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New studies confirm existence of galaxies with almost no dark matter
- Zenbit_UX 8y agoThis leads to an interesting question the article didn't mention... If a galaxy can exist without any dark matter and others with a balance of dark and normal matter, can one exist made entirely of dark matter?
- unfunco 8y agoIt surely wouldn't be a galaxy then? I think a galaxy has to have stars in order to be a galaxy.
- squirrelicus 8y agoI am not a physicist, just someone with an armchair interest but... If dark matter is theorized to explain more gravity than there would be without it, that implies dark matter has a gravitational effect, which means it too might coalesce into very dense things. Perhaps dark matter also forms star-like gravitational bodies, but whatever makes it "dark" hides those too. It seems weird because I'm almost suggesting... Cloaked stars. Which on the face seems terribly unscientific, given what we know about EM radiation of things. But i think dark matter implies things that don't radiate in the EM spectrum. Part of me wants to speculate that dark matter is a gravitational force echoing from 4th dimensional matter. Like if a cube affected a plane over which it hovered. But that's just me being an ignorant speculator.
- FreeFull 8y agoThe thing about dark matter is that because it doesn't seem to interact in any way other than gravitationally, there is no way for it to lose energy and clump together. So, collections of dark matter tend to stay rather diffuse. The way you could notice such a collection without any visible matter is through gravitational lensing.
- peanutz454 8y agoWhy is loosing energy required for particles to clump together?
- smsx 8y agoIf they don't lose energy by interacting through some force other than gravity when they get close together, then they will whip right past each other with nothing to slow them down.
- dagss 8y agoThink about two dark matter clouds crashing into each other. Each cloud just continues with the same speed in the direction it had. They dont interact other than gtavitationally. After passing each other they start to pull on each other gravitationally and slow each other down. Eventually stop and accelerate towards each other again. And so they can oscillate forever back and forth, if left to themselves without other clumps. But at no point would they "clump"... they are only close to one another in space when their speeds are most opposite one another. Being clumped means being im the same location and having the same speed.
- roywiggins 8y agoI expect they would lose energy and clump together due GR effects- the system will emit gravitational waves. https://en.wikipedia.org/wiki/Gravitational_wave#Binaries https://en.wikipedia.org/wiki/Gravitational_wave#Binaries
- ohazi 8y agoIf you could play God and place blobs of dark matter in the right places and moving at the right speeds, it seems plausible that such a dark matter Galaxy could physically exist and be stable. But I don't think we know of a mechanism for such a structure to form naturally. Normal particles collide and transform some of their gravitational energy into heat. If dark matter only interacts gravitationally, blobs of it would pass right through each other without losing energy. I think(?) you need these sorts of collisional energy losses to get matter to coalesce into a galaxy.
- Retric 8y agoYou can get the equivalent of evaporative cooling in an unstable multi body system. You occasionally kick out an object which takes more energy away than the average body that remains. The formation of a dark matter galaxy in such a fashion is plausible, but would take a long time.
- marcosdumay 8y agoThat require that the bodies exchange energy to get near the lower average, and transfer some energy to a few hot particles. Otherwise, you won't get any cooling.
- stcredzero 8y agoThat could be done entirely through gravitational mechanisms. There's something like that which happens to globular clusters.
- selimthegrim 8y agoYou might be interested in Lynden-Bell’s papers
- stcredzero 8y agoThanks. I'm just going by a tidbit from DeepSkyVideos on YouTube. They specifically mention a similarity to evaporative cooling. I just found the video! ("Gravothermal Catastrophe") https://www.youtube.com/watch?v=LYnoV2n5ptg https://www.youtube.com/watch?v=LYnoV2n5ptg
- gus_massa 8y agoThey are called "Dark Galaxies", there are a few candidates, but IIUC they are not confirmed. https://en.wikipedia.org/wiki/Dark_galaxy https://en.wikipedia.org/wiki/Dark_galaxy
- Avshalom 8y agohttps://en.m.wikipedia.org/wiki/Dragonfly_44 https://en.m.wikipedia.org/wiki/Dragonfly_44 might be?
- Zenbit_UX 8y agoYup, that seems to be it, similar mass to our own with 1% off the light emitted.
