I Dark Matter Simulation in Today's APOD

[Buzz Bloom]

Today's APOD

https://apod.nasa.gov/apod/ap171031.html​

shows a simulation picture of the developing universe involving galaxy formations and dark matter thin threads.

I am wondering if any PF participant can answer the following questions:

1. Why does dark matter form thin thread structures rather than clumps?
2. Do the simulations assume other processes in addition to gravity, such as the loss of orbital energy of ordinary matter via photons due to electromagnetic interactions?
3. Are there any similar simulation assumptions about non-gravity processes affecting dark matter?​

 

[Drakkith]

2. Do the simulations assume other processes in addition to gravity, such as the loss of orbital energy of ordinary matter via photons due to electromagnetic interactions?

For dark matter? I wouldn't think so since it doesn't interact via the EM force.

3. Are there any similar simulation assumptions about non-gravity processes affecting dark matter?

Other than possibly the expansion of the universe, I doubt it. I don't even know of any non-gravitational processes that would affect dark matter.

 

[Buzz Bloom]

Hi @Drakkith:

Thanks for your post.

Regarding (2), I was guessing that in the simulation I cited the ordinary matter would likely include loss of orbital energy via photons but I have not been able to find any sources to say that. Regarding (3), I understand that dark matter does not interact with photons, or at least they don't at any level significant enough to have been observed do so up to now. However there seems to be quite a few astronomical physicists who seem to have conjectured that there is some unspecified gravitational mechanism that causes dark matter to loose orbital energy since they have modeled the formation of a disk of dark matter as part of our galaxy. I am also guessing that the simulated formation of thin threads of dark matter in the galaxy would also require such a mechanism.

Regards,
Buzz

 

[Drakkith]

Regarding (2), I was guessing that in the simulation I cited the ordinary matter would likely include loss of orbital energy via photons but I have not been able to find any sources to say that.

When you say "orbital energy", what exactly do you mean? The Earth isn't losing any energy as photons, but gas and dust molecules can lose energy as photons after colliding and it is this which allows a large gas cloud to collapse under gravity.

Regarding (3), I understand that dark matter does not interact with photons, or at least they don't at any level significant enough to have been observed do so up to now. However there seems to be quite a few astronomical physicists who seem to have conjectured that there is some unspecified gravitational mechanism that causes dark matter to loose orbital energy since they have modeled the formation of a disk of dark matter as part of our galaxy.

I see your point. My understanding was that this dark matter lost energy through gravitational interactions with other dark matter, much like how one object in a system of 3 objects can be swung about and sent flying by the gravity of the other two, allowing the other two objects to lose energy and draw closer together in their orbits. However I admit I really don't know much about this. I'd be interested to hear from someone who knows what they've done in this simulation.

 

[Buzz Bloom]

When you say "orbital energy", what exactly do you mean?

Hi Drakkith:

I apologize for using confusing terminology. I would prefer to use proper terminology, but I don't know what a proper phrase would be.

I am not thinking about large masses, but gas and dust particles. When a structure is being formed, like a galaxy or a solar system, the individual gas and dust particles are gravitationally bound to the total mass into orbits/paths (?) relative to the mass distribution. If we choose the center of mass as a reference, each particle has a velocity vector relative to this center of mass. As a particle interacts with others by EM forces, then it's kinetic energy corresponding to it's velocity vector is reduced by the energy of photons radiated away. If the mass has a significant angular momentum, then on the average the likely reduction will be in the direction of the axis of the angular momentum. This will cause a flattening of the mass towards a disk shape.

If dark matter (DM) particles have no mechanism to significantly interact non-gravitationally, then the DM mass cannot flatten. One of the conjectured candidates for DM particles is primordial black holes.

See 4.11 in http://www.slac.stanford.edu/econf/C040802/papers/L002.PDF​

However, they would not have such a mechanism and therefore would not flatten.

Regards,
Buzz

 

[Drakkith]

I apologize for using confusing terminology. I would prefer to use proper terminology, but I don't know what a proper phrase would be.

Ah, okay. I'm not sure what the correct terminology is either. Perhaps gravitational potential energy?

I am not thinking about large masses, but gas and dust particles. When a structure is being formed, like a galaxy or a solar system, the individual gas and dust particles are gravitationally bound to the total mass into orbits/paths (?) relative to the mass distribution. If we choose the center of mass as a reference, each particle has a velocity vector relative to this center of mass. As a particle interacts with others by EM forces, then it's kinetic energy corresponding to it's velocity vector is reduced by the energy of photons radiated away. If the mass has a significant angular momentum, then on the average the likely reduction will be in the direction of the axis of the angular momentum. This will cause a flattening of the mass towards a disk shape.

If dark matter (DM) particles have no mechanism to significantly interact non-gravitationally, then the DM mass cannot flatten.

Well, like I said in my previous post, I don't know what causes dark matter to form filaments and sheets. I don't even know if scientists know.