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I've read that Hawking radiation is implicitly linked with the existence of an apparent horizon (1). This seems a slightly less onerous than linking Hawking radiation with a genuine bona fide event horizon and its dependence on the global spacetime properties.

However I've also read a number of papers that seem to me to suggest than Hawking radiation occurs during stellar collapse and can occur in, albeit physically implausible models, before an apparent horizon forms or even when an apparent horizon is miraculously avoided (2).

So my question is: what is the connection between Hawking radiation and the apparent horizon and for a far-off observer watching a star collapse, when would they observe Hawking radiation to "switch on"?

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    $\begingroup$ Speaking of papers regarding Hawking radiation during stellar collapse . . . arxiv.org/abs/1406.1525 and arxiv.org/abs/1409.1837, both mentioned in a previous question (physics.stackexchange.com/q/137128). I don't suggest taking their conclusions too seriously, but perhaps there's something in there you might find interesting. $\endgroup$ – HDE 226868 Sep 28 '14 at 1:07
  • $\begingroup$ As you've probably guessed that paper inspired my question as it seems to suggest Hawking radiation can exist without horizons. I couldn't find anything that countered that which dealt specifically with stellar collapse. $\endgroup$ – John Davis Sep 28 '14 at 1:24
  • $\begingroup$ I actually hadn't guessed, but I'm not surprised. I never thought of asking this, but it does seem like a logical question. $\endgroup$ – HDE 226868 Sep 28 '14 at 1:25
  • $\begingroup$ Strictly speaking, Hawking radiation does not form at a smooth classical event horizon. There has to be some "surface roughness" that is caused by quantum mechanics. One could expect, that there is a rather turbulent phase just before the event horizon settles into its ground state which causes enormous emissions. Someone can correct me, but I don't think we have a believable model for any of that and I would discount any theory paper about these things (for now!) as speculative, at best. If CERN finds hints of physics beyond the standard model, that might change. $\endgroup$ – CuriousOne Sep 28 '14 at 5:48
  • $\begingroup$ On April 11, 2019, a blog called "Mysterious Universe" put Mersini-Houghton on the spot about an April 10 announcement by the Event Horizon Telescope people that the EHT hookup had photographed a black hole, which contradicted M-H's notion that any collapsing star would put out enough Hawking radiation during its collapse to allow it to do some mundane sort of bounce out of the collapse situation. I recall that Sabine Hossenfelder's blog included an attack on M-H's theory back when it was formulated in the Arxiv papers already cited, but good luck scrolling for it. $\endgroup$ – Edouard Jun 16 at 4:22

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