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Hubble finds first galaxy in the local universe without dark matter

• rmcpherson 9y ago
  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.
  • cozzyd 9y ago
    "Strongest candidate" is a bit strong. Certainly WIMPs are convenient theoretically, but so are e.g. axions.
    • rmcpherson 9y ago
      I used "strongest candidate" not as my opinion (I don't know) but that is how physicists see the various possibilities. Certainly WIMP research has received the largest amount of funding in experimental physics as compared to other candidates.
      • cozzyd 9y ago
        (I am a physicist who used to work on a WIMP detector). WIMP funding has certainly gotten the largest share of funding. Part of the reason is probably that we know how to build detectors good enough to reach a swath of parameter space and the relative popularity of supersymmetry. I've heard some people suggest that part of the reason for funding such expensive WIMP experiments is that we know exactly where the limits of the detection technique lie and they are reachable given enough money.
  • noetic_techy 9y ago
    Why am I not surprised. As a engineering-physics major who works in software, I always correct people when they ask me "what do you think dark matter is?" I think the more appropriate question is, and always has been: Does dark matter even exist? Thank you for casting light on MOND. My money has always been on modified gravity. I think we missed a few terms in the equation.
    • dwaltrip 9y ago
      The comment that you are responding to says that this weakens the case for MOND.
      • noetic_techy 9y ago
        I saw that, it was more so a response to Fermi lab Physicist are taking MOND more seriously. MOND has its problems also. Its my opinion that there is something about the black hole at the center of these galaxies that's causing discrepancies that we attribute to "dark matter". The fact that this new galaxy is so sparse and distributed and shows no sign of dark matter seems to re-affirm this. I'd be curious to know if it even has a central black hole.
        • rmcpherson 9y ago
          To clarify, no one's taking MOND nearly as seriously as WIMPs in terms of funding for experiments to test it. That said, the lack of evidence has lead to increased interest in alternative theories that could explain the phenomenon. Some of these are alternative dark matter particle candidates though, just not WIMPs.
          • koheripbal 9y ago
            It's a pointless conversation the argue over which unproven theory is taken most seriously. The reality is that we don't know.
          • ravar 9y ago
  • autokad 9y ago
    I dont know much about it, but that may not necessarily be the case. Lets assume dark matter does not exist and the people talking about it believe it does. It could be that the people describing it are only describing it through the lens of what they already believe. As in, I believe there are invisible blobs that keep cars close together. I observe some cars far apart and i say "hey look at all that missing blobs"
    • anyfoo 9y ago
      That's missing the point entirely, which is that under theories like MOND, you would not expect to see any galaxies that behave like galaxies with no dark matter, as it posits that dark matter does not exist.
      • autokad 9y ago
        > "you would not expect to see any galaxies that behave like galaxies with no dark matter" you miss my point entirely, your explanation is around an assumption of dark matter. You are not saying this galaxy behaves in such a way that MOND cannot explain, you are saying the galaxy behaves in such a way that based on believing in dark matter, this galaxy moves as if it doesnt have it thus MOND is wrong.
        • rmcpherson 9y ago
          This finding certainly weakens the case for MOND. The authors write that in the conclusion of their paper, as quoted in this article: https://arstechnica.com/science/2018/03/galaxy-seems-to-lack-dark-matter-stumping-astronomers/ https://arstechnica.com/science/2018/03/galaxy-seems-to-lack... MOND can not explain a galaxy without dark matter as it is a proposal for the modification of the laws of gravity at large distances, and can explain the already-observed galaxies that behave as if they have dark matter (or, as MOND posits, the laws of gravity are different than what we have observed on earth.)
        • autokad 9y ago
          @rmcpherson I am not defending MOND or attacking dark matter. I am stating based on what they have said, it doesnt make sense unless they provide some basis on why its problematic for MOND
  • poelzi 9y ago
    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.
  • nonbel 9y ago
    >"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." What makes you say this?
    • sulam 9y ago
      The article sketches the explanation — if dark matter doesn’t actually exist and is simply an observational side effect, all galaxies should demonstrate that effect. That this galaxy doesn’t strengthens the case for dark matter being separable from regular (visible) matter.
      • nonbel 9y ago
        See my post here: https://news.ycombinator.com/item?id=16702599 https://news.ycombinator.com/item?id=16702599 From what I understand of MOND, the less attraction to visible mass (as you would expect from a "ultra diffuse" galaxy like this), the smaller the effect should be. So MOND predicts the effect should be relatively small for this galaxy. Further, in the paper that the OP references they do a calculation and get odds of something around 1/20 that a galaxy like this will exist if MOND is correct. The interesting part is that for some reason they interpret that as ruling out MOND, while meanwhile saying "no one has any idea how this thing can exist if the dark matter theory is right". I would think "no idea how it can exist" should correspond to odds less than 1/20.
