13 ms·

New class of galaxy has been discovered, made almost entirely of dark matter

288 points by daegloe 10y ago

Florin_Andrei 10y ago
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.
- niels_olson 10y ago
  Let's just hope the cables were all connected correctly. https://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly https://en.wikipedia.org/wiki/Faster-than-light_neutrino_ano...
- mordocai 10y ago
  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.
  - burkaman 10y ago
    I think dark matter is a synonym for "undetectable-through-normal-means matter". It's just a description of something mysterious causing effects that some kind of matter would cause. It could be a mix of different kinds of undetectable matter, or some fundamental flaw in our understanding of matter and gravity, or something else. The arguments against aliens and mistakes are presumably that the effects appear consistently across the whole visible universe, and through many different measurements.
  - krastanov 10y ago
    "undetectable-through-normal-means matter" is by definition not normal matter. We do not know what it is, so we have a placeholder name for it: "dark matter", i.e. matter that does not shine in the EM spectrum. We have theories about what it is, but most of the mundane ideas (like cold dust) have been ruled out by observations. Now we are left with all these stranger ideas. "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."
    - adwn 10y ago
      > "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth." Unless you made a mistake in your elimination, which is sometimes more probable than the apparently single remaining truth. Not saying this is the case here, just that this basic principle should be used with great care.
  - empath75 10y ago
    > undetectable-through-normal-means normal matter I assume by this you mean dust and planets and such. https://medium.com/starts-with-a-bang/could-dark-matter-just-be-normal-stuff-thats-dark-5e680e7a066e#.xnhcgnbfi https://medium.com/starts-with-a-bang/could-dark-matter-just... That also goes for 'aliens', because presumably, they'd also be made of normal matter, and if they're not, then we're back to some exotic form of matter like dark matter. As far as being a mistake, it could be possible that we are fundamentally misunderstanding something about how gravity works. That would be as interesting a result as dark matter, to be honest.
  - Florin_Andrei 10y ago
    The only thing we're sure it exists is - there's something out there that acts exactly like normal matter with mass, and it interacts gravitationally with itself and with regular matter, but it doesn't seem to interact in any other way. That's it. The name "dark matter" is a bit optimistic, because we don't even know if it's anything like "matter". It's not a fluke in the equations, it's not a fifth fundamental force. All these alternatives have been shown to be very unlikely. It's not "normal" matter because that one would interact in ways other than gravity, too, and it would tend to make "clumps" (stars, planets). This one doesn't make clumps. So, yes, it's pretty certain that it exists, but nobody knows what it is. Its only attribute observed so far is its mass. Maybe we should call it "dark mass" instead.
    - auvrw 10y ago
      upvoted, and not disagreeing (b/c i know nothing a/b this) -- would like a similarly semisimple explain on how "interaction" thru gravity alone makes any sense at all in the relativity framework: light no longer travels along lines of zero acceleration? Edit: on second thought, a book reference is probably better
      - mikeash 10y ago
        In relativity, gravity isn't a force that causes acceleration, it's a change in the shape of space which changes the path objects take. Thus, gravity affects the path light takes, but that's still a "straight line," because what gravity is really affecting is what a straight line is.
        effingwewt 10y ago
        This is my favorite explanation of this, mind if I forward this to an old Astronomy Professor? I think he'd love this too.
        mikeash 10y ago
        Sure! Just beware that I'm far from an expert and may well have gotten something hilariously wrong. If he rips it to shreds, please let me know what he had to say.
  - semaphoreP 10y ago
    Basically, dark matter is something that has gravitational pull, but shows no other evidence of its existence. We have seen evidence for dark matter in galaxy rotation curves, galaxy cluster interactions, and the cosmic microwave background fluctuations, so we know it's not a mistake (each of these measurements are independent and do not share common systematic errors). We know it's not normal matter (e.g. black holes), because we at least somewhat understand the physics of how normal matter works, and we would expect to see something observationally if it is just normal matter. Since it is not normal matter, using Occam's razor, we can only postulate it is something that has gravitational pull but doesn't interact otherwise. Perhaps once we figure out how to study dark matter, we can tease out some of its properties and it won't just be all lumped together as "dark matter".
    - netcraft 10y ago
      There was a recent idea that small, primordial black holes might be related to dark matter http://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter http://www.nasa.gov/feature/goddard/2016/nasa-scientist-sugg... Basically the idea that theyre smaller and more common than previously expected - it would be really hard to observe them if they weren't in-between us and something else or snacking on something.
  - symmetricsaurus 10y ago
    Dark matter can explain: the large scale evolution of the universe (together with dark energy but whatever), rotational velocities of galaxies, gravitational lensing and other phenomena. No other theory explains all of these things as well or simply. Also, dark matter is not really so exotic and weird. We already know of one type of dark matter particle, neutrinos. To think that there are other more massive particles that only interact through gravity and the weak force isn't much of a strectch in that light.
    - gpderetta 10y ago
      Exactly, in fact the hypothetical massive sterile neutrinos, which are part of one of the possible explanations for the neutrino masses, are one of the dark matter candidates.
