
Hubble finds first galaxy in the local universe without dark matter - sampo
http://www.spacetelescope.org/news/heic1806/
======
nate_meurer
To me, the coolest thing about this story is the telescope that was used to
find this galaxy. It's literally a bunch of off-the-shelf Canon telephoto
lenses stuck together on a metal frame, each with a camera sensor, with
coordinated aim and focus.

Apparently, some of the newer Canon lenses have a newfangled anti-reflective
coating with unprecedented performance. So some enterprising Canucks said,
"you know, maybe we can buy a bunch of those and tape them together and make a
world-class scientific instrument, eh?" And then they fucking did it.

That there is the DIY hacker spirit in broad daylight. It's so awesome I can
barely stand it.

You can read about it here, which found via Phil Plait's excellent blog:
[http://www.dunlap.utoronto.ca/instrumentation/dragonfly/](http://www.dunlap.utoronto.ca/instrumentation/dragonfly/)

~~~
kartan
> a bunch of off-the-shelf Canon telephoto lenses stuck together on a metal
> frame

That's an interesting remark. It's hard to beat mass production. As number
produced skyrocket to the millions, it's difficult that a custom device has
the resources that a mass marketed device has.

That's why millionaires have the same phone that you, the military use Xbox
360 controllers in their submarine's periscopes and COTS supercomputers are
common.

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

[https://www.quora.com/What-smartphone-do-tech-
billionaires-u...](https://www.quora.com/What-smartphone-do-tech-billionaires-
use)

"US Navy submarines are getting Xbox 360 controllers to control their
periscopes" [https://www.theverge.com/2017/9/19/16333376/us-navy-
military...](https://www.theverge.com/2017/9/19/16333376/us-navy-military-
xbox-360-controller)

~~~
ktpsns
In fact, a whole generation of HPC students and scientists write codes for
consumer graphics cards (GPGPU). All of this only because they are so cheap.

~~~
sgt101
I think that the most powerful thing is that they offer a low point of entry.
You can start for pretty near free, but for less than £100k you can build a
differentiating capability and start contributing to knowledge... vs
traditional instruments where the onramp starts at £millions and goes from
there.

------
delecti
This makes me wonder what the normal error bars are for how much dark matter
is in galaxies. Do we often see galaxies with 75-125% as much dark matter as
our models expect? Or maybe 99-101%, or even 10-1000%? How far off of the
expected value does it take to become noteworthy?

That said, that this is a headline at all tells me it's probably closer to the
99-101 case than the 10-1000 case. Given that, this seems to be the tipping to
some breakthroughs, because my sense has always been that we have really
little idea what dark matter _is_.

Edit: Through a bit of link hopping, I found this paper [1]. Based on Figure
11 in that paper, it seems that there's a relatively small error bar around
the expected ratios of dark matter to baryonic matter. At the very least, the
error bar is narrow enough that this new galaxy would be _well_ outside it.

But I don't have any formal astrophysics training, so I'd love to be
corrected.

[1]
[https://academic.oup.com/mnras/article/410/1/210/1033029](https://academic.oup.com/mnras/article/410/1/210/1033029)

~~~
erikpukinskis
I’m a lay person, but...

Isn’t dark matter just matter in other dimensions? Not visible in our three,
but obviously having an effect? Like the body of an arch to flatlanders?

~~~
coryfklein
Not sure why an honest question is getting downvotes.

1) We do not yet have any reason to believe there are more than 3 dimensions
plus time [1] 2) It depends on what you mean by "visible". Take a look at this
picture full screen:
[https://en.wikipedia.org/wiki/Dark_matter#/media/File:Gravit...](https://en.wikipedia.org/wiki/Dark_matter#/media/File:Gravitationell-
lins-4.jpg)

That is an image of dark matter bending the light from distant galaxies - take
note of the circularly shaped bent galaxies radiating around the middle. This
effect is called "gravitational lensing", and it is why we know that _there
actually is_ some matter in this picture. We also know that it is invisible
because otherwise the "dark matter" would be blocking view of the galaxies
you're looking at.

3) I highly recommend reading the summary on the Wikipedia article on dark
matter [2]. The whole article is really long, but the summary paragraphs at
the top give a great high-level explanation of "what we know" about dark
matter.

[1]
[https://physics.stackexchange.com/questions/4079/experimenta...](https://physics.stackexchange.com/questions/4079/experimental-
evidence-of-a-fourth-spatial-dimension) [2]
[https://en.wikipedia.org/wiki/Dark_matter](https://en.wikipedia.org/wiki/Dark_matter)

~~~
chrisfinne
Humans have a long history of misunderstanding the geometry of our world. I
don't know jack about astrophysics, but the explanations for dark matter and
dark energy seem so complicated that my bullshit meter goes off. Reminds me of
the people trying to model the walk of the planets in an Earth-centric
universe. I'd put money on another Newton or Einstein (or singularity AI)
giving us a much more elegant solution in the next 50-100 years.

~~~
kiliantics
I feel similarly dissatisfied with the explanations but, given the current
framework, they're actually the simplest possible models that could work. The
rest of physics and the many observations that need to be consistent with it
are just that elaborate.

------
rmcpherson
I was talking with some physicists from Fermilab last week about dark matter
(I'm a physics enthusiast but certainly no physicist) and one thing they were
discussing was how current dark matter experiments are constraining possible
dark matter candidates significantly as they get more sensitive without
finding any. This has lead to increased interest by physicists in alternative
theories, including MOND. This galaxy discovery, though, appears to be strong
evidence against MOND or other modified-gravity theories since the dark matter
is missing in this case. It provides more evidence for dark matter's
existence, even as the strongest candidate, Weakly Interacting Massive
Particles (WIMPs) are becoming more constrained by lack of evidence.

