
New studies confirm existence of galaxies with almost no dark matter - happy-go-lucky
https://news.yale.edu/2019/03/29/new-studies-confirm-existence-galaxies-almost-no-dark-matter
======
nonbel
I didn't look at the other one but NGC1052-DF2 is consistent with MOND:

    
    
      Original sigma      = 7.8 (+5.2/-2.2)
      New sigma           = 8.5 (+2.3/-3.1)
      "Crude" MOND sigma  = 14 (+4/-4)
    

[https://tritonstation.wordpress.com/2018/04/04/the-dwarf-
gal...](https://tritonstation.wordpress.com/2018/04/04/the-dwarf-galaxy-
ngc1052-df2/)

And keep in mind these uncertainties are arbitrary cutoffs...

Also, others claimed the distance they use (20 Mpc) is wrong. It should be 13
Mpc and this throws off the calculations:
[https://arxiv.org/abs/1806.10141](https://arxiv.org/abs/1806.10141)

I see them still using the 20 Mpc value without addressing those concerns.

~~~
nonbel
Thinking more on this... Those are either 90% or 95% confidence intervals
(couldn't tell from the recent paper).

So that would mean if you looked at 100 galaxies you would expect 10 (or 5) to
fall outside the range _if the model was correct_.

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abhinai
_Am I wrong to conclude that this result itself is an argument for the
existence of dark matter?_

Because the theories that argue that physical laws behave differently at
larger distances would have to work similarly for all galaxies. Physicists
please correct me if I am wrong in making this conclusion!

~~~
meowface
I think that is a reasonable conclusion to make (assuming these findings are
correct). The article says the same:

>Ironically, the lack of dark matter in these UDGs strengthens the case for
dark matter, the researchers say. It proves that dark matter is a substance
that is not coupled with normal matter, since they can be found separately.

~~~
mikeash
This reminds me of the discovery of oxygen, which was more about discovering
the _lack_ of oxygen in a vacuum and in combustion products.

~~~
everyone
[https://en.wikipedia.org/wiki/Phlogiston_theory](https://en.wikipedia.org/wiki/Phlogiston_theory)

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Zenbit_UX
This leads to an interesting question the article didn't mention...

If a galaxy can exist without any dark matter and others with a balance of
dark and normal matter, can one exist made entirely of dark matter?

~~~
ohazi
If you could play God and place blobs of dark matter in the right places and
moving at the right speeds, it seems plausible that such a dark matter Galaxy
could physically exist and be stable.

But I don't think we know of a mechanism for such a structure to form
naturally. Normal particles collide and transform some of their gravitational
energy into heat. If dark matter only interacts gravitationally, blobs of it
would pass right through each other without losing energy. I think(?) you need
these sorts of collisional energy losses to get matter to coalesce into a
galaxy.

~~~
Retric
You can get the equivalent of evaporative cooling in an unstable multi body
system. You occasionally kick out an object which takes more energy away than
the average body that remains.

The formation of a dark matter galaxy in such a fashion is plausible, but
would take a long time.

~~~
marcosdumay
That require that the bodies exchange energy to get near the lower average,
and transfer some energy to a few hot particles. Otherwise, you won't get any
cooling.

~~~
stcredzero
That could be done entirely through gravitational mechanisms. There's
something like that which happens to globular clusters.

~~~
selimthegrim
You might be interested in Lynden-Bell’s papers

~~~
stcredzero
Thanks. I'm just going by a tidbit from DeepSkyVideos on YouTube. They
specifically mention a similarity to evaporative cooling.

I just found the video! ("Gravothermal Catastrophe")

[https://www.youtube.com/watch?v=LYnoV2n5ptg](https://www.youtube.com/watch?v=LYnoV2n5ptg)

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wpasc
Can a physics minded HNer confirm something for me (or set me straight)?

One piece of evidence for dark matter is observed when galaxies collide. Two
galaxies collide, but by observation of gravitational lensing on the light
from stars "behind" the colliding galaxies show that much of the matter from
each continues moving. This matter (dark matter) is seemingly unobstructed by
the collision (because of dark matter's lack of electromagnetic interaction).

