[EvaH (OP)]

Listener Rob wants to know

"Is Dark Matter lumpy, or like grains of sand?"

We'll be answering this on our show, but what do you think?

[Origin]

The current observations indicate that dark matter does not clump, it remains individual particles.

[Halc]

It doesn't have a texture at all, neither lumpy nor gritty like sand.
It has the property of mass and little if anything else, so one cannot capture it and put it in a bottle or something since it cannot interact via any other means with the glass nor itself. So it cannot be seen or touched.

[evan_au]

If Dark Matter interacted strongly with itself, the swirls of Dark Matter around a galaxy would collapse into a flat disk shape, like the visible structure of a galaxy.
- However, computer modeling and studies of gravitational lensing suggest that the galactic Dark Matter halo is more like a thicker ellipsoid.
- When a dwarf galaxy is swallowed by a larger galaxy, the Dark Matter halo of the dwarf galaxy is gravitationally bound and orbits the center of the larger galaxy. Over time, the visible stars and the invisible Dark Matter of the dwarf galaxy are eroded by gravitational interactions with the larger galaxy.
See: https://en.wikipedia.org/wiki/Dark_matter_halo#Shape

Some theoretical physicists have speculated that if Dark Matter interacts with itself at all (eg forming some kind of stable "Dark Hydrogen" or "Dark Helium"), then the energy of the collision would need to be dissipated somehow (with some form of "Dark Light", a non-electromagnetic form of radiation).

[Eternal Student]

Hi.

"Is Dark Matter lumpy, or like grains of sand?"

    Don't forget that we don't even have total agreement on whether Dark Matter exists.  There are alternative theories like MOND  (Modified Newtonian Dynamics) that attempt to explain the unusual dynamics of galaxies without the need to assume Dark matter exists.
     Since Dark Matter has not been isolated and examined and most of the theories for the type of particle that might constitute dark matter  (like W.I.M.P.S.) seem to fail to describe all it's properties, all we really know is that our current models of gravity with the matter and energy we can see don't explain the dynamics adequately.   So maybe "Dark matter" isn't any kind of substance at all but just some modification required in our models.

Best Wishes.

[evan_au]

I agree that we aren't really sure what Dark Matter is, except that something is out of alignment.

alternative theories like MOND (Modified Newtonian Dynamics) that attempt to explain the unusual dynamics of galaxies without the need to assume Dark matter exists.

Dark Matter/MACHO theories have severe problems (like we can find them, but there aren't enough of them to matter).
And Dark Matter/WIMP theories also have problems (like we haven't been able to find the WIMPS yet).

MOND also has problems: If you assume that gravity is universal, then Modified Newtonian gravity can explain galaxy rotation curves, but then it doesn't work so well when it comes to galaxy clusters, and explaining the texture of the CMBR.

A local halo of gravitationally-bound WIMPs gives more degrees of freedom (it doesn't need to be a universal law), so it can explain almost anything - but if you can't see the WIMPS, you can't show they are the cause!

See: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Outstanding_problems_for_MOND

[Zer0]

Listener Rob wants to know

"Is Dark Matter lumpy, or like grains of sand?"

We'll be answering this on our show, but what do you think?

Not that my Opinion holds any factual weightage...
Still, I Think Dark Matter might not even be Matter.
More off like Dark Gravity.

Ps - Mass causes Gravity, so does Momentum.
(Equivalence)

[Halc]

Ps - Mass causes Gravity, so does Momentum.
(Equivalence)

[citation needed]
I actually googled this and got fantastic list of sites pushing bad science, but nothing that supports this.

Yes, rotation (like in a spinning space station) causes acceleration, but acceleration isn't gravity, however much they're locally indistinguishable. Plus, the gravity is in a function of neither the momentum nor the angular momentum.  I can generate significant g forces in a system with hardly any angular momentum at all, and I can have zero acceleration in a system with significant angular momentum. So it isn't the momentum, angular or otherwise, that is generating the local equivalent of a gravitational field.

Oh yea: How is this related to a discussion on dark matter?

I Think Dark Matter might not even be Matter.

I guess that depends on your definition of 'matter', on what falls under that term. The physics definition is pretty loose. I got this from Brittanica:
"Matter, material substance that constitutes the observable universe and, together with energy, forms the basis of all objective phenomena."
That groups it in a different bucket than energy, despite the essential equivalence between the two. It does leave open that the 'basis of all objective phenomena' might include something other than matter, but it would then seem to force one to lump dark matter in with 'energy', which I suppose it is, but only in the same way that a rock is.

It has mass, but so does energy, so it isn't matter by default, I agree. Still, few lump it in elsewhere. I can't think of a form of energy that has proper mass and isn't matter, and dark matter definitely has proper mass.

[Eternal Student]

Hi,

    Ps - Mass causes Gravity, so does Momentum.
    (Equivalence)

[citation needed]
I actually googled this and got fantastic list of sites pushing bad science, but nothing that supports this.

Let's help Zer0 out a bit here.  (You don't need any help @Halc , you already know your stuff and should be confident enough already).

I think Zer0 was talking about this kind of thing:
    https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor
The stress-energy tensor takes mass, energy and momentum as it's components.

[Diagram from Wikipedia.  See earlier link]

   Over half of the stress-energy tensor could be considered as something derived from 3-momentum and all of it is if you're considering 4-momentum.