  - T0T0R0 10y ago
    Exoplanets are dark matter. They do not emit light, so they are dark. You can't see an exoplanet, you have to look for stars that wobble. Asteroid belts are also dark matter. If a remote star has an asteroid belt, it wouldn't be easily detectable, since it probably wouldn't elicit the kepler-style wobbles and flickers, or if it did, perhaps less obvious/detectable events. Based on this, the astronomical fad terminology is flawed, since it seems to claim that only stars matter because only stars are matter (and the luminous gas of nebulae too, of course). The layperson finds intrigue in the term Dark Matter, because journalists are trying to sell a story.
    - netcraft 10y ago
      im pretty sure this isnt true - if any of those things were massive enough to substitute for the gravitational effects that have been observed they would emit infrared radiation at least.
      - T0T0R0 10y ago
        So go ahead and observe this all-too-obvious infrared, that exoplanets must surely emit. Meanwhile, their gravity is now well known to induce wobble on their parent stars, which are much more luminous, and probably outshines any exoplanet in the infrared.
        GFK_of_xmaspast 10y ago
        On the other hand, this has already been done: https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla...
        T0T0R0 10y ago
        This method works best for young planets *that emit infrared light* and are far from the glare of the star. In other words, if they are still swirling balls of liquid magma. So, NOT dark matter.
  - mikeash 10y ago
    "Dark matter" just means "stuff we can't detect except by gravity" so that covers your first case. There's a theory that "dark matter" isn't matter at all, but some modification to the currently accepted laws of physics. Wikipedia lists some of these ideas: https://en.wikipedia.org/wiki/Dark_matter#Alternative_theories https://en.wikipedia.org/wiki/Dark_matter#Alternative_theori... Ultimately, it's generally assumed to be some sort of actual matter because that's what best fits the observations. No alternative explanation has been raised that better fits the observations than the explanation that it's some unknown form of matter that doesn't interact with electromagnetic radiation very much, but still participates in gravity. As for what those observations are, galaxies are observed to rotate much differently than if they only had the mass that was visible, galaxies cluster as if there was more mass than can be seen, gravitational lensing indicates that galaxies mass more than what's observed with light, the cosmic microwave background spectrum is different from what you'd expect from only the stuff we can see, and a fair bit more mentioned on that Wikipedia page. Keep in mind that any alternative explanation needs to not only explain the fact that these things are different, but also the exact quantity by which they are different. So far, "dark matter" is the best explanation anyone's come up with.
    - orf 10y ago
      > some unknown form of batter Mmhhhh... I'm pretty hungry right now and that gave me some wonderful imagery.
      - mikeash 10y ago
        Oopsie, stupid hands. Fixed it.
    - sandworm101 10y ago
      I would call 'dark matter' a description rather than an explanation. It seems to effect gravity. So it has mass. So it's 'matter'. But other than that it is unobserved, and so 'dark'. A+B = a very short but accurate description of an observation, rather than an explanation of why that observation happens.
  - tomr_stargazer 10y ago
    (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#Detection https://en.wikipedia.org/wiki/Massive_compact_halo_object#De... I think the wikipedia Dark Matter article is actually super well written and should address these issues, too!
    - semi-extrinsic 10y ago
      Regarding the "no massive dark bodies" result: does this also cover dark dust? Or is that otherwise ruled out (perhaps by optical density)?
      - maverick_iceman 10y ago
        Baryonic dust is ruled out by BBN.
      - okket 10y ago
        If you have four times of visible matter as "dark dust", this dust would clump together, creating stars, becoming visible matter again.
      - tomr_stargazer 10y ago
        Additional dust inside a given galaxy would both absorb optical light and emit infrared light (depending on its temperature, which is set by the Galaxy's ambient starlight among other factors); both effects are readily detectable.
  - maverick_iceman 10y ago
    Big Bang Nucleosynthesis (BBN) says that the amount of baryonic (i.e. usual) matter cannot be much higher than what we directly observe. However, it accounts for only ~16% of the mass necessary to account for galactic rotation curves, gravitational lensing by galaxies etc. So the leftover must be non-baryonic dark matter. One can of course say that BBN is wrong but that's very unlikely as it has been tested to high precision from CMB study.
  - grahamburger 10y ago
    > How do we know it isn't undetectable-through-normal-means normal matter As I understand this is kind of what we think it is, which is why it's called 'dark.'
  - stouset 10y ago
    > 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? "Dark matter" is just a name for "a phenomenon that only appears to interact with the rest of the universe through gravitational fields, and not through elecromagnetic fields, the weak, or the strong nuclear force". There are many competing theories for what dark matter actually "is". The name is independent of any particular theory that describes its true nature, whether that's an exotic form of matter, aliens, or mistakes in our observation.
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  - SmellTheGlove 10y ago
    > undetectable-through-normal-means normal matter I'm not a physicist, but my understanding was that dark matter is undetectable-through-normal-means because it doesn't interact with normal matter. If it were undetectable-through-normal-means normal matter, it would be detectable through normal means.
  - yincrash 10y ago
    Matt O'Dowd, an astrophysics professor, hosts a YouTube series called PBS Space Time that does a really effective job if explaining the evidence of dark matter in this episode: https://youtu.be/z3rgl-_a5C0 https://youtu.be/z3rgl-_a5C0
