Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 02/08/2016 17:53:01
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David asked the Naked Scientists:
Is it possible that a Black Hole could be made up of Dark Matter?
What do you think?
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The dark matter distribution in galaxies is not uniform. The density appears to increase with radial distance from the galactic cetre. This runs counter to the idea that black holes are made of dark matter.
If the clouds of matter that eventually become galaxies were an uneven mix of baryonic and dark matter the distributions are not that mysterious if only the baryonic component can form solid structures. This leaves the dark matter halo intact around the systems comprising the galaxy. The rotation curve is then as it has always been. With some matter clumping and the rest remaining a gas of dark particles.
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At this point in time, we don't actually know what Dark Matter is. But the properties that are suggested by the today's most popular theory (https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles#WIMPs_as_dark_matter) is that it is some subatomic particle which interacts very weakly with matter, and with other dark matter.
So matter can form a star or a black hole by colliding with other matter, and radiating away the heat of the collision.
But the hypothetical dark matter particles (almost always) go straight through each other, so they don't collide to form a large mass.
This suggests that Dark Matter can't form a black hole by itself, but if Dark Matter happens to strike an existing black hole, it will be absorbed, and not escape.
So Dark Matter could help a black hole grow, but not form a black hole in the first place.
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One candidate for dark matter particles comes from super symmetry and is the hypothetical neutralino.
https://en.m.wikipedia.org/wiki/Neutralino (https://en.m.wikipedia.org/wiki/Neutralino)
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So Dark Matter could help a black hole grow, but not form a black hole in the first place.
That's not quite correct. You're thinking only in terms of sub atomic particles. You've left out Massive Compact Halo Object (MACHO). One type of MACHO is a black hole itself. So in this case one form of dark matter is a black hole. Other types are white dwarfs, rogue planets, red and brown dwarfs. If black dwarfs exist then they too would be a form. Black dwarfs are very rarely mentioned in this forum but they do exist in the literature. See: https://en.wikipedia.org/wiki/Black_dwarf
See also: https://en.wikipedia.org/wiki/Massive_compact_halo_object
Rogue planets are exactly what they sound like, i.e. planets which orbit galaxies directly, i.e. they don't orbit a star. They're also known as an unassociated planet, interstellar planet, nomad planet, free-floating planet, orphan planet, wandering planet or starless planet. See:
https://en.wikipedia.org/wiki/Rogue_planet
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The dark matter distribution in galaxies is not uniform. The density appears to increase with radial distance from the galactic cetre. This runs counter to the idea that black holes are made of dark matter.
Why? Is that because you're assuming a uniform distribution of black holes? If so then I'd agree. Nice catch by the way. What I find strange to begin with is that the distribution isn't uniform. This is probably a clue as to the nature of dark matter.
Every year you become noticeably better and better at physics. It's a pleasure and an honor to watch you and others here learn. :)
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The dark matter distribution in galaxies is not uniform. The density appears to increase with radial distance from the galactic cetre. This runs counter to the idea that black holes are made of dark matter.
Why? Is that because you're assuming a uniform distribution of black holes? If so then I'd agree. Nice catch by the way. What I find strange to begin with is that the distribution isn't uniform. This is probably a clue as to the nature of dark matter.
I think it is high likely that black hole distribution is uniform. The most puzzling thing is why dark matter forms halos around the galaxies but does not appear to be distributed between galaxies. If this were the case the galaxies would tend to expand over time. It has a while since I read the text on galaxy formation so I may be in error with these assumptions.
As far as what constitutes dark matter at the particle scale I am not convinced by supersymmetry. At least not by what I have read so far.
Every year you become noticeably better and better at physics. It's a pleasure and an honor to watch you and others here learn. :)
If anyone wishes to participate in meaningful debate on the subject of physics then at least a basic knowledge of the principles is required. If like me you want to find answers then a more in depth study is required. Plus I just love mathematics.
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At this point in time, we don't actually know what Dark Matter is. But the properties that are suggested by the today's most popular theory (https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles#WIMPs_as_dark_matter) is that it is some subatomic particle which interacts very weakly with matter, and with other dark matter.
So matter can form a star or a black hole by colliding with other matter, and radiating away the heat of the collision.
But the hypothetical dark matter particles (almost always) go straight through each other, so they don't collide to form a large mass.
This suggests that Dark Matter can't form a black hole by itself, but if Dark Matter happens to strike an existing black hole, it will be absorbed, and not escape.
So Dark Matter could help a black hole grow, but not form a black hole in the first place.
What you say sounds good. to rephrase it dark matter is comprised of a fundamental sub-particle. Within a black hole the sub-particles add to it and the black hole grows. that makes sense. So the black hole is mostly made up of a fundamental sub-particle. then you say the black hole cannot be made from these sub-particles which tend to pass through each other perhaps like atoms in a gas which are always moving around.
As I see it there is one other possibility. At maximum compression at big bang, all those little sub-particles can be forced to coexist at a single point. thus the black hole is made of the same stuff except that the center of the black hole is a huge amount of the same stuff compressed into a single point.
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I think it is high likely that black hole distribution is uniform (...around the galaxies)
Most galaxies appear to have a massive black hole at the center (millions to billions of solar masses); this is far from a uniform distribution, but you could argue that this is a special case, so lets ignore central galactic black holes here...
Small-scale black holes (from 1.3 to say 60 times the mass of the Sun) are thought to form from the explosion of a massive star, or possibly from the merger of two smaller stars. In this case, I expect small black holes to be formed with approximately the same distribution as visible stars in the galaxy - for spiral galaxies, you expect to find them concentrated in the plane of the galactic disk (with some orbiting the large galaxy outside the disk along with the remains of smaller galaxies previously torn apart by the larger galaxy).
Spiral galaxies are thought to form much like a protoplanetary disk, only on a much larger scale. Clouds of gas which initially has random directions collides with each other, radiating away the heat, and settling into a general rotation which represents the average angular momentum of all the original gas clouds. This produces a "fried egg" protogalaxy.
But there is evidence that an asymmetrical supernova (https://en.wikipedia.org/wiki/Supernova#Asymmetry) could eject the remnant well away from the plane of the galaxy, which would make the current distribution of black holes (and neutron stars) more uniform than the location of their formation.
You've left out Massive Compact Halo Object (MACHO). One type of MACHO is a black hole itself.
I have no doubt that there are numerous "black holes on a diet", neutron stars, burnt-out cinders of stars, brown dwarf stars and rogue planets roaming the galaxy - all galactic citizens that don't show up well at visible wavelengths. There are hopes that the James Webb telescope might help us find more of these.
But the MACHO theory has gone a bit out of fashion in recent years, as searches for microlensing events did not find enough events to account for all the symptoms of Dark Matter. One team suggests that they found enough for 20% of the Dark Matter, while another team suggests that the fraction is much lower.
I expect that the distribution of rogue planets, brown dwarfs and stellar cinders would lie primarily in the plane of our galaxy.
See: https://en.wikipedia.org/wiki/Massive_compact_halo_object#Detection