Does Dark Matter Fall into Black Holes?

[Godot_]

TL;DR Summary

As the Title Says: Does Dark Matter Fall into Black Holes?
And if it does, does it add to their mass?

Does dark matter(DM) fall into black holes(BHs)?
IMHO it should... ...as it does interact with normal matter gravitationally.

Once it's done so, does it add to the BH's mass?
Again, IMHO it should. AFAIK it does have mass...

Would that "quasi-convert" it to normal matter?
...after all, this should expand the BH's event horizon and thus make the DM observable.

What do the professionals say?

References to nice papers a molecular biologist can understand welcome!

[Edit: Found some answers to my first two questions already in the "similar threads" list. Still #3 is somewhat open so far...]

 

[Orodruin]

Yes and yes. But what makes you think that this in any way would convert dark matter to normal matter?

 

[Godot_]

As I wrote above: Once it's in there, and part of the singularity, it's observable.

Also, IIRC there's that local matter-energy-equivalence thing (*wave hands*) in the singularity.

Lastly, that's why I used "quasi-convert"...

...but please do tell me where I erred - after all, that's what I made the thread for.

 

[Orodruin]

As I wrote above: Once it's in there, and part of the singularity, it's observable.

No. You have a misunderstanding of both what the singularity is and that it would be observable. The sigularity is not in any way or form made out of normal matter. It is something appearing in the GR solutions where essentially the math breaks down and cannot tell you exactly what happens. It also is not observable as it is hidden behind the event horizon (or, more technically, it is not in the past light cone of any observer - not even observers falling into the black hole).

Also, IIRC there's that local matter-energy-equivalence thing (*wave hands*) in the singularity.

Lastly, that's why I used "quasi-convert"...

...but please do tell me where I erred - after all, that's what I made the thread for.

Do you have any reference for this? It sounds like pop-sci gibberish.

 

[Bandersnatch]

@Orodruin I think the train of thought is simply along the lines of: DM can't be directly observed -> DM falling into black holes changes the size of the event horizon -> changing size of the EH is an observable that can be used as indirect evidence of DM.

@Godot_ ...which I don't see any problem with, other than practicality. Measuring the precise extent of any EH, let alone its real-time changes at such miniscule rates as can be expected from ongoing matter intake, is beyond current capabilities.
And when you consider that the standard WIMP dark matter is much more difficult to be 'consumed' by a black hole than baryonic matter, owing to it being weakly interacting therefore unable to lose energy in accretion discs (which means that only direct hits, where the orbit of a DM particle intersects the EH, will end up in the black hole), then you have even harder to observe an effect as DM will constitute a very small fraction of matter that at any given moment is hitting such a small target.
Further again, you would have to be able to distinguish the growth of the EH as a result of consuming DM from the same as a result of consuming baryonic matter.

 

[Orodruin]

@Orodruin I think the train of thought is simply along the lines of: DM can't be directly observed -> DM falling into black holes changes the size of the event horizon -> changing size of the EH is an observable that can be used as indirect evidence of DM

That’s a gravitational effect. We have already observed the gravitational effects of dark matter so that would not really be anything new. Even barring the difficulties in observing the effect itself that you mentioned.

 

[Godot_]

[...]The si[n]gularity is not in any way or form made out of normal matter. It is something appearing in the GR solutions where essentially the math breaks down and cannot tell you exactly what happens. [...]

[...] local matter-energy-equivalence thing (*wave hands*) in the singularity.

Do you have any reference for this? It sounds like pop-sci gibberish.

Doh.
a) Which part of "(*wave hands*)" did I write in an incomprehensible way?
b) I seem to remember mentioning in the OP that I'm a MolBiol science guy. Ofc your wording is the proper one, but from my point of view "matter-energy-equivalence", while neither entirely correct nor precise, circumscribes it decently enough... ...please do show some leniency.

But, OK, to subsume it:

DM does fall into BHs and adds to the mass of the singularity. But the effect is as well too small to be of practical relevance, as beyond the realms of GR, and thus non-observable, too.

Which basically answers my question #3. Thanks for the promptness!

 

[Orodruin]

Which part of "(*wave hands*)" did I write in an incomprehensible way?

In physics one generally does not just wave hands. Those arguments tend to be pretty bad and need subsequent backup by better ones.

but from my point of view "matter-energy-equivalence", while neither entirely correct nor precise, circumscribes it decently enough...

Matter-energy equivalence has nothing to do with it. Particularly in the singularity which is not actually part of the spacetime. ”Not correct” would be a better description. So no, it does not describe things at all. I understand that you are not a physicist, but what you said here really made no sense and I think it is fair to point that out.

DM does fall into BHs and adds to the mass of the singularity.

No. Again, the singularity is not something you can think about like this. It is more like a moment in time than it is a place in space. The mass of the black hole (if we are talking the simplest type) is an overall parameter of the spacetime.

