Large-scale cosmic homogeneity in the WiggleZ survey
Scrimgeour, Morag (University of Western Australia)

The most fundamental assumption of LCDM is that the universe is homogeneous on large scales. This is clearly not true on small scales, where clusters and voids exist, and some studies seem to suggest that galaxies follow a fractal distribution up to very large scales (~200 h-1 Mpc or more), whereas the LCDM model predicts transition to homogeneity at scales of ~ 100 h-1 Mpc. The Friedmann-Robertson-Walker (FRW) metric is only valid for a homogeneous distribution, so any large inhomogeneities could affect our distance measurements and distort our measurements of cosmology. Also, inhomogeneities in the matter distribution have been proposed as an alternative explanation of the accelerated expansion, through the backreaction mechanism (a general relativistic effect whereby the evolution of an averaged, inhomogeneous matter distribution is different from the evolution of a homogeneous distribution, and can actually give rise to an accelerating global expansion rate). It is therefore very important to quantify the scale of homogeneity in the Universe. We have made a measurement of homogeneity of the large-scale galaxy distribution using the WiggleZ Dark Energy Survey, a UV-selected spectroscopic survey of ~200,000 luminous blue galaxies up to z=1, with the Anglo-Australian Telescope. We have corrected for survey incompleteness using random catalogues that account for the various survey selection criteria. The large volume and depth of WiggleZ allows us to probe the transition of the galaxy distribution to homogeneity on large scales, and see if this is consistent with a LCDM prediction.