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Some people speculate that the mysterious dark matter in the universe could be tiny black holes. But on the other side, could dark matter particles attract each other by gravity and finally form a black hole? Since dark matter is even more abundant than normal matter, the dark matter black hole should not be rare.

Qmechanic
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Fine Observer
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2 Answers2

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I think the problem with matter that only interacts gravitationally is that it's hard to get it all to stay in one place. Nebula slowly form stars and planets in part because of collisions between particles lead to larger particles, which tend to attract further particles. But particles that just wizz right through each-other can't coalesce without violating conservation of angular momentum. That's not to say that it's impossible, however. Just that the dynamics are different.

lionelbrits
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    You are right. If dark matter only interact gravitationally, then it's hard for them to accumulate. For example, considering two dark particles, when they "collide"( which means overlapping in space time), they pass through each other. If more particles are added in, a chaotic soup may form. This is much easier to simulate in a computer program. However, whether dark matter really doesn't interact through other forces is not clear. – Fine Observer Dec 20 '13 at 21:32
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    @FineObserver In fact this is what we observe in gravity-only simulations. When galaxies collide, so do their dark matter halos, but they mostly passes right through each other. This forms large dark matter filaments through and around galaxies. – Kevin Driscoll Dec 20 '13 at 21:41
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    @KevinDriscoll I think you should maybe expand that comment into an answer as it describes explicit simulations that exactly answer the OP's question. Lionel's answer is good, but you're clearly an expert thinking exactly about the OP's question in your day job: it would be great to get such an authoritative answer on this site. Too many of us are generalists and, whilst some of us have excellent physical intuition and give great answers, it's not quite the same. – Selene Routley Dec 20 '13 at 21:51
  • Here's a synopsis of a Physics review letter on the topic ;-) – Waffle's Crazy Peanut Dec 21 '13 at 10:10
  • Question 1: Does the claim that DM only interacts gravitationally imply that DM does not obey Newton's laws as you are describing a frictionless material. Question 2: Wouldn't two DM particles occupying the same location in space violate the Pauli Exclusion Principle, as in electron degeneracy where no two neutrons can occupy identical states, even under the pressure of a collapsing star of several solar masses? – Kelly S. French Oct 22 '18 at 23:26
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Dark Matter has not been detected within the vicinity of Earth by even extremely sensitive detectors such as the LUX (Large Underground Xenon) detector. Yet scientists believe that the halos surrounding galaxies are replete with Dark Matter.

It seems more likely that the LUX detectors could not detect Dark Matter simply because there isn't any in the vicinity of our Earth. That would essentially mean that Dark Matter distribution is perhaps non uniform across our galaxy. Since it seems to be localized in galactic halos, and perhaps such regions that are beyond the gravitational field of massive bodies (such as planets/stars/galaxies). Therefore it appears that Dark Matter could be gravitationally repulsive with respect to normal matter.

In that case, it makes perfect sense why Dark Matter cannot form Black Holes since they simply cannot be packed close enough (due to their 'soft' mutual repulsion), to form Black Holes. That also would explain why they are found mostly in the outer reaches of galaxies, within their halos.

For more details please see http://urbanyogi2012.blogspot.ca/2014/06/on-dark-matter.html

