Fractals are beautiful, but we wouldn't want to live in one. If the universe is like a giant fractal ? with matter always appearing in clusters nested inside clusters ? that would be a serious blow to modern cosmology.
Now the largest 3D map of the sky ever used to measure large-scale structure shows that if you zoom out far enough, the universe is reassuringly cluster-free.
We can see that the universe was born smooth by looking at its density, as revealed by the cosmic microwave background radiation ? a faint glow emitted just 380,000 years after the big bang.
Over time, gravity pulled matter together so that gas formed stars, those stars gathered in galaxies, and galaxies congregated in clusters. On a large scale, though, the expansion of the universe should trump the influence of gravity and so matter should be distributed more or less evenly, according to the standard model of cosmology.
But previous studies found clusters within clusters on ever larger scales. One paper even hinted that hyperclusters of galaxies exist all the way out to the 3-billion-light-year scale.
Red-shift saviour
Besides throwing a wrench into many theories of cosmology, a fractal universe would cast doubt on some fundamental aspects of astronomy, says team leader Morag Scrimgeour at the University of Western Australia in Perth. For one thing, red shift ? a light-stretching Doppler effect caused by the expansion of the universe ? would no longer be an accurate tool for measuring cosmological distances.
"If the space is not smooth, if it's distorted by large clumps of matter, then the path of light is distorted and its red shift is no longer simply related to the distance to the galaxy," says Scrimgeour.
To probe the issue, her team analysed data from the WiggleZ Dark Energy Survey, which used observations by the Anglo-Australian Telescope in New South Wales to map about 220,000 galaxies in a volume of space equivalent to a cube with 3 billion light years to a side.
Non-random, to a point
They tested for clusters by placing any given galaxy in the centre of an imaginary sphere and counted the number of galaxies within it. If clustering exists, there should be more galaxies within a sphere than if the 220,000 galaxies were distributed randomly throughout the huge cube.
With relatively small spheres ? up to about 330 million light years wide ? they did find galactic clusters. But in larger spheres the number of galaxies met expectations for randomness.
"This is the first survey large enough to? clearly see the approach to homogeneity and then well past it," says team member Tamara Davis of the University of Queensland in Brisbane.
Hard to disprove
Although the idea of a fractal universe is something few people believe, it is very hard to disprove, says Filipe Abdalla of University College London, who was not involved in the study. "This is the nicest result I've seen," he says, but adds that it is not the final word. The results may yet be contradicted by an even larger survey.
Scrimgeour admits her team's work does not rule out clustering at truly epic scales, but she thinks such structures, if they exist, won't be dense enough to cause problems for cosmology.
The next test will come from the Square Kilometre Array, an international radio-telescope array being built in South Africa and Australia that should provide an even bigger survey of galaxies.
Journal reference: arxiv.org/abs/1205.6812
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