreted as the response of the power spectrum to large-scale modes. It shows how fluctuations in the density of neutral hydrogen in the intergalactic medium are affected by large-scale fluctuations in the density of dark matter. The measured signal is compatible with the theoretical approach and simulations run by another group. In a second time, we develop a formalism enabling the joint analysis of the galaxy/quasar density contrast and CMB lensing. Taking cross-correlations between these probes into account reduces error bars on some cosmological parameters by up to 20%, equivalent to an increase in the size of the survey of about 50%. This analysis is completed by CMB temperature anisotropies information in order to constrain all the parameters of the ΛCDM standard model and galaxy biases at once. Finally, it is extended to obtain contraints on the dark energy equation of state as well as the sum of the masses of neutrinos