Could dark matter compress and form black holes? Since dark matter is even more abundant than normal matter, a dark matter black hole should not be rare...right?

  • $\begingroup$ Another explanation to the question if interested, though it doesn't touch on the possibility of dark matter forming super massive black holes at the center of galaxies, which I think is pretty cool. curious.astro.cornell.edu/the-universe/90-the-universe/… $\endgroup$ – userLTK Sep 29 '15 at 7:26
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    $\begingroup$ The term dark-matter black holes makes little sense, see also my answer. $\endgroup$ – Walter Sep 30 '15 at 12:02
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    $\begingroup$ Once it's a black hole, it doesn't matter (pun intended). $\endgroup$ – Florin Andrei Jun 17 at 18:32

The problem with trying to form a black hole with dark matter is that dark matter can only weakly interact (if at all) with normal matter and itself, other than by gravity.

This poses a problem. To get dark matter concentrated enough to form a black hole requires it to increase its (negative) gravitational binding energy without at the same time increasing its internal kinetic energy by the same amount. This requires some sort of dissipative interaction between dark matter and normal matter (or itself).

The following scenario should make this clear. Suppose we have a lump of dark matter that gravitationally attracts another lump of dark matter. As the two approach each other, they accelerate and gain kinetic energy. The kinetic energy gained will be exactly enough to then separate them to a similar degree to which they started, unless some dissipative process takes place.

An example is to suppose that dark matter is weakly interacting massive particles (WIMPs). WIMPs are gravitationally drawn towards the centres of stars. If the weak interactions happen sufficiently frequently then it might be possible for them to accumulate in stars, rather than shoot through and out the other side.

It has been hypothesised that black holes could be made like this near the centre of a Galaxy, seeded by dense neutron stars. The density of neutron star matter, combined with the enhanced density of dark matter near galaxy centres could result in dark matter accumulation in the neutron stars, leading to the formation of black holes.

Once a black hole is formed then any dark matter that enters the event horizon cannot emerge regardless of what kinetic energy it gains in the process. However, there is still a problem. Material in orbit around a black hole has less angular momentum the closer it orbits. To pass inside the event horizon requires the dark matter to lose angular momentum. Normal matter does this via an accretion disc that can transport angular momentum outwards by viscous torques, allowing matter to accrete. Dark matter has almost zero viscosity so this can't happen.

So building a supermassive black hole from a smaller seed would be difficult, but forming small black holes out of neutron stars might be easier. It has been proposed that a relative lack of pulsars observed towards our own Galactic centre could be due to this process.

  • $\begingroup$ I am curious. I just might not know enough about dark matter. We know it does not interact with ordinary matter except by gravity, and it appears to be more diffusely associated with galaxies than ordinary matter. Do we know anything else about it? It may very well have dissapative mechanisms. It might even have some analogue to chemistry. $\endgroup$ – Aabaakawad Sep 30 '15 at 8:18
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    $\begingroup$ Even if some chance concentration of dark matter manages to form a black hold, you're still likely to have regular old hydrogen atoms and the like whizzing by and getting sucked past the event horizon. I expect a hybrid hole, dark plus normal matter, mixed, is the most common form. $\endgroup$ – Wayfaring Stranger Sep 30 '15 at 9:03
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    $\begingroup$ @Aabaakawad We really don't know anything about dark matter (or dark energy), other than that observations indicate that they exist. There are some proposals that there is a "dark sector", which is non-gravitational physics pretty much exclusively between dark matter. Simulations have had varying degrees of success with no dark sector, so the existence of the sector is suggested to see if it can lead to more accurate simulations: a non-zero interaction cross-section ("dark interactions" maybe?) would give dark matter a way to shed gravitational energy in unobservable ways. $\endgroup$ – zibadawa timmy Sep 30 '15 at 9:07
  • $\begingroup$ @zibadawatimmy, see citations in the footnote within my answer. $\endgroup$ – Aabaakawad Oct 9 '15 at 4:30

As pointed out by Rob Jeffries, forming a black hole (BH) from dark matter (DM) is impossible (unless there is a [hypothetical] interaction by which dark-matter can lose energy that evades all detection). Accreting DM into an existing BH is still unlikely (since DM cannot lose its excess energy and angular momentum as easily as gas), but not impossible and a small fraction of matter accreted into the supermassive BHs (SMBHs) at galactic centres was likely dark.

