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This is extremely suspicious for various theories of dark matter, right? How is it that all disk galaxies have just enough dark matter in just the right distri
by 1053r 9y ago
This is extremely suspicious for various theories of dark matter, right?
How is it that all disk galaxies have just enough dark matter in just the right distribution to make up for the various radii and distributions of visible matter and end up rotating at the same rate?
I have no answers, but as they say, the most interesting words in science are, "Huh, that's funny..."
- mikeash 9y agoIsn’t it just as suspicious absent dark matter? Seems just plain weird regardless.
- hammock 9y agoIf you accept the notion that all disc galaxies are oriented on exactly the same plane (which is current scientific consensus I believe), then it doesn't seem that far-fetched to think that they all rotate the same way as well.
- poopchute 9y ago"If you accept the notion that all disc galaxies are oriented on exactly the same plane (which is current scientific consensus I believe)" That is easily disprovable, no? We can see many many different angles of disc galaxies. A simple google search will show tons and tons.
- muthdra 9y agoI agree. It looks pretty random. Its probably even more random since we can't reliably eyeball the chirality in most of photographed galaxies so half the angles in one of the axis are practically mirrored at first glance.
- eximius 9y agoI feel like they might have noticed if it was that simple.
- colanderman 9y agoThat observation in itself is not a disproof. (Imagine standing in a room with a bunch of CDs suspended horizontally from the ceiling. From any vantage point you can see some CDs edge-on, some from the top, some from the bottom, etc. But they're all oriented in the same direction.)
- deleted 9y ago[deleted]
- sosuke 9y agoThat boggles my mind, do you have any sources I can read to start digging into on this? Not sure why a question like this would be voted down but my own searches have only turned up articles about galaxies spinning in the same direction. Someone else commented with some clarification. https://news.ycombinator.com/item?id=16586901 https://news.ycombinator.com/item?id=16586901
- semi-extrinsic 9y agoThis situation is trivially detectable though. Simply plot the inclination angle of disc galaxies as a function of azimuth and inclination, if it's like you say, there will be a very clear banded structure.
- tossandturn 9y agoThat analogy does not hold up when the CD you are observing from is in the same plane as all the others.
- 9y ago
- mkempe 9y agoDid you maybe mean: small, satellite galaxies show up in the same orbital plane as the main galaxy's disk.
- mutagen 9y agoThey're not all aligned on the same plane, they're somewhat aligned along the matter filaments and sheets (including the suspected dark matter) that make up the large scale structure of the universe. "Galaxies are not distributed randomly in the cosmic web but are instead arranged in filaments and sheets surrounding cosmic voids. [...snip...] We found evidence that the spin axes of bright spiral galaxies have a weak tendency to be aligned parallel to filaments. For elliptical/S0 galaxies, we have a statistically significant result that their spin axes are aligned preferentially perpendicular to the host filaments;..." https://arxiv.org/pdf/1207.0068.pdf https://arxiv.org/pdf/1207.0068.pdf "Galaxy shapes are not randomly oriented, rather they are statistically aligned in a way that can depend on formation environment, history and galaxy type." https://arxiv.org/pdf/1504.05465.pdf https://arxiv.org/pdf/1504.05465.pdf https://ned.ipac.caltech.edu/level5/March15/Joachimi/Joachimi6.html https://ned.ipac.caltech.edu/level5/March15/Joachimi/Joachim...
- wuliwong 9y agoGalaxies are distributed throughout a volume of space, they don't even exist on a single plane. Edit: here is a video which clearly shows the orientations of rotation are not aligned. https://www.youtube.com/watch?v=14yG_YER3xc https://www.youtube.com/watch?v=14yG_YER3xc
- tlb 9y agoThe constant rotational speed is the observation that requires dark matter to exist. Otherwise, small galaxies would rotate faster just like close planets do.
- marcosdumay 9y agoDark matter theory tells why galaxies rotate faster than they should. The fact that this speed is constant requires some other explanation.
- deleted 9y ago[deleted]
- avip 9y agoIt's not "constant" though, it's "cosmologically constant". Cosmologists would routinely assert that 1 == 1000 holds true.
- wuliwong 9y agoI don't believe it is just one thing. I believe the observed shape of galaxies is another problem that is dark matter is a proposed solution. Edit--- I was just reading more and there is also the fact that galaxies stay together at all. Apparently without including dark matter, they wouldn't "hold together." Sounds like the problem with the atom back before quantum mechanics. :)
- tzs 9y agoCouldn't a large galaxy rotate as fast as a small galaxy, without the need for dark matter, if the large galaxy had a sufficiently more massive black hole at its center than the small galaxy. The formula for the orbital period around a central mass is: 3 1/2 / R \ T = 2 Pi |-----| \ G M / where T is the period, R is radius of the orbit, G is the gravitational constant, and M is the mass of the central body. That's Newtonian, but I think that is good enough for this. This says that if Big Galaxy has a radius k times that of Small Galaxy, it won't rotate slower than Small Galaxy as long as Big Galaxy's mass is at least k^3 times Small Galaxy's mass.
