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OppenheimerSnyder model of star collapse 
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#19
Nov1212, 05:13 PM

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Plausibility is definitely subjective, so you can choose to disagree. However, to me it is clear that a model which begins from a momentarily stationary sphere of dust is more plausible than a model which begins from a singularity. We have direct experience with things that approximate a momentarily stationary sphere of dust, but not with singularities. So the opposite stance seems tenuous. 


#20
Nov1212, 05:40 PM

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#21
Nov1212, 07:30 PM

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#22
Nov1212, 09:28 PM

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#23
Nov1312, 02:43 AM

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One of this qualifications I tried to explain was that even if not in the OS model the logical causal future of the collapsing model is a BH with a singularity, and for the noncharged, nonrotating case the only mathematical model we have of that is an exact solution of the EFE is the extended Schwarzschild spacetime. 


#24
Nov1312, 03:09 AM

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http://en.wikipedia.org/w/index.php?...ldid=490892346 has a brief discussion. I'm not terribly familiar with the details of the BKL solution other than it's very chaotic, Wiki gives the references. Wiki talks about BKL in the context of the early universe, I'd assume timereversing that gives the solution Kip Thorne is fond of. 


#25
Nov1312, 04:16 AM

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I have not read Thorne's semipopular book, so I don't know how or in what context he applied the BKL model in the BH setting. 


#26
Nov1312, 07:03 AM

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Here's a relevant quote, from pg 473, about BKL sigularities



#27
Nov1312, 07:15 AM

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IMO, physical plausibility of a model is defined by two factors: are the laws governing the evolution of the model consistent with experiment and are the boundary conditions possible. Here, the laws are the same, the EFE, so they are equally plausible wrt the first factor. Second, the OS boundary conditions are an idealized but reasonable approximation of observed situations, the S boundary conditions have never been directly observed. So OS is more plausible wrt the second factor. I simply don't see the relevance of any of the other points you have brought up. Neither the historical development of the models, nor the fact that the models are closely related, nor any other point you have mentioned seem to have any bearing on plausibility, IMO. 


#28
Nov1312, 08:18 AM

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#29
Nov1312, 09:23 AM

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http://relativity.livingreviews.org/...ticlesu11.html There's a lot of highpowered math here, but it at least appears to illustrate that the BKL model is still an active area of research (at least as of 2008) and is still considered valid; from section 2.9 of the article: "We shall assume throughout our review that the BKL description is correct, based on the original convincing arguments put forward by BKL themselves" This sentence is footnoted with a reference to BKL's original paper. 


#30
Nov1312, 09:40 AM

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#31
Nov1312, 07:39 PM

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Excuse me butting in here, but I have a big concern about the OS calculation. You all keep agreeing that it applies to a spherical symmetric collapsing mass. But when you think about how fast a pulsar spins, in general a nearBH mass would spin very much faster, and at best we would have an oblate spheroid. In fact, what I have in mind would look something like an LP record! I realise that the end result BH would be spherical, but I don't see how it can be spherical beore reaching that stage except in very idealised theory.
If this picture is correct, then mass in the polar direction would have very little distance to fall, but there would be very little of it, while the angular momentum of the equatorial regions would delay the collapse significantly. Is this taken into account in any of the calculations that have been done? A further note  such a flat spinning object would qualify as an axisymmetric collapse as described by Saul Teukolsky, in which one could at some stage have a naked singularity before the Black Hole forms completely. I would like to hear your comments. Mike 


#32
Nov1312, 10:13 PM

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http://en.wikipedia.org/wiki/Kerr_me...Kerr_solutions These solutions have no horizon, just a naked singularity. 


#33
Nov1312, 10:37 PM

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Teukolsky's model is theoretically very interesting, but also unrealistic  any deviation, however slight, from perfect axial symmetry removes the naked singularity (this is why Penrose's revised bet is still unclaimed  a naked singularity from initial conditions that are perturbatively stable). The best bet for realism are numerical simulations. Whatever the details of collapse, the final, stable form long after last matter infall, is simply the KerrNewman metric, at least on the outside. The inside is another matter, that has been discussed above by Pervect. Please note: this whole line of siscussion would be distraction for this thread, which was opened specifically to discuss OppenheimerSnyder. 


#34
Nov1312, 10:40 PM

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#35
Nov1312, 11:53 PM

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#36
Nov1412, 06:00 AM

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I thought my post was about OS black hole formation. But anyway, thanks for your replies. I shall read up more on Kerr's ideas and their followons. I am particularly interested in the effects of perturbations in the various models, and in numerical simulations of BH formation, but I don't have the maths to follow the calculations. Mike 


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