
New class of galaxy has been discovered, made almost entirely of dark matter - daegloe
https://www.washingtonpost.com/news/speaking-of-science/wp/2016/08/25/a-new-class-of-galaxy-has-been-discovered-one-made-almost-entirely-of-dark-matter/
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Florin_Andrei
This is significant. Now astronomers can study dark matter with less
interference from the regular matter background. So you have a more broad
range of objects to study, from galaxies with lots of regular matter, to
galaxies with almost none of it.

Hopefully this will accelerate DM research. It will certainly provide lots
more data.

~~~
mordocai
One thing I don't understand and maybe someone can take the time to explain.

Why does it have to be this "Dark Matter" causing the mass differences we've
been seeing? How do we know it isn't undetectable-through-normal-means normal
matter, or aliens, or mistakes in our calculations/observations, or anything
else?

Since we haven't been able to detect dark matter, it seems from my layman
point of view that any of the above "hypotheses" are equally viable.

What am I missing that makes astronomers and physicists so sure that this
"dark matter" exists?

Edit: Thanks for all the great responses! I'm not going to respond to each
response, but the discussion has been enlightening.

~~~
tomr_stargazer
(PhD student in astrophysics answering here.)

This is a great question! You're addressing the idea (roughly along the lines
of Occam's razor) that if there's any way to explain the "extra mass" without
invoking a new form of matter, it should be preferred.

Most of these other possibilities have been largely ruled out via careful
observations, which are detailed here:
[https://en.wikipedia.org/wiki/Dark_matter#Composition](https://en.wikipedia.org/wiki/Dark_matter#Composition)

One of these possibilities, for example, is that the missing matter is
actually contained in a great many small, dark, massive objects scattered
throughout galaxies -- such as failed stars, planets, or even black holes --
rather than in a diffuse, invisible material. This possibility has actually
been largely ruled out through a number of statistical "microlensing" surveys
that are sensitive specifically to the presence of massive, dark bodies (via
their gravitational lensing of background stars -- a rare event, but
measurable statistically).

[https://en.wikipedia.org/wiki/Massive_compact_halo_object#De...](https://en.wikipedia.org/wiki/Massive_compact_halo_object#Detection)

I think the wikipedia Dark Matter article is actually super well written and
should address these issues, too!

~~~
s_q_b
I'm not even close to a physicist, but it has always bothered me that such a
huge percentage of the matter in the universe would be nearly unobservable.
It's the sort of thing that wouldn't pass my normal sanity check in a
scientific computing model.

But I trust that these possibilities are being well considered. Is there any
significant mainstream buy-in with regard to alternative theories of gravity?

~~~
stcredzero
_it has always bothered me that such a huge percentage of the matter in the
universe would be nearly unobservable. It 's the sort of thing that wouldn't
pass my normal sanity check in a scientific computing model._

How about this? -- A large fraction of bugs in a large, long lived computer
system will be difficult to recreate.

~~~
mattmanser
It's not the same thing.

Imagine being told there's loads of bugs in your system and it's constantly
breaking, even though no-one can show you a single one of these bugs and
you've not got a single user complaint.

That's dark matter.

It's obviously bullshit, but it's the 'simplest' explanation.

~~~
stcredzero
That's not quite right. A better analogy would be:

"Imagine being told there's loads of bugs in your system and it's constantly
breaking, and even though no-one can show you a single one of these bugs, tons
of users are complaining."

That would fit the situation better: There's _only one_ kind of indication
that these things exist.

------
Exras
"The researchers who found Dragonfly 44 weren't looking for a dark galaxy.
Another surprise: They found it using a telescope built of camera parts. The
Dragonfly Telephoto Array was built by a group of astronomers at Yale
University and the University of Toronto who realized that telephoto lenses —
so often used for nature photography and sporting events — were well-suited
for spotting the kind of large, dim objects that pose problems for typical
telescopes."

I like it, reminds me of the discovery of the Cosmic background radiation by
Penzias and Wilson with the Holmdel Horn Antenna. Accept this time nobody had
to shovel bird shit :-)

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semaphoreP
Here is the pre-print version of the paper:
[https://arxiv.org/abs/1606.06291](https://arxiv.org/abs/1606.06291)

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officialjunk
came across this paper that investigates how the EM propulsion drives might
generate thrust, and as a side effect, the theory explains certain phenomena
that we attribute to dark matter and dark energy. i only have undergrad
physics degree, but it sounds interesting. anyone with more experience have
any thoughts about this?

