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Comment by cozzyd | Hacker News Reader

The disc of the Galaxy forms due to the self-interactions ("frictional forces") of ordinary matter. WIMPs shouldn't have those forces and therefore the dark matter halo is probably spherical. If we assume that the dark matter and ordinary matter started with the same angular momentum, then the rotational velocity of stars in the disc should be much faster than the WIMP halo.

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The only frictional forces of ordinary matter that matter on a galactic scale are gravity. And that would affect WIMPs equally.

> If we assume that the dark matter and ordinary matter started with the same angular momentum, then the rotational velocity of stars in the disc should be much faster than the WIMP halo.

I don't see how this can be the case. The orbital period of both would have to be identical in order to be in the same place.

Normal matter loses energy through self-interactions (collisions), which is radiated away. Because the emission is isotropic, angular momentum is conserved even though the total energy isn't. This tends to make the normal matter collapse into a disk, which must rotate faster than the original halo to conserve angular momentum. Dark matter, which hasn't dissipated any energy (gravity is conservative), can move around more, never decaying into the orbits that the normal matter does.

> never decaying into the orbits that the normal matter does.

Exactly my point - it's either in the same orbit as the sun, in which case the orbital speed is the same, or it's not, in which case there's no wind interacting with the detector.

You can't have it both ways.

The dark matter is a gas not a solid. It's not orbiting the sun, it's orbiting the galaxy (technically, not even the galaxy, it's orbiting inside it's own halo mostly). The orbits of the particles would be highly irregular, going in all directions, many with very eccentric orbits, etc. The sun is moving through the "rest frame" of the galaxy at a ~constant speed, but the earth is orbiting the sun so it has fractionally varying relative speed compared to the rest frame (speeding up and down every year relative to the rest frame).

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