Origin of the mysterious cosmic dance of satellites eludes our cosmological theories

26 March 2014

Given the complexity of the physics, and immense range of distances, density and temperatures, it's not easy to recreate the Universe in a laboratory. But in recent years astronomers have called on the power of supercomputers to build synthetic universes, using the super-fast calculations to solve the physical equations and reveal how the stars and galaxies formed.

Andromeda Galaxy
Andromeda Galaxy

Last year, international astronomers, including Geraint Lewis from the School of Physics at the University of Sydney, found a curious coordinated dance of small dwarf galaxies about our nearest cosmological neighbor, the Andromeda Galaxy (M31), all orbiting together in a thin plane.

"The discovery of this plane of satellites was quite a surprise, quite unexpected, but given its important implications, we have to check it further" said Professor Lewis.

On the face of it, such a cosmic two-step is completed unexpected, and so Lewis and his collaborators decided to search for similar planes amongst the galaxies in our best synthetic universe, a universe known as Millennium-II. Their new results have just been accepted for publication in the international Astrophysical Journal in March 2014.

Within our favored cosmological models, cold dark matter dominates galaxy formation, accurately describing the large-scale distribution of galaxies in the Universe, accounting for the clusters and voids that make up the cosmic web. On smaller scales such models struggle to account for the dwarf galaxies seen in galactic halo, significantly over-predicting what we see.

The existence of coherent planes of dwarfs would appear to be another problem for this successful cosmological model.

Their conclusion was that, by looking at the many thousands of Andromeda-like galaxies within Millennium-II, such planes of dwarfs are not very rare by chance universes dominated by cold dark matter

As well as the positions of the dwarfs, Lewis and his collaborators also looked at how the dwarfs were moving. Instead of there being nice ordered dances, what they saw was chaos, with dwarf galaxies all heading in different directions.

As the dwarfs we observe around Andromeda are dancing coherently, they will continue their dance long into the future.

"Unlike a fine Vianesse waltz, where every dance needs to move precisely to maintain the performance, the random motions mean that the planes found with Millennium-II are fleeting things, randomly aligning before rapidly dispersing." - Professor Geraint Lewis

So where does this leave the cosmology of the Millennium simulation?

"The fact that Millennium is unable to explain what we see in Andromeda is intriguing, and tells us that we are missing something, probably something fundamental, about our Universe," said Geraint "and that's exciting. That's what science is all about."

Read the full Astrophysical Journal paper online here

Contact: Tom Gordon

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