
All disk galaxies rotate once every billion years: study - clayt6
http://www.astronomy.com/news/2018/03/all-galaxies-rotate-once-every-billion-years
======
1053r
This is extremely suspicious for various theories of dark matter, right?

How is it that all disk galaxies have just enough dark matter in just the
right distribution to make up for the various radii and distributions of
visible matter and end up rotating at the same rate?

I have no answers, but as they say, the most interesting words in science are,
"Huh, that's funny..."

~~~
tlb
The constant rotational speed is the observation that requires dark matter to
exist. Otherwise, small galaxies would rotate faster just like close planets
do.

~~~
marcosdumay
Dark matter theory tells why galaxies rotate faster than they should. The fact
that this speed is constant requires some other explanation.

~~~
avip
It's not "constant" though, it's "cosmologically constant". Cosmologists would
routinely assert that 1 == 1000 holds true.

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Klathmon
I'm only a very small passively interested layperson here, but does this seem
much _faster_ than anyone else thought?

I looked it up and a "galactic year" for our sun is only 250 million years.
Meaning our planet has been around the galaxy 18 times since its formation!
That seems like so much faster than I would have thought!

~~~
Romanulus
... Also, don't the disks spin at the same speed/uniformly? Meaning closer to
the axis things go slower than on the edges?

~~~
davidp
That's the apparent paradox in the galaxy rotation curve:
[https://en.wikipedia.org/wiki/Galaxy_rotation_curve](https://en.wikipedia.org/wiki/Galaxy_rotation_curve)

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YeGoblynQueenne
Mright. Looks like someone left the default value in the universe's config
file.

~~~
evilantnie
So, we are living in a simulation?

~~~
luc4sdreyer
I think it's very likely:
[https://en.wikipedia.org/wiki/Simulation_hypothesis#Ancestor...](https://en.wikipedia.org/wiki/Simulation_hypothesis#Ancestor_simulation)

It's not a testable claim right now, but it might be in the future:
[https://en.wikipedia.org/wiki/Simulation_hypothesis#Testing_...](https://en.wikipedia.org/wiki/Simulation_hypothesis#Testing_the_hypothesis_physically)

~~~
drb91
Your link only provides a way to falsify a theory, not provide evidence for
simulation.... which is impossible by definition.

Not to mention the idea of a simulated universe is, you know, philosophically
boring and implies mostly false things in most peoples' minds.... like an
anthropomorphic scientist god. In reality, it would change virtually nothing
about how we view our universe.

~~~
luc4sdreyer
> not provide evidence for simulation.... which is impossible by definition.

I disagree.

Science cannot prove that a theory is 100% true, only that it is not wrong by
repeatedly testing the predictions made by the theory. The best we can do is
say: this theory (e.g. general theory of relativity) is the best explanation
for data we have so far and every experimental prediction it has made has come
true.

The paper
([https://arxiv.org/pdf/1703.00058.pdf](https://arxiv.org/pdf/1703.00058.pdf))
linked on the wiki page proposes four experiments that seeks to test the
simulation theory. None of these experiments will single-handedly prove the
simulation theory, but if they all pass we can only keep testing and seeking
alternate explanations.

If the experiments keep passing and no alternative theories can be found, then
we're either in a simulation, or the universe just happens to behave exactly
in the way that a simulated universe would, but it isn't. I agree that we can
never truly know which is true, but at that point the difference is reduced to
semantics. It's like saying electrons don't really exist, instead they're just
wave/particles that are exactly like electrons in every way except for some
immeasurable quality.

This assumes the entity running the sim will never directly influence it, of
course if that ever happens (requiring a _very_ high burden of proof) it'll be
proof of the sim.

~~~
drb91
I’m saying the semantics of a simulation are boring. I think the actual tests
are pretty cool. :)

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fooker
Could be an artifact of the preconditions required for the formation of disk
galaxies.

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trhway
A spiral/disk galaxy's mass is proportional to the square of the radius:

Mg = C1 * Rg * Rg

where C1 depends only on shape and is basically the same across all the
spiral/disk galaxies.

Orbital speed of stars in the disk is proportional to the square root of the
galaxy's mass (the observed absence of the other otherwise must-have orbital
velocity equation component - ie. 1/SQRT(r) - is what causes crack pot
theories like dark matter) :

Vdisk = C2 * SQRT(Mg) = C2 * SQRT(C1) * Rg

where C2 also dependents only on shape and is basically the same across all
the spiral/disk galaxies.

Thus Vdisk = C * Rg where the C is pretty close to being the same across all
the spiral/disk galaxies.

