Local dark matter distribution

In summary, we don't seem to have any unexplained orbital/gravitational anomalies within the solar system. This suggests that the dark matter distribution is probably similar to the average dark matter density in a galaxy.
  • #1
Frabjous
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We do not seem to have any unexplained orbital/gravitational anomalies within the solar system. What does that imply for the local dark matter distribution?
 
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  • #2
Dark matter density isn't typically radically different from the average density of visible matter in a galaxy. Reading data for Andromeda off Wikipedia its visible mass is about ##1.5\times 10^{12}## solar masses and its radius is about 76,000 ly, giving a density on the order of ##10^{-3}## Suns per cubic light year. Dark matter makes up about 85% of the universe, so you'd expect a density four or five times higher.

The solar system has a mass of about one solar mass and a radius (to the heliopause) of about 120AU or about ##10^{-3}## ly, for an average density of about 200,000,000 Suns per cubic light year, some two hundred billion times higher than the average density of visible matter in a galaxy.

Dark matter doesn't clump the way normal matter does, so you'd expect its density wouldn't vary anything like as much as normal matter. So (given the back of the envelope numbers above) I don't think there's very much dark matter in the solar system compared to the amount of normal matter. On that basis I would suspect that the impact of dark matter on solar system orbits would be on the low side of negligible.

You could add a uniform spherical mass density to the solar system and see what effect it has on orbital periods fairly easily. Then you could estimate how much mass density you need to have a detectable effect.
 
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  • #3
Ibix said:
Dark matter doesn't clump the way normal matter does,
If I understand correctly, we ”observe” dark matter in galactic clusters and filaments. Does this imply that some kind of weak clumping is going on? Or is it something else?
 
  • #4
Ordinary matter collapses into recognizable shapes by gravitational collapse and by shedding the gravitational energy by some means (usually radiation.) Since dark matter does not radiate, it behaves differently. Dark matter near galaxies is more like the halo shape.

slask-png.png


That is described in the paper, The Potato Radius: a Lower Minimum Size for Dwarf Planets you might have fun reading it.
 
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  • #5
Frabjous said:
If I understand correctly, we ”observe” dark matter in galactic clusters and filaments. Does this imply that some kind of weak clumping is going on? Or is it something else?
The difference between dark matter and regular matter is that dark matter doesn't interact with electromagnetism - hence its darkness. When two bits of normal matter collide, what's actually happening is that their electromagnetic fields interact - if you push against a wall the electrons of your skin surface repel those of the wall. And it's that collision process that leads to matter clumping into spinning gas clouds, as the bits of gas collide and lose energy as heat. Dark matter doesn't collide, so where bits of matter would collide, lose energy, and eventually form a solar system, bits of dark matter just carry on about their days.

That said, dark matter interacts with gravity, so you do expect it to fall into gravitational wells. It will typically go whizzing out the other side, but more will fall in behind it. So yes, you expect an increased density of dark matter in a solar system compared to the galaxy, and the galaxy compared to intergalactic space, etcetera. Indeed, it wouldn't do what it does to galaxy rotation curves if it didn't. So my back-of-the-envelope calculation probably overstates the density ratio, but I doubt it overstates it enough for a lack of detectable effects to be surprising.
 
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Apologies for making this look like 20 Questions.
Do we believe that the electromagnetic interaction is zero or is it very small; i.e., below current levels of detectability?
 
  • #8
Frabjous said:
Apologies for making this look like 20 Questions.
Do we believe that the electromagnetic interaction is zero or is it very small; i.e., below current levels of detectability?
I was describing truly collisionless dark matter, but you can allow very very small cross sections for collision. Exactly how big is a topic of active debate, as far as I know.
 
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  • #9
Frabjous said:
We do not seem to have any unexplained orbital/gravitational anomalies within the solar system. What does that imply for the local dark matter distribution?
I can't give a citation but I remember reading (here on PF I believe) that the total mass of dark matter within out solar system is comparable to that of a very modest sized asteroid but it is diffuse, so it has no effective effect on any orbits or anything.
 
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  • #10
phinds said:
I can't give a citation but I remember reading (here on PF I believe) that the total mass of dark matter within out solar system is comparable to that of a very modest sized asteroid but it is diffuse, so it has no effective effect on any orbits or anything.
That's correct, but let's emphasize that the solar system is not just a small scale model of the galaxy. And that most of the galaxy's dark matter it outside the range of the visible matter. This artist's impression depicts that (except that the artist had to make the dark mater look visible to depict it.) Depicting invisible stuff with pictures is hard. :wink:

1673706375496.png
 
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  • #11
Interesting information! Always interested in astronomy! Black matter is amazing! There is so much more to discover in this and other universes!
 
  • #12
BillyDoster said:
Interesting information! Always interested in astronomy! Black matter is amazing! There is so much more to discover in this and other universes!
:welcome:

It's not so easy to discover things in other universes.
 
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  • #13
Frabjous said:
zero or is it very smal
I don't think this is a very good question. Do we know anything? Do we know that energy is conserved? Or that the violation is too small to measure? Do we know that electric charge is conserved? Or just that the violation is too small to measure? Do we know that reindeer don't fly? Or just that every one we see can't (or won't).

It's not a question science can answer, distinguishing between "true zero" and "too small to measure:" What is known is:
  • There is nothing that can be seen or that blocks objects that can be seen down to the measurement floor
  • There is no evidence for DM refracting light down to the measurement floor
  • There is no evidence for accelerating DM radiating EM radiation down to the measurement floor
  • Searches for millicharged particles (i.e. the sort of thing that DM would need to be made of) have all turned out negative.
 
  • #14
Hmm. Thread is locked for Moderation, but looks like the OP may have walked back his comment that was quoted. Under review...
 
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  • #15
After moderator review, the thread will remain closed as the OP question has been answered.
 

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