- the8472 10y ago
  It's more localized, but we've seen similar things before at a larger scale, e.g. the bullet cluster. https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster
- jomamaxx 10y ago
  " Now astronomers can study dark matter with less interference from the regular matter background" There is no such thing as 'Dark Matter' yet. Would you people please stop talking about it as though it exists? It is, at this stage, a very crude idea. There is no evidence of its existence. We assume 'Dark Matter' exists because our relativistic equations are broken and it's the easiest thing we can imagine to 'fix' the problem - and we conveniently use the theory to make up for some other flaws as well. There could be many other reasons or characterizations for those phenomena. Maybe we should wait until there are some nice direct experiments that strongly confirm that the 'nothing there' is actually 'something' :) If fear 'Dark Matter' is the 21st century equivalent of 'aether'.
  - Retra 10y ago
    >It is, at this stage, a very crude idea. It is an idea. Ideas exist. It does exist. There's nothing here for you to argue with.
    - jomamaxx 10y ago
      A) Yeah, uh, in Science, when we refer to 'things that exist', we usually mean 'materially' and not 'in fantasy'. So no, that it is an 'idea' does not mean 'it exists'. B) Prove to me that dark matter exists. Characterize it. You can't. The only thing we know is that our equations for gravity don't work, and it would be 'nice' if there were this thing called 'dark matter' out there because it would fit nicely with what we previously understand. So - there's plenty for me to argue with. C) Dark Matter/Energy theory basically states the Universe is made up of 96% of this interesting material we don't know for sure exists, we have no direct evidence of it, and we really can't characterize it very well. When a principle is '96% wrong' - maybe it would be better to question the very nature of the principle, instead of trying to fit it to observation? It's highly possible that our understanding of gravity is just plain wrong. That we can make some inferences in our locality, but in the grand scheme it just falls apart. Obviously, it is wrong to the tune of 96%. Which is not good. There's going to be a lot to 'argue about' with Dark Matter/Energy for the next couple of generations at least.
teh_klev 10y ago
Archive.is linkage for those who can't get past the paywall: http://archive.is/b4e7X http://archive.is/b4e7X
semaphoreP 10y ago
Here is the pre-print version of the paper: https://arxiv.org/abs/1606.06291 https://arxiv.org/abs/1606.06291
pc2g4d 10y ago
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?
- tombone12 10y ago
  Not everything can be automated, but there is certainly at least some machine learning in all of the larger surveys these days. As to dark matter surveys explicitly, I found this one: http://kids.strw.leidenuniv.nl/pr_july2015.php http://kids.strw.leidenuniv.nl/pr_july2015.php
- gus_massa 10y ago
  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 10y ago
    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 10y ago
Exras 10y ago
"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 :-)
SolarNet 10y ago
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 10y ago
  No infrared. Already ruled out by earlier infrared surveys looking for Dyson spheres.
  - drjesusphd 10y ago
    Those were only looking in our stellar neighborhood though, right? Would a galaxy of infrared "stars" be detectable?
    - wcoenen 10y ago
      It should be detectable, but the Ĝ Infrared Search[1] didn't find any in the WISE[2] data. Jason Wright talked about this and related topics in a SETI institute video[3]. [1] https://arxiv.org/abs/1408.1134 https://arxiv.org/abs/1408.1134 [2] https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Exp... [3] https://www.youtube.com/watch?v=XEDR-G2EDRM https://www.youtube.com/watch?v=XEDR-G2EDRM
  - bjelkeman-again 10y ago
    We often seem to assume we know "everything", and we often find out there is more to learn. It would be an interesting universe if we found dark matter to be ancient civilisations. Using up much of the mass and energy (even the infrared).
    - hinkley 10y ago
      Oh stop. This is such an unlikely avenue of inquiry that it just distracts from the conversation. If at some point we figure out that the laws of thermodynamics do not apply, that would be such a profound change in how we interact with the universe that speculation over how to best search the night sky for intelligent life would be the last thing on most people's minds for quite some time. We would quickly find ways to harness quantities of energy that would fry us to a literal crisp today due to conversion losses, and that would be such a breakthrough that rather than look for signs of life in nearby systems, we could just go there and look around.
perseusprime11 10y ago
I always thought that dark matter is a trick played by gravity. I wish we can get to these things faster.
dghughes 10y ago
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 10y ago
  If they can't interact with normal matter or light, we're the dark matter to them.
hinkley 10y ago
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 10y ago
  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!)
geertj 10y ago
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?
officialjunk 10y ago
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).
JackFr 10y ago
It's Dyson spheres.
- JackFr 10y ago
  Not sure I get the downvotes. Because I'm not elevating the discussion? I was trying to be funny.
wallace_f 10y ago
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 10y ago
  I don't think anything in your post makes the least bit of sense.
  - asimuvPR 10y ago
    Perhaps you could tell why?
    - vecter 10y ago
      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
Snesker 10y ago
New galaxies made of an unproven concept! Do you seriously believe this shit?
zouhair 10y ago
Dark Matter is really a bad name. Too confusing.
JumpCrisscross 10y ago
How do we rule out dust between the galaxy and us in cases like this?
- okket 10y ago
  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.

