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Lack of runaway black holes hints at dark matter density
In article ,
Yousuf Khan writes: Although the study's authors don't dare to challenge the orthodoxy of Dark Matter belief, reading between the lines, one can say that this may actually be the falsification of Dark Matter's existence. That is a bizarre conclusion to draw. The paper is published in _Monthly Notices of the Royal Astronomical Society_, but there's a preprint at http://lanl.arxiv.org/abs/1002.0553 What the paper shows is that if the dark matter density is high enough, there will be runaway accretion onto the black hole. The limiting density for the most massive black holes (5E9 solar masses) is around 250 solar masses per cubic parsec. Typical dark matter models have maximum densities around 1 solar mass per cubic parsec. This doesn't look like much of a problem to me. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#12
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Lack of runaway black holes hints at dark matter density
Greg Neill wrote:
Escape velocity is irrelevant for objects approaching from outside the system, since conservation of energy dictates a symmetrical energy profile: they gain kinetic energy (speed) as they fall in and shed it as they rise back out of the gravitational well. Any body that has enough speed to approach the well from a long- way-off has enough energy to leave to the same long-way- off distance, barring direct impact. Only if the obects can shed energy by another mechanism (friction, impact, etc.) can they be captured. Yes, true, some objects will just enter and leave the system. But others will have their trajectories bent enough to go into orbit around the blackhole. This in turn will make it part of the overall blackhole's system mass, which will make the escape velocity shell around the blackhole increase some more. The blackhole doesn't have to absolutely capture the Dark Matter inside its event horizon, if the DM just enters into orbit around is enough. As I said a 5 billion Msun blackhole is as big as some dwarf galaxies. Just like a dwarf galaxy might have DM orbiting around it in a shell, a similar massed BH should have an equal amount of DM orbiting it. The combined weight of the DM and BH will in turn encompass more DM, and so on. If the DM density is high enough, of course. Yousuf Khan |
#13
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Lack of runaway black holes hints at dark matter density
Steve Willner wrote:
What the paper shows is that if the dark matter density is high enough, there will be runaway accretion onto the black hole. The limiting density for the most massive black holes (5E9 solar masses) is around 250 solar masses per cubic parsec. Typical dark matter models have maximum densities around 1 solar mass per cubic parsec. This doesn't look like much of a problem to me. Which is basically the point I was trying to make. If DM density was much higher, then there would've been a runaway accretion process. So this sets an upper limit on the density of DM. Yousuf Khan |
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Lack of runaway black holes hints at dark matter density
Yousuf Khan wrote:
Greg Neill wrote: Escape velocity is irrelevant for objects approaching from outside the system, since conservation of energy dictates a symmetrical energy profile: they gain kinetic energy (speed) as they fall in and shed it as they rise back out of the gravitational well. Any body that has enough speed to approach the well from a long- way-off has enough energy to leave to the same long-way- off distance, barring direct impact. Only if the obects can shed energy by another mechanism (friction, impact, etc.) can they be captured. Yes, true, some objects will just enter and leave the system. But others will have their trajectories bent enough to go into orbit around the blackhole. Only objects that get tied up in the highly relativistic curved space very close the black hole can be so captured, and then only very particular trajectories. Otherwise, they're stuck with the same old Newtonian type trajectories: hyperbolic or parabolic. The net effect is that the black hole represents a very slightly larger "target" than its event horizon. |
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Lack of runaway black holes hints at dark matter density
Dear Yousuf Khan:
On Apr 1, 3:03*pm, Yousuf Khan wrote: Greg Neill wrote: Escape velocity is irrelevant for objects approaching from outside the system, since conservation of energy dictates a symmetrical energy profile: they gain kinetic energy (speed) as they fall in and shed it as they rise back out of the gravitational well. *Any body that has enough speed to approach the well from a long- way-off has enough energy to leave to the same long-way- off distance, barring direct impact. Only if the obects can shed energy by another mechanism (friction, impact, etc.) can they be captured. Yes, true, some objects will just enter and leave the system. But others will have their trajectories bent enough to go into orbit around the blackhole. If Dark Matter is anywhere associated with a galaxy containing this supermassive black hole, it is "in orbit" around it. This in turn will make it part of the overall blackhole's system mass, which will make the escape velocity shell around the blackhole increase some more. Not if its Dark Matter, it won't. The blackhole doesn't have to absolutely capture the Dark Matter inside its event horizon, if the DM just enters into orbit around is enough. It *can't* work like this. As I said a 5 billion Msun blackhole is as big as some dwarf galaxies. Just like a dwarf galaxy might have DM orbiting around it in a shell, a similar massed BH should have an equal amount of DM orbiting it. No "should have"... "might have". The combined weight of the DM and BH will in turn encompass more DM, and so on. If the DM density is high enough, of course. No, only if the Dark Matter is suddenly not Dark, but has friction, will this occur. David A. Smith |
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