The universe is smooth

The universe is smooth – or at least the way luminous material like galaxies are seen across a large volume is, such that at scales of ~300 Mpc and larger the universe is homogeneous. These are the recent results of a recent paper by Scrimgeour et al. from the WiggleZ Dark Energy Survey, a large scale galaxy redshift survey which used 273 nights of Anglo-Australian Telescope time to measure the redshift of over 200,000 galaxies. The survey volume of WiggleZ was 1000 square degrees, with look-back times up to 8 billion years (or a redshift of z=1).

GiggleZ Simulation. (Credit: Greg Poole)

If we look at our local environment, we see that our Galaxy is in a group of ~50 galaxies, called the Local Group. The Local Group is a typical collection of galaxies, a few Mpc in diameter. Many of these galaxy grouping, taken together, form clusters of galaxies, comprising several thousands of galaxies which span 10s of Mpc. And clusters tend to be found near other clusters of galaxies, forming superclusters that are typically 100s of Mpc in size.

A question that has been posed for a long time, and is seemingly answered by the results of Scrimgeour et al. using over 200,000 galaxies in the WiggleZ Dark Energy Survey, is do superclusters cluster? One of the predictions of the current Lambda CDM model of the universe is large-scale homogeneity. In other words, at some distance or “horizon” scale galaxies should be distributed in a statistically homogenous way. As well, we expect any transition from “clumpiness” (on small scales) to “smoothness” (on larger scales) will be gradual.

This is where the science of fractals can be invoked and tested. A universe with a fractal nature would show “self-similar” patterns – that is the clustering pattern of galaxies, clusters and superclusters (and perhaps hyperclusters – clusters of superclusters) would look the same.

A ‘Julia set’ fractal distribution (Credit: Paul Bourke)

Therefore no matter what scale in the universe you looked at, small or large, the pattern would be the same! Not only would this be a major result in structural analysis and theories of cosmology, but it would have physical implications on the transmission of light. Redshifts of galaxies would no longer be a simple proxy for distances, as the path of light would be distorted by large amounts of matter.

However as the WiggleZ team analysed the numbers of galaxies in a succession of spheres of increasing size, the number of galaxies showed that on scales larger than ~300 Mpc no appreciable clustering was found, and the distribution was random, and the case for homogeneity was well founded.

The distribution of galaxies currently observed in the WiggleZ Survey fields. The observer is situated at the origin of the co-ordinate system. The radial distance of each galaxy from the origin indicates the observed redshift, and the polar angle indicates the galaxy right ascension. The faint patterns of galaxy clustering are visible in each field. (Credit: WiggleZ team)

Of course, astronomers would always like larger samples – especially for a test like this – and future galaxy surveys on even larger scales, using instruments like the SKA, will allow this.

For more information:

[Glen Mackie & Sarah Maddison]

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