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Eddington Limit cited on PBS Nova
The most recent program of PBS Nova considered how massive
black holes could have been formed in the early universe. It was concluded that they could not have been formed by simple accretion because Eddington had found that radiation pressure would limit the rate of accretion. Eddington was unaware of dark matter which, I suppose, would not have emitted or been affected by such radiation. Is my supposition correct and, if so, does that mean that super-massive black holes could have been created in the early universe by accretion of dark matter? Thanks. |
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Eddington Limit cited on PBS Nova
On 14 Jan 2018 19:29:55 +0000 (GMT), root wrote:
super-massive black holes could have been created in the early universe by accretion of dark matter? "Dark matter" does not "accrete" because accretion is baryonic behaviour and dark matter, whatever it is, is known not to be baryonic and acts more as a halo. How's that for a one-sentence summary? [[Mod. note -- Excellent. The underlying physics of matter accreting onto a BH is that we imagine a cloud of matter (whether dark or otherwise, baryonic or non-baryonic) around the BH, the matter far from the BH having some random distribution of velocities with respect to the BH. In order to accrete the matter must physically reach the BH's event horizon, which is very small, i.e., the matter must be an an almost radial orbit with respect to the BH, i.e., the matter must be on a very low-angular-momentum orbit about the BH. For any reasonable velocity distribution of the matter far from the BH, most of the matter in the cloud will have far too much angular momentum to reach the event horizon. So, matter needs to lose a significant amount of angular momentum before it can accrete. This requires friction, a magnetic field, and/or some similar process, to transpare the angular momentum out away from the BH as the matter moves inward (typicaly in an accretion disk). Dark matter is uncharged (and hence unaffected by magnetic fields) and essentially collisionless (and hence unaffected by friction), so there's no easy (quick) way for it to lose large amounts of angular momentum and accrete onto the BH. In other words, only a tiny fraction of the ambient dark matter (that fraction which happens to be on a trajectory that takes it to the event horizon) will accrete onto the BH. -- jt]] |
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