Can dark matter skew Earth-based gravitational measurements?

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Here's a thought experiment. An otherwise empty space is filled uniformly with dark matter. We introduce a test particle.

Choose an arbitrary spherical region of space. According to your argument, mass of all dark matter inside that region will attract the test particle, and therefore test particle will orbit that region (or fly on hyperbolic trajectory). I.e. test particle won't fly in a straight line.

Which is obviously ridiculous.
This looks like a paradox, but it has a simple solution in GR: if your whole universe is filled with matter, it contracts. There is no orbit with a single particle, but two test particles would still move together faster than without the dark matter - or get different orbits.

You don't need inhomogeneous distributions. A finite ball with constant density (like a galactic halo) gives the same effect - two test masses feel tidal gravity which looks like an attraction between them.
 

FlexGunship

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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.
I think I'm good with this; makes sense. It's not quite intuitive.

I guess this is because, now second-nature, image of mass accreting gravitationally includes the constant bumping of particles into each other leading to an averaging in their momentum over time.

Is this also true of orbital bodies which have cleared their orbits of material? Do they ultimately collide with co-orbital matter? Or is that operation purely gravitational? (Keeping in mind that clearing an orbit can mean accreting the matter or flinging it out of the orbit.)
 
Is this also true of orbital bodies which have cleared their orbits of material? Do they ultimately collide with co-orbital matter?
Yes, they do.
 

Orodruin

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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.
In this scenario there is no mechanism (apart from the very weak gravitational interaction) for the dark matter to get rid of a significant amount of energy in order to become gravitationally bound to the standard matter.
 
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Is this also true of orbital bodies which have cleared their orbits of material? Do they ultimately collide with co-orbital matter? Or is that operation purely gravitational? (Keeping in mind that clearing an orbit can mean accreting the matter or flinging it out of the orbit.)
This is purely gravitational (apart from collisions of course), but they don't have to collide. Objects close to two lagrange points can be in a stable co-orbit, see Trojan (astronomy) at Wikipedia.
 

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