288 points by daegloe 10y ago

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.

niels_olson 10y ago
Let's just hope the cables were all connected correctly. https://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly https://en.wikipedia.org/wiki/Faster-than-light_neutrino_ano...
mordocai 10y ago
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.
- burkaman 10y ago
  I think dark matter is a synonym for "undetectable-through-normal-means matter". It's just a description of something mysterious causing effects that some kind of matter would cause. It could be a mix of different kinds of undetectable matter, or some fundamental flaw in our understanding of matter and gravity, or something else. The arguments against aliens and mistakes are presumably that the effects appear consistently across the whole visible universe, and through many different measurements.
- krastanov 10y ago
  "undetectable-through-normal-means matter" is by definition not normal matter. We do not know what it is, so we have a placeholder name for it: "dark matter", i.e. matter that does not shine in the EM spectrum. We have theories about what it is, but most of the mundane ideas (like cold dust) have been ruled out by observations. Now we are left with all these stranger ideas. "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."
  - adwn 10y ago
    > "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth." Unless you made a mistake in your elimination, which is sometimes more probable than the apparently single remaining truth. Not saying this is the case here, just that this basic principle should be used with great care.
- empath75 10y ago
  > undetectable-through-normal-means normal matter I assume by this you mean dust and planets and such. https://medium.com/starts-with-a-bang/could-dark-matter-just-be-normal-stuff-thats-dark-5e680e7a066e#.xnhcgnbfi https://medium.com/starts-with-a-bang/could-dark-matter-just... That also goes for 'aliens', because presumably, they'd also be made of normal matter, and if they're not, then we're back to some exotic form of matter like dark matter. As far as being a mistake, it could be possible that we are fundamentally misunderstanding something about how gravity works. That would be as interesting a result as dark matter, to be honest.
- Florin_Andrei 10y ago
  The only thing we're sure it exists is - there's something out there that acts exactly like normal matter with mass, and it interacts gravitationally with itself and with regular matter, but it doesn't seem to interact in any other way. That's it. The name "dark matter" is a bit optimistic, because we don't even know if it's anything like "matter". It's not a fluke in the equations, it's not a fifth fundamental force. All these alternatives have been shown to be very unlikely. It's not "normal" matter because that one would interact in ways other than gravity, too, and it would tend to make "clumps" (stars, planets). This one doesn't make clumps. So, yes, it's pretty certain that it exists, but nobody knows what it is. Its only attribute observed so far is its mass. Maybe we should call it "dark mass" instead.
  - auvrw 10y ago
    upvoted, and not disagreeing (b/c i know nothing a/b this) -- would like a similarly semisimple explain on how "interaction" thru gravity alone makes any sense at all in the relativity framework: light no longer travels along lines of zero acceleration? Edit: on second thought, a book reference is probably better
    - mikeash 10y ago
      In relativity, gravity isn't a force that causes acceleration, it's a change in the shape of space which changes the path objects take. Thus, gravity affects the path light takes, but that's still a "straight line," because what gravity is really affecting is what a straight line is.
      - effingwewt 10y ago
        This is my favorite explanation of this, mind if I forward this to an old Astronomy Professor? I think he'd love this too.
        mikeash 10y ago
        Sure! Just beware that I'm far from an expert and may well have gotten something hilariously wrong. If he rips it to shreds, please let me know what he had to say.
- semaphoreP 10y ago
  Basically, dark matter is something that has gravitational pull, but shows no other evidence of its existence. We have seen evidence for dark matter in galaxy rotation curves, galaxy cluster interactions, and the cosmic microwave background fluctuations, so we know it's not a mistake (each of these measurements are independent and do not share common systematic errors). We know it's not normal matter (e.g. black holes), because we at least somewhat understand the physics of how normal matter works, and we would expect to see something observationally if it is just normal matter. Since it is not normal matter, using Occam's razor, we can only postulate it is something that has gravitational pull but doesn't interact otherwise. Perhaps once we figure out how to study dark matter, we can tease out some of its properties and it won't just be all lumped together as "dark matter".
