[1053r]

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..."

[tlb]

The constant rotational speed is the observation that requires dark matter to exist. Otherwise, small galaxies would rotate faster just like close planets do.

[marcosdumay]

Dark matter theory tells why galaxies rotate faster than they should. The fact that this speed is constant requires some other explanation.

[avip]

It's not "constant" though, it's "cosmologically constant". Cosmologists would routinely assert that 1 == 1000 holds true.

[tzs]

Couldn'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.

[salty_biscuits]

You are right that this could explain the velocity at a particular radius, but it's the radial velocity profile as you go out that hints at dark matter. The velocity as a function of radius is strange.

[wuliwong]

I 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. :)

[Retra]

Dark matter also has a measurable gravitational lensing effect, as I understand it.

[da_chicken]

I 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]

Dark 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.'

[raattgift]

I'll try to help:

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.

[godelski]

When I learned about dark matter I remember it being discussed about how the spin more like a solid rigid disk (ie: homogeneous mass distribution) vs a non-rigid (non-homogeneous) disk. That the distribution of visible matter did not predict the observed rotation behavior (moment of inertia problem, not center of mass. Though angular momentum is key).

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).

[ben509]

There is some evidence that dark matter is self-interacting: https://arstechnica.com/science/2015/04/new-evidence-that-da...

[avaku]

Love the ant on frisby analogy ;)

[ajross]

For 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]

No, 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/

[teamonkey]

It'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.

[LinuxBender]

Perhaps we are all in a simulation and the galaxy rotation is tied to a single clock source. Developers are sometimes a little lazy.

[leggomylibro]

I'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."

[flukus]

  function getRotationSpeed() {
    //just make it a billion for now, we'll randomize in the next version
    return 1000000000;
  }

[prewett]

Probably 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]

> 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...

[TomK32]

Spec said "make things spin" and so they did. After all, this isn't a rogue-like game with random procedurally generated content and permadeath,

[jkw]

or computation and/or memory space is limited! so trade-offs are needed

[mikeash]

Isn’t it just as suspicious absent dark matter? Seems just plain weird regardless.

[hammock]

If 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.

[mutagen]

They'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

"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...

[poopchute]

"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]

I 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.

[colanderman]

That 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.)

[semi-extrinsic]

This 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]

That analogy does not hold up when the CD you are observing from is in the same plane as all the others.

[jl6]

I suspect that in your example you would observe a number of different apparent orientations, but that there would be some types of apparent orientation that could never occur.

[sosuke]

That 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

[drdebug]

Ok, my mind is boggled too... Anything we can read on the topic ?

[eximius]

I feel like they might have noticed if it was that simple.

[wuliwong]

Galaxies 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

[wuliwong]

Maybe their projections? Like all their axis of rotation are parallel?

[zaarn]

We see some galaxies from top-down so their axis of rotation cannot be parallel to any galaxy we see side-on (like the milky way)

[mkempe]

Did you maybe mean: small, satellite galaxies show up in the same orbital plane as the main galaxy's disk.

[JKCalhoun]

"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]

Actually 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]

In 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]

Well... 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_par...

[jessriedel]

I think the radial scales on which galaxies are frisbee-like (i.e., the tangential velocity is roughly proportional to radius so that rotation period is constant) are much smaller than the scales on which we notice the discrepancy from naive expectations that suggests dark matter. The discrepancy in the velocity curve is at radiuses where both dark-matter and non-dark-matter models predict that velocity increases sub-linearly with radius. (It's true the dark-matter models are closer to linear than the non-dark-matter models, both are clearly distinct for it, and hence show whirlpool-like motion rather than frisbee-like motion). See the third figure here:

https://physics.stackexchange.com/questions/134159/what-is-a...

[hartror]

Dark 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.

[jaekwon]

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.

[jagger27]

I just hope when the next simulation starts my mind gets carried forward.

[psyc]

I hope there's a reroll option. My build is interesting but I can't figure out how to play it.

[mindfulmonkey]

it won't, these are stateless containers

[redler]

It could also be that dark matter is distributed proportionately with observed regular matter.

[stcredzero]

This 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/

[ajnin]

Thanks for reporting this bug, it'll take just a few moments to fix it and restart the simulation ...

[ben509]

It'll be a day, though, it's Wednesday and the CI only runs on Thursdays.

[stcredzero]

Nah. It should definitely be a Monday!

[pbhjpbhj]

So, now it has restarted perhaps the Dev can tell us what the fix was?