Can dark matter skew Earth-based gravitational measurements?

  1. FlexGunship

    FlexGunship 739
    Gold Member

    I'll be cautious in asking my my question because I'm out of familiar territory. But...

    Given the following:
    • that dark matter interacts with baryonic matter (exclusively?) via gravity,
    • that evidence for dark matter shows that it exists largely near baryonic matter, and
    • we are (almost?) certain we're entirely unable to measure it presently...

    How do we know dark matter hasn't influenced our current measurements of gravitation on earth?

    It's true that we can be confident is our mass-matter relationship understanding, but couldn't our gravity-mass relationship be skewed if we're getting a constant hum of "background gravitation"?
  2. jcsd
  3. phinds

    phinds 8,096
    Gold Member

    Dark matter interacts with both baryonic matter AND itself, gravitationally.

    Well, yes, but it may be that you have stated that backwards in the sense that formation of baryonic matter into galaxies is probably PRECEDED by the formation of large regions of dark matter. Not known for sure but what I'm saying is that it might be more correct to say that baryonic matter clumps in the present of large regions of dark matter (which itself, does not clump the same way)


    First, dark matter density is so low that even if it had the density around Earth that it has in galactic halos, it would be insignificant.

    Second, for reasons that are not understood, it appears that the density of dark matter in the region of our solar system is unexpectedly low. This is a fairly recent discovery and I don't know if any measurements have show that conclusion to be incorrect.
  4. Bill_K

    Bill_K 4,160
    Science Advisor

    Depends on what it is, but if the dark matter particle is the neutralino, it can also self-annihilate.

    Earthbound experiments to detect dark matter hope it is a WIMP, meaning it has weak interaction with baryons.
  5. phinds

    phinds 8,096
    Gold Member

    Didn't know that. Thanks.

    Yeah, I should have pointed that out. Thanks for that as well.

    On rereading, I think the "exclusively" that I was responding to might well have been questioning whether gravity is the exclusive interaction. I was responding as thought it was asking if gravitational interaction was exclusively with baryonic matter.
  6. What do you mean by measurement of gravity? Making that point clear is crucial for a sensible answer.
  7. Can you point to a paper about that lower than expected dark matter density around the Earth? I would like to know more about that.
  8. phinds

    phinds 8,096
    Gold Member

    Regrettably I cannot, but the comments about it that I saw were here on this forum, I'm pretty sure. I did a quick forum search and don't see what I was looking for. Sorry.

    Apparently, the results have been up and down. Here's an article that mentions both:

    And here's one that says it is much HIGHER in our solar system:

    And another that mentions the point of view I expressed but says more recent data says it's wrong:
    Last edited: May 21, 2014
  9. Seems like people are grasping at straws given the absence of actual data.
  10. mfb

    Staff: Mentor

    This is certainly an interesting point, but it should be highlighted that all articles discuss very small influences on the scale of the solar system. Compared to that, our earth is tiny, so the amount of dark matter in our earth is completely negligible.
  11. phinds

    phinds 8,096
    Gold Member

    Yeah, one of the articles said that if it is a high density in the solar system it's about equivalent to a small asteroid and utterly negligible.
  12. Bill_K

    Bill_K 4,160
    Science Advisor

    For those with an interest in clever acronyms, it is worth mentioning two other alternatives that have been proposed: FIMPs (Feebly Interacting Massive Particles) and EWIPs (Extremely Weakly Interacting Particles)!
  13. It's likely that dark matter distribution has characteristic structure scales on the scale of a small galaxy.

    This means that on the scales of Solar System or ever a star cluster dark matter has essentially uniform density. There are no "small denser clouds" of it around.
  14. FlexGunship

    FlexGunship 739
    Gold Member

    Yes, I did mean to convey that. I only meant that it interacts exclusively via gravity (i.e. not EM).

    Comprising ~85% of total matter (, you'd expect that dark matter would be the DOMINANT source of gravity. Although, this assumes that dark matter's distribution roughly matches that of "regular" matter.

    Again, this might be the smoking gun that address my question. But it seems to beg the question (in the actual use of the term:

    My Question: "Couldn't dark matter skew our understanding of "atomic" (normal matter) gravitation?"
    Your Response: "No, because it's uniformly everywhere."

    So... if it's uniformly everywhere... couldn't it skew out understanding/measurement of "atomic" gravitation?