~~~
poelzi
I follow the BSM-SG model, which also has no dark matter to explain galaxy
rotations.

Disclaimer: I choose my models after reduction of assumptions and stricter
logic is better (Solomonoff Induction). Secondly, I understood when studying
physics, that there seems something wrong, but could not put my finger on it.
So, naturally it made the most sense for me, to first check all the
assumptions currently used and drop those that could easily be seen
differently. In fact, the BSM-SG model assumes so less, that I actually
accepted it after a year of heavy studying and thinking - the first model ever
for me.

It however has some "dark matter" called filaments which are the residue of
the galactic crystallization egg so to say. Read some paper last year that
found them, but of course, they thought it's dark matter. It is actually black
black - absolutely no photon emissions even when heated to millions of
degrees.

In the BSM-SG model galaxies crystallize, it does not have a big bang theory.
The galactic redshift has a very different explanation.

The filaments are only located around the super massive black hole in every
well formed galaxy (not globular clusters). They are the hardest and densest
hard matter in the universe. Everything we know is super soft compared to this
stuff. Our hole star system is maybe a cubic centimeter of this stuff.

If you take all the fragments of the fillaments and put them together they
build a perfect hollow sphere that fits perfectly around the black hole.
Another really wired property: if you take a cube of it and cut it in half,
the mass of both parts will be more then the mass of the original cube. Sounds
paradoxical at first, but once you understand how Newtonian mass is derived
and that it is basically an surface property it makes sense.

If you separate ordinary matter, you don't increase to surface of atomic
structures towards the vacuum. You get relativistic changes due the
microcurviture, but the surface stays the same. This stuff you can cut and
increase the surface until the particle is smaller then the CL node distance
~10^-20 m at which point it will loos its Newtonian mass and only the
intrinsic mass will manifest. Same effect as some Neutrinos have.

But year, I'm a nut job who does not believe in what most think is true, but
to be honest, I don't care. It works so wonderfully and if you can interpret
papers from this models perspective, I have not found a single paper that even
shakes on it's fundamentals. Some days ago a paper was posted here in which
they found "magic angles" in graphene which was doted. I predicted 2 years ago
that there will be some angle influence between the graphene layers.

To predict something more, you know, because I'm nuts: most of the grapheme we
use is quite unordered. There are 2 possibilities in which the carbon atom can
be positioned. To get a really good conductor the atoms need to be properly
aligned together with the alignment of the 2 layers.

The standard perspective on the atomic nucleus is totally oversimplified and
this is something quite some scientists found trough different ways. The BSM-
SG model was the first model where the periodic table stared to make sense for
me, together with all the isotopes.

~~~
eloff
Well, you know when you think everyone else is wrong and you're right - that's
the definition of crazy. I'm not insulting you, as text doesn't convey that I
say that with a smile, but you should consider that it's far more likely that
you are actually the one who's wrong.

~~~
seanmcdirmid
Going or not going with the crowd is a poor definition of craziness. Instead,
consider the candidate thinking logically (e.g. do they believe P&&!P is true)
or critically. Do they have reasonable arguments or do they go in circles,
etc...

~~~
eloff
You're taking me too literally. If I say you're crazy with a smile, I don't
mean your mentally ill, I mean your ideas are out of the ordinary.

------
DrBazza
Phil Plait's take on it: [http://www.syfy.com/syfywire/what-is-this-galaxy-
doing-witho...](http://www.syfy.com/syfywire/what-is-this-galaxy-doing-
without-a-dark-matter-halo)

~~~
scribu
Thanks!

That article offers much needed context for laypeople, such as myself.

Among other things, it points out that, in the image, only the bluish glow is
the galaxy under consideration. All other points are distant galaxies.

------
komali2
Man do I feel dumb when these things hit the front page. I managed to make it
through a couple "layman's guides to the impenetrable world of theoretical
physics," including all variations of "A brief history as time" as well as a
noob's introduction to string theory, title of which escapes me.

