Redshift space: a hunt for Omega

Michal Chodorowski

Copernicus Astronomical Center, Varsovie

Wed, Nov. 08th 2006, 14:15

Salle Claude Itzykson, Bât. 774, Orme des Merisiers

Analyses of the large-scale structure of the Universe provide estimates of cosmological parameters complementary to those obtained from the anisotropy of the cosmic microwave background. In particular, comparing the large-scale distribution of galaxies to their peculiar velocities (deviations from the Hubble flow) enables one to constrain the combination $\Omega_m^{0.6}/b$. Here, ``$\Omega_m$'' is the cosmological matter density parameter and ``$b$'' is the linear bias of galaxies that are used to trace the underlying mass distribution. This is possible because the peculiar velocity field is induced gravitationally, so is tightly coupled to the matter distribution. In the linear regime, the peculiar velocity of a galaxy is proportional to its gravitational acceleration, and the coefficient of proportionality is equal to $\Omega_m^{0.6}$. Since the gravitational acceleration is in practice estimated from the galaxy distribution, the second parameter, $b$, appears. \par
A major problem with velocity-gravity comparisons is that in 3-D galaxy surveys, the third coordinate of a galaxy is not its distance but redshift. Since galaxies have peculiar velocities, redshift is a biased estimator of distance. I will briefly review standard methods to deal with this problem. Finally, I will describe a novel method, based on an analysis of the velocity-gravity relation directly in redshift space.