The universe appears to be clumpier than astronomers expected, according to the largest galaxy survey to date. The extra clumps could call for a redesign of the standard model of cosmology, and maybe a new understanding of how gravity works.

"Maybe on very large scales, Einstein's general relativity is slightly wrong," said cosmologist Shaun Thomas of University College London, lead author of a new paper in Physical Review Letters. "This potentially could be one of the first signs that something peculiar is going on."

When viewed close up, the matter in the universe bunches up into stars, galaxies and galaxy clusters. But as you zoom out, cosmologists expect the universe to look more and more smooth, sort of the way details in an earthly landscape blend together when viewed from an airplane.

What clumpiness there is comes from tiny fluctuations in the density of matter in the early universe. As the universe expanded, spots with a little bit of extra matter gathered more and more matter through gravitational attraction. Based on the best model of how gravity works and what the universe is made of, cosmologists can extrapolate out from the Big Bang to get a pretty good idea of how lumpy the universe should be on every scale.

But Thomas and colleagues looked at the most zoomed-out view of the universe yet acquired, and found more lumpiness than models predict.

"It's a little like discovering that the Earth had more continents than you thought it did," said physicist Michael Hudson of the University of Waterloo in Canada, who was not involved in the new work.

Thomas and colleagues used data from the Sloan Digital Sky Survey, which covers about a fifth of the entire sky, to make a rough 3-D map of 723,556 galaxies that are at least 4 billion light-years away. The researchers calculated how evenly distributed, or smooth, the galaxies appear on length scales of 2 billion light-years.

"These are scales which are starting to approach a reasonable fraction of the size of the universe, and they haven't really been measured before," Hudson said.

The clumpiness of the universe is expected to vary by about 1 percent from one spot to another on these length scales. The new analysis saw a universe that varies by nearly double that amount. It's still basically smooth, but much clumpier than current cosmological models predict.

The result could mean cosmologists need to reassess their understanding of dark energy, the mysterious force that drives the universe outward at an ever-increasing rate. Dark energy itself is supposed to be almost perfectly smooth, but clumps of dark energy could draw clumps of visible matter around them.

The extra lumps could also mean dark energy doesn't exist at all. Instead, gravity could behave differently on very large scales than it does on smaller scales, meaning Einstein's theory of general relativity needs an overhaul.

"General relativity has proved right time and time again, but it's been tested over the same scales," Thomas said. "These are new scales, so it could be that something breaks down. And then you need some new theory."

Of course, the clumpiness could also come from systematic errors in the observations, like stars masquerading as galaxies or dust in the Milky Way blocking distant galaxies from view. Thomas and colleagues checked both of these possibilities and think they're unlikely to be a problem, but there's still some room for questions.

Bigger and newer data sets, like that offered by the upcoming Dark Energy Survey, could help resolve lingering doubts.

"It will be interesting to see if this does bear out, if the standard picture can be changed with a few minor tweaks, or if we need a whole-scale revision," Hudson said. "We'll have to wait and see what happens."

Image: Simulations based on the standard cosmological model indicate that galaxies should be uniformly distributed. The length bar represents about 2.3 billion light years. (Volker Springel/Max-Planck-Institute for Astrophysics, Garching, Germany)

Citation: "Excess Clustering on Large Scales in the MegaZ DR7 Photometric Redshift Survey." Shaun A. Thomas, Filipe B. Abdalla, and Ofer Lahav. Physical Review Letters, 106, 241301, June 13, 2011. DOI: 10.1103/PhysRevLett.106.241301

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