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dark matter question


hoola

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There have been numerous estimates using a wide variety of methods generally involved in measuring the total mass and factoring out all known baryonic mass sources. This article for example estimates 90% the total mass  be dark. However the values I have come across are fairly varied. the link is more the textbook answer than a research paper. I don't know the mass estimate bounds are

https://sites.astro.caltech.edu/~george/ay20/eaa-darkmatter-obs.pdf

 

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with these estimations, and if the distribution is inconsistent on small scales, does our particular vicinity within the galaxy allow the possibility of a "dark matter weather", in that within small regions, perhaps "clouds"  of DM pass through the earth at various times, and contribute to the small variations in the value of G measurement?

Edited by hoola
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10 hours ago, hoola said:

with these estimations, and if the distribution is inconsistent on small scales, does our particular vicinity within the galaxy allow the possibility of a "dark matter weather", in that within small regions, perhaps "clouds"  of DM pass through the earth at various times, and contribute to the small variations in the value of G measurement?

The first thing to keep in mind is that while 90% of the mass of our galaxy is estimated to be dark matter, This includes the entire DM halo or a spherical volume that extends well beyond the visible matter disk of the galaxy.  Once you spread it's mass throughout that huge volume, you end up with an extremely low density. 

The other thing is that even though, if you were to take the total mass of the solar system and spread it out evenly throughout a spherical volume enclosed by Neptune's orbit, you would end up with a overall density that would put a man-made vacuum to shame,  it would still be many many times denser on average then, say, a 10 parsec radius sphere in our part of the galaxy. And that 10 parsec sphere would, still contain more regular matter than DM.

It is estimated that the total mass of DM in the Solar system is equivalent to that of 1 small asteroid. Even a 10 fold increase in this density would be insignificant gravitationally to the Solar system.

If this is the case, then how is it that DM can cause discrepancies in the rotation curves or galaxies? 

The visible matter in galaxies like the Milky Way is concentrated in its central bulge and thin disk.  So if you calculate orbits based on visible matter, you need to take this distribution into account.  DM however, is spread out spherically, and the vast majority is "above" and "below" the galactic disk.  And any mass closer to the center of the galaxy than a given star, has a gravitational effect on that star's orbit around the galaxy.

So, for example, if we take that 1 small asteroid's amount of mass spread out throughout the Solar system, and apply that density to the volume of the sphere contained within the Sun's galactic orbit, you get a total mass of DM that is a significant fraction of the total mass of the visible mass of the Milky way; enough to have a noticeable effect on the Sun's galactic orbit.  

The upshot is that star systems like the Solar system are "matter rich dense spots", which makes their internal orbital mechanics essentially immune to the  kind of DM density variation likely to occur.

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thanks mordred, If the overrall density tends to a certain smoothness, and due to an inherent property of non clumpiness, cannot form "clouds" anywhere in the galaxy, I see that as correct. The question of why G is somewhat indeterminate was the source of the question. I had thought that low freq. gravity waves might be a factor sometime back, but was shown to be incorrect.

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5 hours ago, Janus said:

The upshot is that star systems like the Solar system are "matter rich dense spots", which makes their internal orbital mechanics essentially immune to the  kind of DM density variation likely to occur.

Thanks for explaining that relationship, which I did not know.  +1

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On 1/1/2023 at 7:29 PM, Janus said:

The first thing to keep in mind is that while 90% of the mass of our galaxy is estimated to be dark matter, This includes the entire DM halo or a spherical volume that extends well beyond the visible matter disk of the galaxy.  Once you spread it's mass throughout that huge volume, you end up with an extremely low density.  ...

You seem to know a lot about DM, so maybe you can tell us more:

DM particles attracted by massive objects, like stars and planets, may form DM atmospheres around them?

If not, why not?

If yes, can we make a distinction between the mass of the planet/star and the mass of its close DM atmosphere, the denser part (assuming that DM density increases towards the planet/star center, as for regular matter atmosphere), in order to account for all the dark matter?

Since DM does not interact (except gravitational) with regular matter, it is possible that the above mentioned hypothetical DM atmosphere to be not only around the planet/star but also inside it? The estimated 90% includes that DM?

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14 minutes ago, DanMP said:

DM particles attracted by massive objects, like stars and planets, may form DM atmospheres around them? If not, why not?

It may not because there is no friction to hold it there.