harismind
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    "It seems more likely that the LUX detectors could not detect Dark Matter simply because there isn't any in the vicinity of our Earth." - I disagree. The ability of LUX and other experiments to detect DM depends on how stongly DM interacts/decays etc. There is no strong reason to conclude that there are no DM close to Earth - just that it does not interact/decay stong enough to allow for a detection. – Winther Jun 20 '14 at 01:31
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    "Therefore it appears that Dark Matter could be gravitationally repulsive with respect to normal matter." - This would be very hard to recogncile with our very successful models of structure formation - which treats DM as a purely collisionless fluid - and which interacts with gravity just as normal matter. – Winther Jun 20 '14 at 01:34
  • The 'strong reason' is they haven't been detected despite such powerful detectors! Dark Matter is supposed to be more than 5 times as abundant as normal matter, yet not a trace has been detected in the vicinity of Earth. Simple logic dictates that they have not been detected simply because DM is not present on Earth. – harismind Jun 20 '14 at 01:52
  • Your 'successful models' don't explain anything about DM. If it gravitationally attracts and is 5 times more abundant than normal matter then you should be seeing DM everywhere. You don't. You only see it in Galactic halos far removed from the gravitational ambit large bodies. Ask yourself why this pattern is observed consistently from galaxy to galaxy. – harismind Jun 20 '14 at 01:59
  • Firstly, we cannot "see" things that don't emmit light. The effect we "see" is though its gravitational effect. Right, such models do not explain what DM really is, but they tell us a lot on how it must act and we can say with great certainty that DM is not replusive as you suggest. That would have huge observable consequences on many observables such as 1) the cosmic microwave background 2) clustering statistics of galaxies and 3) galaxy rotation curves to mention some for which we have high quality observations of. – Winther Jun 20 '14 at 02:20
  • I did not mean "see" in a literal sense. If DM were uniformly distributed, we would be observing gravitational anomalies even locally (the motions of our own planets) that can be accounted only by factoring in the effects of DM. No such anomalies have been detected and all the motions of our planets and moon can be perfectly explained by the amount of normal matter they contain. Thus it is clearly not uniformly distributed. I don't see how a repulsive DM would be inconsistent with CMB. I have also explained in my website how it perfectly explains galactic clustering and rotation curves. – harismind Jun 20 '14 at 02:36
  • Nobody claims that DM is uniformly distibuted. For a halo or a galaxy we expect a density profile that falls off with distance from the center - as found from simulations and observationally from, say, rotation curves. "I don't see how a repulsive DM would be inconsistent with CMB" Scientist don't just speculate about this: we calculate and compare with observations. The CMB predictions for our standard DM models are in perfect agreement with the data. Repulsive DM is very much not! Also its not enough to say "it perfectly fits data" without having done the computations. – Winther Jun 20 '14 at 02:57
  • It's surprising that you base your conclusions on some simplistic computer models. One can model things that look like trees with fractal equations, that doesn't mean trees grow according to these equations. There are a whole series of complex processes. Remember, the computer is not the universe. – harismind Jun 20 '14 at 10:42
  • You say "Repulsive DM is very much not!"-what exactly do you know about repulsive DM other than what I have said here? You make such sweeping statement without any knowledge of what you're talking about. Scientists don't do that. You have not answered the glaring logical inconsistencies of your ridiculous model, by hiding behind some computer model that may have several random parameters, that can be selectively chosen to fit any observation. – harismind Jun 20 '14 at 10:48
  • Have you even thought that if DM was attractive in nature, as our planets move around our sun, they would be like snowballs sweeping up the DM along the way and growing in mass. And the effects of this additional mass could be clearly observed in the planetary motions. Yet there is nothing. After I pointed out the fallacy in your model, you are saying "Nobody claims that DM is uniformly distibuted." Actually, that is exactly what scientists are saying. – harismind Jun 20 '14 at 11:00
  • "what exactly do you know about repulsive DM" - from your outline, enough to say I have strong doubts it can ever work. Anyway, the burden of proof is on you. "simplistic computer models" - Not at all. "that can be selectively chosen to fit any observation" - Nope. Most parameters are constrained to 1% accuracy. "they would be like snowballs sweeping up the DM" - To a very small extent. Too large velocities and avg. density too low to ever change mass of planets. "Actually, that is exactly what scientists are saying" - Nope. You must be confusing it with dark energy. – Winther Jun 20 '14 at 11:38
