Known Dark Matter Isn't Enough | Hacker News Reader

Known Dark Matter Isn't Enough

briankoberlein.com

30 pointshelpbygrace16 comments

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.

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.

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.

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.

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.

Black holes are relative. Inside you there is a black hole at your center of mass (its just that your event horizon is very far inside you). To things moving at relativistic speeds in relation to you, your mass is huge (to them), and you look like a black hole.

There is not a black hole at your center of mass. Protons and neutrons (let alone atoms) are not dense enough to form a black hole. In all likelihood, your center of mass is empty space between a nucleus and some electrons.

if you don't consider the effects of quantum mechanics then all objects have an event horizon.

No- if you think about a 'classical' particle (that is, ignore quantum effects), once you are inside the particle the gravitational pull is proportional to the distance from the center.

In other words, if you are outside a spherical mass the pull of gravity gets stronger as you get closer (~1/r^2). But once you are inside the mass, the pull gets weaker. Hence there is no black hole at the center of the earth, for example.

you are correct. But what about objects moving past me at near light speeds? Would they see my mass increase to the point that I have an event horizon?

No, because gravitational mass is not the same as (apparent) relativistic mass. I wrote a post on it a while back, but I'm not sure of the protocol regarding comment links.

I would love to read it.

It's here: http://briankoberlein.com/2013/11/07/massive-issues/

Apparently not, but to be honest the reasons are beyond my full understanding.

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

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

Especially for the graph taken from http://arxiv.org/abs/1112.1320

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