Still don't fucking get it lol. Between this, AI, and even just the drumset, I
wish there was more time in the world to learn all the things I wanna learn :(

~~~
nate_meurer
You're not dumb; sometimes you just need something to click, and you never
know what that something might be.

Try this: when we look at a galaxy, we can see how fast it's rotating. We can
do this using spectrometers that can see where light from stars is red-shifted
(which indicates those stars are away from us) or blue-shifted (moving toward
us).

The faster a galaxy is rotating, the more massive it must be, because without
that gravity the stars would just be flung out into space like toddlers on a
malfunctioning carousel.

We can also make a pretty good guess at the amount of "normal" matter in a
galaxy (i.e., not dark matter) by measuring how bright a galaxy is, since
nearly all light comes from stars, which are, as far as we know, composed
entirely of normal matter.

So, putting those two things together, we encounter a mysterious thing: having
observed thousands of galaxies, nearly all are rotating much faster than is
expected given their estimated mass. One possible explanation for this is that
there's another kind of matter in there that doesn't emit any light, which we
call dark matter. To be more precise, not only can this dark matter not emit
radiation, it cannot interact with radiation at all -- it can't absorb or
reflect it. Otherwise we'd see it, just as we see clouds of dust in galaxies
(which is accounted for in our mass estimates).

Other proposed explanations involve changing the way gravity works. These are
very interesting and compelling ideas, however this newly discovered galaxy
would appear to weaken the case for modified gravity theories, because under
those theories we would expect this galaxy to be rotating as fast as all other
galaxies, since gravity is a function of mass. But that's not what we see;
this new galaxy is rotating much slower. It is, in fact, rotating at the rate
we would predict if it didn't contain any mysterious invisible "dark" matter.
Which is why we think it doesn't!

~~~
baddox
Things like this in theoretical physics, cosmology, quantum physics, etc.
aren't really supposed to "click," are they? My impression is that you just
learn the math and the predictions made by the mathematical models, then learn
about the observations we've made that fit that model's predictions, and
that's really it. "Click" implies to me that you can achieve some sort of
"intuitive" grasp of these concepts, perhaps in terms of more familiar
everyday phenomena, which from what I've heard isn't the case even for the
experts in these fields.

~~~
keldaris
(Theoretical physicist, working in a different area)

"Intuitive" means two different things in this context. One is that, over
time, theoreticians certainly gain a lot of intuition about how the relevant
math works. That's no substitute for doing the work, but it does cut down on
the number of mistakes you make, you often have a decent sense of what's
definitely not going to work, etc.

The other meaning is what people usually imply - that some combination of
analogies with everyday phenomena finally "clicks" and you "understand" the
physics all of a sudden. That's just nonsense. Past the natural limits of our
everyday intuitions (which are very narrow indeed, consider how something as
trivial as terminal velocity is often unintuitive to students in basic
mechanics) physics is applied math, everything else is a crude approximation
at best.

~~~
baddox
Thanks for the insight. I was just going off of what I’ve heard, and it sounds
like you mostly agree.

I will say that what can become intuitive to us (in your second sense) can
certainly change over time, at least after generations. Netwton’s first law
must not have been intuitive, mostly because of the ubiquity of friction in
everyday life, but I think it is fairly intuitive now for people with a solid
basic science education. Maybe more advanced concepts will get more intuitive
over the generations, as science concepts get taught at younger ages.

------
scottcanoni
Make sure to watch the video (in 4K) and see just how far away this galaxy is:

[http://www.spacetelescope.org/videos/heic1806a/](http://www.spacetelescope.org/videos/heic1806a/)

Mind blowing far.

~~~
coryfklein
Makes me wonder how they found this one. Did they just point it at one random
place and happened to find this galaxy there, or did they have to look around
for a long time.

If we take that telescope array and point it at a new random place will we
just find another one of these galaxies in short order?

------
wanda
(I'm no astrophysicist, so forgive me if my comment is stupid or naive)

From the Wikipedia article [0] on NGC 1052:

    
    
        NGC 1052 shows also two small jets emerging from its 
        nucleus as well as a very extended disc of neutral 
        hydrogen, far larger than the galaxy itself, 
        all these features suggesting a gas-rich galaxy 
        collided and merged with it 1 billion years ago
        producing all the above features.
    

Is it possible that NGC 1052-DF2 is simply the remnant of the aforementioned
gas-rich galaxy suspected of having collided with NGC 1052, and that the
apparent dark matter deficiency is a result of that collision?

Or do we know what happens when galaxies collide well enough to conclude with
reasonable certainty that this is not the case?

[0]:
[https://en.wikipedia.org/wiki/NGC_1052](https://en.wikipedia.org/wiki/NGC_1052)

~~~
sddfd
The point is that the galaxy shows that dark matter exists /independently/
(i.e. as its own particles) of normal matter. This is huge.

Up until now, there was a branch of physics that tried to explain dark matter
as a phenomenon that is caused by normal matter. This discovery shows that
such an approach is unlikely to work.

For all of this, it does not matter /why/ there is no (not a lot) dark matter
in that galaxy. Although that is probably the next question physicists are
after!

~~~
CydeWeys
Minor clarification in case someone else similarly misreads your post at first
glance:

Dark matter is not normal matter, and does not exist as particles of normal
matter. It's completely distinct, and we don't even know what it is made of.
This finding gives good evidence that it's actually real, rather than either
simply not existing ("gravity behaves strangely at long distances" hypotheses)
or just being normal matter that isn't easily detectable, thus contributing to
a galaxy's gravity but not what we can see of it.

This new discovery is of a galaxy whose mass does correspond to what we can
see, meaning that we aren't missing obvious explanations for other galaxies,
which rules out a lot of hypotheses that might otherwise explain the mass that
we can't see for other galaxies.

I know you know this, but it's possible to misinterpret your comment.