- ajross 9y agoFor clarity: the finding is about the rotation of the galatic structure (e.g. the spiral arms), not the orbital speeds of the individual bodies, which is different. Google the "density wave" theory of galactic structure for details. Mass distribution is obviously a factor in that analysis, but it's not as critically determined as you think.
- mef51 9y agoNo, the measurements are of the redshift of hydrogen in the galaxy so it's talking about the orbital speeds. Also this quote from the press release: "if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round.”[1] [1]: https://www.icrar.org/cosmic-clocks/ https://www.icrar.org/cosmic-clocks/
- teamonkey 9y agoIt's a bit more abstract than that. The finding is actually that there's an approximately linear relationship between the measured distance of the outermost edge of a galaxy from its centre, and the average linear velocity of objects at that distance. Define a rotating circle based on those two measurements, for each galaxy. These hypothetical circles will all have similar angular velocities no matter how big they are, hence they all take roughly the same amount of time to make one rotation.
- redler 9y agoIt could also be that dark matter is distributed proportionately with observed regular matter.
- JKCalhoun 9y ago"which led them to conclude that the outer rims of all disk galaxies take roughly a billion years to complete one rotation" I was curious about "outer rims". I know little about astrophysics, but is the "outer rim" a definable thing? Because my sense is that, like a whirlpool (and planetary physics), material within a galaxy have different rotational rates depending upon their distance from the center of mass. It seems odd to describe "the spin" of a galaxy at all — but since they specify "outer rim" I expect that to have an agreed-upon definition.
- danielsju6 9y agoActually a galaxy's spin curve significantly diverges from a what we'd expect, like a "whirlpool" or say a solar system. It turns out stars on the rim do not complete orbits significantly slower than ones nearer to the center. It acts more like a frisbee. They're moving faster than they should but not getting "flung" out. It is one of the reasons why dark matter was originally postulated; basically the galaxy system is much larger/massive than what we can see. Adding a "Halo" of massive particles that are weakly interacting explains the curves fairly well--hence Dark Matter.
- philipov 9y agoIn case anyone gets the wrong idea, when dark matter is referred to as weakly interacting in the literature, as in WIMPs, they are specifically discussing particles that interact via the Weak force, not merely particles that don't interact very often. It's a good idea to keep the usage of that phrasing limited to avoid confusion.
- jwfxpr 9y agoWell... Sort of. The 'weakly interacting' means it interacts via gravity and another force no stronger than the Weak force. As per Wikipedia [0]: "There exists no clear definition of a WIMP, but broadly a WIMP is a new elementary particle which interacts via gravity and any other force (or forces), potentially not part of the standard model itself, which is as weak as or weaker than the weak nuclear force, but also non-vanishing in its strength." [0] https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles https://en.wikipedia.org/wiki/Weakly_interacting_massive_par...
- da_chicken 9y agoI don't think so. Dark matter still behaves like any other massive object, as far as I'm aware, beyond the obvious issues with detecting it. It may be more an artifact of the size of each disk galaxy being related to the amount of mass in that galaxy, and perhaps nearly all the angular momentum each galaxy still has is from one source: the big bang. In other words, a galaxy with X amount of mass will have Y amount of angular momentum from the origin of the universe, which will naturally disperse to a size Z as a function of X, Y and the time passed since the origin of the universe. So maybe it's all one big exercise in conservation of angular momentum.
