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Astronomy

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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
    Commented Feb 14, 2017 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
    Commented Feb 14, 2017 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
    Commented Feb 14, 2017 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
    Commented Feb 15, 2017 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
    Commented Feb 15, 2017 at 4:55