~~~
Avshalom
Dark matter is (probably) pretty damn normal, while we don't know precisely
what particles make up the majority of it there's half a dozen particles
predicted by the Standard Model (how much more normal than standard can you
get?) that are dark.

~~~
CydeWeys
Normal aka ordinary aka baryonic matter. Dark matter is decidedly not normal.
The Standard Model encompasses everything that could theoretically exist,
while normal matter (the kind of stuff you and I are made of) is a small
subset of that.

------
mabbo
My father has a wonderful old book published in 1896. It's a textbook for
University students on physics. What makes it interesting is that the electron
was discovered in 1897.

I can't say I've studied the book in detail, but what I did find was something
that reminds me a lot of where we are today with dark matter. We understand
the problem, and where _something_ ought to fit into it and there's lots of
good theories being tried out- but we just don't know what the answer is yet.

I suspect we're writing this generation's version of that book right now, and
we call the subject "dark matter".

------
AgentME
I wonder if there are also galaxies that are just dark matter. (Of course,
it'd be hard to observe them. Maybe it would gravitationally affect galaxies
near it.)

~~~
netcraft
for some definition of "galaxy" \- sort of
[https://news.nationalgeographic.com/news/2012/03/120306-dark...](https://news.nationalgeographic.com/news/2012/03/120306-dark-
matter-galaxies-mystery-space-science/)

------
c-smile
Could it be just because that galaxy has no central black hole? And if so that
rises more questions:

Does each galaxy have black hole associated with it? What if black hole is not
a "point mass source" (a.k.a. singularity) but rather something with different
curvature of space/time that we treat as a dark matter effect?

------
mikhailfranco
And _Dragonfly 44_ is only dark matter ...

[http://www.wired.co.uk/article/dark-matter-galaxy-
dragonfly-...](http://www.wired.co.uk/article/dark-matter-galaxy-dragonfly-44)

If there are many galaxy collisions like the Bullet Cluster, which separate
dark from visible, then it's not surprising that many such pure galaxies
exist, and their creation cannot be _unknown_ (except for the purposes of
melodramatic popsci quotes).

------
thaumasiotes
> Merritt remarks: "There is no theory that predicts these types of galaxies
> [without dark matter] — how you actually go about forming one of these
> things is completely unknown."

Is there a theory that predicts galaxies _with_ dark matter, or states how
they come to be that way? I thought the whole dark matter concept was "we
don't know anything about it".

~~~
raattgift
> Is there a theory that predicts galaxies with dark matter, or states how
> they come to be that way?

Yes, the concordance cosmology (because it concords with all the available
evidence), also called the standard cosmology, has an evidence-matching theory
of structure formation. The European Space Agency has a good overview at
[http://www.esa.int/Our_Activities/Space_Science/Planck/Histo...](http://www.esa.int/Our_Activities/Space_Science/Planck/History_of_cosmic_structure_formation)

The evidence for the concordance cosmology comes from terrestrial laboratory
physics, tests of general relativity and the standard model at various other
places in our solar system, and astrophysical observation. It continues to
deliver good predictions (notably about small-scale fluctuations in the cosmic
microwave background) but strictly speaking it is an effective theory (it is
not complete for the extremely early universe, and it is coarse-grained at
length scales larger than galaxies) and it is frequently updated as new
information arises, so it's not so much falsifiable (barring falsification of
General Relativity, statistical mechanics, thermodynamics, or quantum
mechanics, which underpin the cosmology and give it self-consistency) as
requiring completion. [ one starting point:
[https://en.wikipedia.org/wiki/Lambda-
CDM_model](https://en.wikipedia.org/wiki/Lambda-CDM_model) ]

"CDM" stands for cold dark matter, and its name represents the key features
for the (cosmological scale) fluid implied by the behaviour of visible matter:
it is [a] moving slowly compared to the speed of light (cold, rather than hot)
[b] oblivious to electromagnetism, having and feeling no charge and not
decaying to visible matter, and [c] some form of matter rather than an
adaptation of General Relativity that amounts to an alternative theory of
gravitation.

[a] and [b] also imply that CDM is essentially collisionless, meaning that it
(at least mostly) interacts only gravitationally with other matter and itself,
which means that it cannot ditch angular momentum that would let it fall into
a structure like a galaxy or star (note that normal visible matter -- mostly
protons by mass -- can collide, releasing photons and other daughter products,
and that lots of collisions make matter hotter and brighter generally, and
also more likely to densify into e.g. stars, which are ultimately mostly very
hot very frequently colliding very visible matter).

Cold dark matter mostly behaves like heavy neutrinos. Known neutrinos are
"hot" because they are so low mass that they are hard to keep from
accelerating to speeds comparable to the speed of light, and thus can't keep
their (individually low, but collectively less low) mass localized around a
galaxy cluster. Neutrinos also interact only via the weak force, so while
there are trillions passing through you as you read this, probably none at all
will be "felt" by any of the regular matter in your body. But they are matter,
and they have been observed in laboratory experiments since 1959, and solar
neutrinos have been spotted in detectors in more recent decades.

There are also almost certainly cold regular neutrinos that have redshifted as
a relic field, like the cosmic microwave background (a relic field of
neutrinos). They will be much much harder to detect because the recoil of
atomic nuclei encountering a cold regular neutrino will be much smaller than
one encountering a neutrino moving relativistically.

We may at some point be able to doppler-cool neutrino emitting substances,
producing cold regular neutrinos in terrestrial laboratories.

So the missing piece is preventing already-cold neutrinos from heating up
(extra mass can do that, so can a lower interaction cross-section, e.g.
gravitation-only interaction rather than weak force interaction; both can
combine). Consequently, the existence of a CDM particle is not much of a
stretch, particle physicists are doing what they can to try to find it (and
block out interaction cross-section vs mass areas that have been probed
without finding a CDM particle), and observational astrophysicists are also
looking for evidence from cosmic rays and their sources (e.g. supernovae).
There is still a lot of room for a CDM particle to be hiding, and there is a
lot of not-so-low-hanging fruit that will have to be probed. :-(

There are quite a few proposed extensions of the standard model (of particle
physics) that have suggested places to look first, since a variety of sterile
heavy neutrinos could fix some problems specific to the standard model (i.e.,
not involving gravitation at all). The LHC has killed off a few such particle-
physics proposals, but certainly nowhere near all; it could still end up
finding a particle (or more than one) that would be a good candidate for at
least some of the observationally-inferred dark matter.