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2 hours ago, DanMP said:

You seem to know a lot about DM, so maybe you can tell us more:

DM particles attracted by massive objects, like stars and planets, may form DM atmospheres around them?

If not, why not?

If yes, can we make a distinction between the mass of the planet/star and the mass of its close DM atmosphere, the denser part (assuming that DM density increases towards the planet/star center, as for regular matter atmosphere), in order to account for all the dark matter?

Since DM does not interact (except gravitational) with regular matter, it is possible that the above mentioned hypothetical DM atmosphere to be not only around the planet/star but also inside it? The estimated 90% includes that DM?

Genady has a point.    Gravity causes planets,etc. to form because regular matter interacts electromagnetically. Collisions, friction etc. is a result of this electromagnetic interaction.  A secondary result of this interaction is the production of electromagnetic radiation.  The production of this comes at the expense of kinetic energy from the matter involved. Two particles collide, emit some EMR and separate, but at a slower speed than they met at. This happens enough and a clump of matter of matter forms.

DM does not interact electromagnetically, not only does that mean it doesn't "collide" like regular matter, but it doesn't have the same mechanisim to shed KE.  A DM particle can approach a planet, pass right through it, and fly off with the same speed it started with.  There's is nothing to hold it in the vicinity.

Having said that, There are ways for DM to clump. Gravitational interactions can cause such distributions.  But compared to electromagnetic interaction, they are very,very, very, weak,  and produce results much slower.  The Universe just hasn't been around long enough for small compact collections of DM to form, Just much, much larger and diffuse collections like galactic halos.

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3 minutes ago, Janus said:

Genady has a point.    Gravity causes planets,etc. to form because regular matter interacts electromagnetically. Collisions, friction etc. is a result of this electromagnetic interaction.  A secondary result of this interaction is the production of electromagnetic radiation.  The production of this comes at the expense of kinetic energy from the matter involved. Two particles collide, emit some EMR and separate, but at a slower speed than they met at. This happens enough and a clump of matter of matter forms.

DM does not interact electromagnetically, not only does that mean it doesn't "collide" like regular matter, but it doesn't have the same mechanisim to shed KE.  A DM particle can approach a planet, pass right through it, and fly off with the same speed it started with.  There's is nothing to hold it in the vicinity.

Having said that, There are ways for DM to clump. Gravitational interactions can cause such distributions.  But compared to electromagnetic interaction, they are very,very, very, weak,  and produce results much slower.  The Universe just hasn't been around long enough for small compact collections of DM to form, Just much, much larger and diffuse collections like galactic halos.

This is very interesting.

If, as you say, DM does not interact with the EM force, does it interact with either the strong force or the weak force ?

If not, would that imply that it does not form dark atoms or other particles ?

Leading on from that, there are four  (known) forces that influence our ordinary matter,  so could there be forms of 'matter' that do not interact with one or more of these forces ?

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1 hour ago, studiot said:

If not, would that imply that it does not form dark atoms or other particles ?

I suspect we wouldn’t call them atoms.

Surely someone has solved the Schrödinger equation for a gravitational potential to see what the bound states would look like.

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1 hour ago, Janus said:

Having said that, There are ways for DM to clump. Gravitational interactions can cause such distributions.  But compared to electromagnetic interaction, they are very,very, very, weak,  and produce results much slower.  The Universe just hasn't been around long enough for small compact collections of DM to form, Just much, much larger and diffuse collections like galactic halos.

But do we have any estimation how much denser those galactic halos are compared to intergalactic space (regarding the DM density)? I mean, did galactic DM halos just started to form or is it that all the DM is already clumped in galactic halos?...  I guess, from galaxy rotation curves we can only infer the difference in those densities, not the absolute densities (thus we can only infer the lower limit for the amount of DM)?

I think Genady once argued that general relativity can set an upper DM density limit, even if the DM is near-uniformly distributed through the whole space (although, I am not sure if the dark energy factor would mess up such an calculation).

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1 hour ago, studiot said:

does it interact with either the strong force or the weak force ?

I understand that it is not expected to be color charged hence it doesn't interact strongly. It is expected to interact in some "weak" way, but I don't know what it means. I don't know if anybody does. I'm looking forward to a better answer, too.

41 minutes ago, Danijel Gorupec said:

do we have any estimation how much denser those galactic halos are compared to intergalactic space (regarding the DM density)? I mean, did galactic DM halos just started to form or is it that all the DM is already clumped in galactic halos?

