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nameunknown

Dark Matter Black Hole

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Ok, here's an odd thought, we have some issues with "large black holes" in the early Universe. What if they form of "Dark Matter". Can this happen? They might be a little different to normal black holes...

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Well so called dark matter, if it exists, dose not appear to interact with much if anything at all.  So my guess is that if they already do exist, they  just don't do anything, not with normal matter anyway.  

Edited by saac

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I don't know much about this. I have read that for normal matter to form black holes it needs mechanisms to cool as it contracts and "stick" together. This is mediated by the electromagnetic force which dark matter does not seem to have and so will  it find it hard to reach a critical density. Based on this it seems unlikely. I have read about various simulations based on the LCDM model of the cosmos and they never mention the formation of DM black holes.

Regards Andrew

Just seen saac's reply. If they did form we would not be able to tell them apart from normal matter black holes as the dominant effect is gravity where it has the same properties as normal matter.

Edited by andrew s

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I did realise the "cooling" problem - which was the root of my comment about "they may be a little different" - i.e. the accretion process could not be the same. Of course, the hole itself (forgetting for a moment the  disk, jets etc) should be the same  taking the classiic, mass, spin and perhaps some "charge" model (if DM can have "charge" as EM seems out - but I doubt any "normal" BH has appreciable charge anyway). It was just a thought as to why galactic core BH's could have gotten so big (i.e so different) so early on - otherwise they don't seem to have enough time to form

I did find this just now: http://www.scientificamerican.com/article/dark-matter-black-holes-destroying-pulsars/

and this... http://arxiv.org/pdf/1502.05043v1.pdf

 

Edited by nameunknown
adding refs

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Of course DM may not necessarily be bound by the same thermal constraints as "normal" matter, likewise would accretion necessarily need the same mechanics if it would take place at all - what would be accreted?  As far as I am aware we do not know how DM interacts with gravity, indeed any of the fundamental forces. 

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2 minutes ago, saac said:

Of course DM may not necessarily be bound by the same thermal constraints as "normal" matter, likewise would accretion necessarily need the same mechanics if it would take place at all - what would be accreted?  As far as I am aware we do not know how DM interacts with gravity, indeed any of the fundamental forces. 

We do know how dark matter interacts with gravity that is in effect how it is defined in the LCDM cosmological model. It interacts just as ordinary matter. Further, by definition it does not interact with electromagnetism. These may turn out to be wrong but so could any theory with new evidence.

Regards Andrew

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Thanks andrew I wasn't aware of that.  Is the mode of interaction a function of the definition of DM; in other words we have defined how it interacts?

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In the LCDM model the DM and DE have to have certain properties to allow the model to be fitted to experiment. These include the details of the CMB, gravitational lensing, galaxy rotation etc. This limits the candidate particles in the WIMP model of DM and excludes others like neutrinos which are "hot" i.e. move at relativistic speeds.

I assume that given the current physics we put into the LCDM models DM BHs don't form as they are not discussed e.ghttp://www.ras.org.uk/images/stories/press/Papers/stw077.pdf

Regards Andrew

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Hi Guys,

Thanks for the comments. Not sure we can say that LCDM acts just as ordinary matter under gravity. It certainly partakes of a gravitational interaction, we can see that by its effect on galactic rotation, but whether the Higgs mechanism gives it mass in quite the same way and to the same extent is possibly open to doubt (i.e. no reason to assume it is the same). Also, if DM falls into a black hole one has to resolve the "information paradox" Quantum Physics says information "cannot be lost", while Relativity says that BH's have "no hair" and any information chucked-in is lost (the ultimate "Del *.*"). So either the iinformation that the BH was formed (perhaps in part) from DM is lost or it isn't - there is no clear resolution of that one. Some theories say that the information can be regained from the Hawking Radiation (which isn't normally considered to be possibly DM)....

P

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1 minute ago, nameunknown said:

. Not sure we can say that LCDM acts just as ordinary matter under gravity. It certainly partakes of a gravitational interaction, we can see that by its effect on galactic rotation, but whether the Higgs mechanism gives it mass in quite the same way and to the same extent is possibly open to doubt (i.e. no reason to assume it is the same). 

P

In the LCDM cosmological model based on standard GR it is assumed that it has the same gravitational interaction i.e. a unit mass of dark matter and a unit mass of normal matter bend space-time by the same amount. (This is assumed, in say, gravitational lensing when we calculate how much Dark Matter there is and where it is from the deflection of light.)

Until we identify the WIMP that is dark matter (assuming that is what it turns out to be) we can but speculate of how the Higgs mechanism gives it mass. But in conventional GR if it has a mass then that mass will have the same effect on space-time as and other mass of the same amount. If not then we are into a very different theory of general relativity.

Regards Andrew

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I read an interesting article in New Scientist this week.  The recent LIGO detection falls within the mass range predicted for Primordial black holes formed during the big bang - this would need confirmation by analysis of the type of orbit (primordial black holes would apparently have an elliptical orbit whereas stellar black holes would have a circular orbit) and this will probalby have to wait for LIGOs successor, the Einstein Telescope.  If this all pans out then this would confirm the existence of primordial black holes and if the detection rates match predictiions, which is looking likely, this in turn may give an answer to the problem of the missing mass of the universe and answer the dark matter question.

https://www.newscientist.com/article/mg22930664-600-ligo-could-catch-dark-matter-made-of-black-holes/

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