    Or, asked another way: given that we can't directly observe dark matter, how do we factor it out of local gravitation calculations? Or has it always been implicitly included?
  15. Bill_K

    Bill_K 4,160
    Science Advisor

    The SuperCDMS Group at Berkeley maintains a dark matter FAQ, which has this to say on this question:

  16. mfb

    Staff: Mentor

    This assumption is not true.
    To clump on small scales similar to regular matter, dark matter would have to interact via the electromagnetic or strong force, and then it would not be "dark" any more.
  17. If it is *uniformly* everywhere in the Solar System, then it will be undetectable even if its density is not low.
  18. Bill_K

    Bill_K 4,160
    Science Advisor

    Not true, of course. The attractive mass that determines your orbit at radius r is the total mass inside that radius. If the density is uniform, this will increase with r. Consequently there will be an effect on planetary orbits, and especially highly elliptical orbits such as comets.

    This is also why the gravitational effects of dark matter were first noticed in the outer regions of galaxies.
  19. FlexGunship

    FlexGunship 739
    Gold Member

    If gravity is the sole governing force, then why WOULDN'T it concentrate in areas where electromagnetically bonded gravitational bodies (i.e. the Earth, on a local scale) exist? There's nothing about the Earth that would tend to nudge it out of the way. Instead, you have the last 4.5 billion years of attractive force concentrating dark matter into an orbital ring.

    Is there a fifth fundamental force, that causes it to re-disperse?

    This is still where I'm at (and willing to understand why I'm wrong). Since it'll pull equally in all directions, there's a cancelling effect over large areas with a distributed dark mass.

    Or... if it weren't uniformly distributed and it huddled close to other gravitationally significant locales, all it would do is skew you're metrics for determining mass at a distance. You would say: "ah, based on it's orbit, that body must have a mass of 5x1023 kg" when (perhaps, in reality) 0.5% of that mass is not atoms, but dark matter.

    Running this same test at home, (keeping in mind that dark matter could not influence a laboratory scale because it cannot exert force on the scale) the apparent mass of iron on a scale (in the presence of Earth's gravity + dark matter gravity) would vary from that of iron without the dark matter present.

    Imagine the following two scenarios:

    1. A comet in orbit around the sun as we understand it with mass 2x1030 kg of ordinary matter
    2. The same comet in an identical orbit around the sun with a concentration of 1% dark matter (which is constantly falling towards the gravitational center of the sun) instead of regular matter

    What test would you perform to differentiate these two scenarios?

    If your response is: "Well, we know the mass of the sun because we know what it's made of and how much of it there is. And there's nothing significant missing for dark matter to make up." I'd argue that's a tautology; you're assuming the conclusion as a premise.

    The sun is massive. It formed SPECIFICALLY because of gravity. Why wouldn't non-orbital dark matter fall into it? Why wouldn't dark matter in our orbit tend to fall to the gravitational epicenter like all of the other matter that originally formed the Earth?

    Only Cavendish's ORIGINAL experiment to determine the constant of gravitation would be unaffected by dark matter in this way.
  20. Yes, such configuration is not prohibited by laws of physics.
    It is prohibited by laws of statistics :)

    In order to clump up into a planet-sized body, particles need to concentrate in planet's volume *and then lose part of their energy*. If they don't, they will just fly out of that volume back to infinity.

    For normal matter, it's easy. Gas and solids can't penetrate each other - they collide. In doing so, they heat up. Heat gets radiated away, part of the energy is lost, and they can't escape to infinity anymore. Thus, you have accretion process (planetary, stellar, even galactic).

    Dark matter can't do that. Even if dark matter particle falls into already existing dense mass (e.g. Jupiter or Sun), it doesn't collide with the mass - it zips through it on a hyperbolic trajectory, and goes back to infinity.

    Dark matter *can* form dense object only by having N-body gravitation interactions which exchange kinetic energy in a way that small fraction of dark matter particles gets most of it and flies away, leaving a gravitationally bound "globular cluster" of dark matter particles. Such an event is statistically possible, but exceedingly unlikely: about as likely as cup of water spontaneously boiling because air molecules just happened to give their kinetic thermal energy to the water.

    What would *slow down* incoming dark matter particle so that it enters a stable orbit around Earth in order to eventually form a ring? Nothing.

    Sure thing, dark matter probably does fall from infinity into the Sun as we speak. And then it flies right through the Sun and happily continues on its hyperbolic trajectory back to infinity.
  21. phinds

    phinds 8,096
    Gold Member

    It WOULD. Then it would keep going and end up as far away on the other side as it was far away in the first place. then it would do it again. Regular matter doesn't do that because it collides and thus ends up clumping.

    You clearly have not yet gotten your head around the ramifications of the fact that dark matter ONLY interacts gravitationally with ordinary matter.
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