
Is dark matter made of primordial black holes? - dnetesn
https://phys.org/news/2018-04-dark-primordial-black-holes.html
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cozzyd
Arxiv link:
[https://arxiv.org/abs/1710.05032](https://arxiv.org/abs/1710.05032)

I recently went to a talk from the author of
[https://arxiv.org/abs/1709.06576](https://arxiv.org/abs/1709.06576), which
claims that PBH's can't make up the majority of the dark matter because the
merger rate would be much higher than what LIGO observes.

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philipov
The Primordial Black Holes idea is "probably not totally dead." [0]

Try these lectures on for size:

[0]: linked directly to primordial black holes

[https://youtu.be/vPHfvMQd73A?t=31m39s](https://youtu.be/vPHfvMQd73A?t=31m39s)

[1]: previous lecture

[https://www.youtube.com/watch?v=NJhANpBtUyQ](https://www.youtube.com/watch?v=NJhANpBtUyQ)

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stevemk14ebr
How would this work? What does it mean to be "made of" a black hole

~~~
mabbo
"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.

~~~
jrq
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?

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mabbo
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!

~~~
empath75
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.

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dflock
That would be fairly mind boggling - > 84% of the matter in the universe is
black holes - we'd be living in a universe that's mostly black holes.

~~~
LifeLiverTransp
But wouldnt be all visible light bend and wobble like looking through a pond
of glass pearls when looking at the stars?

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rbanffy
I doubt we could detect the lensing of these black holes so close to the
galaxies they orbit. I think we would, perhaps, eventually see something
falling into one.

edit: someone pointed out the gas in the galactic halo is not dense enough to
form accretion disks around PHBs, so, no. We wouldn't see anything glowing.

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bladedtoys
Wouldn't we expect to see a great many black holes interacting with regular
matter and so emitting a great deal of radiation?

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the8472
black holes do not emit appreciable amounts of radiation themselves. Their
accretion discs do. That in turn requires mass influx large enough to form an
accretion disk with enough friction to radiate. The gas in the galactic halo
and for that matter even the interstellar gas in the center of the galaxy[0]
is just too thin on average to provide that. We only see them when they're
draining mass from a binary companion or swallowing a gas cloud.

[0] [https://www.sciencenews.org/article/dozen-new-black-holes-
fo...](https://www.sciencenews.org/article/dozen-new-black-holes-found-milky-
way-center)

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pokemongoaway
"Could my one fudge factor be used to legitimize my other fudge factor?"
"Could this legitimize my career as philosopher of fudge factors?" Lots of
important questions...

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edem
What if they are not PBHs but particles produced by evaporated PBHs? Does this
make sense at all? I'm not a physicist.

~~~
antonvs
Particles like that would be ordinary matter. In the quantities necessary to
produce the effects attributed to dark matter, we would be able to detect them
easily.

Even if they remained "dark" \- not emitting any light of their own - they
would block or otherwise interfere with the light coming from everything
behind them.

The reason PBHs can be a dark matter candidate is because they can compress
enormous amounts of matter into a very small space, and are difficult to
detect when they're not interacting with ordinary matter.