~~~
thaumasiotes
[https://en.wikipedia.org/wiki/Lambda-
CDM_model](https://en.wikipedia.org/wiki/Lambda-CDM_model) has this to say
about the role of dark matter in the theory, which I find worrying:

> Dark matter is _postulated_ in order to account for gravitational effects
> observed in very large-scale structures (the "flat" rotation curves of
> galaxies; the gravitational lensing of light by galaxy clusters; and
> enhanced clustering of galaxies) that cannot be accounted for by the
> quantity of observed matter.

(My emphasis.)

This sounds very much like someone wanted a theory that would predict the
observed rotation of galaxies, and created one that _assumed_ the existence of
dark matter as a way of doing that. (This was also basically my understanding
of where the theory of dark matter came from.)

I'm asking about a theory that predicts dark matter, from some other set of
assumptions.

Have I understood this right?

~~~
neolefty
I think you're basically right, but I'd encourage you to feel comfortable with
it as a normal part of the discovery process.

I would phrase it as "There's something happening that we see only
indirectly—by its diffuse gravitational influence. Let's give it a name."

That's happened many times before in science. For example, the measurable
perturbation of Mercury's orbit due to special relativity—it was measured
before it was explained, and there were multiple competing explanations.

Or, long before that, the causes of disease. What we believe now (invisibly
small germs) was laughed at.

~~~
thaumasiotes
I agree that this is a normal part of the discovery process.

But I don't agree that it constitutes a theory of dark matter, any more than
the observation of the precession of Mercury constitutes a theory of general
relativity. I was contrasting the claim from the article, "there is no theory
that predicts galaxies without dark matter" with the implied claim that there
_is_ a theory that predicts galaxies _with_ dark matter.

If the extent of our theory is "we conclude that the galaxies we've observed
are permeated by a substance which is massless[1] but nevertheless generates
gravitational fields, because their gravity is all messed up", that is not a
theory that predicts the nonexistence of galaxies that lack this substance and
display normal gravity, nor is it a theory that predicts how a galaxy would
acquire any of this weird substance. If it doesn't do those things, what's
surprising about the quote I highlighted?

If the dark matter concept is purely observational -- "I'm going to call the
weird gravity of this galaxy 'dark matter'" \-- it cannot possibly conflict
with any other observations. As far as this theory is concerned, it _does_
predict the existence of galaxies without weird gravity, because we observe
those, and that is how we've defined "predictions" as generated by this
theory. If that sounds stupid to you -- and it does to me -- either we
shouldn't be talking about "predictions" or there's something better qualified
to be called a "theory". Is there?

[1] I'm using "massless" to refer to the property called "collisionless" in
the wikipedia article. Physicists may define mass as the property by which
gravitational fields are generated, but I think the ability to collide with
other masses is more fundamental to the basic concept.

~~~
neolefty
Haha, I'm no physicist, so take anything I say with a grain of salt. Also, I
don't think I can address everything you wrote, so apologies for picking and
choosing. I'll say what I can:

> the ability to collide ... is more fundamental

That's exactly the funny thing—when physicists say "dark matter" they're
saying matter that attracts but doesn't collide. They can only detect it
indirectly. I think they normally want to agree with you that matter interacts
with light and other matter. "Dark matter" would _not_ be their first choice
of explanations.

> the observation of the precession of Mercury constitutes a theory of general
> relativity

I'm going to get pedantic here and separate _observation_ from _theory_ ,
because I think it helps talk about it. Mercury was _observed_ to precess
oddly, and nobody knew why. For quite a while, the _theory_ was that an unseen
planet was doing it. So I'd phrase it only slightly differently:

> the observation of the precession of Mercury constitutes _evidence for_ a
> theory of general relativity

I draw the distinction because it's central to the debate about dark matter.
Weird gravity isn't a theory at all—it's an observation. And it's one
physicists pretty much agree on—galaxies are definitely acting weird, with
respect to their visible mass.

One possible _theory_ to explain it is dark matter. Another is MOND. Any of
them could be true—it's kind of probabilistic. For some physicists this new
observation is shifting the probabilities in favor of dark matter, but it's
obviously still flawed because we can't _really_ explain _dark_ because we
have no positive confirmation of its existence outside of gravity. We'd love
to find a WIMP, for example, but we haven't.

------
mtreis86
Here is the article referenced. The link on spacetelescope isn't working.
[http://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_...](http://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_attachment/317/van_Dokuum_paper_submitted_to_Nature.pdf)

------
acqq
Main message:

“Although counterintuitive, the existence of a galaxy without dark matter
negates theories that try to explain the Universe without dark matter being a
part of it [3]: The discovery of NGC 1052-DF2 demonstrates that dark matter is
somehow separable from galaxies. This is only expected if dark matter is bound
to ordinary matter through nothing but gravity.”

