dark matter question

[Domobran7]

20 hours ago, Genady said:

I don't think so. The galaxy rotates because of the collapse of an original cloud into the flattened disk and conservation of angular momentum. No such collapse occurs with DM halo.

x with swansont

But considering that one of reasons why galaxy rotates is the black hole at its center (and black holes formed naturally all seem to rotate, as far as I'm aware), and the dark matter is somewhat responsive to gravity, wouldn't that incite dark matter halo to at least somewhat flatten and rotate? Or the effect is simply too weak?

[Genady]

29 minutes ago, Domobran7 said:

one of reasons why galaxy rotates is the black hole at its center

Is it?

[swansont]

32 minutes ago, Domobran7 said:

wouldn't that incite dark matter halo to at least somewhat flatten and rotate?

Why would it flatten?

Rotation requires angular momentum. What is the origin of that?

[Domobran7]

55 minutes ago, Genady said:

Is it?

Looking at it now, it seems that the effects of the black hole are too localized to really matter. Still, if black hole had no influence at all, why are there so many spiral/disc/etc. shaped galaxies? I always assumed it had to do with influence of the black hole, but could it be the natural result of galaxy's rotation? And if so, would that mean that original galaxies were all roughly spherical, similar to the dark matter halo? But if it is the case, why is the Large Magellanic Cloud an irregularly shaped galaxy? Could it have been a small spiral galaxy that got distorted by the Milky Way, or there is a lower mass limit on spiral / disc galaxies?

[Genady]

1 hour ago, Domobran7 said:

so many spiral/disc/etc. shaped galaxies... could it be the natural result of galaxy's rotation?

These shapes and the rotation are a natural result of gravitational collapse of the cloud of dust and gas which had a higher density of matter than surrounding space.

 

1 hour ago, Domobran7 said:

original galaxies were all roughly spherical, similar to the dark matter halo?

No.

You can find an overview of these processes and related ideas here: Galaxy formation and evolution - Wikipedia

[Mordred]

Another theorem that's a good study is density wave theorem this will cover the flattening effect due to mass rotation. it also covers metallicity distributions as it applies to star formations. as well as how the spiral arms come about. The theory also applies to the rings of Saturn, as the momentum terms apply the same in both cases.

[mistermack]

On 1/4/2023 at 12:24 AM, Genady said:

I don't think so. The galaxy rotates because of the collapse of an original cloud into the flattened disk and conservation of angular momentum. No such collapse occurs with DM halo

I'm not sure I'm with you on that. I know an ice skater speeds up by pulling their arms in, but they pull them in in the plane of spin, not vertically at 90 deg to the rotation. If a sphere collapses down to a disk, I don't see why that should speed up the spin. The reduction in the diameter of a cloud would accentuate any spin that was there, but not the change of shape from sphere to disk. 

I think spin might have originated when two galaxies 'collide' by passing through each other, like the Milky Way and Andromea are forecast to do in the future. Something like that is sure to set off a few swirls.

[Genady]

21 minutes ago, mistermack said:

The reduction in the diameter of a cloud would accentuate any spin that was there, but not the change of shape from sphere to disk. 

You are right. Changing of a shape from sphere to disk does not accelerate the rotation. The reduction in the diameter does. The original cloud of dust and gas is thousands of times larger than the galaxy.

[sethoflagos]

So while the dark matter does experience some gravitational attraction to the galactic disc, it just ploughs straight through it and out the other side without losing momentum whereas normal matter will inevitably dissipate heat due to friction during transit, and ultimately after a few transits will join the disc. Have I got that right?