Angular speed of a star in the disk at the distance "r" from center is

Vdisk / (2pi * r) = (C * Rg) / (2pi * r)

which at the edge of the galaxy, ie. where r = Rg, becomes

(C * Rg) / (2pi * Rg) = C / 2pi

ie. the same constant across all the spiral/disk galaxies. The end.

~~~
xr4ti
Possibly naive question from a non-physicist... does this not follow from
relativity as a natural a consequence of the effect gravity/mass has on
spacetime?

By that I mean that the edges of galaxies experience a "reference" spacetime
(e.g. constant period), while the interiors experience a stretched out version
spacetime (i.e. inverse time dilation and length constriction)?

~~~
trhway
Also not a physicist. At the galactic scale gravity forces are too weak to
take into account any non-Newtonian considerations. At least for myself, most
things observed so far at galactic scale, like flat disk star orbital speed
for example, can be easily explained by straight 6th grade Newtonian physics.

~~~
titzer
_sigh_ I wouldn't have replied to this if you hadn't posted half a dozen
comments. When I was 16 I thought special relativity was a hoax and I tried to
demonstrate this with triangular diagrams scribbled in pencil in my notebooks.
I ended up just rediscovering the Doppler effect. I never showed it to anyone,
but when I think back, I still cringe a little. Relativity and time dilation
are real things with real theory and real experiments.

You'd do well to do a little more research on the topic before posting so many
comments on HN.

There is a _ton_ of evidence for dark matter. We still don't know exactly what
it is, but we do know lots about it (e.g. it is not baryonic matter because
observation differs from theoretical models of how it would interact with
ordinary baryonic matter). As for all that _other_ evidence, the first hit on
Google is for laymen: [https://medium.com/starts-with-a-bang/five-reasons-we-
think-...](https://medium.com/starts-with-a-bang/five-reasons-we-think-dark-
matter-exists-a122bd606ba8)

TLDR: gravitational lensing, missing mass, and three other really good reasons
that dark matter is a serious subject of study.

Disclaimer: I am not a physicist. And I'm trying not to represent myself as
one on HN.

Talking about this stuff is fun, sure. It's great to learn! But please don't
spread ignorance with such high confidence. As for "just" Newtonian physics,
there's a whole research field called "Modified Newtonian Dynamics" (MOND).
[https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics](https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics)

TLDR; there are lots of problems with MOND, and it doesn't explain
gravitational lensing and other effects of dark matter that we see.

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kakarot
> However, the researchers note that further research is required to confirm
> the clock-like spin rate is a universal trait of disk galaxies and not just
> a result of selection bias

Exactly, and that's why the article's title is misleading and clickbaity. The
article also basically copies the press release [0] line for line.

Still, if this is true that's a pretty groundbreaking discovery.

Perhaps we are witnessing a previously undetected force at work, one that
operates on much larger scales with much larger masses.

[0] [https://www.icrar.org/cosmic-clocks/](https://www.icrar.org/cosmic-
clocks/)

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OrganicMSG
This is strikingly scale invariant. I know it seems insane to ask it of
something that big, but could it possibly be an effect that is bootstrapping
up from the smallest scales?

~~~
JeremyBanks
What do you mean?

~~~
OrganicMSG
These are galaxies of very different sizes but yet sharing an oddly similar
time signature which have another similarity, that is that they have developed
without a major collision since the big bang, given they are disk galaxies.

I am asking whether there could be a scale invariant function acting here that
makes the rotation of the galaxy more dependent on age rather than size, that
got started when the vortices that ended up as galaxies were much closer
together and at much smaller scales.

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drexlspivey
That's 3.17 * 10^-17 Hz

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rwallace
Most accounts claim that thanks to dark matter, stars orbit at the same speed
regardless of distance from the galactic center. They don't say whether this
is linear or angular velocity, but even if we assume it's linear velocity, how
can it be that the sun (which is half way out) orbits every 0.25 Gyr and stars
at the rim orbit every 1 Gyr?

~~~
trhway
Sun at 26 Kly and the outer edge stars at 80 Kly have the same 200km/s orbital
speed, and as result have 3 times difference in the orbital period.

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muthdra
Seriously, can't the sheer speed of the inner components of a galaxy cause the
gravity variance we attribute to dark matter? Wouldn't the speed increase the
mass of the objects and therefore also their gravity?

~~~
saagarjha
The sheer speed you're talking about isn't nearly high enough for mass to
change significantly. You need to get very close to the speed of light before
this is the case.

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JoeAltmaier
Perhaps, due to folding in a higher dimension, all galaxies are one galaxy in
different 3D 'cross section'

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lawlessone
is this not normal? Like how planets will orbit a mass of star or black hole
at a certain distance depending on their speed?

~~~
samschooler
I’d actually love to get an explanation for this. Do galaxies work similar to
solar systems in that all objects are in orbit around a central mass or is it
a different phenomenon that binds matter together into a galaxy?

~~~
Merad
I believe the current theory is that most (all?) galaxies have a supermassive
black hole at their heart.

