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

TL;DR

This study measures the transition to large-scale cosmic homogeneity using the WiggleZ survey, introducing a robust method for defining the homogeneity scale and confirming the universe's uniformity at large scales across different epochs.

Contribution

It presents a new, more robust method for defining the homogeneity scale and applies it to the largest-volume galaxy survey to date, covering multiple cosmic epochs.

Findings

Homogeneity scale exceeds 70 Mpc/h at all studied redshifts.

Results agree with LCDM predictions and simulations.

Fractal distribution with D_2<2.97 is excluded at large scales.

Abstract

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 (Gpc/h)^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%, or equivalently the fractal dimension of the sample is within 1% of D_2=3, at radii larger than 71 \pm 8 Mpc/h at z~0.2, 70 \pm 5 Mpc/h at z~0.4, 81 \pm 5 Mpc/h at z~0.6, and 75 \pm 4 Mpc/h at z~0.8. We demonstrate the robustness of our results against selection function effects, using a LCDM N-body simulation and a suite of inhomogeneous fractal distributions. The results are in excellent agreement with both the LCDM N-body simulation and an analytical LCDM prediction. We can exclude a fractal distribution with fractal dimension below D_2=2.97 on scales from ~80 Mpc/h up to the largest scales probed by our measurement, ~300 Mpc/h, at 99.99% confidence.