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Comment by openasocket | Hacker News Reader

How dense is the concentration of dark matter? I mean, it's affected by gravity, so could it/does it come together to form "dark stars" of some form? And what would happen if I had a chunk of dark matter in a lab: what would it look like? Would it be invisible? Would I be able to pick it up in my hand, or would the particles just slip through ordinary matter like a ghost? I'm guessing the answer to most of these is "we don't know," but I'd love to know if we have any guesses.

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Assuming dark matter is a WIMP, I think it would be invisible and fall through your hand (and the earth) when you tried to pick it up. Stars require the particles in question to bang around against each other and interact with each other. WIMP particles wouldn't even be able to interact with each other except gravitationally - ie. they wouldn't "collide" or create pressure in an enclosed space. A cloud of dark matter is likely to stay a cloud for a very very long time rather than collapsing in on itself.

I think of it like an N-body gravity simulation[1] with very very large N and no collision detection.

[1] http://justfound.co/gravity/

> I think it would be invisible and fall through your hand (and the earth)

Why would't it fall to the center of the earth and stay there, since that is the local gravitational center?

No, it would speed past the center until losing moment on the surface of the other side of the planet, and oscillate back and forth. Because there's no "friction" slowing down the dark matter it will continue like this forever, by conservation of energy.

Check out http://physics.stackexchange.com/questions/214950/if-dark-ma...

Aha, DM particles are in eternal orbits!

Thanks!

Wouldn't it -very- slowly lose energy due to gravitational radiation?

If the cloud was inhomogeneous, yes. However, (unless I am mistaken, which is fairly likely,) the more symmetric a bulk collection of particles (such as WIMPs), the less gravitational radiation it would emit as it oscillated. In particular, for every particle falling in from one side of the planet, you have another particle falling in from the other side of the planet, so the mass distribution of the system does not change over time. I don't know how gravetomagnetic effects would come into play.

> speed past the center

What caused these particles to accelerate up to the speeds they are at in the first place, then? Something must be interacting with them quite significantly if they have non-zero velocity?

the force of gravity

Gravity.

Actually, almost the only thing that we know about dark mater is that gravity affect it.

(Actually, we only know that there is something that is making a gravitational pull of normal mater, and we call this mysterious thing "dark mater".)

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