All disk galaxies rotate once every billion years: study
astronomy.com
astronomy.com
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..."
The formula for the orbital period around a central mass is:
3 1/2
/ R \
T = 2 Pi |-----|
\ G M /
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.
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. :)
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.
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.'
The paper itself is at http://s3-ap-southeast-2.amazonaws.com/icrar.org/wp-content/...
From the very first paragraph it takes cold dark matter as a given, and refers back to its reference MMW98 many times.
Two key sentences in the Conclusions: "While R_max appears to mark a sharp truncation in the [luminous] disc of galaxies, it does not enclose all baryons. Stars in the halo are distributed to much larger radii, and their kinematics indicate the dark matter also extends further, likely to the virial radius."
So this is the wrong paper to be looking for departures from the standard structure formation (a good overview of which is http://www.esa.int/Our_Activities/Space_Science/Planck/Histo... )
> that must mean that all the dark matter halos are of roughly the same size
No, the issue here is that the luminous "surface" of the carefully selected sample of galaxies is nearer the cores than a number of previous numerical simulations of galaxies firmly rooted in the standard structure formation. The paper discusses a number of possible reasons for this, including (very bluntly) "Baryonic physics is messy" [introduction, second paragraph] and "Theory and observations indicate that feedback from star formation ... or active galactic nuclei ... can rearrange the distribution of baryons, and in the process drag along dark matter ... into an altered distribution, affecting all scaling relations" [ibid].
As to the coreward surface, "... our results are best explained by a true physical truncation of discs. Whlie the formalism presented this far implies continual accretion limits the extent of discs, section 5.4 considers other scenarios for limiting the extent of discs [.......] [including] the limitations in the angular momentum in an initial proto-galactic collapse ... truncation in star formation due to disc stabilization ... ionization by the UV background ... and spreading of the disc due to angular momentum transfer."
So baryonic gas and dust falling (back) onto galaxies is their favoured model for squashing the luminous matter inwards, and they look for evidence of old (and likely to explode) stars beyond the luminous edge (but well inside the edge of the CDM halo) as a source for some of that gas and dust. They have a number of ideas about why such stars may be in the halo in the first place, consistent with the standard structure formation model (but not precluding at least one or two other models).
> Essentially all we know about dark matter is that 'it falls down', in that it interacts gravitationally and not in really any other way.
It doesn't fall down much because it can't radiate away its angular momentum in a scattering interaction with other DM or baryons. So particle DM in the halo tends to stay in the halo, rather than migrating inwards. Assuming the CDM is collisionless, inward migration is solely because of gravitational interactions, which are extremely weak and thus a very slow way to ditch enough momentum to descend to a lower orbit. The lingering DM constrains the lower orbits available to baryons when they collide and ditch momentum radiatively, which is why there's still so much luminous matter outside the core.
> opposed to so-called dark energy (the stuff driving the acceleration of the cosmos)
In the standard cosmological gauge (which takes a specific slicing of 4-spacetime into 1+3 time+space, and treats all the stress-energy in the bulk as strictly inertial in that slicing, represented as compressible homogeneous fluids (each with a particular density/pressure relationship, remembering that pressure is the inverse of tension) under constant spatially isotropic tension, and imposes a set of coordinates that fix on Eulerian objects in the baryonic matter fluid) the "comoving" coordinates from one spatial slice to the next are related by the cosmological constant. In this representation, all the matter fluids (baryons, dark matter, radiation) dilute away -- their density decreases -- with the metric expansion. However, one of the fluids has constant density and tension (i.e., negative pressure): it does not dilute away.
If we consider vacuum de Sitter space in the cosmological gauge, i.e., if we take the above and remove all the distracting matter and radiation fluids and focus on an expanding spacetime which is empty except for this constant (positive) rest density and (negative) pressure, then we can work out that the pressure must be -1/3 of the energy density. The strictly timelike worldlines of Eulerian obsevers in this setup diverge with the expansion of the universe. There is no acceleration felt by any of the observers, but they calculate mutual recession distances that rise extremely high at large spatial distances (and large spatial distances are more common in the future).
> dark energy 'makes things fall up.'
The critical point is in the previous paragraph: if our galaxy clusters are practically always Eulerian observers, they don't feel the effects of the expansion as they do their own internal gravitational interactions. If we have a galaxy cluster of non-radiating dark matter, nothing falls up and away from it during its history from the beginning of the dark energy dominated epoch. (Real clusters in the DE-dominated epoch will radiate photons at the very least, but would do so even with no expansion; any ejected matter is thrown out into inter-cluster space by internal processes, not by dark energy.)
