How come we know what a neutron star is made up of but do not know what's inside black holes? A genuine question...
The issue is rather that we don't know what all that star stuff becomes once it goes into the black hole, though there are theories. The problem is that the defining feature of a black hole, its even horizon, by necessity means there is no way of knowing what theory is correct since nothing happening inside is supposed to matter to us.
So before we can make any real progress on the interior of black holes we thus need to figure out exactly in what sense the event horizon leaks information, if it does.
In contrast, while we don't really know what happens to matter inside a neutron star, its surface remains visible to us and so we have been able to discard many different theories about what goes inside it. There is still debate about the precise equation of state, which correspond to different internal structure the various models represent and so different ideas of how a neutron star is made up.
Honestly, I don't know how much difference there are between the various models, but pretty crazy sounding ideas are still popping up, so there must be at least some space left.
Right. As far as "an observer at infinity" is concerned, all that in-falling matter gets asymptotically closer to the event horizon and never crosses it.
One way to reframe the situation which I quite like: Black holes have no insides.
Also, the process that compresses matter to the point it collapses to a black hole is very destructive; there is no reasonable way to form a black hole in the middle of the star.
So we need to find the Meltdown/Specter-like vulnerabilities of black holes? I wonder how many problems that would cause the universe.
(There are nit-picky theoretical ways in which a black hole can evaporate, keyword "Hawking Radiation", but as far as I know that hasn't been observed, because it's too weak and too far away. But even then we learn about its surface/event horizon, not its inside).
https://iopscience.iop.org/article/10.1088/1367-2630/10/10/1...
This is a "numerical observation", so a simulation. And not of a black hole, but of a model of a black hole in one dimension in a Bose-Einstein-Condensate.
Nobody has created a black hole in a lab and measured its radiation.
But sure, if you want, we can discount that specific study and focus on others that use different methods to detect it that aren't "just simulations"
https://arxiv.org/abs/1009.4634 https://arxiv.org/abs/1401.6612
That way the space expansion in the origin of the universe could just be the black hole forming and quickly eating more mass of its surrounding and thus expanding. Also would explain a lot of things related to lack of anti-matter and such since the part of the outer universe our blackhole ate wouldn't necessarily have an even distribution.
Well, regardless of whether or not our universe is contained within a black hole, basically no one thinks there is literally a singularity of infinite density inside of black holes. It's just what relativity states (and we already know relativity isn't a complete explanation, especially in regards to some of these incredibly extreme situations) and treating black holes like they contain incredibly massive quantities of mass while basically being a point-particle works for all of our day to day purposes.
String theory and loop quantum gravity both explain black holes without needing a singularity, and every astrophysicist that I've spoken to a read comments from discussing this topic has basically said "Yeah, it's not a singularity, we just don't know what it is, and that whole singularity thing is close enough"