~~~
TillE
That does seem fairly convincing, though it also highlights the lack of
precise theories about dark matter: what is it, and how did it get there?
Hopefully these are answerable questions some day.

~~~
acqq
The theory is as precise as we can observe it, the match is quite good
compared to the alternatives:

[https://www.duo.uio.no/bitstream/handle/10852/52374/FinalRep...](https://www.duo.uio.no/bitstream/handle/10852/52374/FinalReport.pdf)

------
Zamicol
How do we know that gravity is constant in proportion to mass and not a
function of time? General relativity tells us mass slows down time, but we
assume slowing down action doesn't affect gravity. If gravitons, or other
quanta of gravity, exists wouldn't general relativity infer that massive
objects have less gravity in proportion to mass than less massive objects?

We know from general relativity that massive galactic cores have less time in
proportion to their mass compared with galactic arms, which have more time
(action) relative to their mass.

Why wouldn't general relativity's slow down in time also mean less gravity in
proportion to mass?

~~~
semi-extrinsic
Well, if the strength of gravity was linear in redshift, astronomers would
have detected it decades ago.

~~~
jboy55
Apologies if this is naive, I was looking for a free book for a flight a week
ago and I downloaded 'The Einstein theory of relativity; a concise statement'
by H A Lorentz, 1920, available here
[https://archive.org/details/einsteintheoryr00einsgoog](https://archive.org/details/einsteintheoryr00einsgoog)
as well.

On page 22, Lorentz states a 3rd experimental test for the theory, "If his
(Einstein) theory is correct as it stands, there ought, in a gravitational
field, to be a displacement of the lines of the spectrum towards the red." He
adds, "No such effect has been discovered. ... there is no way of accounting
for this failure if Einstein's theory in its present form is assumed."

He goes on to say that some modification would be necessary. What eventually
happened to this 3rd test?

~~~
whatshisface
Here's the wikipedia page for the test you're talking about:
[https://en.wikipedia.org/wiki/Gravitational_redshift](https://en.wikipedia.org/wiki/Gravitational_redshift)

> _The first accurate measurement of the gravitational redshift of a white
> dwarf was done by Popper in 1954, measuring a 21 km /sec gravitational
> redshift of 40 Eridani B._

The effect has been seen in experiments. (See the section on experimental
verification.)

------
rpedela
Does the galaxy spin correctly according to our current understanding of
gravity?

~~~
sampo
Looks like. So this would be the first galaxy that actually seems to rotate
following the laws of gravity as we know them.

 _The data on the 10 globular clusters the team tracked showed them moving
much more slowly than would be expected. That led to an estimated mass that
was extremely low for a galaxy—on the order of 10^8 solar masses. Using the
amount of light emitted by the galaxy produced an estimate of the total mass
of stars in the galaxy that was also in the neighborhood of 10^8. Normally, we
infer that there 's dark matter around because the galaxy appears to have a
lot more matter than the amount provided by the stars we can see. But in this
case, there's a minimal difference between the two._

[https://arstechnica.com/science/2018/03/galaxy-seems-to-
lack...](https://arstechnica.com/science/2018/03/galaxy-seems-to-lack-dark-
matter-stumping-astronomers/)

Edit: 10^8, not 108.

~~~
Zamicol
That's 10^8, not 108.

You made me check the definition of a galaxy.

------
wuliwong
First of all, this is super cool but I'm confused about these sentences:

>The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow
separable from galaxies. This is only expected if dark matter is bound to
ordinary matter through nothing but gravity.

Why is this the only explanation? Was this a prediction of dark matter
theorists that they will find galaxies without any dark matter and that proves
it only interacts via gravity?

Lately, I've pondered that the observed behavior dark matter explains could be
the result of a new force that only has an appreciable effect in regimes of
extremely high mass/energy. Or possibly gravity is more complex than predicted
by GR. Just like newtonian gravity needed to be extended by GR possibly GR
still needs further extension to account for unexpected behavior in these
extreme mass/energy regimes? If all the forces are ultimately just one force
than these two ideas I've been thinking about are essentially the same.

I would assume there has been some work exploring this avenue of thought? Has
it been fruitless?

~~~
enkid
The idea of modifying general relativity is called as MOND and has been
thoroughly investigated by physicists. There are still proponents, but I think
the models are not matching the observational data. Also, the fact that
Galaxies with similar (visible) masses have different levels of rotational
momentum seems to hint that dark matter is a thing. I don't understand why
people think having something that interacts with fewer forces than normal
matter is more complicated than Jerry rigging a change to the elegant general
relativity.

