The WiggleZ Dark Energy Survey: the transition to large-scale cosmic homogeneity

We have made the largest volume measurement to date of the transition to large-scale homogeneity in the distribution of galaxies. We use the WiggleZ survey, a spectroscopic survey of over 200 000 blue galaxies in a cosmic volume of ∼1 h^(−3) Gpc^3. A new method of defining the ‘homogeneity scale' is presented, which is more robust than methods previously used in the literature, and which can be easily compared between different surveys. Due to the large cosmic depth of WiggleZ (up to z = 1), we are able to make the first measurement of the transition to homogeneity over a range of cosmic epochs. The mean number of galaxies N(< r) in spheres of comoving radius r is proportional to r^3 within 1 per cent, or equivalently the fractal dimension of the sample is within 1 per cent of D_2 = 3, at radii larger than 71 ± 8 h^(−1) Mpc at z ∼ 0.2, 70 ± 5 h^(−1) Mpc at z ∼ 0.4, 81 ± 5 h^(−1)  Mpc at z ∼ 0.6 and 75 ± 4 h^(−1) Mpc at z ∼ 0.8. We demonstrate the robustness of our results against selection function effects, using a Λ cold dark matter (ΛCDM) N-body simulation and a suite of inhomogeneous fractal distributions. The results are in excellent agreement with both the ΛCDM N-body simulation and an analytical ΛCDM prediction. We can exclude a fractal distribution with fractal dimension below D_2 = 2.97 on scales from ∼80 h^(−1) Mpc up to the largest scales probed by our measurement, ∼300 h^(−1) Mpc, at 99.99 per cent confidence.