  - netcraft 10y ago
    There was a recent idea that small, primordial black holes might be related to dark matter http://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter http://www.nasa.gov/feature/goddard/2016/nasa-scientist-sugg... Basically the idea that theyre smaller and more common than previously expected - it would be really hard to observe them if they weren't in-between us and something else or snacking on something.
- symmetricsaurus 10y ago
  Dark matter can explain: the large scale evolution of the universe (together with dark energy but whatever), rotational velocities of galaxies, gravitational lensing and other phenomena. No other theory explains all of these things as well or simply. Also, dark matter is not really so exotic and weird. We already know of one type of dark matter particle, neutrinos. To think that there are other more massive particles that only interact through gravity and the weak force isn't much of a strectch in that light.
  - gpderetta 10y ago
    Exactly, in fact the hypothetical massive sterile neutrinos, which are part of one of the possible explanations for the neutrino masses, are one of the dark matter candidates.
- T0T0R0 10y ago
  Exoplanets are dark matter. They do not emit light, so they are dark. You can't see an exoplanet, you have to look for stars that wobble. Asteroid belts are also dark matter. If a remote star has an asteroid belt, it wouldn't be easily detectable, since it probably wouldn't elicit the kepler-style wobbles and flickers, or if it did, perhaps less obvious/detectable events. Based on this, the astronomical fad terminology is flawed, since it seems to claim that only stars matter because only stars are matter (and the luminous gas of nebulae too, of course). The layperson finds intrigue in the term Dark Matter, because journalists are trying to sell a story.
  - netcraft 10y ago
    im pretty sure this isnt true - if any of those things were massive enough to substitute for the gravitational effects that have been observed they would emit infrared radiation at least.
    - T0T0R0 10y ago
      So go ahead and observe this all-too-obvious infrared, that exoplanets must surely emit. Meanwhile, their gravity is now well known to induce wobble on their parent stars, which are much more luminous, and probably outshines any exoplanet in the infrared.
      - GFK_of_xmaspast 10y ago
        On the other hand, this has already been done: https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla...
        T0T0R0 10y ago
        This method works best for young planets *that emit infrared light* and are far from the glare of the star. In other words, if they are still swirling balls of liquid magma. So, NOT dark matter.
- mikeash 10y ago
  "Dark matter" just means "stuff we can't detect except by gravity" so that covers your first case. There's a theory that "dark matter" isn't matter at all, but some modification to the currently accepted laws of physics. Wikipedia lists some of these ideas: https://en.wikipedia.org/wiki/Dark_matter#Alternative_theories https://en.wikipedia.org/wiki/Dark_matter#Alternative_theori... Ultimately, it's generally assumed to be some sort of actual matter because that's what best fits the observations. No alternative explanation has been raised that better fits the observations than the explanation that it's some unknown form of matter that doesn't interact with electromagnetic radiation very much, but still participates in gravity. As for what those observations are, galaxies are observed to rotate much differently than if they only had the mass that was visible, galaxies cluster as if there was more mass than can be seen, gravitational lensing indicates that galaxies mass more than what's observed with light, the cosmic microwave background spectrum is different from what you'd expect from only the stuff we can see, and a fair bit more mentioned on that Wikipedia page. Keep in mind that any alternative explanation needs to not only explain the fact that these things are different, but also the exact quantity by which they are different. So far, "dark matter" is the best explanation anyone's come up with.
  - orf 10y ago
    > some unknown form of batter Mmhhhh... I'm pretty hungry right now and that gave me some wonderful imagery.
    - mikeash 10y ago
      Oopsie, stupid hands. Fixed it.
  - sandworm101 10y ago
    I would call 'dark matter' a description rather than an explanation. It seems to effect gravity. So it has mass. So it's 'matter'. But other than that it is unobserved, and so 'dark'. A+B = a very short but accurate description of an observation, rather than an explanation of why that observation happens.
- tomr_stargazer 10y ago
  (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#Detection https://en.wikipedia.org/wiki/Massive_compact_halo_object#De... I think the wikipedia Dark Matter article is actually super well written and should address these issues, too!
