One
of the most amazing facts about the large-scale universe is the very
large number of beautiful spiral galaxies. Most of the larger
galaxies in the local universe are spiral galaxies. In my previous
post The Unsolved Mystery of Why So Many Galaxies Are
Beautiful Spiral Galaxies, I
discussed the inadequacies of current attempts to explain the
existence of so many spiral galaxies.
The Whirlpool Galaxy (Credit: NASA)
Galaxies
rotate, taking about 200 million years to rotate. For spiral galaxies
this rotation leads to what is called the winding problem. This is
the problem that the rotation of spiral galaxies should cause the
spiral arms of galaxies to be ruined after a few rotations (in less
than a billion years), due to a “winding up” effect. But somehow
the spiral arms of spiral galaxies have apparently persisted for
more than 10 billion years. Scientists have attempted to explain the
persistence of spiral
arms using a theory called the spiral density wave theory. But that
theory is not in good shape, and is not well-supported by evidence. A
scientific paper with the phrase "A case against density wave theory" in its title mentions “further
negative evidence for density wave spirals.”
When
I wrote my original post, I had never heard of “super spiral”
galaxies. In the spring of 2016, news articles started to appear on this
extra-large type of spiral galaxies. A NASA press release was
entitled “Scientists Discover Colossal 'Super Spiral' Galaxies.”
It said that the newly discovered type of spiral galaxy was as big
and as bright as the biggest and brightest galaxies previously known.
The
new type of “super spiral” galaxy is ten times more massive
than our own galaxy. This apparently makes the spiral galaxy
explanation problem ten times worse than it previously was.
Page
4 of this paper shows 53 of the “super spiral” galaxies. They
look pretty much like spiral galaxies we are used to seeing in photos
of distant space. Referring to a major attempt to simulate galaxy
evolution with a computer simulation, section 7.2 of the paper says,
“"Even the largest galaxy evolution simulations to date, such
as the Illustris simulation...are not big enough to manufacture a
significant number of super spirals."
The
Illustris project was the largest attempt to simulate the universe
using a supercomputer, and used 8000 CPU's running in parallel. I
searched all 4 scientific papers published by the Illustris team, and
found no evidence that their simulation had produced an outcome in
which a large fraction of the galaxies are spiral galaxies. The
authors made no attempt to categorize how many of their simulated
galaxies were spiral galaxies. One of the papers claims that the
simulation produced “ a reasonable population of ellipticals and
spirals,” but from that statement we cannot tell whether the number
of spiral galaxies was 10%, 1%, or .0001%.
I
also tried using the “Infinitely Scrolling Galaxy Explorer” of
the Illustris project, at this location. This allows you to scroll
through simulated galaxies produced by the simulation. Very few of
the simulated galaxies had clear spiral arms like that of the
Whirlpool galaxy. Almost all the galaxies shown looked like
elliptical galaxies or irregular galaxies or disk-shaped galaxies with random concentrations of stars but not spiral arms. It seemed that less than
2% of the simulated galaxies were spiral galaxies, and the number
could have been less than 1%. In our universe ring galaxies are
rare, but in the Illustris simulation there seemed to be many times
more ring galaxies than spiral galaxies. In the Illustris simulated galaxies, in the rare cases in which
there was something what looked like a spiral arm, there was almost
always just one spiral arm, rather than the two or three spiral arms
we see in real spiral galaxies.
I
therefore find the way in which the Illustris project reported its
outputs to be misleading in regard to the issue of whether the
project was able to produce a universe in which a large fraction of
the galaxies are spiral galaxies. Here is what an MIT press release
of the project stated (a press release reproduced on the Illustris
web site):
But
this statement does not match what you see when you scroll through
the galaxies produced by the project, using the “Infinitely
Scrolling Galaxy Explorer” on the Illustris web site. While a large
fraction of the simulated galaxies are disk-shaped, only a tiny
percentage (perhaps as few as 1 percent) look like spiral galaxies
with one or more spiral arms (spiral galaxies have 2 or 3 spiral arms). Who should we blame here for this misleading
statement? Given the fact that the MIT press release writer has
inserted in brackets the word “spiral” (a word Vogelsberger
apparently did not use), we perhaps cannot directly blame
Vogelsberger. But we can fault him for failing to correct the
modified version of his statement. With the insertion of the word
“spiral,” the reader is left with the very misleading impression
that the Illustris simulation “got the ratio right” by creating a
simulated universe in which the number of spiral galaxies was similar
to the ratio in the known universe. The simulation did no such thing.
Another
press release of the Illustris project claimed that the project
created “a realistic mix of spiral galaxies like the Milky Way and
giant elliptical galaxies.” That phrase (repeated by many other
news sources that used the press release) is not accurate in light of
the fact that in our universe a large fraction of the galaxies are
spiral galaxies, but in the Illustris simulation only a tiny fraction
are spiral galaxies (as little as 1% or less).
It
seems that our scientists do not actually have a credible explanation
for the high occurrence of spiral galaxies in the universe. The
recent discovery of spiral galaxies ten times bigger than any
previously observed underscores this explanatory shortfall.
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