[http://arxiv.org/pdf/1604.03449v1.pdf](http://arxiv.org/pdf/1604.03449v1.pdf)
>McCulloch (2007) has proposed a new model for inertia (MiHsC) that assumes
that the inertia of an object is due to the Unruh radiation it sees when it
accelerates, radiation which is also subject to a Hubble-scale Casimir effect.
In this model only Unruh wavelengths that fit exactly into twice the Hubble
diameter are allowed, so that a greater proportion of the waves are disallowed
for low accelerations (which see longer Unruh waves) leading to a gradual new
loss of inertia as accelerations become tiny. MiHsC modifies the standard
inertial mass (m) to a modified one (m_i) as follows:

m_i = m (1-(2c^2)/(|a|Θ)) = m (1 - λ/4Θ) (1) where c is the speed of light, Θ
is twice the Hubble distance, ’|a|’ is the mag- nitude of the relative
acceleration of the object relative to surrounding matter and λ is the peak
wavelength of the Unruh radiation it sees. Eq. 1 predicts that for terrestrial
accelerations (eg: 9.8m/s2) the second term in the bracket is tiny and
standard inertia is recovered, but in low acceleration environments, for
example at the edges of galaxies (when a is small and λ is large) the sec- ond
term in the bracket becomes larger and the inertial mass decreases in a new
way so that MiHsC can explain galaxy rotation without the need for dark matter
(McCulloch, 2012) and cosmic acceleration without the need for dark energy
(McCulloch, 2007, 2010).

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dghughes
Sometimes I have to wonder maybe we are the weird ones and dark matter being
more common is normal.

Maybe there are trillions of beings looking at us and our weird matter and are
amazed we can survive.

~~~
sevenless
If they can't interact with normal matter or light, _we 're_ the dark matter
to them.

------
SolarNet
Is it possible that an alien race has simply covered that galaxy with dyson
spheres? That life is more common than we thought - or perhaps more rapidly
advancing once intelligence is hit - and most galaxies are "dark" from their
interference?

~~~
aab0
No infrared. Already ruled out by earlier infrared surveys looking for Dyson
spheres.

~~~
3pt14159
Ruled out? From Wikipedia:

Identifying one of the many infrared sources as a Dyson sphere would require
improved techniques for discriminating between a Dyson sphere and natural
sources. Fermilab discovered 17 potential "ambiguous" candidates, of which
four have been named "amusing but still questionable". Other searches also
resulted in several candidates, which are, however, unconfirmed.

~~~
yongjik
I think aab0 means that this particular galaxy is ruled out as a Dyson sphere,
not the possibility of there being one somewhere in the universe.

A Dyson sphere should still emit infrared, unless the aliens can subvert
thermodynamics, at which point anything is possible.

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teh_klev
Archive.is linkage for those who can't get past the paywall:

[http://archive.is/b4e7X](http://archive.is/b4e7X)

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pc2g4d
I find myself wondering if the type of inference required to make this claim
could be automated. Given the right observations, couldn't dark matter be
detected (or at least hypothesized) algorithmically? Couldn't this be used to
create a "dark matter scope" by which the dark matter in the universe can be
"seen" and visualized?

~~~
gus_massa
I'm guessing here, but they probably detected this automatically. There are
millions of millions of galaxies and you can't put a even graduate student to
look at each one.

My guess is that they were preparing a boring paper, like "Analysis of
Normal/Dark Matter ratio in WhAtEvEr type galaxies near SoMeWhErE". They put a
telescope, some processing and then they transfer the data to Excel to make a
nice graphic. Then got some outliers, and with more analysis they were
discarded as error. But they got one nasty outlier that were not easy to kill.
They measure it again, and again, and probably made another team double check
it.

(Perhaps they were looking for faint galaxies, and it was not too much luck.)

Anyway, probably most of the calculation was automated, and probably now some
other teams will try to find similar objects.

~~~
gammarator
This was definitely a high-priority, by-hand, single object reduction. They
put 33 hours of Keck time into it--you don't do that unless you're certain you
have something very interesting.

(For reference, the capitalized value of one night (~8 hours) of 10-m
telescope time is roughly $100k.)

~~~
gus_massa
Ups. Nice.

More questions. How did they (you?) select this object to give it more
attention?

------
hinkley
From the article it seems to me that it's not the telephoto lenses that were
the breakthrough here but the lens coating on the lenses. So is the next step
to work with the manufacturer to get those coatings available on purpose-built
astronomy equipment?

And is it only applicable to refractory telescopes?