Edited January 5, 2023 by sethoflagos

[Mordred]

1 minute ago, sethoflagos said:

So while the dark matter does experience some gravitational attraction to the galactic disc, it just ploughs straight through it and out the other side without losing momentum whereas normal matter will inevitably dissipate heat due to friction during transit, and ultimately after a few transits will join the disc. Have I got that right?

right DM can readily pass by other particles in the galaxy without interaction while baryonic matter will interact and those baryonic particles are more likely to join the galaxy as you described

[Genady]

29 minutes ago, sethoflagos said:

So while the dark matter does experience some gravitational attraction to the galactic disc, it just ploughs straight through it and out the other side without losing momentum whereas normal matter will inevitably dissipate heat due to friction during transit, and ultimately after a few transits will join the disc. Have I got that right?

Yes. There is some loss in the dark matter also because of gravitational radiation going away when its particles accelerate and decelerate, but that is many orders of magnitude smaller than the loss to collisions in the normal matter.

[Mordred]

as this subject is on Dm being the sterile neutrino here is some useful tidbits to chew on. In the standard model model SU(3) gauge used to describe neutrinos. The left hand (particle) is a doublet. However the right hand neutrino is a singlet. This has consequence which takes quite a bit of math to explain However let me know if anyone would like to see the related formulas. I will gladly post them. Anyways this has consequences in the weak mixing angles of the CKM matrix via the Higgs seesaw mechanism will have a higher mass term than the left hand neutrino ( a massive partner.) hence cold dark matter (vs warm or hot).

 so as matter doesn't generate pressure p=0 equation of state. While neutrinos do w=1/3 

[DanMP]

23 hours ago, swansont said:

If there were a really large amount of DM in the atmosphere (but not elsewhere nearby) we would probably notice a discrepancy in the gravitational acceleration we have at the surface vs in orbit.

Imagine a scenario in which Earth's atmosphere (the air) is as "visible" as DM. We would notice it due to discrepancy in the gravitational acceleration we have at the surface vs in orbit, or in other way? If the answer is yes, we would still notice it if its mass would be much much smaller and distributed with decreasing density from the center of the planet (not just from the surface) upward?

 

23 hours ago, swansont said:

Another way to look at things is that we do know of particles that interact via the weak force and gravity, but not via EM or strong. Neutrinos.

We do not have an “atmosphere” of neutrinos.

The analogy is not good for many reasons:

1. neutrinos are emerging with relativistic speeds from nuclear reactions, while DM particles don't
2. neutrinos are not forming a galactic halo
3. there is much more DM
4. DM particles may be much lighter than neutrinos

•  

[studiot]

15 minutes ago, DanMP said:

neutrinos are emerging with relativistic speeds from nuclear reactions, while DM particles don't

 

Citation please, from someone who has actually measured DM 'particle' velocities.

Edited January 5, 2023 by studiot

[DanMP]

2 minutes ago, studiot said:

17 minutes ago, DanMP said:

neutrinos are emerging with relativistic speeds from nuclear reactions, while DM particles don't

 

Citation please, from someone who has actually measured DM 'particle' velocities.

I meant DM particles don't emerge from nuclear reactions, otherwise we could say "that one is a DM particle", and we would know much much more about them.

About speed, we don't know much. At least I don't. Do you know someone who has actually measured DM 'particle' velocities?

[MigL]

1 hour ago, DanMP said:

1-neutrinos are emerging with relativistic speeds from nuclear reactions, while DM particles don't

If DM was part of the Standard Model we would expect them to be also.
So either the SM is wrong and DM has to be included, or, DM was generated in the initial 'genesis' from the hot dense state of the early universe. 
Either way, wth few interactions to to slow it down, DM moves at relativistic speeds.

1 hour ago, DanMP said:

2-neutrinos are not forming a galactic halo

Why not ?
They do have mass, but their high speeds would require extremely large orbits.
Depending on the mass of the sterile neutrino, one could calculate the orbital radius and its magnitude compared to galactic radii.

1 hour ago, DanMP said:

3-there is much more DM

You can look up the amount of neutrinos emitted by the Sun each second, multiply that by the number of stars in a typical galaxy, multiply by the amount of galaxies in the visible universe, and multiply by 13 billion years worth of seconds.
Does that seem like a small number to you ?