  - semi-extrinsic 10y ago
    Regarding the "no massive dark bodies" result: does this also cover dark dust? Or is that otherwise ruled out (perhaps by optical density)?
    - maverick_iceman 10y ago
      Baryonic dust is ruled out by BBN.
    - okket 10y ago
      If you have four times of visible matter as "dark dust", this dust would clump together, creating stars, becoming visible matter again.
    - tomr_stargazer 10y ago
      Additional dust inside a given galaxy would both absorb optical light and emit infrared light (depending on its temperature, which is set by the Galaxy's ambient starlight among other factors); both effects are readily detectable.
- maverick_iceman 10y ago
  Big Bang Nucleosynthesis (BBN) says that the amount of baryonic (i.e. usual) matter cannot be much higher than what we directly observe. However, it accounts for only ~16% of the mass necessary to account for galactic rotation curves, gravitational lensing by galaxies etc. So the leftover must be non-baryonic dark matter. One can of course say that BBN is wrong but that's very unlikely as it has been tested to high precision from CMB study.
- grahamburger 10y ago
  > How do we know it isn't undetectable-through-normal-means normal matter As I understand this is kind of what we think it is, which is why it's called 'dark.'
- stouset 10y ago
  > 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? "Dark matter" is just a name for "a phenomenon that only appears to interact with the rest of the universe through gravitational fields, and not through elecromagnetic fields, the weak, or the strong nuclear force". There are many competing theories for what dark matter actually "is". The name is independent of any particular theory that describes its true nature, whether that's an exotic form of matter, aliens, or mistakes in our observation.
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- SmellTheGlove 10y ago
  > undetectable-through-normal-means normal matter I'm not a physicist, but my understanding was that dark matter is undetectable-through-normal-means because it doesn't interact with normal matter. If it were undetectable-through-normal-means normal matter, it would be detectable through normal means.
- yincrash 10y ago
  Matt O'Dowd, an astrophysics professor, hosts a YouTube series called PBS Space Time that does a really effective job if explaining the evidence of dark matter in this episode: https://youtu.be/z3rgl-_a5C0 https://youtu.be/z3rgl-_a5C0
the8472 10y ago
It's more localized, but we've seen similar things before at a larger scale, e.g. the bullet cluster. https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster
jomamaxx 10y ago
" Now astronomers can study dark matter with less interference from the regular matter background" There is no such thing as 'Dark Matter' yet. Would you people please stop talking about it as though it exists? It is, at this stage, a very crude idea. There is no evidence of its existence. We assume 'Dark Matter' exists because our relativistic equations are broken and it's the easiest thing we can imagine to 'fix' the problem - and we conveniently use the theory to make up for some other flaws as well. There could be many other reasons or characterizations for those phenomena. Maybe we should wait until there are some nice direct experiments that strongly confirm that the 'nothing there' is actually 'something' :) If fear 'Dark Matter' is the 21st century equivalent of 'aether'.
- Retra 10y ago
  >It is, at this stage, a very crude idea. It is an idea. Ideas exist. It does exist. There's nothing here for you to argue with.
  - jomamaxx 10y ago
    A) Yeah, uh, in Science, when we refer to 'things that exist', we usually mean 'materially' and not 'in fantasy'. So no, that it is an 'idea' does not mean 'it exists'. B) Prove to me that dark matter exists. Characterize it. You can't. The only thing we know is that our equations for gravity don't work, and it would be 'nice' if there were this thing called 'dark matter' out there because it would fit nicely with what we previously understand. So - there's plenty for me to argue with. C) Dark Matter/Energy theory basically states the Universe is made up of 96% of this interesting material we don't know for sure exists, we have no direct evidence of it, and we really can't characterize it very well. When a principle is '96% wrong' - maybe it would be better to question the very nature of the principle, instead of trying to fit it to observation? It's highly possible that our understanding of gravity is just plain wrong. That we can make some inferences in our locality, but in the grand scheme it just falls apart. Obviously, it is wrong to the tune of 96%. Which is not good. There's going to be a lot to 'argue about' with Dark Matter/Energy for the next couple of generations at least.

