I can see why Dark Matter (DM) is unlikely to form concentrations that would trigger the formation of a "Dark Matter Black Hole" but Dark Matter and Atomic ("normal") Matter (AM) each seem to feel the other's gravitational attraction, which would lead us to suppose that some DM would find is way into Black Holes (BHs). The question is whether DM might behave differently when it enters a BH and whether there might be observable consequences, which might shed some hazy light on the nature of DM. We'd need to find a BH in an area known to be rich in DM and also at an observable distance from Earth.
Odd behaviour might include a change in character of the force of attraction between AM and DM such as becoming repulsive, or being overcome by another force, at short range.
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Of course it can; but, once the black hole has formed, the properties of the matter that made it aren't relevant for the properties of the black hole itself, at least within general relativity, that can accommodate dark matter.
Thanks for all your quick responses! Is it not interesting that we never find agglomerations of dark matter, despite its clear susceptibility to gravitational forces, which have brought about large agglomerations of ordinary matter.
If the DM particles experience repulsive forces at closer range then might we expect to see, or be able to detect, haloes of DM particles around black holes, just outside the Schwarzschild radius? Possibly all large agglomerations of normal matter would have such haloes.
Well yes, if DM is subject to repulsive gravity, as some speculation goes, it would stay
away in the halos. This might also explain why it is not readily found.
There's no such notion as ``repulsive gravity''-the term is meaningless, because the exchange of spin 2 particles-which is what defines gravity-is, always, attractive.
The reason no ``agglomerations'' of dark matter have been found is due to the fact that dark matter interacts with ordinary matter, mainly, through gravitational interactions-that are very weak.
Contrary to popular claims, black holes do not produce extraordinarily strong gravitational fields-in any case fields different than those of stars or planets of mass equal to that, which produced the collapse.
Currently cosmologists believe in ΛCDM model, so observational data are used to estimate model parameters (fractions of dark matter, visible matter, dark energy etc) . This is also called standard cosmological model. When we make a statement that dark matter and dark energy exists, we should remember that we are talking in the context of this model. This model is based on the general theory of relativity which applies equally to the dark sector as well. Existence of black holes is a prediction of general relativity; consequently also that of the ΛCDM cosmology. In such a scenario, a black hole can attract visible as well as dark matter via it's gravitational interactions.
Stam NIcolis
Of course you are right and I apologise for my clumsy phraseology. When you write “dark matter interacts with ordinary matter, mainly, through gravitational interactions-that are very weak.” Do you mean
1. “because of the dispersed nature of DM and large separations between DM and AM” or
2. “because it is possible that the force of gravity experienced and/or generated, by DM, is intrinsically weaker than that between two agglomerations of AM of similar mass to the DM”?
If #1: then what can we learn about DM?: For example that, despite the fact that it clearly experiences gravitational forces, which are attractive, the absence of any evidence of agglomeration over a period of up to 14 billion years argues for the presence of principle(s) (such as something akin to the Pauli Exclusion Principle) or forces (such as something akin to the strong nuclear force) that prevent the close approach of DM particles to each other. We don’t even know the density of DM. the average density of atomic matter is., or was, I believe, equivalent to about one atom of hydrogen m-3. I presume there are estimates of the proportion of atomic matter dispersed throughout space compared with the quantity bound in agglomerations. If so, they would provide a basis for comparison.
If #2: then where does that leave us, and where does it leave the TGR?
Biswajoy Brahmachari Thanks for your reply. Does the ACDM cosmological model predict the agglomeration of dark matter? Either way, what assumptions are made about its intrinsic properties?
As you must gave guessed I am not a Physicist. Actually I read Chemistry at Oxford in 1958-62 and then did a D.Phil in Biochemistry in 1962-4. I am 79 (80 in March) and asked this question in the pursuit of knowledge and understanding.
I came across this from 2014 just now:
https://medium.com/starts-with-a-bang/ask-ethan-100-why-doesn-t-dark-matter-form-black-holes-c5b6d90b1883
Ethan doesn't have any answers either, but one question is worth asking: Is DM (in)compressible?
IS there a limit on compressibility as far as Black hole formation is concerned?
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