1 hour ago, DanMP said:

4-DM particles may be much lighter than neutrinos

So you know what DM particles are ?
You seem to 'know' their mass and typical speeds ...

[studiot]

38 minutes ago, DanMP said:

I meant DM particles don't emerge from nuclear reactions, otherwise we could say "that one is a DM particle", and we would know much much more about them.

About speed, we don't know much. At least I don't. Do you know someone who has actually measured DM 'particle' velocities?

The problem, as I see it, is that we don't know anywhere near enough.

These guys know heaps more than I do, yet they readily note that they are still guessing.

https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question59.html

 

We postulate 'particles', we don't know if DM has particles or if these particles can form DM atoms, or what those DM atoms might look like.

So we don't know if there are such reactions as DM nuclear reactions.

 

 

[Genady]

4 hours ago, studiot said:

The problem, as I see it, is that we don't know anywhere near enough.

About 15 years ago I watched an MIT seminar on issues in cosmology. A professor leading it has offered his prediction that in 10 years we will know what dark matter is and we will know that dark energy doesn't exist.

15 years later, we don't know much more than we did then, do we?

[studiot]

1 minute ago, Genady said:

About 15 years ago I watched an MIT seminar on issues in cosmology. A professor leading it has offered his prediction that in 10 years we will know what dark matter is and we will know that dark energy doesn't exist.

15 years later, we don't know much more than we did then, do we?

Yup.

[swansont]

19 hours ago, DanMP said:

The analogy is not good for many reasons:

1. neutrinos are emerging with relativistic speeds from nuclear reactions, while DM particles don't
2. neutrinos are not forming a galactic halo
3. there is much more DM
4. DM particles may be much lighter than neutrinos

If DM particles are much lighter than neutrinos then it would be fairly easy for them to be relativistic.

”relic” neutrinos from about a second after the big bang have very low energy - less than a milli-eV

https://en.m.wikipedia.org/wiki/Cosmic_neutrino_background

[DanMP]

2 minutes ago, swansont said:

If DM particles are much lighter than neutrinos then it would be fairly easy for them to be relativistic.

”relic” neutrinos from about a second after the big bang have very low energy - less than a milli-eV

This is a good point, but for me (thank you). Since we don't know any current process that creates DM particles (as we do for neutrinos), we may asume that they are "relic", having low energy/speeds.

 

18 hours ago, MigL said:

So you know what DM particles are ?
You seem to 'know' their mass and typical speeds ...

No, but nobody knows, so we can not rule out DM atmospheres just saying that neutrinos are not forming atmospheres. We don't even know that they don't, if we can't detect "relic" neutrinos.

A DM/neutrino atmosphere may be much lighter than air atmosphere and "penetrating" under the surface, so it would be very hard to detect using just their gravitational influence. Just see how rapidly air density decreases as you go up ...

 

[DanMP]

19 hours ago, studiot said:

We postulate 'particles', we don't know if DM has particles or if these particles can form DM atoms, or what those DM atoms might look like.

So we don't know if there are such reactions as DM nuclear reactions.

So you suggest that DM particles can form atoms? This is new. What force would keep the nuclei and the "electrons" together? And why not molecules and so on?

[studiot]

2 minutes ago, DanMP said:

So you suggest that DM particles can form atoms? This is new. What force would keep the nuclei and the "electrons" together? And why not molecules and so on?

Please read more carefully the quote you made from my post.

I did not suggest that DM can form atoms, or any other particles.

I said explicitly we don't know if.......

 

[swansont]

3 hours ago, DanMP said:

No, but nobody knows, so we can not rule out DM atmospheres

But you can blithely rule out higher energy DM? No, that’s not how this works.

Formulate a model where you form an atmosphere with DM, rather than declaring by fiat

[hoola]

is the galactic DM halo proposed to orbit along with the same speed and direction of the visible matter in our galaxy?