- Balgair 9y agoDark Matter was first discovered because the galaxies are not rotating at the correct amount, if you only consider the 'light' matter we can see. That all the dark matter halos should rotate at this rate, despite the decoupling from light matter, is strange given what we know about dark matter [0]. Obviously, more data needs to be taken, but the result that all the galaxies are rotating at the same rate is not something we should suppose from first principals. AFAIK, dark matter is kinda thought of as a sparsely non-interacting gas. Over the eons, the dark matter particle with enough momentum to have escape velocity have mostly evaporated from the galactic halos, leaving the light matter galaxies and large clouds/halos of dark matter. As we have seen in the Bullet Galaxy collision, the halos really don't interact, thus dark matter is non interacting. What then makes the similar spinning rates of galaxies interesting is that the halos of dark matter should not be the same size, given that they don't interact. The halos should not all be the same size, they should be different sizes. If they are different, then the rotation rates should be different too. Just like a figure skater pulling in her arms to spin faster, the smaller galaxies should be spinning faster. This was the original problem that made us look for dark matter, the galaxies are spinning faster than we think they should. Big galaxies should fling themselves apart. We now think that all the galaxies are just embedded in dark matter halos, solving the angular momentum issue. Essentially, the light matter is like an ant on a spinning Frisbee. But if the galaxies are now all spinning at the same rate, and that the amount of light matter in a galaxy is independent from spin rate, that must mean that all the dark matter halos are of roughly the same size. Which sounds crazy, thus the news article. If anyone with a better understanding is out there, PLEASE let me know where I am making a mistake. Thank you! [0] Essentially all we know about dark matter is that 'it falls down', in that it interacts gravitationally and not in really any other way. This is opposed to so-called dark energy (the stuff driving the acceleration of the cosmos) in that the dark energy 'makes things fall up.'
- LinuxBender 9y agoPerhaps we are all in a simulation and the galaxy rotation is tied to a single clock source. Developers are sometimes a little lazy.
- leggomylibro 9y agoI'd love to see that datasheet. "To use the PLL (Planck-Locked Loop) as an oscillator source, use the 1-billion year galaxy tick as a source and set the PLLMUL register to divide by its maximum scaling factor of 1.855 * 10^52."
- jkw 9y agoor computation and/or memory space is limited! so trade-offs are needed
- TomK32 9y agoSpec said "make things spin" and so they did. After all, this isn't a rogue-like game with random procedurally generated content and permadeath,
- flukus 9y agofunction getRotationSpeed() { //just make it a billion for now, we'll randomize in the next version return 1000000000; }
- prewett 9y agoProbably a bug in using that function, though: 1 billion years is the rotation period, not the rotation speed. The rotation speed is 2 * pi * r / (1e9 * 365.25 * 86400), where r is the distance from the center of rotation (in meters). Maybe that's the problem with dark matter: objects get heavier when speed increases, so since the speed is 18 orders of magnitude too large, everything is heavier than it ought to be. All because of an incorrectly named function.
- aktau 9y ago> Maybe that's the problem with dark matter: objects get heavier when speed increases, so since the speed is 18 orders of magnitude too large, everything is heavier than it ought to be. All because of an incorrectly named function. Reminded me of "Tlön, Uqbar, Orbis Tertius" by Borges: > By 1942, Tlönian objects began to inexplicably appear in the real world. One of the first instances in which this occurs is when Princess Faucigny Lucinge received via mail a vibrating compass with a Tlönian scripture. Another instance is witnessed by Borges himself: a drunk man, shortly after dying, dropped coins among which a small but extremely heavy shining metal cone appeared. It is suggested that these occurrences may be forgeries, but yet products of a secret science and technology. https://en.wikipedia.org/wiki/Tl%C3%B6n,_Uqbar,_Orbis_Tertius https://en.wikipedia.org/wiki/Tl%C3%B6n,_Uqbar,_Orbis_Tertiu...
- stcredzero 9y agoThis is extremely suspicious for various theories of dark matter, right? Whoever wrote the procedural generation for this universe forgot to update the debugging scaffold code in the galaxyRotationRate() function. https://xkcd.com/221/ https://xkcd.com/221/
- ajnin 9y agoThanks for reporting this bug, it'll take just a few moments to fix it and restart the simulation ...
- ben509 9y agoIt'll be a day, though, it's Wednesday and the CI only runs on Thursdays.
- stcredzero 9y agoNah. It should definitely be a Monday!
- pbhjpbhj 9y agoSo, now it has restarted perhaps the Dev can tell us what the fix was?
- hartror 9y agoDark matter is considered a likely candidate for catalysing galaxy creation. This could lead itself to keeping the ratios of regular matter to dark matter densities being largely the same across most galaxies.
- jagger27 9y agoI just hope when the next simulation starts my mind gets carried forward.
- psyc 9y agoI hope there's a reroll option. My build is interesting but I can't figure out how to play it.
- mindfulmonkey 9y agoit won't, these are stateless containers
- jaekwon 9y agohttps://twitter.com/jaekwon/status/974265708325539841 https://twitter.com/jaekwon/status/974265708325539841 Theory: If #gravity were modeled as a constant egress flow of spaceelevator harpoons coming out of each atomic planet, the scale invariance of the rotational frequency of #galaxies would not be surprising. No need for #DarkMatter, gravity isn't a field, but it can come in waves.