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Warpine t1_iv5stev wrote

Reply to comment by wthulhu in Closest known black hole to Earth spotted by astronomers by lunarmoonr

Signs are pointing to this being a very unlikely candidate for dark matter

They’re a lot of things Cold Dark Matter theories can explain than stellar mass black hole models can’t

CDM explains baryonic acoustic oscillations, why some galaxies are missing DM while most have a halo of it, the evolution of the structure of the universe, etc

This discounts the mounting evidence against swarms of stellar mass black holes. Dark matter outweighs luminous matter approximately 6 to 1. That means for every star in the sky, there should be a black hole ~6x as massive (on average). That’s a lot of huge black holes!!

We’ve surveyed the night sky for thousands of hours, looking for any signs of stellar mass gravitational lensing, but we just don’t find it in sufficient quantity that could suggest any number of any size black hole can even come close to the mass required to be dark matter

tl;dr: there’s a lot of evidence that suggests DM isn’t black holes

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Nidungr t1_iv601jo wrote

But if dark matter is affected by gravity and does not undergo nuclear reactions, wouldn't much of it form black holes anyway?

Dark matter would attract other dark matter and cluster together, but instead of forming stars, just collapse in on itself?

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sticklebat t1_iv6mmtt wrote

No. Clumping together requires dissipative forces (like friction) — there needs to be some way of removing kinetic energy from a system for it to clump together. The leading theory explaining dark matter is that it is made of weakly interacting massive particles (WIMPs). They interact through gravity and the weak force, but nothing else. Gravity is a conservative force and cannot cause clumping on its own, and the weak force is too weak to cause it to any significant extent, either.

That also explains why dark matter forms roughly spherical halos around galaxies that extend farther out than the regular matter. It also explains observations like the mass distribution of the bullet cluster. Two galaxy clusters collided, and all the ordinary matter slowed down as all the gas and dust collided, but the dark matter just passed right through unimpeded, separating out the regular and dark matter of the clusters.

TLDR: gravity doesn’t cause clustering. It needs help from dissipative forces that dark matter doesn’t really experience.

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Warpine t1_iv7s0fh wrote

I suppose that, technically, gravity isn’t conservative. Energy is lose to gravitational waves as WIMPs pass one another and they eventually COULD collapse into black holes, given sufficient (spitballing number here, no math done) quintillions of years

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sticklebat t1_iv8q5yd wrote

That’s true; GR is not conservative. Your estimate of time is by probably too small by tens, if not hundreds, of orders of magnitude, though. For example, the Earth’s orbit is decaying due to gravitational wave emissions, to the tune of about 200 Watts. Assuming everything else magically stayed the same, it would take about 10^23 years (100,000 quintillion years) for Earth to hit the sun. The rate at which gravitational waves would extract energy from a cloud of dark matter would be unimaginably smaller than that.

Also, if WIMPs actually do interact via the weak force, that would probably stop - or at least further delay - black hole formation as they become more densely packed.

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Warpine t1_iv8vj6q wrote

I figured it would be some absurdly long time scale. I didn't know about the timescale of earth losing energy to gravitational waves & crashing into the sun though; thanks :)

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