~~~
noetic_techy
MOND is only one theory in a whole class of modified gravity theories:

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

------
nonbel
> _" Based on these data the team discovered that NGC 1052-DF2 larger than the
> Milky Way, but contains about 250 times fewer stars, leading it to be
> classified as an ultra diffuse galaxy."_

Interesting, the mass discrepancy (ie, "dark matter") has been noted to be
proportional to the predicted acceleration due to the visible mass (according
to Newtonian mechanics): [https://arxiv.org/abs/astro-
ph/0403610](https://arxiv.org/abs/astro-ph/0403610)

How does this galaxy fit into that relationship? If the galaxy is ultra
diffuse it sounds like very little deviation would be predicted, but I am no
expert here.

Edit:

To clarify since the relationship is somewhat tortured: According to my
understanding this observation sounds surprising in light of dark matter, but
100% consistent with what MOND proponents have been saying. I'd love to hear
from someone who knows more.

~~~
marcosdumay
Honestly, MOND proponents seem to be saying everything and its inverse.

What is not really a complaint, because they should be looking everywhere. But
yes, that will invalidate some MOND theories, while keeping other ones
unharmed, and will be evidence of some other set of them. And I don't think
anybody should be surprised by any of that.

(But I'm no expert either, so if one wants to correct me, it will be welcome.)

~~~
nonbel
I could be a MOND proponent only in that "dark matter" looks exactly like
epicycles to me. I make no claims to understand the details of MOND, so please
do not blame them for any of my errors.

That said, what do you mean by "MOND proponents seem to be saying everything
and its inverse"?

~~~
marcosdumay
There are many MOND theories, and you will find some that fit almost any
observation you can imagine.

~~~
nonbel
The paper considers the class of MOND theories that made a precise prediction
of deltaV = 20 km/s for this galaxy. I guess I am referring to only those.

------
mehrdadn
> missing most — if not all — of its dark matter

Is it just me or does this seem a bit like saying a landline is a wired cell
phone? I thought dark matter represented what's missing, so if it's not there
then nothing is missing, right?

~~~
teraflop
We know that dark matter makes up a large fraction of the universe's overall
mass. And we observe dark matter (indirectly) in just about every galaxy we
look at. This makes sense, since we believe that galaxies are formed when
local concentrations of matter collapse under their own gravity; you'd expect
this process to affect both types of matter the same way. When we see a galaxy
with no dark matter, it suggests that something unusual is going on, in a way
that affects dark matter differently.

~~~
lugg
> We know that dark matter makes up a large fraction of the universe's overall
> mass.

Do we?

I thought it was that we know a significant amount of mass is missing from our
observations and we explain it with dark matter.

Dark matter to me is a crutch to explain things until we really know.

Is it not at all possible our understanding is just wrong and that nothing is
missing?

~~~
philwelch
It's possible, but I think it's worthwhile to examine the intuitions that so
many people have that leads them to dismiss the notion of dark matter, or at
least the more "exotic" sounding theories like WIMPs.

We know that every particle we've been able to track down and verify the
existence of responds to the gravitational force. We also know that some
particles (like neutrinos) have virtually no interaction with anything other
than gravitational force, to the point where the vast majority of them will
pass through the entire earth undetected and having virtually no effect on
anything or anyone.

The notion of particles that are even _harder_ to detect than neutrinos
doesn't seem that absurd. It's kind of anthropocentric to assume that most of
the mass in the universe is going to be the same kind of mass that we are, or
that we are at least capable of easily observing.

And think about what it would imply if there _were_ no dark matter. It would
almost imply that, as of the late 20th century, humanity finally had a basic
comprehension of _all_ of material existence.

The universe that humanity understood before Copernicus and Galileo is less
than 1% of what we can see now. If our math is right and there really is
enough dark matter and dark energy to throw off our models, then the universe
we can see now is about 5% of everything that exists. That seems pretty
plausible for a civilization that can barely get out of its own atmosphere.

------
sudosteph
Woah!

I found this article from 2007 where Hubble found a "ring" of dark matter that
seemed to ripple out from a collision :
[https://www.nasa.gov/mission_pages/hubble/news/dark_matter_r...](https://www.nasa.gov/mission_pages/hubble/news/dark_matter_ring_feature.html)

Could this galaxy be an example of what the galaxy with the "ring" might look
like some day far in the future? Fascinating stuff.

------
mrfusion
Wouldn’t this finding mean we can rule out modified gravity theories?

if Modified gravity were real wouldn’t all galaxies have to show the effects?

~~~
noetic_techy
What about lack of super massive black hole singularity?

~~~
hannasanarion
Smbcs are tiny on the scale of galaxies. You can measure their mass, they have
a negligible impact on the rest of the galaxy.

Smbcs exist because crap tends to fall towards the middle of galaxies and pile
up. Galaxies do not exist because of SMBCs

~~~
noetic_techy
I'm not even talking about their mass gravitation, I'm saying its possible
that central singularity has some sort of unknown gravitational effect that
ripples outward. When you talk about dark matter, you cant limit yourself to
conventional gravitation theory. Maybe the SMB's are the heavy neutrino/WIMP
source just as our sun is the source of the neutrino's we detect. We still
have no clue what happens at the center of these singularities.

