[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 ;)