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Our future, our universe, and other weighty topics

Tuesday, June 17, 2014

Vast Question Mark in Space Highlights Cosmic Structure Mystery

Galaxies are collections of millions or billions of stars. The two largest galaxies in what is called the Local Group of galaxies are our galaxy (the Milky Way) and the Andromeda galaxy, both of which are spiral galaxies containing hundreds of billions of stars. Both of these galaxies are surrounded by plane-like distributions of much smaller dwarf satellite galaxies. A recent scientific paper says these structures do not fit in with the prevailing theory of galactic structure formation. 

 Andromeda Galaxy

The plane of dwarf galaxies that surrounds the Andromeda galaxy is called the Great Plane of Andromeda. The plane is about 400 kiloparsecs wide, but only about 14 kiloparsecs thick. On this web site is an animated 3D model simulating this vast structure. As you can see from the animation, when you view the plane from a particular angle, the dwarf galaxies in the middle of this huge plane make a gigantic question-mark shape, which reminds me of the giant question mark formed by the stars of the Big Dipper. 

cosmic question mark

The structure of dwarf galaxies and globular clusters that surrounds our galaxy is called the Vast Polar Structure or VPOS. It is called polar because it is oriented above our galaxy and below our galaxy, without any matching structure on the sides of our galaxy. In the chart below (from this scientific paper), each of the blue or brown dots is one of the dwarf satellite galaxies or globular clusters in the VPOS (and our galaxy is in the middle of the chart). 


A recent scientific paper points out that both of these two structures (the Great Plane of Andromeda and the Vast Polar Structure) are not what we would expect to exist if the prevailing theory of galaxy formation (the lambda cold dark matter theory) is correct. That theory (which also goes by the ridiculously nerdy name of ˄CDM) has been criticized for being centered around the assumption that there exists a mysterious substance called cold dark matter, the existence of which is still unproven.

The [lamda cold dark matter] model predicts that dwarf galaxies should form inside of small clumps of dark matter and that these clumps should be distributed randomly about their parent galaxy,” says one of the paper's authors, David Merritt. “But what is observed is very different. The dwarf galaxies belonging to the Milky Way and Andromeda are seen to be orbiting in huge, thin disk-like structures.”

Merritt's scientific paper concludes (page 18) that under the assumptions of the most popular galaxy formation theory (the lambda cold dark matter theory), there would be only about 2 chances in a million that we would see arrangements of dwarf satellite galaxies such as we observe in the Great Plane of Andromeda and the Vast Polar Structure, occurring near both our galaxy and the Andromeda galaxy. Instead, that theory predicts that dwarf satellite galaxies would form in a more random arrangement around a larger spiral galaxy.

So where does that leave our galactic astronomers? It leaves them pretty much caught with their pants down (to use an idiomatic expression meaning to be found in an embarrassing situation). Our galactic astronomers have spent at least 60 years trying to get a good predictive theory of the origin of galactic structure, and they apparently haven't got there yet.

We know there are at least four fundamental forces (the strong nuclear force, the weak nuclear force, electromagnetism, and gravitation), but we try to explain the origin of galaxies by imagining that only one of those forces (gravitation) was involved. But the laws of nature we have discovered may be the mere tip of the iceberg. It could be there are major undiscovered laws of nature that played a role in the formation of galactic structure; and instead of there being only four fundamental forces, there may be five, six, seven, or more than ten. If so, our current attempts to explain the origin of galactic structure may be as premature as the attempts of ancient scientists to explain the origin of life. 

Postscript: See this post for a new related discovery that seems to greatly deepen the mystery of cosmic structure formation.