------
nogbit
Can you imagine being in a civilization in that galaxy, they must really think
they are the center of the universe.

~~~
AlphaWeaver
Sounds like a cause for the Krikkit Wars...

------
CelestialTeapot
Interesting, though I wonder if using multiple $1k lenses is as cost effective
as buying one 16" telescope (<$3k) and a sensor/camera ($1k?) that would give
~3x more light gathering capacity than the array of 24 lenses.

------
viraptor
> local Universe

What? What's the alternative of non-local universe that Hubble could possibly
see?

~~~
mentalpiracy
I believe the headline is using local in the plainest sense of proximity. NGC
1052-DF2, the galaxy in question here, is only 65 million light-years away.
That's not terribly far away, cosmically speaking.

------
robbomacrae
Dumb question here but how do we know dark matter isn't just regular ol matter
that is inside the event horizon of black holes? It's something I've always
wondered and this discovery reminded me of it..

~~~
arduinomancer
I’m pretty sure that one piece of evidence for dark matter is that galaxies
with dark matter have flat rotation curves. This means as you move outwards
from the centre of the galaxy, it turns out counterintuitively that objects
have the same orbital speed. Because of Kepler’s laws, this implies that each
farther orbit is encompassing more and more mass to give the farther object
the same speed as the inner ones. Dark matter is spread out through the whole
galaxy extending far out past the luminous matter like stars. If this mass was
all in the centre inside the event horizon like you’re saying, the rotation
curve of the galaxy would not be flat. The orbital speeds would drop off like
they do in our solar system.

------
crimsonalucard
I have a noob question. How do astronomers know that the stuff they don't see
is this exotic "dark matter" rather than just regular matter that their
telescopes can't see?

~~~
cozzyd
Microlensing excludes large unseen objects and the CMB power spectrum suggests
most matter is nonbaryonic.

~~~
crimsonalucard
Hi just curious. What is nonbaryonic matter and what is the CMB power
spectrum?

Also doesn't microlensing involve gravity? Thus the effects of dark matter on
microlensing would be identical to regular unseen matter which also interacts
with gravity in the same way? What's the difference?

~~~
cozzyd
The CMB is the cosmic microwave background, essentially the earliest light in
the universe from when atoms are formed. The power spectrum of the temperature
fluctuations in the CMB contains information about the constituents of the
universe because each constituent (radiation, baryonic matter, non-baryonic
matter) behaves differently in the early universe. Here is a Scientific
American article that offers some explanation:
[http://background.uchicago.edu/~whu/SciAm/sym1.html](http://background.uchicago.edu/~whu/SciAm/sym1.html)

Microlensing involves gravity. The difference is that normal matter that could
be dark matter would be relatively things massive but compact things like
black holes and brown dwarfs as opposed to a diffuse halo. The compactness
would make them detectable by lensing.

------
nerfhammer
so if dark matter effects are missing from tiny galaxies is that evidence that
dark matter is not undetectable massy particles but that gravity just behaves
differently at huge scales?

~~~
Avshalom
Honestly it's probably _more_ evidence for dark matter; even tiny galaxies are
large enough that MOND formulations should have effects.

------
erikpukinskis
James Webb James Webb James Webb

------
kraig911
It must be heaven.

------
StanislavPetrov
Seems like just further confirmation that we actually don't know how gravity
works. Inventing some magic modifier that makes our theories (and formulas
based on those theories) "work" (dark matter) was always rather absurd.

~~~
InclinedPlane
It's not "magic" it's a model that fits the observations: otherwise known as
science. Let's be clear here, every other model does not explain the
observations, by a pretty significant margin. Moreover, the concept of weakly
interacting massive particles is not outlandish, we already know of some
particles that behave very similarly (neutrinos).

~~~
StanislavPetrov
Except dark matter isn't an observation. Its an unobserved variable that's
been inserted to make models that don't match observations add up.

~~~
InclinedPlane
Dark matter is a theory that fits the observations. Prior to the 1980s nobody
had ever been able to image a single atom as well, atoms are just theories
that fit the observations. Proton-proton fusion has never been observed
directly experimentally, and yet we confidently say that it is the primary
method of energy generation inside our own Sun and many other stars, because
it's the only theory that fits the observations. This is how science works.

~~~
StanislavPetrov
>Dark matter is a theory that fits the observations.

You are mistaking "theory" and "hypothesis". Dark matter is a hypothesis. It
is unproven, it has not been observed. Describing this as a "galaxy devoid of
dark matter" rather than "a galaxy where the dark matter hypothesis does not
add up" is absurd. There is a difference between speculation about potential
causes for observed phenomena and observed phenomena themselves.

~~~
kolinko
Au contraire - in that galaxy the dark matter hypothesis really adds up. It's
existence kills theories that "dark matter" is just an artefact of how gravity
works.

------
chinmayatyc
Crazy uniformed but fun theory !: Quantum theory suggests that universes split
when a decision is taken. This is my crude understanding of parallel universe
theory. So, while universe splits, it creates a superimposing copy of itself
where decision is different. Like a git repository, where only changes are
saved, instead of copying everything. Gravity however continues to interact
with this changes and so we observe the dark matter. If their is any weight to
this theory,: 1\. We can say where changes occurred 2\. We can predict dark
matter 3\. We might be able to jump to parallel universe as we know where they
are! Only if we can solve how gravity exactly works!

