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Are black holes expected to contain the same ratio of dark matter to regular matter as the rest of the universe? I've heard that dark matter is distributed in halos around galaxies. Does that make it less likely to be ingested into a black hole?

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    $\begingroup$ Probably not, cause regular matter forms a neat accretion disk and dark matter (to my knowledge) does not. The Milky way is 88% dark matter but the solar system is 99.999999999% regular matter. One clumps, the other doesn't. But, I'll let someone smarter than me answer this one with a reference to a real scientific stud or estimate. I will add that there might be some uncertainty on primordial black holes and how much dark matter went into their formation. Stellar black holes are made up of essentially all regular matter $\endgroup$ – userLTK Feb 14 '17 at 2:54
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    $\begingroup$ @userLTK I might modify your comment to read "all current observed effects indicate that darkmatter is uniformly distributed" and so on, just to emphasize that right now we know diddly-squat about dark matter. $\endgroup$ – Carl Witthoft Feb 14 '17 at 13:38
  • $\begingroup$ But we do think that black holes CAN hold dark matter, correct? Dark matter is still affected by gravity, if nothing else right? $\endgroup$ – joseph.hainline Feb 14 '17 at 14:19
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    $\begingroup$ @CarlWitthoft - that's a good point. It was probably a bad comment anyway cause it's an answer, but I was hoping someone smarter than me would give an official answer then I'll delete the comment. And, Jospeh - yes, it's likely that dark matter gets eaten by and can't escape black holes but until we know what it is, I don't think anyone can say with certainty. Dark matter is affected by gravity, $\endgroup$ – userLTK Feb 14 '17 at 17:28
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(Short answer: No, scroll to the last point.)

  • It is irrelevant to an external observer whether the matter that fell into the black hole was dark matter or baryonic, by the no hair theorem. The only properties of a black hole from our point of view are mass, electric charge and angular momentum. (But of course we don't understand quantum gravity.)
  • From the point of view of matter which has fallen into the black hole, nothing special happens upon crossing the event horizon. This means that dark matter stays dark and baryonic matter stays baryonic when viewed from inside the black hole.
  • There is some controversy about how much dark matter exists in the universe. This recent article, for example, indicates in the abstract that more accurate modelling of galactic rotation curves could eliminate a large percentage of the expected non-baryonic dark matter. (Note as @pela indicated in the comments, that this author's papers have not been peer reviewed and could be suspect.) Obviously, the amount of dark matter in the universe would greatly affect the question's answer. I should note that the controversy is mostly composed of a small number of vocal scientists who appear disproportionately in the media. Following the mainstream news science sections, I get the impression that the death of dark matter seems to be announced once a month or so.
  • The formation of supermassive black holes is poorly understood. One hypothesis is that they may form by successive merger of stellar mass black holes. As there have recently been gravitational wave observations of such mergers, and as candidates for intermediate mass black holes have also been observed recently, I will assume here that this is how they form and that supermassive black holes are therefore made of roughly the same stuff as stellar mass black holes.
  • Black holes lose most of their mass during the formation process. It is important to always keep in mind whether we are talking about the mass of the stellar core which collapsed to form the black hole (this is often the "mass" of a black hole that is referred to when speaking about e.g. the minimum size black hole that can form from core collapse) or the mass of the black hole as seen by a distant observer after the supernova.
  • Dark matter particles can not lose much orbital energy by interacting with other matter nor by radiation, therefore will remain in orbit around a black hole rather than falling in, unless they happen by unlikely chance to hit it near the event horizon. This paper indicates that simulated supermassive black holes derive no more than about 10% of their mass from dark matter.

However, it must be said that some scientists suspect that dark matter is made of primordial black holes in the first place. There is also the theory of MACHOs (Massive Compact Halo Objects), that dark matter is composed of large compact bodies such as black holes, but it is believed by most that this theory can not account for the dark matter in the universe.

