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why doesn't dark matter collapse into black holes?

Posted: Sat Feb 02, 2013 3:21 am UTC
by idobox
I understand dark matter is not well understood. For example we don't know what it is, but the most popular hypothesis appears to be WIMPS.

In star formation, a cloud of gas collapses due to gravity, but pressure, then photonic pressure, then degeneration pressure counteract it.
If WIMPS do not interact with matter, they shouldn't be subject to the first two, and quickly form very dense objects. No?

Re: why doesn't dark matter collapse into black holes?

Posted: Sat Feb 02, 2013 4:18 am UTC
by starslayer
Dark matter is collisionless and cannot easily get rid of energy or angular momentum. You have to remove lots of energy from gas to get it to collapse and form stars. Usually, this energy takes the form of photons the gas radiates as it collapses - for a long time, gas cloud collapse is isothermal. When it stops being isothermal, the collapse slows down dramatically. It basically doesn't collapse for the same reason globular clusters don't collapse to a single point (stars in a GC behave much the same way as DM WIMPs do in their interactions with each other).

Re: why doesn't dark matter collapse into black holes?

Posted: Sat Feb 02, 2013 11:39 am UTC
by idobox
I didn't realize WIMPS do no interact with each other. I suppose gravitational waves take a long time to extract energy.

Re: why doesn't dark matter collapse into black holes?

Posted: Sat Feb 02, 2013 5:44 pm UTC
by starslayer
The way they interact is in the name: Weakly Interacting, as in, via the weak interaction. The interaction cross section, should two meet up, is tiny, and their number density is similar to that of typical interstellar gas. It takes quite a while for them to run into each other, and they usually just pass right on by when they do.

Re: why doesn't dark matter collapse into black holes?

Posted: Sat Feb 02, 2013 7:03 pm UTC
by idobox
I didn't get that weakly meant that, I just thought it meant like, not a lot.

Aren't neutrinos exactly like that, except they're very light (if they even have a mass)? Is there room in the standard model for what would essentially be a heavy neutrino?

Re: why doesn't dark matter collapse into black holes?

Posted: Sun Feb 03, 2013 1:34 am UTC
by Diadem
idobox wrote:I didn't get that weakly meant that, I just thought it meant like, not a lot.

Aren't neutrinos exactly like that, except they're very light (if they even have a mass)? Is there room in the standard model for what would essentially be a heavy neutrino?

Yes. Neutrinos have in fact long been one of the possible explanations for what dark matter is, though I think they are discredited as a possible explanation these days.

Re: why doesn't dark matter collapse into black holes?

Posted: Sun Feb 03, 2013 10:29 am UTC
by eSOANEM
Diadem wrote:
idobox wrote:I didn't get that weakly meant that, I just thought it meant like, not a lot.

Aren't neutrinos exactly like that, except they're very light (if they even have a mass)? Is there room in the standard model for what would essentially be a heavy neutrino?

Yes. Neutrinos have in fact long been one of the possible explanations for what dark matter is, though I think they are discredited as a possible explanation these days.


IIRC vanilla-neutrinos have been discounted, but "sterile" neutrinos which don't even interact weakly (so only interact gravitationally) have not.

Re: why doesn't dark matter collapse into black holes?

Posted: Sun Feb 03, 2013 3:45 pm UTC
by Aiwendil
Neutrinos have in fact long been one of the possible explanations for what dark matter is, though I think they are discredited as a possible explanation these days.


That's right. The reason that ordinary neutrinos are not considered a possible explanation of dark matter is that astronomical and Cosmological observations indicate the dark matter must be 'cold' - that is, non-relativistic. But since neutrinos are so light, they are essentially always ultra-relativistic.