        • sulam 9y ago
          This galaxy is down in the 1 in 100_000 parts, not 1 in 20 fwiw. I am not going to debate MOND, I am just explaining the reasoning.
          • nonbel 9y ago
            I drew the blue vertical line at 20 km/s: https://image.ibb.co/m38AH7/galaxy.png https://image.ibb.co/m38AH7/galaxy.png Are you claiming this is cutting off 1e-5 of the area of that Gaussian curve they fit? It looks very close to 1/20 to me. I don't care about .05, just to get an order of magnitude and compare it to the "this is an unbelievable result" quotes.
  • yiyus 9y ago
    I do not think this is an argument in favor or against the existence of dark matter. I think it is a very surprising result either way. Only thing we know is that this galaxy is a very special fluke. It will surely be difficult to explain by non-dark-matter theories, but I do not think dark matter advocates are going to have any easier to explain why this galaxy (and, as far as we know, only this galaxy) doesn't have any dark matter. It will be exciting to see what explanations both camps come up with.
    • simias 9y ago
      I'm no physicist or astronomer but as far as I can tell I don't think what you said makes complete sense. While it's true that it remains to be explained why a certain galaxy doesn't have the same amount of DM as the others at a glance it doesn't seem particularly implausible that such dramatic variation could theoretically happen depending on what DM truly is and how it ends up in galaxies in the first place. On the other hand you'd expect a modification to the theories of physics themselves like MOND would apply to all objects in the universe in the same way, if you find a galaxy where the model breaks down then the model is not good enough. That would be like finding a galaxy far away where gravity itself works differently, that'd be wild. So as far as I can tell it does seem to discredit something like MOND more than DM because you expect physical laws to be universal while it's plausible that DM could not be distributed equally everywhere.
• komali2 9y ago
  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 :(
  • zitterbewegung 9y ago
    Just try to dedicate some of your time with the basics. It really just takes you putting some time out of your day to understand. Also feeling dumb is part of the process .
  • nabla9 9y ago
    http://theoreticalminimum.com/courses http://theoreticalminimum.com/courses
  • nate_meurer 9y ago
    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!
• nonbel 9y ago
  >"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.
  • nonbel 9y ago
    I see the paper says: >"[MOND predicts] the expected velocity dispersion of NGC1052–DF2 is ... 20 km s−1, a factor of two higher than the 90% upper limit on the observed dispersion." http://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_attachment/317/van_Dokuum_paper_submitted_to_Nature.pdf http://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_... So they chose to use a 90% CI along with assuming the error is normally distributed, etc. Looking at figure 3b it seems that if they instead arbitrarily chose to use a 95% CI that 20 km/s would be within the interval. Ie, these results appear to be moderately unlikely if MOND were correct but extremely unlikely if "dark matter" is correct. Thus, according to Bayes' rule, the probability that MOND is correct has increased.
    • nonbel 9y ago
      Not sure where the downvotes are coming from. The reasoning that this evidence is moderately unlikely given that MOND is true can be found in the parent post. The authors worked it out to ~1/10 to 1/20. They don't put a number on how unlikely it would be given dark matter is true, but it sounds pretty extreme: >Merritt remarks: "There is no theory that predicts these types of galaxies — how you actually go about forming one of these things is completely unknown." http://www.spacetelescope.org/news/heic1806/ http://www.spacetelescope.org/news/heic1806/ >"And NGC 1052-DF2 appears to have none. Or at best, about as much dark matter as normal matter. And that just doesn't make sense." http://www.syfy.com/syfywire/what-is-this-galaxy-doing-without-a-dark-matter-halo http://www.syfy.com/syfywire/what-is-this-galaxy-doing-witho... The denominator of Bayes rule for Pr(MOND|data) is just the sum of all Pr(Theory) x Pr(data|Theory) where the main theory is "dark matter" and second is MOND (everything else is some small value epsilon), while the numerator is Pr(MOND) x Pr(data|MOND). Where is the mistake?
• viraptor 9y ago
  > local Universe What? What's the alternative of non-local universe that Hubble could possibly see?
  • mentalpiracy 9y ago
    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.
• decacorn 9y ago
  This blows my mind.
• mtreis86 9y ago
  Here is the article referenced. The link on spacetelescope isn't working. http://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_attachment/317/van_Dokuum_paper_submitted_to_Nature.pdf http://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_...
• acqq 9y ago
  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 9y ago
    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 9y ago
      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/FinalReport.pdf https://www.duo.uio.no/bitstream/handle/10852/52374/FinalRep...
• rpedela 9y ago
  Does the galaxy spin correctly according to our current understanding of gravity?
  • sampo 9y ago
    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-dark-matter-stumping-astronomers/ https://arstechnica.com/science/2018/03/galaxy-seems-to-lack... Edit: 10^8, not 108.
