B Dark matter and extreme gravitational regimes

[member 563992]

If dark matter really lives up to it's name and truly is some form of matter, then wouldn't it feed black holes given extreme gravitational regime in a black hole?

 

[Bandersnatch]

It should, as long as it can hit such a small target.

 

[Ibix]

If dark matter really lives up to it's name and truly is some form of matter, then wouldn't it feed black holes given extreme gravitational regime in a black hole?

Dark matter only interacts gravitationally, so it has trouble losing energy, so the cross-section for capture of dark matter by a black hole is much smaller than for normal matter. But yes, it can be swallowed by a black hole like anything else.

 

[member 563992]

I presume it's difficult to test since dark matter doesn't emit light so we can't see it accruing near a black hole's horizon?

 

[Ibix]

I presume it's difficult to test since dark matter doesn't emit light so we can't see it accruing near a black hole's horizon?

It doesn't particularly accumulate, that's the point. To accumulate it needs some mechanism to dump kinetic energy, which ordinary matter does by collision. But dark matter is (near) collisionless, which is why it forms diffuse halos around galaxies.

Dark matter would only drop into a black hole if it happens to be very nearly on a collision course with the hole. It doesn't form accretion discs, visible or otherwise.

 

[member 563992]

Is there a way to observe Sgt A*, calculate it's size based on normal matter and derive how much dark matter it has swallowed? I imagine there would've been plenty of dark matter that was very nearly on a collision course with the black hole.

 

[Vanadium 50]

If I have a black hole of mass M, how do I know how much of M was originally ordinary matter and how much was originally dark?

 

[PeroK]

If I have a black hole of mass M, how do I know how much of M was originally ordinary matter and how much was originally dark?

It's written on the label!

 

[Orodruin]

I imagine there would've been plenty of dark matter that was very nearly on a collision course with the black hole.

I don’t. As has been mentioned already, dark matter lacks a way to radiate away energy and clump. This makes it relatively less abundant in matter-dense regions.

 

[Ibix]

Is there a way to observe Sgt A*, calculate it's size based on normal matter and derive how much dark matter it has swallowed?

If you could account for its entire history and track every bit of normal matter that fell in, sure. But the thing is older than the Earth and we've only just started to get blurry pictures of it. The data isn't there and can't ever be.

In principle you could track its growth, but I suspect the error bars around how much normal matter it swallows (even if we get vastly better images) would be much larger than the supposed dark matter infall.

 

[Ibix]

imagine there would've been plenty of dark matter that was very nearly on a collision course with the black hole.

Black holes are small. Even Sagittarius A* is only about twenty million kilometres across, less than twice the size of the Sun. You have to hit within 1.5 times that to fall in. It's a really small target on a galactic scale.

 

[PeroK]

If dark matter really lives up to it's name and truly is some form of matter, then wouldn't it feed black holes given extreme gravitational regime in a black hole?

A black hole has the same gravitational attraction as a star of the same mass. From that point of view, the concept of an "extreme gravitational regime" is a misconception. It's only when you within where the surface of the star would have been that the gravity keeps increasing - but you would have crashed into the original star at that point in any case.

 

[member 563992]

If you could account for its entire history and track every bit of normal matter that fell in, sure. But the thing is older than the Earth and we've only just started to get blurry pictures of it. The data isn't there and can't ever be.

In principle you could track its growth, but I suspect the error bars around how much normal matter it swallows (even if we get vastly better images) would be much larger than the supposed dark matter infall.

Darn. I was hoping it would lead to clarifications on the JWST observations of black holes in the early Universe and their sizes.

Edit: Also, it'd have been nice to measure dark matter with the tiny black holes generated by LHC even though they evaporate before having the chance to be observed (of course, still want them to evaporate before growing so they won't swallow Earth but that's a conundrum).

 

[member 563992]

A black hole has the same gravitational attraction as a star of the same mass. From that point of view, the concept of an "extreme gravitational regime" is a misconception. It's only when you within where the surface of the star would have been that the gravity keeps increasing - but you would have crashed into the original star at that point in any case.

My point precisely by saying "extreme gravitational regime" as in: past the event horizon.

 

[PeroK]

My point precisely by saying "extreme gravitational regime" as in: past the event horizon.

That's irrelevant for dark matter "feeding the black hole". The dark matter would have to be local to get sucked in.

 

[member 563992]

That's irrelevant for dark matter "feeding the black hole". The dark matter would have to be local to get sucked in.