Here's a quote from Wiki:
     The stress–energy tensor, sometimes called the stress–energy–momentum tensor or the energy–momentum tensor, is a tensor physical quantity that describes the density and flux of energy and momentum in spacetime,........... This density and flux of energy and momentum are the sources of the gravitational field in the Einstein field equations of general relativity, just as mass density is the source of such a field in Newtonian gravity.

    However, it's a bit of jump to claim that momentum is equivalent to mass.  (I mean, it just isn't in the wider sense of it but let's just confine our attention to gravitation for the moment).      Even as regards being a source of gravitation or curvature for spacetime there are limits on our ability to exchange momentum for rest mass.  The momentum of objects in space does influence the curvature of spacetime but not in the same way as rest mass would:   Just by looking at the stress-energy tensor we can see that rest mass will contribute to the  0,0  (or time-time) component of the stress energy tensor while 3-momentum contributes to other components.  So there are going to be some solutions to the Einstein Field Equations (some manifolds and metrics) for spacetime that can't be re-created by trading rest mass for momentum.  (This is not a trivial or obvious fact.   It's obvious that the solutions will be different but less clear if there would be a change of co-ordinates which make the solutions physically equivalent - i.e. the metric solutions are just tensor transformations of each other).
    I can't really be bothered to check if it might be possible to identify the different solutions you obtain as being equivalent but in different co-ordinates but I don't think we need to.  The following example seems sufficient to show one situation where we could not hope to find such a transformation.
   
    Brief example with diagrams    Consider space that is empty apart from 3 particles that are initially at rest in a line    •   •   •     That should provide us with a certain metric as a solution to Einsteins Firled Equations.  The particles will move in response to this curvature but we can see that their movement will effectively bring them together and in the natural co-ordinates in which we are regarding the situation, the centre particle never moves and we will be able to identify an event in spacetime where all the particles collide.
   Now we could reduce the mass of the centre particle  to get    •    ^•    •        and we could try and give the centre particle some momentum, possibly enough to compensate for the reduction in mass.  This may give use the same solution for the metric initially but it will only last for one instant, since the centre particle was given a non-zero momentum initially it must move relative to the other two particles and the curvature of space will change in response.  Depending on how much momentum the centre particle has and the initial direction of that momentum we can obtain drastically different results as time evolves.  For example the centre particle may come into contact with one of the particles before the other (and not both simultaneously) or it may be travelling off at some angle to the line of particles with sufficient momentum that it will escape to infinity and never coalesce with either of the big particles.  These are events in spacetime where only 2 out of the 3 particles will occupy the same space at the same time.   I can't see any way in which a change of co-ordinates would be sufficient to make the metric or the curvature of spacetime in this situation equivalent to the metric in the previous situation - but let's go slow and check it a bit more....
     The events we can identify in spacetime are fundamentally different and no change of co-ordinates can map events from one situation to events that don't exist in the second situation.  As compelling as that seems, it's not quite enough to be certain that the curvatures of spacetime won't remain the same.  All we know is that our overall systems must be physically different and not just co-ordinate transformations of each other.  The geography of the landscape might be the same even though we had different collisions between vehicles driving over the landscape - we knew the vehicles were different in mass and momentum initially anyway.  We do need to check carefully that the geography of the landscape must be different.  Since there are some significantly different events (such as when and which particles collide or occupy the same space) we can drop in a test mass at key times and see that it's movement differs   (for example if all the particles have coalesced, as in the first situation, then regardless of where you drop the test mass in, it can only be attracted to one point in space but this is not true if you're in the second situation and only 2 out of 3 particles have coalesced).  From the different movement of test particles we really can conclude that, at least at some time, the curvatures of the manifolds will be physically different and not just co-ordinate transformations of each other.

   In summary:  Mass and momentum are both sources of gravitation.  However, they are not quite equivalent, there are limits on our ability to exchange some rest mass for momentum and retain the same curvature of spacetime.  At best, we could initially obtain a space with the same physical curvature but in most situations this would not be sustained as time evolves.

Best Wishes.

(Final thoughts:  The example has turned out to be longer than I expected and perhaps I should have just gone straight for the Maths but nevermind.... most people prefer some waffle to mathematics).

[Zer0]

Listener Rob wants to know

"Is Dark Matter lumpy, or like grains of sand?"

We'll be answering this on our show, but what do you think?

Not that my Opinion holds any factual weightage...
It ain't Factual.

Still, I Think Dark Matter might not even be Matter.
More off like Dark Gravity.
Unaccounted Gravity seems a better choice of words.

Ps - Mass causes Gravity, so does Momentum.
(Equivalence)
Was referring to G.R.
My Bad!

Ps - If any of the Above makes even little logical sense, then Be Proud, that You All have taught me Well!
🙏
(& If Not, then just spank me & try harder, but please don't call it quits)
🙃

[Eternal Student]

Hi again.

"Is Dark Matter lumpy, or like grains of sand?"

  This podcast has been quite a while coming.  Anyone know if it's out yet?

Best Wishes.

[Bored chemist]

From the point of view of an ant, sand is lumpy.

[evan_au]

Listen/read the answer here: https://www.thenakedscientists.com/articles/questions/qotw-dark-matter-lumps-or-sand