- deleted 8y ago[deleted]
- BurningFrog 8y agoI think the main theory for how the dark matter free galaxies form is that they somehow lost their dark matter component. Perhaps in a collision with another galaxy. So in that scenario it's quite possible that the dark matter part of those galaxies is drifting around on it's own somewhere. I don't know that we'll have any way of detecting them soon though...
- tropo 8y agoThat would fit nicely with dark matter being perfectly ordinary stuff like chunks of rock that happens to be difficult to observe from afar.
- BurningFrog 8y agoThat's a great common sense theory that's sadly been thoroughly disproven.
- cb_ 8y agoAs another commenter has noted, they wouldn't then be galaxies. And the question of whether there are (or how common and at what mass) galaxyless dark matter overdensities (think of it as a clump, in the literature they are called "halos") is still open. One of the problems with the current best model of the universe (Lambda CDM - a universe that is primarily make up of some cosmological constant driving expansion and cold dark matter whose gravity drives structure formation) is that it predicts that there should be far more of these dark matter overdensities than we see galaxies. This is the "Too big to fail" problem - https://arxiv.org/abs/1103.0007 https://arxiv.org/abs/1103.0007 is a pretty readable description. Basically the model predicts lots of dark matter clumps orbiting the milky way and we don't see enough satellite galaxies to populate them. So there are now two options: 1) Galaxy formation becomes stochastic at lower dark matter halo masses - and so there are lots of these galaxyless overdensities of dark matter. 2) There is something wrong with our model and there might not be these overdensities. 3) The milky way is an outlier. I think the short summary is that, at very low masses (say ~1e6 solar masses) we wouldn't be surprised if not all dark matter halos form galaxies. At high masses (1e12 solar masses, roughly that of the milky way halo) we would be very surprised if there weren't galaxies. But in between we are not sure. Edit: To be clear, these structures would still likely contain baryons (normal matter), they just haven't formed stars (or would have formed only a very few stars).
- z3phyr 8y agoHow does dark matter interact with another dark matter? I know that dark matter can only interact with normal matter by gravitational force..
- angel_j 8y agoI don't believe in the dark matter as they are currently selling it. I think we have small eyes on the universe and are missing information we'll probably never get at our scale. The missing energy could be giant elementary particles (in the standard model), and these "particles" go to bigger-than-black-holes energy, which we could never gauge in a lab, tho I'm pretty sure they can be "predicted" along an energy scale.
- Causality1 8y agoWhat I've never understood about dark matter is if it doesn't interact with the electromagnetic force, why is it found in voluminous clouds instead of collapsing into dense discs or balls? Radiation pressure doesn't affect it, it experiences no impact or friction effects, no thermal effects, so why doesn't it fall to the middle of the galaxies and stay there?
- mkl 8y agoI think it's like this: Imagine you have a cloud of dark matter, with a combined centre of gravity. Now imagine a "piece" of dark matter falling down to that centre of gravity. It doesn't impact, or slow down with friction, or anything like that, it just keeps going, through the centre, and out the other side, just as far as it fell before. There's no way to clump together, or stay in the centre.
- gizmo686 8y agoWhy should it stay there? Imagine you had a sphere of particles with an initial velocity of 0 that interact only through gravity. At first, these particles would all accelerate towards the center of gravity. Once they reach the center of gravity, they would have accumulated some velocity, and so continue to travel away from the center of gravity (but still be accelerating towards it). Eventually, the acceleration towards the center of gravity would 0 out their velocity again, and they would start falling back towards it. The end result is that your original sphere oscillates indefinantly, but always returns to its original state. As your initial configuration becomes more random, this discernible time structure begins to fade, and you end up with a voluminous cloud. What allows regular matter to collapse into dense structures is that it interacts with itself to shed velocity.
- Causality1 8y agoSo then why is its structure and location correlated with galaxies at all? Shouldn't it be relatively even throughout the universe?
- improbable22 8y ago
- sdenton4 8y agoPossibly dumb question: Do we know the dark matter proportion of our own galaxy? (in other words, how hard is it to take a selfie, at galactic scales?)
- lordnacho 8y agoIsn't it to do with the rotational velocity? If it isn't what you'd expect from the matter that you see, you need DM for it to add up.
