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