Supposedly, the galactic halos don't end, and there is no specific intergalactic DM:

Quote

Instead galaxies have long outskirts of dark matter that extend to nearby galaxies

(Missing dark matter located: Intergalactic space is filled with dark matter (phys.org))

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At one time, certain species of neutrinos were considered candidated for 'dark' matter.
( don't know if they still are )

We could then examine the interaction cross-section of neutrinos to see what dark matter would act like.

I certainly doesn't clump together to form structures, and easily passes through electromagnetically bound matter.

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Sterile neutrinos are still in consideration as they are predicted by the standard model. I have been looking into the literature to find examples of what the projected cross section would like. However as we have never observed sterile neutrinos it's all naturally conjective 

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23 hours ago, Genady said:
23 hours ago, DanMP said:

DM particles attracted by massive objects, like stars and planets, may form DM atmospheres around them? If not, why not?

It may not because there is no friction to hold it there.

How do we know that?

 

20 hours ago, Janus said:

Genady has a point.    Gravity causes planets,etc. to form because regular matter interacts electromagnetically. Collisions, friction etc. is a result of this electromagnetic interaction.  A secondary result of this interaction is the production of electromagnetic radiation.  The production of this comes at the expense of kinetic energy from the matter involved. Two particles collide, emit some EMR and separate, but at a slower speed than they met at. This happens enough and a clump of matter of matter forms.

DM does not interact electromagnetically, not only does that mean it doesn't "collide" like regular matter, but it doesn't have the same mechanisim to shed KE.  A DM particle can approach a planet, pass right through it, and fly off with the same speed it started with.  There's is nothing to hold it in the vicinity.

Having said that, There are ways for DM to clump. Gravitational interactions can cause such distributions.  But compared to electromagnetic interaction, they are very,very, very, weak,  and produce results much slower.  The Universe just hasn't been around long enough for small compact collections of DM to form, Just much, much larger and diffuse collections like galactic halos.

DM particles may interact (collide?) with other DM particles or other weakly interactive particles:

18 hours ago, Mordred said:

The common feeling is that DM doesn't interact with the strong or EM field. It may interact with itself or other weakly interactive particles. All particles obviously interact with gravity

 

So, we know for sure that DM particles can not form atmospheres around massive objects (star, planets)? Galactic halos are not similar with atmospheres?

We don't know how exactly DM particles are behaving when they get very close (in collision course) to other DM particles, nor to regular particles. It is not possible to "collide" with regular particles as a mosquito with a locomotive, not being noticed at all?

Anyway, besides that we don't know exactly how DM atmospheres would/could form, we have some (observational?) evidence against them?

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1 hour ago, DanMP said:

How do we know that?

Because there is no way to dissipate the energy, except via gravitational radiation.

1 hour ago, DanMP said:

DM particles may interact (collide?) with other DM particles or other weakly interactive particles:

Which would be elastic collisions.

 

1 hour ago, DanMP said:

So, we know for sure that DM particles can not form atmospheres around massive objects (star, planets)? Galactic halos are not similar with atmospheres?

Similar in some ways, perhaps, but not atmospheres.

1 hour ago, DanMP said:

We don't know how exactly DM particles are behaving when they get very close (in collision course) to other DM particles, nor to regular particles. It is not possible to "collide" with regular particles as a mosquito with a locomotive, not being noticed at all?

We can only go by the physics we know.

1 hour ago, DanMP said:

Anyway, besides that we don't know exactly how DM atmospheres would/could form, we have some (observational?) evidence against them?

We know the mass distribution that must exist for the rotation curves we observe.

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10 minutes ago, swansont said:
1 hour ago, DanMP said:

Anyway, besides that we don't know exactly how DM atmospheres would/could form, we have some (observational?) evidence against them?

We know the mass distribution that must exist for the rotation curves we observe.

We can make a distinction between the mass of the massive object and the mass of its potential DM atmosphere in order to know that DM atmosphere is present or not? 

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1 hour ago, DanMP said:

We can make a distinction between the mass of the massive object and the mass of its potential DM atmosphere in order to know that DM atmosphere is present or not? 

The amount of DM that would be near a planet is well outside the precision of planetary mass measurements. (see the comments by Janus on this matter)

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.

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.

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