Why is there no DM halo around the solar system?

[nikkkom]

Hi @nikkkom and @newjerseyrunner:

I confess that I still find the article unintelligible, but after scanning through it a few times, looking for sections I could more-or-less understand, I have formed a strong impression that the the intensified density regions formed as "hairs" are not gravitationally bound to the solar system, although I failed to find an explicit statement saying that. My interpretation is that the hairs are more-or-less stable, in that they are continuously formed from the input streams of DM, while the DM in the hairs continues to flow through and out of the solar system. Do you think that this interpretation is physically possible?

Hairs won't form, there is nothing to stop DM particles from dispersing after passing the focus.
Take ordinary lens and let parallel rays of light be focused with it. Do you see "light hair" forming when rays of light get focused?

 

[Buzz Bloom]

Hairs won't form, there is nothing to stop DM particles from dispersing after passing the focus.
Take ordinary lens and let parallel rays of light be focused with it. Do you see "light hair" forming when rays of light get focused?

Hi nikkkom:

Thanks for you post.

I think we are not on the same page here.

Regarding

"Do you see "light hair" forming when rays of light get focused?"​

do you think that it might be possible that the phenomenon of (1) gravitational lensing of a stream of DM particles being bent gravitationally while moving through a massive body, might be sufficiently different than (2) light paths being bent by the change of index of refraction at the surface of a lens, so that (1) could produce "hairs" while (2) doesn't?

Regarding

Hairs won't form, there is nothing to stop DM particles from dispersing after passing the focus.​

Why do you think it is impossible for hairs to form continuously, AND then these DM in these hairs disperse and move pass the focal point and on through the solar system into interstellar space?

Regards,
Buzz

 

[nikkkom]

Regarding

"Do you see "light hair" forming when rays of light get focused?"​

do you think that it might be possible that the phenomenon of (1) gravitational lensing of a stream of DM particles being bent gravitationally while moving through a massive body, might be sufficiently different than (2) light paths being bent by the change of index of refraction at the surface of a lens, so that (1) could produce "hairs" while (2) doesn't?

No, I don't think so. In both cases, we have non-interacting particles converging nearly to a point, passing through it and dispersing. (A beam of *electrons* would be different.)

 

[Buzz Bloom]

No, I don't think so. In both cases, we have non-interacting particles converging nearly to a point, passing through it and dispersing. (A beam of *electrons* would be different.)

Hi nikkkom:

Thanks again for your prompt post.

I think we have different interpretations of what a "hair" is.

I believe that you are thinking that a "hair" consists of a specific collection of DM stuff in the shape described in the article as a "hair". I agree that this interpretation of "hair" is impossible.

I am thinking of a "hair" as an oddly hairlike shaped region of space (which may slowly move over time in the general region of the gravitating body), and that there is a continuous flow of DM through (in and then out of) this shape. What makes the shape distinctive is that the density of DM inside this shape is very much larger than the density of the incoming stream of DM.

Regards,
Buzz

 

[nikkkom]

I am thinking of a "hair" as an oddly hairlike shaped region of space (which may slowly move over time in the general region of the gravitating body), and that there is a continuous flow of DM through (in and then out of) this shape.

Yes. And I'm saying that's not going to happen. No "hair", no "tube" and no "cylinder".

 

[nikkkom]

DM particles would have approximately these trajectories:

 

[Buzz Bloom]

DM particles would have approximately these trajectories:

Hi nikkkom:

The following is Figure 1 from the article.

Why do you think the DM paths produced by the gravitational lensing from the article's simulations are impossible?

Regards,
Buzz

 

[nikkkom]

Why do you think the DM paths produced by the gravitational lensing from the article's simulations are impossible?

Gosh.

Because in the picture you posted, something inexplicable bends DM particles' path (where the small box is) and they somehow stop moving sideways. This should not be possible, as DM particles do not interact with themselves.

 

[Buzz Bloom]

Because in the picture you posted, something inexplicable bends DM particles' path and they somehow stop moving sideways.

Hi nikkkom:

As I discussed previously, the DM is not assumed to stop moving.

I am thinking of a "hair" as an oddly hairlike shaped region of space (which may slowly move over time in the general region of the gravitating body), and that there is a continuous flow of DM through (in and then out of) this shape. What makes the shape distinctive is that the density of DM inside this shape is very much larger than the density of the incoming stream of DM.

The focusing just continuously moves a stream of DM through a smaller region of space, so that the density of DM in that smaller region is much increased compared with the input stream. The figure does not show the stream exiting the hair, just entering it, but the interpretation I am describing includes exiting as well, and I think this interpretation is what the article intended.

I apologize that I don't seem to be able to describe this interpretation in a way that communicates clearly.

Regards,
Buzz

 

[Jonathan Scott]

It's not true that DM doesn't interact at all; it's that it only interacts gravitationally.

It seems at least theoretically possible that if some effect causes it to focus into a narrower stream, then the self-gravitational attraction of that stream would refocus it weakly again later, possibly with a sort of diffraction pattern where directional oscillations reinforce and cancel, until it disperses back to a random flow. However, I very much doubt that a sufficiently coherent uniform flow could exist to start that effect in the first place, and I suspect that you'd require very special (physically implausible) conditions to focus the flow sufficiently accurately for the self-gravity to have enough effect to be noticeable.

 

[nikkkom]

It's not true that DM doesn't interact at all; it's that it only interacts gravitationally.

It seems at least theoretically possible that if some effect causes it to focus into a narrower stream, then the self-gravitational attraction of that stream would refocus it weakly again later

I'm googling "DM density"...

http://arxiv.org/abs/1205.4033

"we find that the data imply a local dark-matter density of 0.008 +/- 0.003 Msun/pc^3 = 0.3 +/- 0.1 GeV/cm3"

This is way, way too low for gravitational self-interaction.

 

[Buzz Bloom]

I suspect that you'd require very special (physically implausible) conditions to focus the flow sufficiently accurately for the self-gravity to have enough effect to be noticeable.

Hi Jonathan:

I am not sure I understand the implications of self-gravity here. Are you saying that if a hair structure were to form (as apparently predicted by the Prezeau paper nikkkom cited in post #47) with the DM moving out of the space occupied by the hair as new DM moved into this space, that the DM in the hair would gravitationally influence the later DM being focused towards the hair in such a way that the hair structure would cease to exist?

Regards,
Buzz

 

[Jonathan Scott]

I'm googling "DM density"...

http://arxiv.org/abs/1205.4033

"we find that the data imply a local dark-matter density of 0.008 +/- 0.003 Msun/pc^3 = 0.3 +/- 0.1 GeV/cm3"

This is way, way too low for gravitational self-interaction.

But if you assume an unrealistically ideal uniform flow being focused in an unrealistically ideal mathematical way, you can achieve far higher densities. I'm not saying that's realistic physics, but it might account for the temporary "hair" formation in this simulation. However, I wouldn't expect it to be stable even in theory, so the flow would disperse more randomly afterwards.