teh_klev 10y ago

Archive.is linkage for those who can't get past the paywall: http://archive.is/b4e7X http://archive.is/b4e7X

semaphoreP 10y ago

Here is the pre-print version of the paper: https://arxiv.org/abs/1606.06291 https://arxiv.org/abs/1606.06291

pc2g4d 10y ago

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?

tombone12 10y ago
Not everything can be automated, but there is certainly at least some machine learning in all of the larger surveys these days. As to dark matter surveys explicitly, I found this one: http://kids.strw.leidenuniv.nl/pr_july2015.php http://kids.strw.leidenuniv.nl/pr_july2015.php
gus_massa 10y ago
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 10y ago
  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 10y ago

Exras 10y ago

"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 :-)

SolarNet 10y ago

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 10y ago
No infrared. Already ruled out by earlier infrared surveys looking for Dyson spheres.
- drjesusphd 10y ago
  Those were only looking in our stellar neighborhood though, right? Would a galaxy of infrared "stars" be detectable?
  - wcoenen 10y ago
    It should be detectable, but the Ĝ Infrared Search[1] didn't find any in the WISE[2] data. Jason Wright talked about this and related topics in a SETI institute video[3]. [1] https://arxiv.org/abs/1408.1134 https://arxiv.org/abs/1408.1134 [2] https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Exp... [3] https://www.youtube.com/watch?v=XEDR-G2EDRM https://www.youtube.com/watch?v=XEDR-G2EDRM
- bjelkeman-again 10y ago
  We often seem to assume we know "everything", and we often find out there is more to learn. It would be an interesting universe if we found dark matter to be ancient civilisations. Using up much of the mass and energy (even the infrared).
  - hinkley 10y ago
    Oh stop. This is such an unlikely avenue of inquiry that it just distracts from the conversation. If at some point we figure out that the laws of thermodynamics do not apply, that would be such a profound change in how we interact with the universe that speculation over how to best search the night sky for intelligent life would be the last thing on most people's minds for quite some time. We would quickly find ways to harness quantities of energy that would fry us to a literal crisp today due to conversion losses, and that would be such a breakthrough that rather than look for signs of life in nearby systems, we could just go there and look around.

perseusprime11 10y ago

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

dghughes 10y ago

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 10y ago
If they can't interact with normal matter or light, we're the dark matter to them.

hinkley 10y ago

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 10y ago
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!)

geertj 10y ago

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?

officialjunk 10y ago

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).

JackFr 10y ago

It's Dyson spheres.

JackFr 10y ago
Not sure I get the downvotes. Because I'm not elevating the discussion? I was trying to be funny.

wallace_f 10y ago

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 10y ago
I don't think anything in your post makes the least bit of sense.
- asimuvPR 10y ago
  Perhaps you could tell why?
  - vecter 10y ago
    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

Snesker 10y ago

New galaxies made of an unproven concept! Do you seriously believe this shit?

zouhair 10y ago

Dark Matter is really a bad name. Too confusing.

JumpCrisscross 10y ago

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

okket 10y ago
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.