But the effect is as well too small to be of practical relevance, as beyond the realms of GR, and thus non-observable, too.

Even if it were not too small to detect, it would tell you nothing new about dark matter.

 

[ohwilleke]

Summary:: As the Title Says: Does Dark Matter Fall into Black Holes?
And if it does, does it add to their mass?

Does dark matter(DM) fall into black holes(BHs)?
IMHO it should... ...as it does interact with normal matter gravitationally.

Once it's done so, does it add to the BH's mass?
Again, IMHO it should. AFAIK it does have mass...

For what it is worth, in theory, this would provide a way to distinguish between explanations of dark matter phenomena involving dark matter particles and those involving modifications or new understandings of gravitational laws.

If one could quantify everything observable going into a black hole, and the black hole changed in mass faster than that, this would suggest that a dark matter particle solution was correct, particularly if the excess change in black hole mass corresponded to the expected amount of dark matter trapped gravitationally into falling into the black hole.

Measuring all that would be hard, but it would not be impossible in principle.

A related subject of active investigation in quite a few published papers is whether dark matter particle capture by highly dense objects made of visible matter like neutron stars would change their "equation of state" and hence their observable properties and the mass at which they collapse into black holes.

This is tricky, however, because the expected effects are very model dependent upon the properties of the dark matter, and because precisely describing the status quo without dark matter in such a large and complex system where gravity has relativistic properties is very challenging. Also, there are other non-dark matter hypotheses (e.g. quark stars and boson stars) that also have to be distinguished from what you would expect from a neutron star or something similar with some proportion of dark matter in it.

 

[PeterDonis]

Once it's in there, and part of the singularity, it's observable.

It's observable because of the black hole's gravitational effects. But it was equally observable before it fell in because of its gravitational effects. No change.

there's that local matter-energy-equivalence thing (*wave hands*) in the singularity.

Don't wave your hands. Either give a valid reference (and I doubt you can) or don't put personal speculations (which is what this looks like to me) in your posts.

DM does fall into BHs and adds to the mass of the singularity.

No, it adds to the mass of the black hole. The singularity is not "the place where the hole's mass is". It's not even a place at all. The mass of the hole is a global property of the hole's spacetime geometry; it can't be localized.

 

[PeterDonis]

A related subject of active investigation in quite a few published papers

Can you provide links?

 

[ohwilleke]

Can you provide links?

See, for example, the following recent papers from 2021 and 2022:

* Reliable quark-nuclear hybrid EoS for neutron stars under modern observational constraints (considering the quark star possibility, an alternate model that would have to be ruled out).
* Capture of Dark Matter in Neutron Stars
* Compactness bounds in General Relativity (general theoretical considerations related to this problem)
* Inferring the neutron star maximum mass and lower mass gap in neutron star-black hole systems with spin (a no new physics baseline analysis)
* Improved Spectral Representations of Neutron-Star Equations of State (another no new physics baseline analysis)
* Revisiting constraints on asymmetric dark matter from collapse in white dwarf stars
* Neutron Stars with Baryon Number Violation, Probing Dark Sectors
* Constraints from compact star observations on non-Newtonian gravity in strange stars based on a density dependent quark mass model (considering the "strange star" alternative)
* On dark stars, Planck cores and the nature of dark matter (more of a thought experiment)
* Dark matter admixed neutron stars
* Strange stars with a mirror-dark-matter core confronting with the observations of compact stars
* Nuclear-Physics Multi-Messenger Astrophysics Constraints on the Neutron-Star Equation of State: Adding NICER's PSR J0740+6620 Measurement (experimental data for analysis by others)
* Multicomponent multiscatter capture of Dark Matter
* Measurement of high energy dark matter from the Sun at IceCube (conceptually similar)
* Solar Reflection of Dark Matter (conceptually similar)
* Constraining Time Dependent Dark Matter Signals from the Sun (conceptually similar)
* Measuring the dark matter environments of black hole binaries with gravitational waves (addressing the ways to study dark matter properties using black holes).
* Constraints on the neutron star's maximum densities from postmerger gravitational-waves with third-generation observations (collecting raw data)
* Simulation of energy transport by dark matter scattering in stars

 

[Ibix]

Would that "quasi-convert" it to normal matter?
...after all, this should expand the BH's event horizon and thus make the DM observable.

Restating something said above, the problem here is that we don't think the singularity is a real thing. We think that singularities in our models of black holes actually suggest that our understanding of gravity is incomplete and we need a better theory (and there are several other lines of argument with the same conclusion), but it's proving trickier to develop one than expected. So the reality is that matter (dark or otherwise) falling into black holes does something we don't understand yet.

We do expect dark matter falling into a black hole to add to its mass, but due to its expected lack of electromagnetic interaction it won't be swept up at anything like the rate of normal matter so the effect may not be as large as you think. However, as ohwilleke points out, there is still a theoretical lever there that might be exploited to detect dark matter if it is a particle of some kind.