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I'm assuming that the answer to the question in the title is a resounding yes. Since Baryonic matter and dark matter interact via gravitational forces.

If this is the case how is information not lost if, as is presumed, dark matter does not interact via the electromagnetic force?

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  • $\begingroup$ The second part of your question is unclear. What does EM interaction (specifically) have to do with information? $\endgroup$ – DilithiumMatrix May 12 '16 at 20:18
  • $\begingroup$ I was thinking perhaps that Hawking radiation allows information to be "saved" as it were, and it is, as I understand, related to the evaporation of a black hole. Hence, it seems inextricably linked with the EM force. Would there be a similar mechanism for dark matter? I presume this is perhaps unanswerable given that these are active areas of research... $\endgroup$ – DarthPlagueis May 12 '16 at 20:36
  • $\begingroup$ Okay, I think I see what you're saying --- my answer below still applies. Without a definitive mechanism by which information is preserved, we can't say how it might differ between DM and 'normal' matter. $\endgroup$ – DilithiumMatrix May 12 '16 at 20:39
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On top of the other excellent answers I'd like to point out that the accretion rate of dark matter particles is believed to be much smaller. The reason matter in accretion disk is being accreted rapidly is because they lose energy from electromagnetic radiation.

For dark matter particles, in practice the only way it can be accreted is if the particle happens to be on a trajectory that falls into the black hole. Close to the black hole the gravity is completely dominated by black hole so gravitational interaction between baryonic and dark matter is very small.

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  • $\begingroup$ Is it possible to add an answer to favourites? $\endgroup$ – DarthPlagueis Jun 12 '16 at 9:19
  • $\begingroup$ ^^ I dont think so, but I appreciate the comment $\endgroup$ – Otto Jun 18 '16 at 7:14
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Yes. Black Holes (BH) can grow from accreting anything with energy --- including dark matter (DM).

I'm not entirely clear on the second part of your question, but probably the most important thing to keep in mind is that the black hole information paradox is still unresolved. Answering how information is not lost for any type of particle, including DM, after being accreted by a BH is a very active area of research, and doesn't have an established answer.

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  • $\begingroup$ The question whether black holes conserve information is nonsensical. A cup of coffee doesn't conserve information. I have no idea how adult men with that kind of brains ever got into this nonsense. One should think they have better things to do... like extracting at least a single strong prediction from string theory. $\endgroup$ – CuriousOne May 12 '16 at 21:01
  • $\begingroup$ @CuriousOne There's nothing about the information paradox intrinsic to string theory. Many think that BH's provide the optimal place to merge gravity with quantum theory, as they are very real, physical objects in which both are strongly at play. Understanding black holes and information, is trying to work towards a unified theory, and one of the only ways with a clear starting point and some progress. Saying it is nonsensical is exactly analogous to the same claims about SR, GR and thought experiments of lights and lasers in elevators and spaceships. $\endgroup$ – DilithiumMatrix May 12 '16 at 21:13
  • $\begingroup$ I didn't say there was, I said that the folks who brought this up have more important things to do. Good physics always starts with observable phenomenology and the information paradox is not observable. I kind of doubt that one can even construct a Gedankenexperiment around it, as gravity can not be shielded, at least not without an infinite amount of matter. I also said that there are useful questions around the topic, e.g. the one if black holes respect any quantum numbers. I have a hunch that they do not, which gives rise to an important prediction that is observable. $\endgroup$ – CuriousOne May 12 '16 at 21:17
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The information paradox has no particular connection to electromagnetism. Hawking radiation is not just photons, it's any sort of particle. And Hawking radiation in itself doesn't solve the information paradox - the problem is that Hawking radiation is supposed to be thermal, so quantum information of infalling objects has been irreversibly lost, but that would violate unitarity.

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