- Pharmakon 8y agoThe existence of dark matter in galactic halos is inferred from the velocity dispersion of stars in a galaxy (sigma). It’s important to realize that this is a very rough kind of estimate because every aspect of it (as in the case of the M-sigma relation) is roughly estimated. Virial mass is inferred from average temperatures of gas by way of detexting X-rays. Central black hole mass is inferred through the M-sigma relation, and stellar masses are inferred through luminosity. Every so often a new paper comes out revising the overall estimates, but the proportion of DM to luminous matter stays pretty constant. Now, in a sense this makes a “selfie” easier, because you just need a series of snapshots which help you make all of these inferences. On the other hand because we’re in the disk of our galaxy it is difficult to image because we have to deal with all of that hot interstellar gas and dust, and the extremely bright galactic core. I’m not sure how much, when you take into account a long period of time and many wavelengths can be observed, but I’d say we can observe the majority of our galaxy in at least one wavelength. Still, we’re not talking about “selfies” here, just really well educated guesses based on observations of our own galaxy and similar spiral galaxies. So as far as inferring the existence of a dark matter halo in our own galaxy we’re pretty certain, and it seems to make up about 95% of the galactic mass. It’s based on something a lot more tenuous and complex than a “selfie” though. Once you’ve done your best to estimate the total luminous mass of the galaxy and it’s rough distribution though, and ruled out MACHOs with lending surveys all that’s left in the remaining narrow window is dark matter. The only other theory with any chance at all is MOND, but it has to thread crushingly narrow passages between observation and tests of GR.
- 8y ago
- wpasc 8y agoCan a physics minded HNer confirm something for me (or set me straight)? One piece of evidence for dark matter is observed when galaxies collide. Two galaxies collide, but by observation of gravitational lensing on the light from stars "behind" the colliding galaxies show that much of the matter from each continues moving. This matter (dark matter) is seemingly unobstructed by the collision (because of dark matter's lack of electromagnetic interaction). I heard this explained to me once, wondering if it is true.
- joshvm 8y ago> because of dark matter's lack of electromagnetic interaction This is still an assumption, because we don't know for sure that dark matter is only weakly interacting, but this is what was observed in the Bullet Cluster collision: https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster There's a good explanation here: https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoking-gun-for-dark-matter/ https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoki... The lensing was found to be strongest near the visible matter (stars), and the amount of observed lensing couldn't be accounted for with the total stellar mass alone. So the assumption is that the dark matter, like the stars, wasn't slowed much by the collision. The gas, on the other hand, interacts electromagnetically and slows down in the middle. That's not to say that the stuff that stars are made up of isn't affected by electromagnetism, but it's a short-range effect and even in a collision, interstellar separation is large (i.e. contrary to what you might think, a galaxy collision doesn't involve all the stars smashing into each other, most of them will just sail right on through). Gravity will dominate in that case.
- wpasc 8y agoFascinating, thank you for the specific example. With a memorable name like "Bullet Cluster" I won't forget it.
- ncmncm 8y ago"Gas", here, as usual, means "plasma". Gas does not interact electromagnetically. Use of the term "plasma" invites banishment from the community of astronomers, and loss of access to telescopes.
- nonbel 8y agoI didn't look at the other one but NGC1052-DF2 is consistent with MOND: Original sigma = 7.8 (+5.2/-2.2) New sigma = 8.5 (+2.3/-3.1) "Crude" MOND sigma = 14 (+4/-4) https://tritonstation.wordpress.com/2018/04/04/the-dwarf-galaxy-ngc1052-df2/ https://tritonstation.wordpress.com/2018/04/04/the-dwarf-gal... And keep in mind these uncertainties are arbitrary cutoffs... Also, others claimed the distance they use (20 Mpc) is wrong. It should be 13 Mpc and this throws off the calculations: https://arxiv.org/abs/1806.10141 https://arxiv.org/abs/1806.10141 I see them still using the 20 Mpc value without addressing those concerns.
- nonbel 8y agoThinking more on this... Those are either 90% or 95% confidence intervals (couldn't tell from the recent paper). So that would mean if you looked at 100 galaxies you would expect 10 (or 5) to fall outside the range if the model was correct.
