Would a black hole made of dark matter differ from one made of ordinary matter?

[Spas Stoilov]

let's imagine that we can create a black hole from dark matter. Is it going to be different from the ordinary black hole(which is created by ordinery matter)?

 

[Nugatory]

let's imagine that we can create a black hole from dark matter. Is it going to be different from the ordinary black hole(which is created by ordinery matter)?

No ( as far as we know).

 

[Ibix]

Black holes in GR have three fundamental properties - mass, charge, and angular momentum. Nothing else (they "have no hair"), so their formation history is irrelevant beyond how much mass, how much charge, and how much angular momentum went into them. Hence @Nugatory's answer.

 

[Spas Stoilov]

So if we imagine that all matter is returned somehow after black hole evaporation. How it would be returned as a ordinary matter or dark matter?

 

[Bandersnatch]

As radiation. Black holes evaporate by radiating.

 

[Orodruin]

As radiation. Black holes evaporate by radiating.

"Radiation" is an ambiguous term here. Hawking radiation a priori contains anything that couples to gravity, but unless the temperature is very large (i.e., the BH is very small) in comparison to the mass, emission of massive particles will be severely suppressed.

 

[phinds]

So if we imagine that all matter is returned somehow after black hole evaporation. How it would be returned as a ordinary matter or dark matter?

Mostly as photons and neutrinos up until the BH gets quite small when it gets hot and as Orodruin points out, massive particles can be emitted.

 

[.Scott]

These BHs eat dark matter and eventually emit it as photons and neutrinos (with maybe bigger stuff if the BH fully evaporates).
So, I would guess that there must be a way to convert neutrinos and photons back into dark matter. Do we have any clues about this?

There is another issue: As regular matter spirals through the accretion disk, a great deal of its mass is emitted before ever reaching the event horizon.
But dark matter won't react the same way. It will pass through the accretion disk without loosing any angular momentum or emitting any radiation. So it could only fall into the hole if it had very little angular momentum relative to the BH or if it lost energy through some other mechanism (gravity waves?).

Hmmm... Short of a black hole, can dark matter pass through neutron starts without interacting with them? I wonder if that would be measurable?

 

[phinds]

These BHs eat dark matter and eventually emit it as photons and neutrinos (with maybe bigger stuff if the BH fully evaporates).
So, I would guess that there must be a way to convert neutrinos and photons back into dark matter.

I think you're making an unwarranted leap there.

Hmmm... Short of a black hole, can dark matter pass through neutron starts without interacting with them? I wonder if that would be measurable?

Interesting point. Since DM is known to interact gravitationally, it would seem possible that a slow-moving DM particle (assuming DM IS particulate matter) might be captured by a neutron star and that fast moving ones would not.

 

[Orodruin]

Since DM is known to interact gravitationally, it would seem possible that a slow-moving DM particle (assuming DM IS particulate matter) might be captured by a neutron star and that fast moving ones would not.

Interacting gravitationally is certainly not enough to capture a dark matter particle. You need to additionally get rid of some kinetic energy to become gravitationally bound. This possible process has been very well studied over the last decades, not only in terms of capture on neutron stars, but even more interestingly for capture in the Sun. A dark matter overdensity in the centre of the Sun could possibly lead to DM-DM annihilations into standard model particles. Neutrino telescopes are looking for high-energy neutrinos from the Sun in order to search for this indirect dark matter signature.

 

[phinds]

Interacting gravitationally is certainly not enough to capture a dark matter particle. You need to additionally get rid of some kinetic energy to become gravitationally bound.

Yes, that's exactly why I specified a "slow moving" DM particle; that is, one that would have little kinetic energy.

 

[Orodruin]

Yes, that's exactly why I specified a "slow moving" DM particle; that is, one that would have little kinetic energy.

This does not matter at all. It still needs to get rid of excess kinetic energy to become gravitationally bound.

 

[phinds]

This does not matter at all. It still needs to get rid of excess kinetic energy to become gravitationally bound.

OK, I see what you mean but if it were traveling slow enough would it not at least just oscillate back and forth THROUGH the neutron star?

 

[Orodruin]

OK, I see what you mean but if it were traveling slow enough would it not at least just oscillate back and forth THROUGH the neutron star?

There is just no way for the DM particle to be captured like this without somehow transferring kinetic energy. DM particles from the halo come from very far away, where the potential is basically zero, so essentially regardless of their halo velocity, they will not be gravitationally bound to the star (in other words, they gain enough velocity from falling into the gravitational potential in order not to be bound). They need to get rid of some energy in order to become gravitationally bound. Once this happens through some interaction with the matter in the star, they will have an orbit that at least partially passes through the star and be subject to further interactions. This is the type of processes that eventually would gather the DM particles in the star's core. You can find some of the relevant information along with some more references in one of my latest papers that studied the behaviour of inelastic dark matter in the Sun.

 

[phinds]

There is just no way for the DM particle to be captured like this without somehow transferring kinetic energy. DM particles from the halo come from very far away, where the potential is basically zero, so essentially regardless of their halo velocity, they will not be gravitationally bound to the star (in other words, they gain enough velocity from falling into the gravitational potential in order not to be bound). They need to get rid of some energy in order to become gravitationally bound. Once this happens through some interaction with the matter in the star, they will have an orbit that at least partially passes through the star and be subject to further interactions. This is the type of processes that eventually would gather the DM particles in the star's core. You can find some of the relevant information along with some more references in one of my latest papers that studied the behaviour of inelastic dark matter in the Sun.

Got it. Thanks.