I heard this explained to me once, wondering if it is true.

~~~
joshvm
> because of dark matter's lack of electromagnetic interaction

This is still an assumption, because we don't know for sure that dark matter
is only weakly interacting, but this is what was observed in the Bullet
Cluster collision:
[https://en.wikipedia.org/wiki/Bullet_Cluster](https://en.wikipedia.org/wiki/Bullet_Cluster)

There's a good explanation here: [https://astrobites.org/2016/11/04/the-
bullet-cluster-a-smoki...](https://astrobites.org/2016/11/04/the-bullet-
cluster-a-smoking-gun-for-dark-matter/)

The lensing was found to be strongest near the visible matter (stars), and the
amount of observed lensing couldn't be accounted for with the total stellar
mass alone. So the assumption is that the dark matter, like the stars, wasn't
slowed much by the collision. The gas, on the other hand, interacts
electromagnetically and slows down in the middle.

That's not to say that the stuff that stars are made up of isn't affected by
electromagnetism, but it's a short-range effect and even in a collision,
interstellar separation is large (i.e. contrary to what you might think, a
galaxy collision doesn't involve all the stars smashing into each other, most
of them will just sail right on through). Gravity will dominate in that case.

~~~
ncmncm
"Gas", here, as usual, means "plasma". Gas does not interact
electromagnetically.

Use of the term "plasma" invites banishment from the community of astronomers,
and loss of access to telescopes.

~~~
joshvm
It's referred to as plasma in the primary literature:
[https://arxiv.org/abs/astro-ph/0608407](https://arxiv.org/abs/astro-
ph/0608407)

Many people use hot/ionised gas and plasma interchangeably though. For
example:
[https://academic.oup.com/mnras/article/389/2/967/974617](https://academic.oup.com/mnras/article/389/2/967/974617)

------
Causality1
What I've never understood about dark matter is if it doesn't interact with
the electromagnetic force, why is it found in voluminous clouds instead of
collapsing into dense discs or balls? Radiation pressure doesn't affect it, it
experiences no impact or friction effects, no thermal effects, so why doesn't
it fall to the middle of the galaxies and stay there?

~~~
gizmo686
Why should it stay there? Imagine you had a sphere of particles with an
initial velocity of 0 that interact only through gravity. At first, these
particles would all accelerate towards the center of gravity. Once they reach
the center of gravity, they would have accumulated some velocity, and so
continue to travel away from the center of gravity (but still be accelerating
towards it). Eventually, the acceleration towards the center of gravity would
0 out their velocity again, and they would start falling back towards it. The
end result is that your original sphere oscillates indefinantly, but always
returns to its original state.

As your initial configuration becomes more random, this discernible time
structure begins to fade, and you end up with a voluminous cloud.

What allows regular matter to collapse into dense structures is that it
interacts with itself to shed velocity.

~~~
Causality1
So then why is its structure and location correlated with galaxies at all?
Shouldn't it be relatively even throughout the universe?

~~~
improbable22
I think this is roughly the question why are there galaxies (and clusters, and
stars) at all? And the answer is that the primordial soup wasn't perfectly
smooth, and gravitational attraction has the tendency to make clumpy things
more clumpy. Precisely how un-smooth the soup was, and how clumpy galaxies are
today, is quite a large part of the information we have about the history of
the universe -- the CMB is a snapshot of the soup right when it came out of
the kitchen.

If dark matter was just like matter, then you'd expect it to go along for the
ride, and exist now mostly in the same places (and at the same contrast) as
normal matter. If it were something else, then its distribution could be much
less clumpy, and I think this is a constraint on what kinds of things are
plausible candidates.