Note that we are not required by nature to use the standard cosmological gauge, but we do have to preserve the central observables of the spacetime geometry and the observables of galaxy clusters: stitched into the expanding spacetime (well-modelled by a Robertson-Walker metric) are concentrations of matter that source real metrics that asymptotically decay to (near-enough-to-be-practically-indistinguishable-from) Schwarzschild at reasonably short spatial distances. The crucial thing in that is that these Schwarzschild spacetime patches DO NOT EXPAND, but the (effectively vacuum) RW spacetime they're stitched into does. If you start playing around with the expanding part you can choose a bunch of different ways to "explain" the features observed by an astronomer in one Schwarzschild patch examining the radiation originating at other Schwarzschild patches, but the standard cosmological gauge is hard to beat in terms of simplicity.
An analogy for a non-rigid disk would be like swinging a flimsy plastic pipe. You'll notice that it is not straight when spinning and the far end lags the hand holding onto the rod. Vs if you spin with a metal pipe, the whole thing is rigid and your hand and the tip of the rod are in the same place.
The figure skater analogy is usually used in discussions about center of mass and moment of inertia (figure skater pulls arms in and the center of mass changes, but the energy is transferred into spin energy). Which rigid disks have a different moment of inertia than a non-rigid disk (eg: hoops spin different than disks).
Mass distribution is obviously a factor in that analysis, but it's not as critically determined as you think.
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.
"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."
function getRotationSpeed() {
//just make it a billion for now, we'll randomize in the next version
return 1000000000;
}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_Tertiu...
"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
"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://ned.ipac.caltech.edu/level5/March15/Joachimi/Joachim...
That is easily disprovable, no? We can see many many different angles of disc galaxies. A simple google search will show tons and tons.
(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.)
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
Edit: here is a video which clearly shows the orientations of rotation are not aligned.
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.
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.
[0] https://en.wikipedia.org/wiki/Weakly_interacting_massive_par...
https://physics.stackexchange.com/questions/134159/what-is-a...
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.
Whoever wrote the procedural generation for this universe forgot to update the debugging scaffold code in the galaxyRotationRate() function.
I looked it up and a "galactic year" for our sun is only 250 million years. Meaning our planet has been around the galaxy 18 times since its formation! That seems like so much faster than I would have thought!
It doesn't, which is a lot of the reason why we think dark matter exists.
Here's a video on the subject: https://www.youtube.com/watch?v=Dx1Wf84bC2M
The problem is, they still end up needing some dark matter before they work very well. I think every bright person assumes dark matter must be fake when they hear about it, but nobody has succeeded in getting rid of it. Dark matter plus the physics we know about is actually a really great fit to what astronomers see, and it has been hard to improve on.
https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...
Personally I'm just fine with either. Particle physics makes it pretty clear IMHO that the idea of a weakly interacting massive particle is perfectly reasonable, because the particle zoo that we have is already rich far beyond "normal matter" and adding a few more isn't conceptually too difficult. (It'll break our theories, of course, but we kinda already know there must be something wrong with them, so that's not a huge shocker.) And the idea that physical laws may not be exactly perfectly the math we thought they were is also not that shocking to me, for all kinds of reasons, again not least of which is that we know something is wrong with our current theories.
If you plot mass extinction events against when the solar system transits through the arms, eleven extinction events in the last 500 million years occurred while going passing through one of these high density regions. [2]
[1]: https://en.wikipedia.org/wiki/Density_wave_theory [2]: https://arxiv.org/abs/1309.4838
No, where I think they really screwed up was in level of detail -- you know, where you get to smaller scales and start generating details procedurally rather than pulling geometry out of memory. I mean, I understand why it's necessary to do that -- who wants to store the position and momentum of every particle in the observable universe? -- but they could have at least faked some kind of continuity between observations, rather than calling Math.random() every time!
You're guaranteed that for any other particle of the remaining set, the state vectors are orthogonal to the state you just read out. If you do the experiment with two entangled particles, by reading one, you'll immediately know the state you're going to read for the other one.
If you do it for more than one particles, for each state you read you reduce the size of the remaining set of vectors that my come out.
From a compression/encoding point of view it's kind of neat. If you do it for lots and lots of systems of many particles in a certain microstate, on average you're going to end up with nearly identical results for each total readout process, although the precise values and the order in which they appear will vary wildly.