~~~
gammarator
Large astronomical telescopes are necessarily reflecting: it's more cost
effective to build large mirrors than large lenses. But that necessarily
implies that there will be obscuration (secondary mirrors, etc.) in the light
path, which complicates the focused image and makes it difficult to look for
diffuse, low-surface brightness features.

The other challenge in looking for these sorts of objects is scattered light.
What van Dokkum & co realized was that commercial telephoto lenses are
exquisitely designed to minimize scattered light, and as refractors have no
central obscuration. So a small array of such lenses is actually more
sensitive to faint, diffuse features than much larger telescopes. (Plus
cheaper!)

------
wallace_f
I sometimes enjoy to daydream about the possibilities of dark matter, but
probably the simplest explanation, and the least spectacular one, is the
answer here: dark matter likely is a product of black holes, and probably just
an even distribution of black holes. There is already growing a little bit of
evidence that this is the case. I would be thrilled to learn about gravity
leaking from other dimensions or alternate forms of matter, but it is probably
just normal matter trapped inside black holes.

~~~
andrepd
I don't think anything in your post makes the least bit of sense.

~~~
asimuvPR
Perhaps you could tell why?

~~~
vecter
Almost every statement he makes is pure nonsense.

> dark matter is likely a product of black holes

what product, specifically, and through what mechanism?

> and probably just an even distribution of black holes.

That just contradicts the first half of the sentence, which says that dark
matter is a product of black holes, but now he's saying that they are black
holes themselves.

We understand black hole behavior quite well. They can't be microscopic black
holes because they would evaporate almost instantaneously due to Hawking
radiation. So they must be bigger. But we know there's dark matter in our own
galaxy, and there certainly isn't a uniform ubiquitous spread of black holes
in our galaxy. And if they were so uniformly distributed, they would probably
merge and become many fewer black holes. But we clearly don't see that either
in our own galaxy or elsewhere.

Then some ridiculous vague statement about "gravity leaking from other
dimensions"? It's insane, it's like he used a random grammar generator with
physics words.

What he said was so crackpot that it would easily qualify as "not even wrong".
[0]

[0]
[https://en.wikipedia.org/wiki/Not_even_wrong](https://en.wikipedia.org/wiki/Not_even_wrong)

~~~
JumpCrisscross
I read it as meaning "the gravitational effects we attribute to dark matter
may be a product of even more massive black holes at galactic cores". I once
wondered this. Then I learned our studies of how star revolution rates taper
off as a function of distance from the core were not explicable with a giant
mass at the core; there needed to be a diffuse mass.

~~~
wallace_f
What I intended as the meaning was close, it was basically this (hopefully it
makes more sense): the gravitational effects we attribute to an unknown
source, dark matter, is likely the product of matter inside of black holes.

In other words, there may be weird quirks of physics that are not yet
discovered. what they are I don't know. Maybe mass density or gravitational
density has an upper limit, and the universe conserves mass by redistributing
it in some strange way.

The point is also just this: fantastic explanations that explain what dark
matter is (alternate universe's, leaking gravity) are probably the least
likely to to be true. Instead, the most likely explanation is the one which
uncovers the least fantastic possibilities -- it likely is explained using the
least modifications to our current observations/understanding of the universe.

I don't know how it didn't make any sense, but I'm sorry that it didn't - Can
someone please enlighten me?

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perseusprime11
I always thought that dark matter is a trick played by gravity. I wish we can
get to these things faster.

------
JumpCrisscross
How do we rule out dust between the galaxy and us in cases like this?

~~~
okket
The amount needed would be huge and we could see the effects. This "dark dust"
would dim/absorb the light emitted by stars behind it and turn it into
infrared light, that we could see.

Also, since it is regular ("baryonic") dusk, the slightest distortion would
make it clump together, create stars, turn into visible matter.

The last reason is the cosmic microwave background (CMB). We can infer from it
how much baryonic mass in the universe should be and the huge amount of dark
dusk does not fit in this calculation by a factor of roughly four.

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zouhair
Dark Matter is really a bad name. Too confusing.

------
JackFr
It's Dyson spheres.

~~~
JackFr
Not sure I get the downvotes. Because I'm not elevating the discussion? I was
trying to be funny.

------
geertj
Would this mean that there's a lot more dark matter in the universe than we
previously thought? And consequently that we may escape a terrible heat death
but instead get some real action near the end?