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sdenton4
Possibly dumb question: Do we know the dark matter proportion of our own
galaxy? (in other words, how hard is it to take a selfie, at galactic scales?)

~~~
Pharmakon
The existence of dark matter in galactic halos is inferred from the velocity
dispersion of stars in a galaxy (sigma). It’s important to realize that this
is a very rough kind of estimate because every aspect of it (as in the case of
the M-sigma relation) is roughly estimated. Virial mass is inferred from
average temperatures of gas by way of detexting X-rays. Central black hole
mass is inferred through the M-sigma relation, and stellar masses are inferred
through luminosity. Every so often a new paper comes out revising the overall
estimates, but the _proportion_ of DM to luminous matter stays pretty
constant.

Now, in a sense this makes a “selfie” easier, because you just need a series
of snapshots which help you make all of these inferences. On the other hand
because we’re in the disk of our galaxy it is difficult to image because we
have to deal with all of that hot interstellar gas and dust, and the extremely
bright galactic core. I’m not sure how much, when you take into account a long
period of time and many wavelengths can be observed, but I’d say we can
observe the majority of our galaxy in at least one wavelength. Still, we’re
not talking about “selfies” here, just really well educated guesses based on
observations of our own galaxy and similar spiral galaxies.

So as far as inferring the existence of a dark matter halo in our own galaxy
we’re pretty certain, and it seems to make up about 95% of the galactic mass.
It’s based on something a lot more tenuous and complex than a “selfie” though.
Once you’ve done your best to estimate the total luminous mass of the galaxy
and it’s rough distribution though, and ruled out MACHOs with lending surveys
all that’s left in the remaining narrow window is dark matter. The only other
theory with any chance at all is MOND, but it has to thread crushingly narrow
passages between observation and tests of GR.

~~~
nonbel
>"[dark matter] seems to make up about 95% of the galactic mass...crushingly
narrow passages between observation and tests of GR"

GR only matches observation because you added in 20x more stuff that is
undetectable other than as a deviation from the predictions of GR.

~~~
Pharmakon
That’s true, but it’s also true that hypothesis is preferred because of the
abject failure or other theories and hypotheses to match those observations.
The window for something other than DM is exceedingly narrow and only MOND has
a hope of fitting through, and currently not in a form that’s very compelling.

~~~
nonbel
The point is this "window" isn't narrow at all once you allow dark matter.

Practically any theory plus 95% "dark matter" can match the data.

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_bxg1
Wow. From my armchair it seemed to me much more likely that physics just works
a different way at that scale than that there was an invisible substance
magically sprinkled evenly throughout most of the universe.

~~~
wallace_f
Also from my armchair view as well.

Both of these galaxies are UDG's, keep in mind.

In my uneducated opinion dark matter is an outcome from cluttering too much
gravitational force in too small a space.

------
jcims
Are there any alternative theories that invoke strange interactions of
matter/gravity/etc with time? I guess the gravitational wave results out of
LIGO would indicate that it propagates across time in a fashion similar to
electromagnetic radiation, but if gravitational effects extended beyond 'now'
there would seemingly be a possibly small cumulative effect.

~~~
danwills
One alternative is 'Process Physics' which is not widely accepted but has a
fundamentally different explanation for dark matter: Turbulence in the in-flow
of space into matter. This flow is PP's model for gravity itself, as if matter
continuously 'consumes' space to continue its existence. It's an interesting,
quite different and generally totally rejected point of view.

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snarfy
I had a naive theory that there is a limit to black hole size/mass that once
reached causes gravity to have a more linear strength instead of the inverse
square. This discovery blows out that pet theory.

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fallingfrog
This would seem at first glance to be evidence against entropic gravity?

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axilmar
Could it be that we simply cannot observe all of the actual objects a galaxy
contains, and thus we maybe do not need another theory or dark matter?

~~~
bufferoverflow
Pretty much all massive objects except black holes emit light/radiation, and
black holes, afaik, are detectable. Thus there's something else that we give a
placeholder name "dark matter". It's possible it's not a matter of any kind at
all, but just some very smooth but irregular field that interacts with the
space-time curvature (gravity).

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zyxzevn
Today it is dark matter, but tomorrow it is...
[https://i.imgur.com/YcsVKGX.png](https://i.imgur.com/YcsVKGX.png)

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angel_j
I don't believe in the dark matter as they are currently selling it. I think
we have small eyes on the universe and are missing information we'll probably
never get at our scale.

The missing energy could be giant elementary particles (in the standard
model), and these "particles" go to bigger-than-black-holes energy, which we
could never gauge in a lab, tho I'm pretty sure they can be "predicted" along
an energy scale.