  • "- from your outline, enough to say I have strong doubts it can ever work."-nope. you know next to nothing about DM, let alone repulsive DM. "Most parameters are constrained to 1% accuracy"-you have less than a keyhole view of the universe and then make these grand models that you think explains it. It's like looking out though your kitchen window and making a map of the city. – harismind Jun 20 '14 at 11:50
  • "To a very small extent"?? That's a laugh. You have DM 5 times as much as normal matter. It's like having 5 DM suns, 5 earths, 5 jupiters etc. in our solar system. And you are saying only to a very small extent. Do you even know how gravity works?. No I am not confusing DM with DE. The LUX detectors were built with the specific purpose of finding DM not DE. If they didn't think they could find DM on earth, why else would they have looked for it here? – harismind Jun 20 '14 at 11:55
  • "You have DM 5 times as much" Yes, on average in the universe! At the scale of the solar system DM is spread out fairly evenly with density roughly $10^6$ of the avg. DM density in the universe. A planet has a density $10^{30}$ times the avg. matter density in the universe. "why else would they have looked here" Experiments look for DM interacting/decying into matter - not via mass-increase of planets. DM can affect planet orbits to some extent and this could be detected, but this is not because planets absorb DM - its due to all the mass within the orbit giving a larger gravitaitional force. – Winther Jun 20 '14 at 12:40
  • If DM is gravitationally attractive, over time (13.8 billion years) and with the continuous motion of the stars, planets & galaxies, DM would have to be accumulating pretty tightly around these massive bodies. Why is that not happening? Remember DM is 5 times as abundant as normal matter. "DM can affect planet orbits to some extent and this could be detected"-- then why is that not detected? Please don't cite some computer simulation to back up your argument. Give observational evidence. – harismind Jun 20 '14 at 12:58
  • "would have to be accumulating pretty tightly" No. DM has no pressure, i.e. nothing to slow it down, so large velocities prevent this from happening (to a large extent). "then why is that not detected?" DM density in solar-system is just tiny. Some works have tried to estimate how much the solarsystem has accumulated - and this seems to indicate that less than around 0.002% of the mass of the Earth has been accum. so far (http://arxiv.org/abs/0806.3767). This is tiny! – Winther Jun 20 '14 at 13:29
  • "DM has no pressure"-On what do you base this on? You don't even know what DM is, yet you make these wild assumptions about what it should be. Agreed that it says, as per its model (which is based on many questionable assumptions) that the DM capture is around 0.002% (1.78x10^-5) of Earth mass. It also concludes by saying "we find that dark matter in our Solar System is far more important than previously thought". In any case these are just models about something scientists have very little knowledge about. It is hard to take this seriously. – harismind Jun 20 '14 at 16:58
  • Repulsive gravity as a result of Dark Energy has been proposed before by a theoretical physicist, Massimo Villata. I have mentioned that in my write-up. However, he has proposed antimatter as causing this repulsive gravity, to which I don't quite agree. I think it is extradimensional matter trapped in closed 2d M-branes (something like tiny bubbles, not of this universe, though within), whose positive curvature on our space-time would cause repulsive gravity. For Villata's paper, see http://arxiv.org/abs/1201.3810 – harismind Jun 20 '14 at 16:59
  • I don't think I want to prolong this discussion any more, since it is taking too much of my valuable time. Hence I will not be responding to any further posts. – harismind Jun 20 '14 at 17:01
  • Reading this convo provokes a lovely laugh, thank you both. @harismind you say "It's surprising that you base your conclusions on some simplistic computer models" which exposes your ignorance of all of this perfectly... It's unfortunate that the OP accepted your answer, since it is severely lacking in evidence-based content. The simulations we use in astrophysics are empirically driven - we don't just stick whatever we want in them and then see what happens. – Daddy Kropotkin Sep 27 '18 at 18:16
  • You're willing to make claims that are totally unfounded, just pure conjecture. Astrophysical simulations make minimal assumptions about DM to try to reproduce the observational effects, which do not include repulsions, electromagnetic nor mechanical, and these simulations do a great job at reproducing observations. – Daddy Kropotkin Sep 27 '18 at 18:16
  • DM forms in haloes around galaxies, and that's about as far as we really know, but the Lambda CDM concordence cosmology explains the growth of structure very well, even on very large scales i.e. the Cosmic Web. Also, you seem to need desparately to educate yourself on this further, because there is a difference between Dark Energy and Dark Matter, but you've been saying that Dark Matter is repulsive and then cited something about Dark Energy..... Get a clue! – Daddy Kropotkin Sep 27 '18 at 18:16
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    Lastly, absence of evidence is not necessarily evidence of absence, and this is especially true when we are talking about possible new forms of matter. That is, it's non-detection is not necessarily proof of it's non-existence, especially when it's something as allusive as Dark Matter. – Daddy Kropotkin Sep 27 '18 at 18:16