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Comment by mabbo | Hacker News Reader

"Dark matter" is just a word for the missing mass we can detect gravitationally, but can't otherwise detect. We know there's something out there from it's effects on other normal matter. But it doesn't seem to have any other effect apart from gravitation.

What the authors are suggesting is that this "dark matter" effect is actually the result of many small black holes orbiting galaxies. That they aren't exotic particles, just something we already know about in an unexpected place.

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That's such a good explanation! I chose the spreadsheet life instead of the space life, but I especially love difficult ideas explained simply!

Is the unexpected element of this that it is weird to think that very small black holes interact with each other? What does it mean that they don't collapse into each other and rather stay individual?

The same reason stars or planets don't collide for billions of years. Space is mostly empty and heads-on collisons are rare.

So objects usually need to be captured into mutual orbit first via 3-body interactions and then slowly move closer to each other via further perturbations or gravitational waves.

I'm not certain- I too chose the spreadsheet life, but am a space nerd. Someone correct me where I'm wrong!

I think it has to do with the number of black holes that would be required, the rate that black holes evaporate naturally(?), and our current best theories of the history and mechanisms of the universe. Like maybe in a universe where all these black holes form, we wouldn't expect to see [something we definitely do see]. So you need to adjust more variables to make that fit the puzzle, but now more things we do see don't make sense.

It's a bit like a big Sudoku, but with way more constraints!

Black holes of any realistic size evaporate very slowly, and will be absorbing more mass than they will be radiating for most of the lifetime of the universe.

You’re talking billions of billions of billions of years before most of them even start shrinking.

To be more technically correct. "Dark matter" is the word we use to describe the thing that would explain why galaxies and the stars within them do not move in accordance with the gravitational model we believe explains what we see.

We assume that our gravitational model is correct and that an additional item is needed to bring the model into agreement with the observations made. When a model seems to work so well, we are generally unable to go back and look at the basis (and assumptions) on which our theories are made. This applies in all areas of our lives, including all of our science. That is the nature of who people are.

Wouldn’t these black holes have evaporated by now?

Reasonable sized black holes evaporate extremely slowly. A black hole the mass of the moon would take 10^45 years to evaporate, so none would have evaporated yet in the lifetime of this universe which is about 1.4*10^10 years old. See https://en.wikipedia.org/wiki/Hawking_radiation. Only black holes around 10^11 kilos or less will have evaporated by now. Couldn't think of any objects around that size to compare to, but there are some here: https://en.wikipedia.org/wiki/Orders_of_magnitude_(mass)

The radius of the biggest black hole that would have evaporated by now is about a femtometer, or the size of a proton.

Of course a black hole’s temperature is inversely proportional to its mass, so a tiny black hole would be terrifically hot. Ironically a much more massive black hole is undetectable unless it’s lensing something, or “feeding” on something. A small black hole would have a very high temperature just from Hawking radiation.

Aren‘t those the most energy dense objects known to us? Not sure how large the black holes in the ‚Kugelblitz‘ idea are, but supposedly if we could harvest the energy output of a significant fraction of the sun, we could manufacture them and then use it to travel to other stars rather easily... or destroy entire planets, as those two things unfortunately go hand in hand (in that regard Star Wars may be more realistic than other SciFi funnily enough).

Vehicles are too easily weaponized. The amount of energy it takes to move a human and a cabin is more than enough energy to destroy a small building. People have driven their cars into houses and businesses for as long as cars have existed. Not to mention aircraft. And driving cars into cars is a too common tragedy.

The odd part about Star Wars is the relative weakness of their weapons. Star Trek have "warp core breaches in a bottle" AKA the photon torpedo, which is much better at damaging starships than ramming.

So next question how does this account for the required spin? won't black holes that small would be spinning beyond what is possible through GR? can these black holes be just normal ones or would they be a "naked black hole/singularity"?

According to https://phys.org/news/2016-09-cold-black-holes.html a black-hole greater than the mass of the moon would have an apparent temperature less than that of the cosmic background radiation, so it wouldn't have started shrinking yet. And it'd be very hard to see. Temperature is inversely proportional to mass ( https://en.wikipedia.org/wiki/Hawking_radiation ) so heavier black holes are even colder.

According to https://what-if.xkcd.com/129/ a moon-mass black hole would be as big as a grain of sand.

I don't know how far out LIGO can detect small-mass black-hole mergers, nor how likely these mergers would be. Nor do I know how detectable micro-lensing events would be.

Could anti-matter form a blackhole?

Yes. But it wouldn't be an "anti black hole" or an "antimatter black hole", just a "black hole" like any other.

But as far as we know there are no "anti stars" out there that could collapse and form a black hole. We haven't observed any at least (don't ask me how we could tell an antimatter star from a regular one - but we could tell if one met a regular one).

I haven't studied astronomy as much as I would like, so this is probably a stupid question, but is it possible that something other than star collapses caused a bunch of anti-matter in the early universe to form black holes, both explaining dark matter and answering the question of "where did all the antimatter go?"

It’s not impossible, but we’d have no way of knowing because the result would be just another black hole. What I can say with some confidence is that unless some wildly unexpected physics emerges, there’s no way antimatter only occurred in dense clumps and collapsed before it could annihilate. So could there have been a freak occurrence yielding a black hole from a dense cloud of antimatter? Unlikely, yet possible, but it would still do nothing to explain why matter ended up dominating antimatter.

Yes, antimatter has mass just like normal matter and therefore can contribute to a black hole like normal matter does.

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