- abhinai 8y agoAm I wrong to conclude that this result itself is an argument for the existence of dark matter? Because the theories that argue that physical laws behave differently at larger distances would have to work similarly for all galaxies. Physicists please correct me if I am wrong in making this conclusion!
- meowface 8y agoI think that is a reasonable conclusion to make (assuming these findings are correct). The article says the same: >Ironically, the lack of dark matter in these UDGs strengthens the case for dark matter, the researchers say. It proves that dark matter is a substance that is not coupled with normal matter, since they can be found separately.
- acqq 8y agoAlso: https://medium.com/starts-with-a-bang/mysterious-galaxy-measured-exquisitely-and-contains-no-dark-matter-at-all-753338968df6 https://medium.com/starts-with-a-bang/mysterious-galaxy-meas... “Ironically, the discovery of a galaxy without dark matter — amidst a sea of galaxies that require huge amounts of it — helps prove the validity of the dark matter-rich picture of the Universe. Only if normal matter can be separated out from the dark matter and left to form its own structures would such a galaxy be possible.”
- mikeash 8y agoThis reminds me of the discovery of oxygen, which was more about discovering the lack of oxygen in a vacuum and in combustion products.
- everyone 8y agohttps://en.wikipedia.org/wiki/Phlogiston_theory https://en.wikipedia.org/wiki/Phlogiston_theory
- cmroanirgo 8y agoI've been very reluctant to go 100% with the Dark Matter theories, preferring to think that Kepler's Law was somehow the root problem. Note, I had no real basis for these thoughts! These findings do indicate that the Dark Matter theory is increasingly reliable, in that some galaxies are shown to have different densities (& thereby there's nothing wrong with Kepler's Law, and further proof that my "going with the gut" when compared with empirical science is just silly!) These findings help reinforce that Kepler's Law are working well and seem to indicate that that dark matter is not necessarily tied with matter, but is hanging around in different densities in different galaxies. > Ironically, the lack of dark matter in these UDGs strengthens the case for dark matter, the researchers say. It proves that dark matter is a substance that is not coupled with normal matter, since they can be found separately. To me, it's a bit like how our planet is not uniformly the "same stuff", but we have deposits of Gold and Coal and Granite spread around the place in an uneven manner.
- jcims 8y agoAre there any alternative theories that invoke strange interactions of matter/gravity/etc with time? I guess the gravitational wave results out of LIGO would indicate that it propagates across time in a fashion similar to electromagnetic radiation, but if gravitational effects extended beyond 'now' there would seemingly be a possibly small cumulative effect.
- posterboy 8y agoi dont understand what you are saying. What would accumulate? At least I can say, everything propagates across time. No time, no propagation.
- danwills 8y agoOne alternative is 'Process Physics' which is not widely accepted but has a fundamentally different explanation for dark matter: Turbulence in the in-flow of space into matter. This flow is PP's model for gravity itself, as if matter continuously 'consumes' space to continue its existence. It's an interesting, quite different and generally totally rejected point of view.
- fallingfrog 8y agoThis would seem at first glance to be evidence against entropic gravity?
- snarfy 8y agoI had a naive theory that there is a limit to black hole size/mass that once reached causes gravity to have a more linear strength instead of the inverse square. This discovery blows out that pet theory.
- zyxzevn 8y agoToday it is dark matter, but tomorrow it is... https://i.imgur.com/YcsVKGX.png https://i.imgur.com/YcsVKGX.png
- _bxg1 8y agoWow. From my armchair it seemed to me much more likely that physics just works a different way at that scale than that there was an invisible substance magically sprinkled evenly throughout most of the universe.
- axilmar 8y agoCould it be that we simply cannot observe all of the actual objects a galaxy contains, and thus we maybe do not need another theory or dark matter?
- bufferoverflow 8y agoPretty much all massive objects except black holes emit light/radiation, and black holes, afaik, are detectable. Thus there's something else that we give a placeholder name "dark matter". It's possible it's not a matter of any kind at all, but just some very smooth but irregular field that interacts with the space-time curvature (gravity).
- Farradfahren 8y agoImagine if those where anti-matter galaxys, as if dark-matter was not attracted to anti-matter. We could never visit them.