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This 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
by Zenbit_UX 8y ago
This 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..