Dark Matter Distrbution in Galaxies

[MikeeMiracle]

TL;DR Summary

Could there potentially be Dark Matter stars and Dark Matter planets within our galaxy?

Do we understand Dark Matter enough to know how it is concentrated within a galaxy. For example, would it be evenly distributed like hydrogen would be or could it be concentrated like most matter is? For example could there potentially be Dark Matter stars and Dark Matter planets within our galaxy?

 

[Vanadium 50]

Dark Matter does not clump, so the answer is "no".

 

[MikeeMiracle]

Ok, so the observations indicate a roughly even spread of Dark Matter across the galaxy? Or is it like a halo like the stars concentrated near the center of our galaxy with gradualy less of it as you get further from the galactic center? Just trying to get a rough understanding of where it is.

Thanks in advance.

 

[Janus]

Summary:: Could there potentially be Dark Matter stars and Dark Matter planets within our galaxy?

Do we understand Dark Matter enough to know how it is concentrated within a galaxy. For example, would it be evenly distributed like hydrogen would be or could it be concentrated like most matter is? For example could there potentially be Dark Matter stars and Dark Matter planets within our galaxy?

To form the type of structures you mention requires an interaction between the matter involved beyond just gravity. The matter has to interact via electromagnetism. This allow the particles to "collide" stick together, and also provides a means of ridding itself of energy.
So when two Visible matter particles meet, it is their electromagnetic fields that are interacting, they determine whether the particles stick together, or bounce off each other. Remove this interaction and the particles would pass right through each other like they weren't even there.
With dark matter, even when it meets up with visible matter, it has no "handle" with which to interact.
A dark matter particle falling towards the Earth, would pick up speed as it fell, pass right through the Earth as if there had been a hole drilled through it, and then climb away on the other side. If it didn't pass right through the center of the Earth it would have its path deflected by gravity, it just wouldn't lose any speed.

That is not to say that dark matter avoids all clumping all together. The various gravitational interactions between dark matter and visible matter can and does cause dark matter form large loose structures like the galactic halos, Dark matter is also expected to be slightly denser near the center of galaxies. But without the electromagnetic interaction to speed things up, this is an extremely slow process, And dark matter simply has not had time to condense more.

 

[MikeeMiracle]

Thank you Janus. I understand your statement about the difference between two particles of matter interacting via their electromagnetic fields and that this process does not occur when a dark matter particle and matter particle come close. In the same vein as the two matter particles colliding, I assume we have no reason to believe some similar interaction would occur when two dark matter particles colide?

I.e. The distribution of dark matter in the galaxy has to be roughly even to account for our observations regarding the galatic rotation being what it is?

Thanks again.

 

[Janus]

Thank you Janus. I understand your statement about the difference between two particles of matter interacting via their electromagnetic fields and that this process does not occur when a dark matter particle and matter particle come close. In the same vein as the two matter particles colliding, I assume we have no reason to believe some similar interaction would occur when two dark matter particles colide?

I.e. The distribution of dark matter in the galaxy has to be roughly even to account for our observations regarding the galatic rotation being what it is?

Thanks again.

Two dark matter particles meeting wouldn't be any different in the final result.
Think of particles like skaters on ice. When visible matter particles get close enough, it is like the skaters reaching out and linking hands. Dark matter has no "hands". So a when it nears a visible matter "skater", even though the visible matter has hands to grasp with, the dark matter skater has no hands to grasp. With two dark matter skaters, neither one has hands to grasp with or to be grasped.

Now, that is not to say that dark matter can't interact at all except for gravity. They might be weakly interacting particles, that under the right circumstances, interact via the weak nuclear interaction. But the weak interaction isn't a binding force, but an interaction that changes the nature of the participants (it is involved in nuclear decay, for example) But while we can't rule this out entirely, we also don't have any evidence that points to it either.

 

[lomidrevo]

It is not directly related to your original question, but this recent article demonstrates that our understanding of dark matter and galaxy formation is far from to be complete.
http://www.astronomy.com/news/2019/12/astronomers-find-19-more-galaxies-missing-their-dark-matter

 

[MikeeMiracle]

Interesting aritcle. Personally the whole concept of Dark Matter does not sit well with me, my gut tells me we are looking at some form of modified gravity instead. When the first galaxy was found without any apparent dark matter it was presented as a "blow" to those those believe modified gravity is the answer but as you say there appears to be more to galaxy formation than we curently believe.

Do we know if any of these galaxy's without dark matter have "recently" passed close to another galaxy that might have slowed them down / changed their trajectory. This could explain the lack of dark matter as that would have carried on on it's previous path and left the galaxy's which we find with no dark matter.

 

[lomidrevo]

Interesting aritcle. Personally the whole concept of Dark Matter does not sit well with me, my gut tells me we are looking at some form of modified gravity instead. When the first galaxy was found without any apparent dark matter it was presented as a "blow" to those those believe modified gravity is the answer but as you say there appears to be more to galaxy formation than we curently believe.

There is obviously something missing in our current knowledge (if the observations are correct), but it doesn't mean that dark matter is an incorrect concept. In general, DM is still the best explanation for the observed rotation curves, afaik. The article doesn't dispute the DM concept. It just shows that our understanding of how is DM affecting the formation of galaxies is not complete yet.
I am not an expert, but I think MOND is having even bigger troubles to explain this exceptional cases, ie. galaxies with rotation curves matching "standard/unmodified" gravity.

 

[Buckethead]

Keep in mind that the idea that dark matter exists is a hypothesis. MOND is also a hypothesis and it is likely that there are or will be other hypothesis that may show themselves to be superior. I get weary of hearing "We know dark matter exists but...". We do not know dark matter exists. Period!

 

[Vanadium 50]

"We know dark matter exists but...". We do not know dark matter exists. Period!

Given that we live on a lump of some, it's hard to say it doesn't exist.

Now, if you mean non-baryonic dark matter sufficient to approximately close the universe, I might be closer to agreeing with you.

 

[Buckethead]

Given that we live on a lump of some, it's hard to say it doesn't exist.

Now, if you mean non-baryonic dark matter sufficient to approximately close the universe, I might be closer to agreeing with you.

I do mean non-baryonic dark matter, and I'm kinda pleased you almost agree.

 

[PeroK]

Personally the whole concept of Dark Matter does not sit well with me, my gut tells me we are looking at some form of modified gravity instead.

What have you been eating?

 

[Vanadium 50]

Or worse...who?

 

[berkeman]

Sigh. Thread closed with no warnings or infractions. Unless somebody wants to PM me with a nomination... It IS that time of the year...