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The universe is smooth, not a fractal at large scales, say astronomers

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A new paper provides robust evidence for a homogeneous universe when viewed at large scales. The finding validates a number of assumptions made by general relativity.

fractal brocoflower (flickr)
fractal brocoflower (flickr)

A new paper confirms a conventional picture of the universe, one that is smooth and homogeneous at very large scales, losing its fractal structure when we zoom out far enough. The finding means that astronomers are justified in measuring average quantities over large spatial volumes — there’s no hidden lumpiness messing up the math. This is good news, since a fractal structure at large scales would mean that "Einstein’s equations are being wrongly applied, and our understanding of things like dark energy could be deeply flawed," says the paper’s lead author, Morag Scrimgeour (video below).

Our current picture of the universe goes like this: hundreds of billions of stars group together to form galaxies, galaxies clump together to form clusters, and clusters clump together to form superclusters. In other words, the universe exhibits properties of a fractal — the same "lumpy" pattern is visible whether we’re looking at the galaxy scale or the supercluster scale.

"We can say with a high degree of certainty that our picture of the large-scale Universe is correct."

The question is what happens when we zoom out farther? To find the answer, the team used data from the WiggleZ Dark Energy Survey; a map of a large chunk of the cosmos containing some 240,000 galaxies. In order to determine how smooth the universe looks at very large scales, the team picked galaxies pinpointed by WiggleZ and drew imaginary spheres around them, counting how many other galaxies were nearby. It then calculated how many galaxies you would expect in a sphere the same size assuming that galaxies are uniformly sprinkled through the cosmos. If the universe has a fractal structure, you would expect the former number to be larger than the latter. On the other hand, if the universe is smooth at large scales, the two should be equal for big spheres — the composition of the real universe should be indistinguishable from a random distribution.

And that’s exactly what the team found. The results indicate that in spheres 600 million light years in diameter or larger, about 6,000 times the size of our Milky Way galaxy, there’s no evidence of clustering. They also show a gradual transition from lumpiness to smoothness as you move out from smaller to larger scales. In that sense, the universe looks a lot like snow — made up of fractal flakes, but transitioning to a uniform sea of white as you step back.