Galaxies Might Cluster via the 'Cheerios Effect'


A new paper in the Dec. 1 issue of Astrophysical Journal Letters announces the discovery of a strange, boomerang-shaped galaxy nestled in a filament of hot gas connecting two massive galaxy clusters (Abell 1763 and Abell 1770).

Astronomers believe similarly shaped galaxies could serve as signposts to help identify other filaments, which in turn are an indicator of fertile regions for the formation of stars.

This unusual galaxy may offer some tantalizing clues about how galaxy clusters and superclusters form and evolve. Meanwhile, other physicists are looking a bit closer to home for clues about the deep structure of our universe — namely, your morning bowl of cereal.

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Clark University’s Arshad Kudrolli and Michael Berhanu published a paper in Physical Review Letters this fall describing the so-called “Cheerios effect” and its implications for galaxy clusters.

Stars congregate in galaxies. Galaxies tend to bunch together in clusters, which in turn clump near other clusters to form so-called “superclusters”, joined by long thin filaments of super-heated gas connecting the massive clusters. The result: huge, gravitationally linked walls of galaxies with vast empty spaces in between — a gigantic cosmic web.

(In September, the Planck satellite obtained its very first images of galaxy clusters, and follow-up analysis by the XMM-Newton mission revealed a previously unknown supercluster of galaxies.)

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These clusters and superclusters evolved from denser patches of material as the universe rapidly expanded after the Big Bang, about 13.7 billion years ago, researchers said. Over time, the clumps and threads of this primordial matter eventually cooled, condensing into the galaxies we see today. A lot of the leftover gas is strewn in the filaments between galaxy clusters.

What could this possibly have to do with a bowl of Cheerios? Those delicious little “O”s floating in milk tend to clump together in the bowl, making it easier to scoop up any stragglers with a spoon.

“If you put Cheerios in a bowl, they aggregate,” Kudrolli told Physics Buzz. “That’s because of surface tension.”

Specifically, the floating Cheerios are “milk-philic,” creating tiny depressions in the surface of the milk, which causes them to fall towards each other — much like the mass of of the sun, for instance, curves spacetime, giving rise to a gravitational pull.

The Cheerios will also tend to clump against the edges of the bowl, since the milk will curve slightly against those edges. So you get a dual attraction: between Cheerio and Cheerio, and between Cheerios and the edges of the bowl.

Kudrolli and Berhanu didn’t use actual Cheerios in milk to study this well-documented effect. Instead, they used glass spheres floating in a funnel-shaped container filled with water. Simply by changing how much water was in the container, they could make the spheres clump together or spread apart. The result was a surprisingly accurate recreation of the Cheerios effect.

Among their findings: if you just have a few Cheerios in a large bowl of milk, they clump together into hexagonal shapes, almost like a honeycomb. When there are many more Cheerios widely dispersed throughout the bowl, the tasty “O”s will form dense clusters with large empty spaces between them, looking very much like the vast, interconnected web of galaxy clusters and superclusters in our universe.

So the cosmos can be likened to a bowl filled with milk, in which matter floats and gradually clumps together to form large structures.

Ponder that over your morning cereal.

Image (top): Cheerios, in a bowl (iStockPhoto). Image (middle): The Bullet Cluster, located 3.8 billion light-years away (NASA/STScI/Magellan/U.Arizona/D.Clowe et al.)

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