Now because all this iterative state unfolding more or less comes down to be a kind of hash function you want to make sure, that users of the middleware don't rely on hidden internal state, or assume some kind of hidden seed. The downside to this is, that this particular implementation detail destroys locality, which kind of goes against the whole idea of the fixed-event-propagation-differential system, that aims to isolate high energy processes from neighboring parts of the simulation by easing their timespace metric.
There are a few corner cases (which actually came to happen in a lot of instances in the simulation), where out-of-bounds stress-energy densities are (successfully) isolated from the rest of the simulation, leaving visible to the rest of the simulation only a meta-description of the contents inside the region, that boils down to mass, charge and spin (where due to some interesting interaction charge and spin happen to have the same kind of visible effects on the outside timespace metric) and the surface area of the boundary region. However right at the boundary region, the cursor iterating over the aforementioned state vector unfolding may cross into the isolated region. At first it looked as if this could break the simulation. But it allowed for a wonderful hack for an incremental garbage collection inside the isolated regions, by treating the whole isolated region as a single meta particle, holding N instances of the state vector, where N is proportional to the surface area of the boundary region. Randomly selecting one of the quasi frozen states from inside the isolated area, we can call its destructor, be unfolding its complement an entangled particle that happens to by just outside of the boundary region.
This goes nicely with another hack, introduced early in development: The on-demand spawning of entangled particle/antiparticle pairs, which can be used to transmit forces between the actual particles you want to simulate.
By applying these on-demand spawns on the isolated regions, it turns out, these regions can be garbage collected, by kind of "evaporating" their contents through a entropy maximizing process, thereby avoiding the need to faithfully reconstruct the original information; instead the remaining hash value is uniformly distributed over the simulation and used to seed the entropy pool from which random numbers for the unfolding process are taken.
Kind of neat, don't you think?
I wonder...how many times have they deleted our memories already?
But this reminds me of another memory wipe situation I've wondered about. One of the common things in many tales of alien encounters is the aliens messing with the memories of those who interact with them.
Unlike with simulation operators, who are essentially gods to us with omniscience and omnipotence and completely unconstrained by any laws of science or logic that we know, aliens would presumably be constrained. They would not be omniscient and omnipotent--just more advanced than us.
So with aliens we would have hope of combating their memory wiping, or at least detecting it.
For example, if you often drive at night in isolated areas where you might be particularly vulnerable to alien abduction, you could keep some innocuous physical item in your car, such as a book with a bookmark in it, or a Rubik's cube, or a cassette tape of a band you hate. Pick an item that you can put in a certain state that is alterable, and have a standard state you keep it in. The bookmark is always on page 100, the Rubik's cube is always solved, the tape is always rewound.
If you ever see anything strange that even suggests "UFO", you alter the item. Move the bookmark to page 110. Add a couple twists to the cube. Start playing the cassette.
If you remain aware of the possible UFO until it goes away or you figure out what mundane thing it is you are seeing, you fix the item.
If, however, you either remember that you saw something but don't remember what happened, or you don't remember anything weird but find you have lost time, you can check the item and if it is out of its normal state you know that you thought you saw a UFO and now you don't remember it.
The idea here is that the stories of aliens usually include something making much of our technology fail so that we can't record them, and they also probably know enough about to us to recognize when someone who sees them writes a note on paper and so deal with that. But unless they have a way to read minds or extract and interpret memories, they probably won't recognize that, say, starting to listen to music is actually a form of note taking, and so won't know they need to rewind the cassette before they let you go with your memories wiped.
So...anyone of you actually do anything like that?
It's not a testable claim right now, but it might be in the future: https://en.wikipedia.org/wiki/Simulation_hypothesis#Testing_...
Not to mention the idea of a simulated universe is, you know, philosophically boring and implies mostly false things in most peoples' minds.... like an anthropomorphic scientist god. In reality, it would change virtually nothing about how we view our universe.
I disagree.
Science cannot prove that a theory is 100% true, only that it is not wrong by repeatedly testing the predictions made by the theory. The best we can do is say: this theory (e.g. general theory of relativity) is the best explanation for data we have so far and every experimental prediction it has made has come true.
The paper (https://arxiv.org/pdf/1703.00058.pdf) linked on the wiki page proposes four experiments that seeks to test the simulation theory. None of these experiments will single-handedly prove the simulation theory, but if they all pass we can only keep testing and seeking alternate explanations.