However, once matter has accreted into a BH, the information about its origins (dark matter or baryonic) is lost. Therefore, it makes no sense to talk about dark matter black holes: black holes don't have any property beyond their mass, spin, and charge (no-hair theorem).

This very property makes primordial black holes interesting as candidates for dark matter, since they would not violate constraints (for example from big-bang nucleosynthesis) about the amount of baryonic matter. However, AFAIK, primordial black holes are out of favour as DM candidates (for reasons beyond the scope of this answer).

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    $\begingroup$ Does DM contribute much to the growth of central black hoes of galaxies? $\endgroup$ – questionhang Sep 30 '15 at 12:38
  • $\begingroup$ @questionhang As I've said a 'small fraction of matter accreted into the supermassive BHs (SMBHs) at galactic centres was likely dark', I'd be surprised if its much more than 1%, likely much less. $\endgroup$ – Walter Oct 1 '15 at 8:35
  • $\begingroup$ why? dm could be difficult to be accreted than baryon matter? $\endgroup$ – questionhang Oct 1 '15 at 11:51
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    $\begingroup$ @questionhang As I've said 'since DM cannot lose its excess energy and angular momentum as easily as gas'. It can only lose it via gravitational interactions (with anything), but that is very inefficient. $\endgroup$ – Walter Oct 8 '15 at 11:43
  • $\begingroup$ @Walter, see citations in my footnote within my answer. $\endgroup$ – Aabaakawad Oct 9 '15 at 4:29

Black holes are the result of mass so concentrated that gravity does not let anything out, including light. Just about the only things we know about dark matter is that it has mass and seems to only interact with ordinary matter through gravity. Since we do not know the physics of dark matter at all, it is impossible to say what processes might concentrate dark matter enough to form a black hole, but theoretically a black hole could be formed from dark matter, or even both ordinary matter and dark matter. Some theorists [footnote] even think that is exactly what super-massive black holes are.

footnote: Jeremiah P. Ostriker of Princeton University, Collisional Dark Matter and the Origin of Massive Black Holes, and Zoltán Haiman of Columbia University, The Formation of the First Massive Black Holes. Also interesting, these papers, from 2011 to now, citing Ostriker's seminal paper.

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    $\begingroup$ @Aabaakaward Can you give several names of "some theorists"? $\endgroup$ – questionhang Oct 8 '15 at 14:51
  • $\begingroup$ @questionhang, Jeremiah P. Ostriker of Princeton University, see arxiv.org/pdf/astro-ph/9912548.pdf (2000), and Zoltán Haiman of Columbia University, see arxiv.org/pdf/1203.6075.pdf (2012), etc. Also interesting, these papers, from 2011 to now, citing Ostriker's seminal paper, _see scholar.google.com/… $\endgroup$ – Aabaakawad Oct 9 '15 at 4:12
  • $\begingroup$ This is interesting material. Apparently, the observational limits on self-interaction are not quite low enough yet to rule this out. $\endgroup$ – Rob Jeffries May 2 '16 at 8:56
  • $\begingroup$ This whole idea of collisional or self-interacting dark matter is just a red herring. There is no compelling reasons to make this assumption, after all cold dark matter (CDM) that at most weakly interacts beyond gravity is in excellent agreement with all data sofar. In case where there may be tensions, the predictions (based on CDM) are not very robust, often relying on numerical simulations of insufficient resolution. Of course, there no direct (or indirect) detection of CDM so far, but that only means that it's presumably not made from WIMPs (or with a cross section well below expectations) $\endgroup$ – Walter Jul 24 at 6:48

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