    • Zamicol 9y ago
      That's 10^8, not 108. You made me check the definition of a galaxy.
    • garmaine 9y ago
      Not necessarily. Globular clusters are sorta like mini galaxies outside the main galaxy. I didn't see anything indicating whether the main collection of stars is itself spinning.
• wanda 9y ago
  (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 9y ago
    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!
    • wuliwong 9y ago
      This feels like a leap in logic to me. It seems that it is possible that a 'dark force' could be not operating as strongly in this galaxy as easily as the possibility that dark matter is not being present. Without a mechanism to explain why either one exists, I don't see how this observation could rule one or the other out. Not sure if this analogy works? Imagine one group postulated a new type of matter to explain the effects of EM radiation. Another group theorized there was a new force. Then there was observed some charge particles moving at constant velocity and the conclusion was the new matter theory had to be correct since we observed regular matter without the effects of EM radiation.
    • wanda 9y ago
      Ah, gotcha! That is huge indeed. I appreciate the clarification.
• delecti 9y ago
  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
  • philwelch 9y ago
    There seems to be much more "dark matter" in our own galaxy than "normal matter", so "no dark matter at all" is an extremely surprising discovery.
  • danielsju6 9y ago
    I believe the "octane" of normal vs. dark matter explains in part the difference shape / sizes of galaxies.
  • erikpukinskis 9y ago
    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?
    • amluto 9y ago
      Not quite. No one knows what dark matter is. Astrophysicists know it by how it behaves. It seems to behave just like a normal gas, or perhaps dust or even moderate chunky stuff, except that it appears to be essentially completely transparent and simply passes through normal matter. (There’s one candidate theory of dark matter called, IIRC, “MACHO”s, which is the idea that dark matter is just a bunch of small, very dense objects dotting the universe. We don’t see them because they’re compact and don’t take up enough space to be noticed.)
• StanislavPetrov 9y ago
  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.
  • mark_edward 9y ago
    Are you a physicist or cosmologist?
  • contravariant 9y ago
    Well, we were right about induction currents, gravitational waves and anti-matter, so it's not as if filling in the gaps is always incorrect.
    • noetic_techy 9y ago
      Right eventually. Before that the explanations were a lot wackier. I could easily see "dark matter" the same way I see "ether".
    • semi-extrinsic 9y ago
      Don't forget neutrinos... or a million other things.
  • macintux 9y ago
    Positing some mystery matter that doesn't behave like anything we've observed strikes me as more absurd than gravity working differently at galactic scales, so I guess it's all a matter of perspective.
    • InclinedPlane 9y ago
      It is more absurd. Firstly, WIMPs do not "behave unlike anything we've observed". They behave similarly to neutrinos except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds they travel at much slower speeds. No new fundamental forces are required to explain WIMPs, they are simply particles we haven't discovered yet. Given that in the 20th century alone we discovered photons, protons, neutrons, anti-particles, quarks (and their whole family of composite particles in the form of baryons and mesons), neutrinos, and weak bosons while in the 21st century we've discovered the Higgs it would be ridiculous to say that new particle discoveries are beyond the realm of possibility. Moreover, we know that the standard model of particle physics is incomplete due to the existence of neutrino oscillations (among other things). So we can be fairly confident there are new particles yet to be discovered, and we have a model of dark matter which fits a hugely diverse set of data fairly well that no other model fits. To deny that dark matter is the best model for the data at present is nothing more than superstition.
• kraig911 9y ago
  It must be heaven.
• thaumasiotes 9y ago
  > 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".
  • wrs 9y ago
    Any theory of what dark matter is (and there are many) would need to explain how galaxies came to be the way they are. Indeed, explaining anomalies in galaxy behavior is the main reason the idea of “dark matter” came about.
  • raattgift 9y ago
    > 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/History_of_cosmic_structure_formation http://www.esa.int/Our_Activities/Space_Science/Planck/Histo... 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.
• AgentME 9y ago
  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 9y ago
    for some definition of "galaxy" - sort of https://news.nationalgeographic.com/news/2012/03/120306-dark-matter-galaxies-mystery-space-science/ https://news.nationalgeographic.com/news/2012/03/120306-dark...
  • lainga 9y ago
    I think Dragonfly 44 is pretty close to all-dark-matter.
• mehrdadn 9y ago
  > 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?
  • wrs 9y ago
    Dark matter is only “missing” in the sense that an explanation of it is missing from established physical theories. It (whatever it is) certainly appears to be present, because it has a major effect on observed galaxy behavior. Otherwise there would be no need to bring up the topic!