If the experiments keep passing and no alternative theories can be found, then we're either in a simulation, or the universe just happens to behave exactly in the way that a simulated universe would, but it isn't. I agree that we can never truly know which is true, but at that point the difference is reduced to semantics. It's like saying electrons don't really exist, instead they're just wave/particles that are exactly like electrons in every way except for some immeasurable quality.
This assumes the entity running the sim will never directly influence it, of course if that ever happens (requiring a _very_ high burden of proof) it'll be proof of the sim.
Mg = C1 * Rg * Rg
where C1 depends only on shape and is basically the same across all the spiral/disk galaxies.
Orbital speed of stars in the disk is proportional to the square root of the galaxy's mass (the observed absence of the other otherwise must-have orbital velocity equation component - ie. 1/SQRT(r) - is what causes crack pot theories like dark matter) :
Vdisk = C2 * SQRT(Mg) = C2 * SQRT(C1) * Rg
where C2 also dependents only on shape and is basically the same across all the spiral/disk galaxies.
Thus Vdisk = C * Rg where the C is pretty close to being the same across all the spiral/disk galaxies.
Angular speed of a star in the disk at the distance "r" from center is
Vdisk / (2pi * r) = (C * Rg) / (2pi * r)
which at the edge of the galaxy, ie. where r = Rg, becomes
(C * Rg) / (2pi * Rg) = C / 2pi
ie. the same constant across all the spiral/disk galaxies. The end.
Out of genuine interest, do you have any citations from reputable physicists who agree dark matter is a "crack pot theory"?
http://iopscience.iop.org/article/10.1086/508162/meta
There is a great deal of evidence for dark matter that does not depend on Kepler's laws, please stop spreading misinformation.
Source: assistant editor of a physics journal, know several astrophysicists personally, currently dealing with a MOND paper this very week...
They do that based on a misapplication of the divergence theorem. They assume that all the mass inside an orbital radius can be treated as a point-mass which is true for uniform spherical distributions. They also assume the mass outside the radius has no net infuence - which is true for uniform spherical shells. Neither of these is valid for non-uniform distributions - a flat disk in not symmetric in 3-space. If they'd just do some electrostatics problems they'd know the different formulas for disks and rings. I was going to google a sample page but there's too many to sift through and I gotta run an errand.
Welcome to the club - it's incredibly small...
"The approximation that the mass distribution is spherical is reasonable for the dark halo, but not for the flat stellar disk. Better models suggest that this estimate is probably high by about 30%, since a disk requires less mass to produce a given centripetal acceleration"
And there you have it. While he notes that a spherical distribution is different than a flat disk, he estimates the error at about 30%. The problem is that the error is not one of a constant factor, it's a different function. This unspecified better model also doesn't seem to care about the matter outside the solar radius - which also affects our velocity in a disk, but not in a sphere.
There is a lot of math in chapter one, but also a lot of hand waving.
If anyone can tell me why Kepler applies galaxies (vs 2-body systems with certain constraints), I'd be happy to show where the error in that analysis is. Kepler doesn't apply to galaxies. It's really that simple.
I can't imagine the amount of superiority you must feel to assume that physicists don't know high school level integration of Newtonian forces.
Every couple days the department chair or some well-known professor would get a letter (this was before most of the crackpots had email accounts, I guess) claiming that some theory--particularly general relativity--was wrong and all it took was a simple application of high school arithmetic to see it.
One of our faculty happened to coin the phrase "dark energy" while I was there, and man did the letters start coming in after that. Of course, the only absolutely universal commonality among all of the letters was that they were all nonsense, like the above.
In general, I felt sort of bad because you could tell that a lot of these people were really not well, but it's still hard for me to see things from someone's perspective who genuinely believes that the entire astrophysics community (save a tiny handful of contrarian actual physicists) has somehow engaged in a massive conspiracy to ignore introductory physics in favor of some other vastly more complicated theory for absolutely no possible benefit. (Especially when these professors are literally teaching introductory physics to students while they're pursuing their research.)
I'm not claiming it is the case now (though it does smack of luminiferous aether a bit) but the scientific community can and has been wrong sometimes in the face of as yet undiscovered principles.
Someone else unwittingly suggested a parallel to Fermat, which I think hits the nail on the head exactly. If someone thinks they can prove Fermat's last theorem with some high school geometry, odds are pretty good they're a crackpot. If they're a well-regarded mathematician and they can build off new mathematics and make a few very complicated leaps by discovering new mathematics of their own, then people take them more seriously.
