One
of the most astounding characteristics of 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. Why is that fact
surprising?
It's
surprising because of what is called the winding problem. Galaxies
rotate, and a galaxy like ours takes about 200 million years to
rotate. But consider a rotating spiral galaxy. The stars closer to
the center of the galaxy will take a much shorter time to rotate
around the center of the galaxy than the stars closer to the edge of
the galaxy (just as planets close to the sun have much shorter years
than planets far away from the sun). That's because the circles of
rotation of stars closer to the galaxy's center have a much smaller radius. Therefore, based purely on
rotation speeds, we should expect that the spiral arms of a spiral
galaxy should “wind up” after only 2 or 3 rotations, and that spiral arms should last less than a billion years.
But
the age of the universe is about 13 billion years, and spiral
galaxies are believed to be about that age, or almost as old. This
means the average spiral galaxy has undergone more than 50 rotations.
Based on simple rotation considerations, it seems that we should not
at all be seeing even a tenth of the spiral galaxies that we see in
the sky.
But
don't worry, scientists have an explanation to cover this: what is
called the density wave theory. But it's not a particularly credible
explanation. It doesn't seem to stand up very well to observations, and it
isn't well-confirmed by computer simulations.
Explanations
of the density wave theory often use an analogy involving traffic
patterns. We are told that just as we can explain concentrations of
cars near freeway exits, we can explain the concentration of stars in
the spiral arms of spiral galaxies.
But
anyone familiar with the distance between stars should be suspicious
with this analogy. Cars on a freeway exit are relatively close to
another. But stars are not relatively close to each other. The
distance to the nearest star is 6 million times the diameter of our
sun. So how can any type of freeway exit car concentration analogy
be appropriate for stars so far apart?
The
“density wave” imagined by the density wave theory is merely an
area where stars are about 10% more common. But given the immense
relative distance between stars, how can anything that far apart
act like a wave?
A
recent scientific paper studied the spiral galaxy M81. The paper
concluded, “Our data therefore provide no convincing evidence for a
stationary density wave with a single pattern speed in M81, and
instead favor the scenario of kinematic spiral patterns that are
likely driven by tidal interactions with the companion galaxies M82
and NGC 3077.” The “tidal interactions” theory is a completely
different one from the density wave theory, and one with its own
plausibility problems (tidal interactions are random gravity tugs
that we should not expect to produce all that often the orderliness of
spiral arms).
The spiral galaxy M81 (Credit: NASA)
In
a paper that calls itself “A case against spiral density wave theory,” some
scientists stated the following:
An
offset is expected between these subsamples as a function of radius
if the pattern speed of the spiral arm were constant - as predicted
by classic density wave theory. No significant offsets are found....The
standard scenario of density wave theory with a
constant pattern speed results in an offset with respect to age for
the distribution
of distances to the spiral arms as one moves from the central
regions... No
significant differences are found in the distribution of these
sources, giving further negative evidence for density wave spirals.
If
scientists actually understand what forms spiral galaxies, they
should be able to create computer simulations that show spiral
galaxies very often forming from random collections of matter, with the spiral
galaxies persisting in sufficient numbers. But the simulations don't
do that. A recent major galaxy evolution simulation was the Eagle project, described
in this paper written by more than a dozen scientists. But the 38-page paper
doesn't mention density waves, doesn't mention spiral arms, and
doesn't even use the word “spiral.”
The
paper has a visual showing some simulated galaxies that resulted
from the simulation, although we have no idea whether the authors cherry-picked
those galaxies that most looked like spirals out of some large batch of simulated galaxies. But even the shown simulated galaxies
don't actually have clear spiral arms (except for one). We see instead disks seeming
to consist of random blobs of matter surrounding a dense core. From
the fact that the paper makes no mention of “spiral” or “spiral
arms,” we can conclude that no notable success was achieved in
frequently creating simulated galaxies with spiral arms like the spiral arms in
spiral galaxies (if such a success had been achieved, I can't see how
the authors would not have mentioned it).
When
asked about the spiral arms in spiral galaxies, scientists will often
speak as if they understand their origin and persistence, without
confessing their lack of understanding on this matter. But
occasionally you will get some refreshing candor on this topic.
The abstract of this 2012 paper
candidly says, “After almost fifty years the origin of
spiral arms in disk galaxies remains one of the major unsolved
problems in astrophysics.”
Here we have another case of scientists trying to explain a mountainous effect (that a large fraction of the universe's galaxies are magnificent spirals) by using a little molehill of an explanation (that it's just "traffic jams" that cause this). I suspect something vastly deeper is going on.
Postscript: The recent discovery of "super spirals" ten times bigger than our galaxy makes it all the much harder to explain spiral galaxies, as discussed here.
Postscript: The recent discovery of "super spirals" ten times bigger than our galaxy makes it all the much harder to explain spiral galaxies, as discussed here.
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