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I am a layman, so I'm not completely up on the latest black hole theories.

However, I have been thinking about how an event horizon can be be penetrated by matter outside of the event horizon. Now, to any outside observer, matter will go towards the black hole, but we see its apparent relative clock slow down drastically before it enters from the horizon. So we can think of matter never entering until "the end of time" in the outside universe. Now, obviously black holes exist and can grow in accumulated matter, but what about the following considerations:

1) If Hawking Radiation works the way we think it does, why would a black hole not evaporate before an in-faller ever reaches the event horizon?

2) What about the rate of expansion of space in our universe? If an in-falling observer sees the apparent rate of time accelerate in this universe, that should include observing an accelerating expansion of space. (unless somehow this rate is actually decelerating for things not near such abnormally shaped space-time) If the observer is still inside this universe "before the end of time", would not the apparent acceleration of the expansion of space eventually have an effect on the path of the in-faller towards the event horizon?

safesphere
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Robert Mashlan
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  • Related: https://physics.stackexchange.com/q/82678/123208 although the answers there don't mention Hawking radiation. – PM 2Ring Apr 21 '20 at 06:08
  • There was a comment here, which since disappeared, that stated that the expansion of space does not effect the space between gravitationally bound objects. If this statement is true, it would mean that our observable universe is not expanding, because any object within cosmic event horizon which projects gravity towards us at the rate of causality would be preventing the expansion of space. – Robert Mashlan Apr 22 '20 at 19:39
  • My comment was, “the expansion of space does not affect gravitationally bound objects”. This is well known and true (for the expansion without a significant acceleration). Your objection is incorrect, “any object within cosmic event horizon which projects gravity towards us” is not gravitationally bound with us. “Bound” essentially means “orbiting”. The largest bound structures normally are galaxies. The expansion of space does not affect a galaxy, but does pull galaxies apart from each other, because most galaxies are not gravitationally bound despite “projecting gravity” onto each other – safesphere May 02 '20 at 04:35
  • Also note that space inside galaxies does expand as anywhere else, but this does not affect any distances. As an intuitive illustration imagine fallen tree branches floating in a river where it expands. The branches float apart, but leaves on each branch stay with the branch despite water flowing through it. Here floating water is expanding space, branches are galaxies, and leaves are stars. Space expands from inside to beyond the galaxy, but the size of the galaxy doesn’t change. In other words, matter doesn’t expand with the expansion of space. Search this site, it has many answers on this. – safesphere May 02 '20 at 04:53
  • To be even more precise, a non-accelerating expansion of space does not necessarily affect non-bound objects either. Consider two light objects in space, so the gravitational attraction is negligible. One object moves on inertia toward the other with a constant speed exactly compensating the space expansion rate there, so the distance remains the same. In this case, space will expand forever, but the distance between the object will never change despite of them not being bound to each other by attraction. With no acceleration, there are no forces in this system, so nothing ever changes. – safesphere May 02 '20 at 05:13
  • The problem that I'm having is in using "negligible" to describe expansion of space effects. As the space near an anomalous region of space like the one near the event horizon of a black hole is extreme, things that are "negligible" shouldn't be unnecessarily dismissed out of hand. An infaller to an event horizon would observe a blue shift of the outside universe due to time dilatation, however, there would be an apparently accelerated red shift due to the expansion of space at the same time. If the rate of change of expansion is decreasing, this would be negligible, but not in the other case – Robert Mashlan May 03 '20 at 08:45
  • You did not put the @ address in your comment, so I was not notified. “As the space near an anomalous region of space like the one near the event horizon of a black hole is extreme, things that are ‘negligible’ shouldn't be unnecessarily dismissed out of hand.” - Correct. “An infaller to an event horizon would observe a blue shift of the outside universe due to time dilatation” - Incorrect. Both the infalling observer and the observed light ate affected by the time dilation, so no blueshift is observed from in the radial direction. In fact it is a 2x redshift at the horizon. – safesphere May 19 '20 at 03:24

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