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    $\begingroup$ Just a small point, but "happen by chance to hit square on", while generally correct, probably isn't 100% right. All massive particles that fly inside 3 radii are destined to fall into the black hole unless an outside force acts on them. They don't have to hit directly just within 3 radii or 2 radii from the event horizon. (I think its 3x, sometimes I read 1.5x). Dark matter should be governed by relativistic effects, so there is a capture region outside the event horizon even if the dark matter doesn't lose any orbital energy by collision. $\endgroup$ – userLTK Feb 14 '17 at 22:01
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    $\begingroup$ Nice answer, but the first paper you link to (Xiaochun & Kuan 2009) I'm a bit suspicious about. It is non-refereed, as is all of Xiaochun's other papers. But as far as I can see from browsing very quickly through it, it actually seems to roughly confirm the accepted dark-to-baryonic matter ratio. I don't think there's much controversy about the amount of DM (unless you're a MONDer). Also, note that your last comment is about primordial black holes, i.e. whatever they're made of is a negligible part of the full-grown BH. But maybe that's what you mean by "in the first place". Anyway, +1. $\endgroup$ – pela Feb 14 '17 at 22:11
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    $\begingroup$ Also, to expand a bit on @userLTK's comment: The 1.5 Schwarzschild radii ($R_\mathrm{S}$) is the "photon sphere", inside which all incoming objects will spiral into the black hole (they could still escape before they reach 1 $R_\mathrm{S}$ if actively "trying to"; e.g. a flashlight crossing $1.5R_\mathrm{S}$ will spiral in, but could still shine light outwards from the BH that could escape). The $3R_\mathrm{S}$ is the "innermost stable orbit", inside which an arbitrarily small perturbation will cause the object to spiral in. $\endgroup$ – pela Feb 14 '17 at 22:22
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    $\begingroup$ I agree absolutely with @userLTK. By hitting the black hole "square on" I did not intend to refer to hitting the event horizon exactly, rather I intended it as seen from afar; that there is a small target region near the centre that they must hit. I'll try to make the answer a little more clear. $\endgroup$ – user25972 Feb 15 '17 at 4:52
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    $\begingroup$ @pela Thanks for your feedback on Xiaochun & Kuan's paper. I must admit I did not check it up in enough detail. I'll also update the answer based on your feedback. $\endgroup$ – user25972 Feb 15 '17 at 4:55
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Dark matter is (thought to be) in halos which extend both to the centers of galaxies and outside most of the normal matter in galaxies (gas, stars, dust). So a black hole inside a galaxy could and undoubtedly will ingest some dark matter. However:

Stellar-mass black holes form from the core-collapse of a massive star. Since stars are almost entirely of regular matter, the initially formed BH remnant would itself have been made almost entirely of regular matter. Such BHs might later grow by accreting gas (e.g., from a close binary companion star), in which case they're gaining mass in the form of regular matter. There would inevitably be some dark matter swallowed by the BH as it orbited within its parent galaxy -- just as the BH would swallow some interstellar dust, for example. But it would still be overwhelmingly formed out of regular matter.

Supermassive black holes in galaxy centers would probably start out from some kind of early-universe collapse of a gas cloud or very massive star, which would again be mostly regular matter. Subsequent growth of supermassive BHs comes primarily from interstellar gas feeding an accretion disk around the BH, plus the occasional star that wanders too close -- so once again it's mostly regular matter that falls in the black hole. (The central regions of galaxies do have some dark matter, but they're dominated by regular matter. Plus, regular matter in the form of gas clouds can easily lose energy via cloud-cloud collisions and sink to the center of the galaxy, where it could feed a supermassive BH; dark matter can't do this.)

(Of course, as user25972 points out, it's largely irrelevant to outsiders like us what kind of matter goes into making a BH. A black hole formed out of dark matter would behave identically to one formed out of regular matter.)

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  • $\begingroup$ Do we know why when the galaxy formed the dark matter didn't get distributed similarly to the regular matter? Why is it out there in halos? Shouldn't gravity pull it in like normal matter into clumps and stars, etc.? $\endgroup$ – joseph.hainline Feb 19 '17 at 23:18
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    $\begingroup$ Regular matter in the form of e.g. gas clouds can collide and lose energy and angular momentum, so that the gas will end up on smaller orbits closer to the center of the (proto-)galaxy. Gas clouds are also supported against self-gravity by internal pressure, which depends on (among other things) temperature; since gas can cool by radiation, gas clouds can end up with lower pressure and thus collapse to form clumps and stars. $\endgroup$ – Peter Erwin Feb 20 '17 at 0:04
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    $\begingroup$ So the answer in a sense is: dark matter and regular matter started out with similar distributions (and collapsed to form halos in the early universe); but regular matter in the form of gas can lose orbital energy and thus collapse further in ways dark matter can't. $\endgroup$ – Peter Erwin Feb 20 '17 at 0:06

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