[machina_ex_deus]

The universe is not inside a black hole. Inside black holes the radial coordinate is time like, which is definitely not true in our universe, where the time coordinate is timelike, and the radial coordinate is space like.

Inside of black holes looks nothing like ordinary spacetime. Inside black hole, everything in your future is with decreasing radial coordinate, which means space is shrinking until you hit the singularity where radial coordinate is zero and you have no future.

[dustingetz]

No, this depends on your choice of coordinate system - has been debunked N times on physics reddit in virtually every thread that it comes up. The EH itself is not a singularity in the observers reference frame as they cross it nor do they particularly notice when they do.

[machina_ex_deus]

I never said EH is a singularity. I said you could notice you're inside as your timelike coordinate becomes the radial coordinate. That's something you could easily notice if you look around, it would correspond to a shrinking universe, and our universe is expanding.

The "you won't notice crossing the event horizon" troupe is true only in a very local sense. If you move around and observe the geometry around you, you can definitely tell you're inside a black hole.

[KolenCh]

As the black hole getting larger, it is more difficult to notice this difference (of crossing the event horizon or “observe the geometry around you”.) and as we are talking about the whole visible universe being inside a black hole, we are in this extreme large scale.

Also, I’m not sure why you’re arguing about the radial coordinate being time-like. You can only measure in your own local reference frame. You wouldn’t necessarily be able to transform between your own local reference frame to the blackhole’s if you don’t know you’re in one.

The universe as a black hole is actually a very old idea: https://en.wikipedia.org/wiki/Black_hole_cosmology

I’m not saying we are in one, but I’m saying it is not as obviously false as you might be arguing.

[dustingetz]

https://www.reddit.com/r/askscience/s/vwCegmNdOE ?

[machina_ex_deus]

The fact that radial coordinate is timelike inside the event horizon doesn't change when you change coordinate systems. The radial direction remains timelike in kruskal coordinates. Direction being space like or time like is independent of coordinate system.

[lostmsu]

I think for this to be a complete explanation, it needs to be shown that what was "the time" coordinate is not expanding in some sense.

Does volume even make sense in GR?

Thought experiment here would be: suppose me and you appeared stationery (from the point of view of the outside reference frame) just above the horizon of a non-rotating black hole in the manner of Boltzmann brains. We immediately start falling and cross the horizon. Now will you see me receding from you or going at you? Will I be red-shifted or blue-shifted?

[gpderetta]

There is a singularity in our past. Are we in a white hole?