Collapse of Dark Matter into Black Holes

[friend]

If Dark Matter is made of particles, then wouldn't it be possible to have clumps of it collapse into a black hole? Would there be any radiation from that collapse that we could see? Would that radiation be distinct from the formation of normal black holes? Shouldn't we be able to measure the dark-matter content of supernovae?

 

[Janus]

Dark matter doesn't tend to clump. Normal matter clumps because it interacts by the electromagnetic forces, which allows it to "stick" together. Dark matter doesn't interact with these forces so it won't stick together.

Also, it is this electromagnetic interaction that allows normal matter to shed excess energy as radiation. Again, DM doesn't react this way, so it can't radiate away energy either (Except gravitationally. But gravitational radiation is so weak that we are still trying to even detect it).

 

[friend]

Dark matter doesn't tend to clump. Normal matter clumps because it interacts by the electromagnetic forces, which allows it to "stick" together. Dark matter doesn't interact with these forces so it won't stick together.

Also, it is this electromagnetic interaction that allows normal matter to shed excess energy as radiation. Again, DM doesn't react this way, so it can't radiate away energy either (Except gravitationally. But gravitational radiation is so weak that we are still trying to even detect it).

OK, suppose that DM does not interact except only through gravitation. Even then, it's easy to imagine a bunch of it very slowing being pulled together until there's enough of it in a small enough place to form a black hole. So what happens when it does collapse? Normal matter has nuclear forces creating pressure to resist collapse. Dark Matter does not. And there is 5 times more Dark Matter than normal matter. I would think that would make Dark Matter collapse much more likely than normal matter collapse. What would that look like?

But if you say that Dark Matter just passes right through everything else so that it returns to the distance it started out at when it first started to fall, then that would mean that higher densities of DM are just a temporary situation so that on average there would be no higher densities, right? I wonder if this all argues against DM being a particle?

 

[Chalnoth]

It doesn't work that way. If an object falls into a potential well, it picks up speed, and then just goes back out the other side. The only way for matter to actually collapse (and stay that way) is for it to lose energy through friction. Dark matter experiences precious little friction, and so tends not to collapse.

 

[AWA]

Hmmm... This Dark matter sure reminds me of the good ol' aether.

 

[Chalnoth]

Hmmm... This Dark matter sure reminds me of the good ol' aether.

Any similarities are exceedingly superficial. A wide body of mutually-corroborating evidence supports the existence of dark matter.

 

[friend]

It doesn't work that way. If an object falls into a potential well, it picks up speed, and then just goes back out the other side. The only way for matter to actually collapse (and stay that way) is for it to lose energy through friction. Dark matter experiences precious little friction, and so tends not to collapse.

It seem entirely possible that enough DM at various distant locations should eventually converge to the same point at the same time, or at least to a certain vicinity at a particular time. This may be enough matter to collapse into a black hole. It's not the chemical or nuclear forces that cause collapse but simply enough mass in a given region. That there is ever a DM density higher in some places than in others argues for the possibility of a density high enough to form a black hole. The question is how likely is it? Or is it possible for DM to escape a black hole, unlikely since it would be bound gravitationally. You have not shown that it is not possible for DM to form a BH.

If DM always passed through everything else without sticking, then the average density would always be constant everywhere including around galaxies. I wonder if the measured density around galaxies can be used to determine the strength of interaction.

 

[Chalnoth]

It seem entirely possible that enough DM at various distant locations should eventually converge to the same point at the same time, or at least to a certain vicinity at a particular time. This may be enough matter to collapse into a black hole. It's not the chemical or nuclear forces that cause collapse but simply enough mass in a given region.

If this were to happen, then the black hole would be rather small, and would therefore rapidly evaporate. The probability of a large black hole appearing in this manner is so absurdly small as to be effectively impossible.

If DM always passed through everything else without sticking, then the average density would always be constant everywhere including around galaxies. I wonder if the measured density around galaxies can be used to determine the strength of interaction.

This isn't entirely the case. Dark matter does experience some friction. The expansion of the universe itself acts as friction, for instance: if an object moves with respect to the expansion, then it will catch up with stuff that's moving faster, which means that it effectively slows down. In this way, various dark matter haloes tend to combine into larger structures.

However, once inside a halo, this friction from expansion does nothing to cause the matter within the halo to collapse further. The dark matter particles just remain in orbit around the center of the halo.

Not forever, of course: these dark matter particles still interact, even if only weakly. And so over very large periods of time the haloes tend to collapse. But this time scale is expected to be many times the current age of our universe.