If the black hole is moving through space, and that space is filled with dark matter, won't dark matter going past the horizon be feeding the black hole?

 

[Orodruin]

You have to hit within 1.5 times that to fall in.

Reference please. For light, the max impact parameter for the geodesic to end up inside the horizon is $3\sqrt{3}R_S/2$ if I don’t misremember. That’s about 2.6 times the Schwarzschild radius.

 

[Orodruin]

If the black hole is moving through space, and that space is filled with dark matter, won't dark matter going past the horizon be feeding the black hole?

Yes it will, but there won’t be so much dark matter for it to matter significantly.

 

[member 563992]

Yes it will, but there won’t be so much dark matter for it to matter significantly.

Won't this allow for study on how dark matter interacts in extreme gravitational regimes?

 

[Ibix]

Reference please.

I was just going with the photon sphere. You seem to have done an actual calculation.

 

[PeroK]

Won't this allow for study on how dark matter interacts in extreme gravitational regimes?

How so?

 

[member 563992]

How so?

If it interacts gravitationally different from visible matter it might give us clues into the nature of dark matter, no?

 

[member 563992]

If it interacts gravitationally different from visible matter it might give us clues into the nature of dark matter, no?

Let me expand on that: if dark matter is throughout our galaxy, and thus our solar system, and, we produce black holes at LHC, couldn't we study their expected evaporation due to visible matter and measure the properties of dark matter if the evaporation rate is different from what we normally would expect just with visible matter?

 

[PeroK]

Let me expand on that: if dark matter is throughout our galaxy, and thus our solar system, and, we produce black holes at LHC, couldn't we study their expected evaporation due to visible matter and measure the properties of dark matter if the evaporation rate is different from what we normally would expect just with visible matter?

Cosmology would take some giant steps forward if we could recreate a galaxy at the LHC and study it in the lab - dark matter and all. Sadly, all attempts even to detect dark matter in the lab have failed. That's why some are now skeptical of its existence!

 

[member 563992]

Cosmology would take some giant steps forward if we could recreate a galaxy at the LHC and study it in the lab - dark matter and all. Sadly, all attempts even to detect dark matter in the lab have failed. That's why some are now skeptical of its existence!

Sure, but we can create tiny black holes in collisions at the LHC and study how fast they evaporate. If dark matter is matter that succumbs to the gravitational field of a black hole, with our solar system going through the galaxy, which implies going through dark matter, shouldn't we be able to measure the influence of dark matter on the evaporation of the tiny black holes?

 

[PeroK]

Sure, but we can create tiny black holes in collisions at the LHC and study how fast they evaporate.

How long have you been working on black hole evaporation at the LHC?

 

[member 563992]

Nevermind, it seems we didn't detect microscopic black holes @ the LHC.

https://cms.cern/news/search-micros...scopic,models that postulate extra dimensions.

I wonder if cosmic rays which happen at higher energies than those LHC can produce create microscopic black holes and thus test dark matter via evaporation.

 

[member 563992]

Nevermind, it seems we didn't detect microscopic black holes @ the LHC.

https://cms.cern/news/search-microscopic-black-hole-signatures-large-hadron-collider#:~:text=No experimental evidence for microscopic,models that postulate extra dimensions.

I wonder if cosmic rays which happen at higher energies than those LHC can produce create microscopic black holes and thus test dark matter via evaporation.

Well, ChatGPT clarifies this one as well.

Theoretically, cosmic rays with extremely high energies, such as those in the exa-electronvolt range, could, under certain models, produce microscopic black holes when they interact with particles in the Earth's atmosphere. This idea is based on some speculative extensions of certain theories, like extra dimensions in string theory.

However, it's important to note that this is highly speculative and not confirmed by experimental evidence. The energies required for such processes are far beyond what current particle accelerators can achieve, making it challenging to test these theories directly. As of my last knowledge update in January 2022, there hasn't been any experimental confirmation of microscopic black hole production through cosmic ray interactions.

 

[member 563992]

Still, wouldn't it be cool to test dark matter via microscopic black hole evaporation?

LE: Seems there are efforts to detect microscopic black holes created in the atmosphere. https://www.mpi-hd.mpg.de/HESS/page...k hole,showers resulting from such collisions.

 

[Orodruin]

You seem to have done an actual calculation.

I had my students do it as homework at some point. I guess that means I did it too since I had to know the correct answer.

Well, ChatGPT clarifies this one as well.

please don’t use ChatGPT as a source of information regarding advanced physics subjects. It is very prone to sounding authorative when in reality it has no clue whatsoever.