    • mehrdadn 9y ago
      I get that, it's not that I don't get the reasoning. It's just that it seems backwards to me (like in the phone example). But maybe my comment was an artifact of the fact that I (layman) haven't really bought into the whole idea that we really ever had any "theory" for dark matter in the first place. To me the appropriate headline would've been more like "astronomers find the first galaxy that actually behaves as theories predict".
      • wrs 9y ago
        I don’t think we do have a working theory that explains all the observed evidence. We know the theory without dark matter is wrong, because the vast majority of galaxies don’t agree with it. But we don’t have a new theory that holds together that does explain the observations, with or without dark matter. Basically “dark matter” just means “something that makes the observations work” and the choice of words “dark matter” is because it would work to have some kind of yet-unexplained matter that doesn’t interact with the matter we know about. It looked like it might also work to change the theory of gravity, in which case that was a bad choice of placeholder name — but now this is a counterexample to that idea.
      • ianai 9y ago
        I don’t know the math, but my intuition based guess is it’s related to the range of interaction possible with other particles through QMs probability functions. Ie it’s important to remember that at quantum scale matter/energy transfer through space as probability density waves. Dark matter are at a low bar of the interaction distribution. Something about the empirical values of those equations gives way to many more particles at that low end than expected, but their existence couldn’t ever be ruled out.
• Zamicol 9y ago
  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 9y ago
    Well, if the strength of gravity was linear in redshift, astronomers would have detected it decades ago.
    • jboy55 9y ago
      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 9y ago
        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.)
• DrBazza 9y ago
  Phil Plait's take on it: http://www.syfy.com/syfywire/what-is-this-galaxy-doing-without-a-dark-matter-halo http://www.syfy.com/syfywire/what-is-this-galaxy-doing-witho...
  • scribu 9y ago
    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.
  • jeremyjh 9y ago
    This is much more informative than the submitted article.
• sudosteph 9y ago
  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_ring_feature.html https://www.nasa.gov/mission_pages/hubble/news/dark_matter_r... Could this galaxy be an example of what the galaxy with the "ring" might look like some day far in the future? Fascinating stuff.
• nerfhammer 9y ago
  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 9y ago
    Honestly it's probably more evidence for dark matter; even tiny galaxies are large enough that MOND formulations should have effects.
• crimsonalucard 9y ago
  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 9y ago
    Microlensing excludes large unseen objects and the CMB power spectrum suggests most matter is nonbaryonic.
    • crimsonalucard 9y ago
      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 9y ago
        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.
  • kevinwang 9y ago
    What's the difference between the two things you described? To my understanding, the two options you gave are the same: Dark matter is a hypothesized form of matter which does not interact with normal matter or light (i.e. It can't be seen).
• wuliwong 9y ago
  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 9y ago
    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.
    • wuliwong 9y ago
      Dark matter is easily the most popular theory for this observed phenomena, so clearly most people do NOT think having "something that interacts with fewer forces than normal matter is more complicated than Jerry rigging a change to the elegant general relativity." I'm not sure if you are saying that I think one is more complicated than the other because I do not. I'm simply wondering about ideas and asking questions. Surely we should still ask questions, right?
    • noetic_techy 9y ago
• mrfusion 9y ago
  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 9y ago
    What about lack of super massive black hole singularity?
    • hannasanarion 9y ago
      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 9y ago
        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.
• chinmayatyc 9y ago
  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!
• scottcanoni 9y ago
  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 9y ago
    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?
• nogbit 9y ago
  Can you imagine being in a civilization in that galaxy, they must really think they are the center of the universe.
  • AlphaWeaver 9y ago
    Sounds like a cause for the Krikkit Wars...
• mabbo 9y ago
  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".
• erikpukinskis 9y ago
  James Webb James Webb James Webb
• c-smile 9y ago
  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?
• robbomacrae 9y ago
  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..
  • grzm 9y ago
    Because we can measure the mass of blackholes and account for it.
  • arduinomancer 9y ago
    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.
• nate_meurer 9y ago
  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/
  • oh_sigh 9y ago
    I think your link is actually the more relevant one for this story on hn.
  • fsloth 9y ago
    "To me, the coolest thing about this story is the telescope that was used to find this galaxy." Uh, we just found out a new principle on the large scale organization of matter in the known universe. The method is surely inspirational, but I really have to question your statement that the method of discovery credits more merit than the discovery itself.
    • nate_meurer 9y ago
      Relax. It's just, like, my opinion man.
      • fsloth 9y ago
        Sure, sorry, you did state "to me". English is not my native language and nuances sometimes escape me. The jury rigged ground based telescope is cool.
        • 9y ago
• CelestialTeapot 9y ago
  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.
• mikhailfranco 9y ago
  And Dragonfly 44 is only dark matter ... http://www.wired.co.uk/article/dark-matter-galaxy-dragonfly-44 http://www.wired.co.uk/article/dark-matter-galaxy-dragonfly-... 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).