Dark matter and microscopic black holes

[phsopher]

What I was thinking is that if dark matter particles don't interact electromagnetically or by nuclear forces then what is there to stop them coming arbitrarily close to each other thus forming microscopic black holes? And shouldn't we then be able to detect them by Hawking radiation? Does the fact that we don't, tell us something about the nature of dark matter (such as that it must interact in some other way) or about Hawking radiation or am completely on the wrong track? Thanks.

 

[mathman]

The major problem with our knowledge of dark matter is that, except for gravity, they don't seem to interact in any way that we know of.

As far as coming arbitrarily close together, they most likely, as far as we know, would just keep moving past each other, so they wouldn't clump together to form anything.

 

[Chronos]

It is generally beieved [based on observational evidence] that dark matter does not clump. It can apparently pass right through a black hole with nary a hiccup - an amazing feat that not even a photon can duplicate [photons do interact with matter].

 

[Chalnoth]

It is generally beieved [based on observational evidence] that dark matter does not clump. It can apparently pass right through a black hole with nary a hiccup - an amazing feat that not even a photon can duplicate [photons do interact with matter].

What? No, I don't think so. There is nothing about the nature of dark matter that let's it avoid relativity, not that we know anyway. Black holes are small, of course, so they aren't going to run into black holes often. But unless the dark matter is made out of something extraordinarily exotic, it'll get trapped within just like any matter would.

Now, it is true that dark matter does not clump. This is simply because it has no electric charge. Normal matter clumps because it experiences friction. From gas rubbing up against other gas, energy is lost in the form of light. From gas just sitting in orbit around a gravity well, energy is lost due to Bremsstrahlung radiation (accelerated charged radiate).

Because dark matter is nearly collisionless, and has no electric charge, dark matter basically has no way to clump. So it never collects tightly enough to form a black hole.

 

[Matterwave]

What I was thinking is that if dark matter particles don't interact electromagnetically or by nuclear forces then what is there to stop them coming arbitrarily close to each other thus forming microscopic black holes? And shouldn't we then be able to detect them by Hawking radiation? Does the fact that we don't, tell us something about the nature of dark matter (such as that it must interact in some other way) or about Hawking radiation or am completely on the wrong track? Thanks.

We have never detected Hawking radiation. Trying to detect black holes by trying to see their "hawking radiation" is kind of like trying to use your eyes to detect the genes that make up your DNA. Also, by the no-hair theorem, there should be no difference between a black hole created by normal matter and a black hole created by dark matter. So, how can we say "oh this one is formed by dark matter, not baryonic matter"?

Think about it. ;)

 

[phsopher]

Now, it is true that dark matter does not clump. This is simply because it has no electric charge. Normal matter clumps because it experiences friction. From gas rubbing up against other gas, energy is lost in the form of light. From gas just sitting in orbit around a gravity well, energy is lost due to Bremsstrahlung radiation (accelerated charged radiate).

Because dark matter is nearly collisionless, and has no electric charge, dark matter basically has no way to clump. So it never collects tightly enough to form a black hole.

But couldn't the dark matter particles once in a while come so close to each other that gravity would become strong enough to trap them together?

We have never detected Hawking radiation. Trying to detect black holes by trying to see their "hawking radiation" is kind of like trying to use your eyes to detect the genes that make up your DNA. Also, by the no-hair theorem, there should be no difference between a black hole created by normal matter and a black hole created by dark matter. So, how can we say "oh this one is formed by dark matter, not baryonic matter"?

Think about it. ;)

I know that we never detected Hawking radiation, but I was under the impression that this is because the Hawking radiation of stellar black holes is very weak. I thought that if dark matter formed microscopic black holes their Hawking radiation should, at least in theory, be detectable, because the Hawking radiation would be stronger and because it would happen all over the place (no need for stars to collapse and all that stuff). As to telling black holes apart, obviously in principle we can't do it, but I thought that it is unlikely for normal matter to form microscopic black holes because electromagnetic and nuclear forces would stop the collapse for small masses. I could be completely wrong of course.

 

[Chalnoth]

We have never detected Hawking radiation. Trying to detect black holes by trying to see their "hawking radiation" is kind of like trying to use your eyes to detect the genes that make up your DNA. Also, by the no-hair theorem, there should be no difference between a black hole created by normal matter and a black hole created by dark matter. So, how can we say "oh this one is formed by dark matter, not baryonic matter"?

Think about it. ;)

That's not entirely true. Apart from the very end of the evaporation of a black hole, we do have very specific predictions for the spectrum and time dependence of Hawking radiation. Furthermore, very small black holes are expected to be extremely bright objects that evaporate in quite a spectacular flash. So yes, if microscopic black holes existed, detecting their Hawking radiation would work.

 

[stevebd1]

Hi phsopher

Here's a previous thread regarding dark matter and black holes-
https://www.physicsforums.com/showthread.php?t=283242

But couldn't the dark matter particles once in a while come so close to each other that gravity would become strong enough to trap them together?

One hypothesis-
'Search for very high energy gamma rays from WIMP annihilations near the Sun
with the Milagro detector'
http://scipp.ucsc.edu/milagro/papers/PhysRevD_70_083516.pdf

 

[Chalnoth]

But couldn't the dark matter particles once in a while come so close to each other that gravity would become strong enough to trap them together?

Well, this is at the boundary of quantum gravity, where we don't yet know enough to say just how many such particles we'd need in close proximity to one another to produce one. My suspicion is very, very many, far more than you'd get by chance through any reasonable frequency.

Furthermore, if a black hole were produced under such circumstances, it would just look like a normal annihilation between dark matter particles.

 

[phsopher]

I didn't think about them annihilating each other. Thanks for your replies, they've been very helpful.