Large surveys of the local Universe have shown that galaxies with different intrinsic properties, such as colour, luminosity and morphological type display a range of clustering amplitudes. Galaxies are therefore not faithful tracers of the underlyin
g matter distribution. This modulation of galaxy clustering, called bias, contains information about the physics behind galaxy formation. It is also a systematic to be overcome before the large-scale structure of the Universe can be used as a cosmological probe. Two types of approaches have been developed to model the clustering of galaxies. The first class is empirical and filters or weights the distribution of dark matter to reproduce the measured clustering. In the second approach an attempt is made to model the physics which governs fate of baryons in order to predict the number of galaxies in dark matter haloes. I will review the development of both approaches and summarize what we have learnt about galaxy bias.
The distribution of angles subtended between pairs of galaxies and the line of sight,which is uniform in real space, is distorted by their peculiar motions, and has been proposed as a probe of cosmic expansion. We test this idea using N-body simulati
ons of structure formation in a cold dark matter universe with a cosmological constant and in two variant cosmologies with different dark energy models. We find that the distortion of the distribution of angles is sensitive to the nature of dark energy. However, for the first time, our simulations also reveal dependences of the normalization of the distribution on both redshift and cosmology that have been neglected in previous work. This introduces systematics that severely limit the usefulness of the original method. Guided by our simulations, we devise a new, improved test of the nature of dark energy. We demonstrate that this test does not require prior knowledge of the background cosmology and that it can even distinguish between models that have the same baryonic acoustic oscillations and dark matter halo mass functions. Our technique could be applied to the completed BOSS galaxy redshift survey to constrain the expansion history of the Universe to better than 2%. The method will also produce different signals for dark energy and modified gravity cosmologies even when they have identical expansion histories, through the different peculiar velocities induced in these cases.