~~~
muthdra
Yeah, or so I heard. What's the cause and effect order for galaxies and their
corresponding supermassive black holes?

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ggm
constant angular velocity is not the same as constant linear velocity. One of
them, things move faster or slower against a fixed reference point depending
on where you are on the disc.

So I'm a bit confused about what 'all' and 'rotate once every billion years'
means here. If they all have the same angular velocity, thats pretty bizarre.
If the net effect of scale is that on average, for a given frame of reference,
stars in the disc move at much the same rate no matter what size the disc is,
depending on how close to the core, thats different, and possibly just about
ytivarg, the thing which makes centipede force work

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mrfusion
Is it strange that it’s a round human number like one billion? Why not
883,259,631 years?

~~~
ralusek
To be clear it's not a human number, because a year is not arbitrary, like an
hour or a minute. It is still weird that it would be a clean decimal
(arbitrary) figure based off of our solar system's year.

~~~
em3rgent0rdr
Well a year is still a human-centric number, because it is related to the
planet humans live on.

However, it is not weird and most likely it means nothing but a simple
coincidence. When you look at the number of time units that it could be a
power of n multiple of, then such a coincidence becomes less surprising.

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isostatic
Milky Way is every 200 million years, as every school child knows:

"The sun, and you and me, and all the stars that we can see Are moving at a
million miles a day In an outer spiral arm at forty thousand miles an hour Of
the galaxy we call the Milky Way

Our galaxy itself, contains a hundred billion stars It's a hundred thousand
light years side-to-side It bulges in the middle, sixteen thousand light years
thick But out by us its just three thousand light years wide

We're thirty thousand light years from galactic central point We go round
every two hundred million years And our galaxy is only one of millions of
billions In this amazing and expanding universe"

\- The Galaxy Song.

~~~
throwaway2048
the rotation of a galaxy isnt the same thing as the rotation of the stars
contained within in. The rotation of the galaxy is measured by the rotation of
arms/other features, which are due to a traveling density wave (think about
how traffic speeds up/slows down)

~~~
wyager
If stars orbit faster than arms, doesn’t this mean that stars have to
occasionally change which arm they’re in? How does that work - some sort of
gradual re-clumping of stars?

~~~
mkempe
Yes, our solar system travels from arm to arm! while the overall shape of the
galaxy remains.

By the way, our transit between and into these galaxy arms is theorized to
coincide with past mass-extinction events, across tens of millions of years.

~~~
civilian
Whaaat! I never heard about the galaxy arm transit coinciding with mass
extinction events. I'm about to google, but do you have a favored source?

~~~
mkempe
I commented here 10 months ago

[https://news.ycombinator.com/item?id=14291004](https://news.ycombinator.com/item?id=14291004)

in relation to "Mass Extinction and the Structure of the Milky Way (2013)
(arxiv.org)"

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trisimix
Stephen :(

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notMick
Can I set my watch by that?

~~~
saurik
> “It’s not Swiss watch precision,” said Gerhardt Meurer, an astronomer from
> the International Centre for Radio Astronomy Research (ICRAR), in a press
> release.

~~~
thaumasiotes
>> “It’s not Swiss watch precision,”

This is a weird case, like "20/20 vision", where the metaphor expresses the
opposite of the reality. Swiss watches are so expensive because they're
clunkier and less accurate than quartz crystal watches.

~~~
hibbelig
I think the idea is that the Swiss mechanical watches are more precise than
other mechanical watches...

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amelius
I'd like to see a time-lapse video of that :)

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Aardwolf
Referring to "All" in the title:

All of them? Even those outside the observable universe? Or only the observed
ones?

~~~
Romanulus
How can we know anything about what cannot be observed?

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ilyagr
How well-defined is the angular velocity of a galaxy? Since it's not a solid
disk, there's no reason for it to be the same for different regions. Is there
any uniformity at all, e.g. do stars at the same distance from the center move
at similar speeds?

Somebody else in the thread points out today our own son makes 4 revolutions
around the galaxy center in a billion years, not 1.

~~~
8bitsrule
You may find this helpful:
[https://en.wikipedia.org/wiki/Galaxy_rotation_curve](https://en.wikipedia.org/wiki/Galaxy_rotation_curve)

If you look at a 33rpm spinning record, you'll see that any point on the edge
of the label is moving a lot slower (linear speed) than the record's edge ...
even though it has the same 'angular velocity' (33 rpm).

But galaxies are not solid like a record, and as the article explains, "most
stars in spiral galaxies orbit at roughly the same speed" (that's linear
speed). So the edges 'fall behind.' (That was not recognized until about
1975.)