Scott Aaronson has a really great take on this here: https://www.scottaaronson.com/blog/?p=304
As an aside that's probably only interesting to me, I went to a Great Books college. It's one of those hippy dippy liberal arts schools where there are no electives, no textbooks, no lectures, just primary sources. You learn mathematics by reading Euclid, Ptolemy, Newton, etc.
At any rate, for first-year "Labratory" class, you trace the atomic theory from the Greeks to roughly Avogadro. Along the way you read papers by and replicate the experiments of many influencial scientists along the way. When I mention that the above crackpots are "wrong", I don't mean it in the sense of the aether or analogous theories along the way to the atomic theory, namely phlogiston or caloric, two previous theories of heat. Those theories did not explain new observations and were rightly discarded along the way. Priestly and Lavoisier were brilliant and tried to make theory match observation, so now even though their theories are discarded, their work still stands on its own for the time. I wouldn't call them wrong. There's a definite difference between them and theories that are just wrong.
Nobody's going to get a Nobel Prize when they just make the world believe that they have obviated the need for dark matter by pulling out some high school math and some Kepler proofs.
Someone will win it if they ever make an observation that proves a MOND theory in a way that shows conclusively that dark matter is not needed to explain all current observations and in fact cannot remain compatible with this new observation.
There's a big difference between the two, I think.
This requirement is impossible to meet for sufficiently flexible theories. Eg no one will ever "win" over "God did it" according to your metric.
Also, if you see how many people in high academic positions are obviously wrong about simple stuff they use every day like p-values, nothing will surprise you any more.
I'm not sure I see how your second point is relevant. You seem to imply that I think that academics are infallible in all areas not even related to their expertise. Do you truly see no difference between non-statistics researchers incorrectly calculating statistical values and dozens of unrelated experiments all leading to the same result?
>"I'm not sure I see how your second point is relevant. You seem to imply that I think that academics are infallible in all areas not even related to their expertise. Do you truly see no difference between non-statistics researchers incorrectly calculating statistical values and dozens of unrelated experiments all leading to the same result?"
This is series of strawmen... I'm saying widespread confusion can, and currently does, exist on a topic even amongst the experts. Therefore it is not unbelievable that it does on other topics as well.
Also, dark matter is an interpretation of a result, not a result itself, so an experiment cannot lead to it directly. Finally, I don't think there are any actual experiments that have supported dark matter, only astronomical observations. Correct me if wrong but I found this after a quick search: https://www.sciencenews.org/blog/science-ticker/results-slew...
Actually another point is that this is not the issue. The calculations are fine, they are just calculating something other than they think: p(Data | Hypothesis) when they want p(Hypothesis | Data).
That is very similar to what the OP claimed about calculating a model that assumes a sphere to describe a disc (no idea if they are correct on that).
Crackpot theories are eccentric and not commonly believed.
I am interested in crackpot theories because sometimes they end up turning out right, and I think within that dynamic there are important lessons for humanity that we end up learning in very hard ways.
For example, the Great Depression paved the way for the acceptance of Keynesian economics--ideas previously written-off as crackpot theories. Another: NASA engineers warned of the exact dangers of a foam strike, under the exact conditions in which it took place, were at the time written-off by senior management. Social scientists that studied this described part of the problem as Groupthink.
By that I mean that the edges of galaxies experience a "reference" spacetime (e.g. constant period), while the interiors experience a stretched out version spacetime (i.e. inverse time dilation and length constriction)?
Or did the entire community of astrophysics simply ignore high school physics in favor or something vastly more complicated because reasons?
In physics, how well your model can predict past and future states from the current state is rather important. Though future states more than past state but any model should be able to also somewhat agree with it's the past it generates.
You can't simply say "this model also predicts that" without giving on how accurate it does predict and comparing to existing models. Otherwise we'd be using flat earth models for building bridges and planning ship routes.
You'd do well to do a little more research on the topic before posting so many comments on HN.
There is a ton of evidence for dark matter. We still don't know exactly what it is, but we do know lots about it (e.g. it is not baryonic matter because observation differs from theoretical models of how it would interact with ordinary baryonic matter). As for all that other evidence, the first hit on Google is for laymen: https://medium.com/starts-with-a-bang/five-reasons-we-think-...
