Dark Matter & Black Holes

The non amateur stuff. Hawking, black holes, that sort of thing

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JohnH
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Dark Matter & Black Holes

Post by JohnH »

Given that there is thought to be far more dark matter than "normal" matter, and that dark matter will be as much affected by gravity, I am not clear why there isn't a constant stream of dark matter falling into the huge black holes at the centre of (all?) galaxies.
If this does happen, would there be any observational way of detecting it?

Or why does it sit there as a halo, rather than clumping together into dark-matter objects, the equivalent of star formation.
Or why do stars, whose overall composition is fairly well known, not contain at least a noticeable proportion of the dark stuff (which would not be repelled by light pressure as a star lights up).

Perhaps there are well understood reasons for dark matter "avoiding" getting tangled up with normal matter in these ways? Or am I just imagining non-existent problems?
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Paul S
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Re: Dark Matter & Black Holes

Post by Paul S »

I think if we knew any of the answers, we wouldn't be calling it dark matter. My assumption is that it does not follow our understanding of physics re gravity, relativity, Newton etc.
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Cliff
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Post by Cliff »

Dear al(L)
Or does Mond (modified newtonian dynamics) make some sense after all ?
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KendalAstronomer
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Post by KendalAstronomer »

Dark matter is believed to form dark matter objects. Their 'influence' (rather than a direct detection of them) is seen in, for example, galactic mergers. Other than that, dark matter seems to have a limited interaction with normal matter, making it difficult to observe. Observers look for other effects such as the geometrical distortion of space by the gravitational influence of dark matter (which affects light travelling through it) or large scale events that depend majoritively on gravitational forces.
ajb
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Post by ajb »

I assume you are thinking of something like an accretion disk of dark matter heating up and emitting electromagnetic radiation (x-rays and alike)?

Well, dark matter will fall into a black hole. However, as there is practically no interaction between the dark matter particles themselves or "normal" matter there is no "friction" to heat the dark matter. Thus the dark matter will not emit radiation.

To put it another way, in order to radiate the dark matter would have to interact significantly with electromagnetism. Almost by definition dark matter has no appreciable electromagnetic interactions.

So, as already stated in this post the most significant interactions of dark matter are gravitational.
JohnH
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Post by JohnH »

ajb:
Well, dark matter will fall into a black hole. However, as there is practically no interaction between the dark matter particles themselves or "normal" matter there is no "friction" to heat the dark matter. Thus the dark matter will not emit radiation.

Yes that seems to knock on the head any observational evidence for dark matter falling into black holes. Pity!

But KendalAstronomer's:
Dark matter is believed to form dark matter objects.
seems to offer the possibility of observational evidence for "star-scale" condensations of dark matter (presumably observable via gravitational lensing by something which, otherwise, "isn't there". But I don't recall seeing anything about this.
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KendalAstronomer
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Post by KendalAstronomer »

Apologies, by "objects", I meant very large nebulae, galactic haloes, things like that that take part in galactic mergers and are big enough for significant gravitational effets. Not dark matter toasters or even star scale objects, though I'm sure people are looking (not for the toasters).
Cliff
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Post by Cliff »

Dear Johnh
This month's (March) BBC Sky @ Night magazine has an article by Govert Schilling about Dark Stars that might interest you.
Best of luck from Cliff
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Post by Cliff »

Dear Johnh
I forgot to mention that there is also an article in this week's "New Scientist" (! March) " X marks the spot in dark matter web" which suggests that two flowing filaments of dark matter travelling in different directions collide they might create a sort of "double" galaxy ie with two discs of stars and gas. This scenario arises according to a computer simulation by Fabio Governato at the University of Washington, Seattle.
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JohnH
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Post by JohnH »

Thanks Cliff. Yes I saw the New Scientist article, and I'll search out the BBC one.
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Earthshine
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Post by Earthshine »

Bear in mind that gravity is the weakest of the forces. Even with the strength of a black hole, it only affects matter up to a distance of, say, 1000's of miles away from it. As I see it, dark matter seems to exist mostly at the edges of galaxies and even with the dark matter that exists near to a black hole, only a certain amount would get drawn in, just as with any other kind of matter.

A typical black hole will only have a diameter of a few miles or so.
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Re: Dark Matter & Black Holes

Post by greg »

JohnH wrote:Or why does it sit there as a halo, rather than clumping together into dark-matter objects, the equivalent of star formation.
Or why do stars, whose overall composition is fairly well known, not contain at least a noticeable proportion of the dark stuff (which would not be repelled by light pressure as a star lights up).
I have just finished reading an article in 'Sky at Night' (March '08)
It basically asks the question about the existence of dark stars.
Assistance Professor of Physics at the University of Utah in Salt Lake City, Paolo Gondolo and his team are working on a theory that these dark stars were formed by dark matter at the beginning of the universe right after the 'Big Bang' and were the first generation stars.
I find this very intriguing and would like to hear your thoughts. Could they have existed? Do they still exist? Who know's but its definately worth thinking about!
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