

Known Dark Matter Isn't Enough - helpbygrace
http://briankoberlein.com/2014/08/19/known-dark-matter-isnt-enough/

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readerrrr
I like the idea of black holes being more common. If there is one close we
could visit it someday and yet it doesn't pose any danger to us, since there
is no chance that it is in our solar system.

Given the ratio of one black hole per 1000 stars, which is the best we can
guess right now, the average distance to the closest black hole is 36
lightyears.

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Tloewald
There's a chance one might _pass through_ , which seems very alarming to me.
It also seems like we'd have observed interactions between such black holes
and other stars pretty frequently.

I don't know if we'd have expected to have seen lensing effects from our
whole-sky surveys, but it seems to me that this would also be probable.

Finally, if you send a line through our galaxy, you'd expect it to pass near a
lot of these black holes -- and thus you'd expect to see a lot of microlensing
effects which the article states don't seem to occur nearly often enough.

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readerrrr
I wouldn't worry myself with that. Since a black hole behaves exactly like a
normal star at a large distance, the chance is very small, even if there are
many of them. For example, when our galaxy merges with Andromeda there will be
only ~1 star collision.

Normal black holes are really small, much smaller than a planet, which we can
barely detect now. And those planets must be orbiting a star and in the
correct inclination to be detected.

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BrianKoberlein
Stellar mass black holes are small in size, but supermassive black holes can
be larger than our solar system. Fortunately for us they only exist in the
centers of galaxies. The closest supermassive black hole to us is 30,000 light
years away.

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Tloewald
Also even small (stellar) black holes are very massive — 5x sol and up —
(since they tend to occur from the collapse of very large stars) so having one
pass near or through the solar system could be catastrophic.

Primordial black holes are posited to be possibly so small as to be able to
pass through the earth without interacting with more than a few atoms, which
would make visiting them both difficult and not enormously interesting.

Edit: clarification and added info on primordial black holes which I looked
up.

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Tloewald
Nice survey of dark matter candidates. I had not heard about the primordial
black hole option.

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acqq
The next part, linked now at the end, is also informative:
[http://briankoberlein.com/2014/08/20/already-know-quite-
bit/](http://briankoberlein.com/2014/08/20/already-know-quite-bit/)

Especially for the graph taken from
[http://arxiv.org/abs/1112.1320](http://arxiv.org/abs/1112.1320)