TLDR: gravitational lensing, missing mass, and three other really good reasons that dark matter is a serious subject of study.
Disclaimer: I am not a physicist. And I'm trying not to represent myself as one on HN.
Talking about this stuff is fun, sure. It's great to learn! But please don't spread ignorance with such high confidence. As for "just" Newtonian physics, there's a whole research field called "Modified Newtonian Dynamics" (MOND). https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
TLDR; there are lots of problems with MOND, and it doesn't explain gravitational lensing and other effects of dark matter that we see.
Mg = C1 * Rg * Rg
Is not a good assumption for the discs we observe in the universe, because they do not have a constant surface density. The picture is further complicated by the fact the many disc galaxies also contain a central bulge, large enough to effect the potential (to say nothing of Dark Matter).
Exactly, and that's why the article's title is misleading and clickbaity. The article also basically copies the press release [0] line for line.
Still, if this is true that's a pretty groundbreaking discovery.
Perhaps we are witnessing a previously undetected force at work, one that operates on much larger scales with much larger masses.
I am asking whether there could be a scale invariant function acting here that makes the rotation of the galaxy more dependent on age rather than size, that got started when the vortices that ended up as galaxies were much closer together and at much smaller scales.
It's a bit like this: https://xkcd.com/1758/
One theory suggests that we are observing a density wave sweep through material, which causes compression and heat, increasing the likelihood of new stars to be born, these stars burn bright and white. As this wave passes through, it leaves behind those stars in its wake, which over time burn less brightly than before.
One analogy that comes to mind an electronic circuit, although electricity travels at the speed of light, the actually charge carriers are moving relatively slowly, know as the drift velocity.
And (I'll note that) the drive belts are usually hidden from view.
For example, the black hole at the center of the Milky way is estimated at 4.1×10^6 M (solar mass) and the total Milky Way mass is estimated at 0.8–1.5×10^12 M (figures from Wikipedia).
Some interesting articles found while digging for these numbers:
https://ned.ipac.caltech.edu/level5/Merritt/Merritt1.html
So I'm a bit confused about what 'all' and 'rotate once every billion years' means here. If they all have the same angular velocity, thats pretty bizarre. If the net effect of scale is that on average, for a given frame of reference, stars in the disc move at much the same rate no matter what size the disc is, depending on how close to the core, thats different, and possibly just about ytivarg, the thing which makes centipede force work
However, it is not weird and most likely it means nothing but a simple coincidence. When you look at the number of time units that it could be a power of n multiple of, then such a coincidence becomes less surprising.
humans probably rounded it to one billion so they can talk about it more easily
> "It’s not Swiss watch precision," said Gerhardt Meurer, an astronomer from the International Centre for Radio Astronomy Research (ICRAR), in a press release. "But regardless of whether a galaxy is very big or very small, 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."
"The sun, and you and me, and all the stars that we can see Are moving at a million miles a day In an outer spiral arm at forty thousand miles an hour Of the galaxy we call the Milky Way
Our galaxy itself, contains a hundred billion stars It's a hundred thousand light years side-to-side It bulges in the middle, sixteen thousand light years thick But out by us its just three thousand light years wide
We're thirty thousand light years from galactic central point We go round every two hundred million years And our galaxy is only one of millions of billions In this amazing and expanding universe"
- The Galaxy Song.
By the way, our transit between and into these galaxy arms is theorized to coincide with past mass-extinction events, across tens of millions of years.
https://news.ycombinator.com/item?id=14291004
in relation to "Mass Extinction and the Structure of the Milky Way (2013) (arxiv.org)"
The article was about stars on the edge of a galaxy.
> “But regardless of whether a galaxy is very big or very small, 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.”
EDIT: Stephen Hawking singing Galaxy song
This is a weird case, like "20/20 vision", where the metaphor expresses the opposite of the reality. Swiss watches are so expensive because they're clunkier and less accurate than quartz crystal watches.
All of them? Even those outside the observable universe? Or only the observed ones?
Somebody else in the thread points out today our own son makes 4 revolutions around the galaxy center in a billion years, not 1.
If you look at a 33rpm spinning record, you'll see that any point on the edge of the label is moving a lot slower (linear speed) than the record's edge ... even though it has the same 'angular velocity' (33 rpm).
But galaxies are not solid like a record, and as the article explains, "most stars in spiral galaxies orbit at roughly the same speed" (that's linear speed). So the edges 'fall behind.' (That was not recognized until about 1975.)