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There are a lot of questions on this site about white holes, but none of them specifically answer my question. Most of these questions talk about observing white holes from outside, but do not talk about the possibility of observing a white hole from inside.

I have read this question:

Is it possible we live in a white hole?

Any theory/author using the white hole model to explain the birth of the universe?

These are the only questions talking about the possibility of being inside a white hole, but none of the answers give an explanation to my question about experimentally telling whether we were inside one.

We have just visually observed a black hole (M87) for the first time, and took a real picture of it.

It is conjectured that they cannot, and the only possible black holes are the hypothetical primordial black holes that existed from the very beginning of the universe.

How can anything ever fall into a black hole as seen from an outside observer?

Now as far as I understand, what we observed is not even a really formed black hole (nor a fully formed event horizon), because that never forms in a finite time when viewed from our frame here on Earth.

Equations of fundamental physics are usually time-reversible (particle physics is time-reversible in a CP or CPT sense, though) which means you can take a certain reversal of the solution of your equations to obtain another solution. A white hole is a complementary object to a black hole in this sense. Every particle in it's field behaves the same as a particle near a black hole only with a reversed arrow of time.

White Hole Formation and Queries

Since white holes are supposed to be time-reversed black holes, does this mean that (when viewed from our frame), these white holes never fully form either?

The proper view is that the universe itself is an inside-out black hole, with a cosmological horizon that surrounds us.

Are we inside a black hole?

So there are a few things that come to mind:

  1. black holes (event horizon) never form in a finite time for an outside observer, and space contracts/squeezes inside into a singularity (the singularity is the future)

  2. white holes are time reversed black holes

  3. white holes (event horizon) never form in a finite time for an observer inside, and space inside them expands like in our universe (the singularity is the past)

So based on the these, and the last answer, that says that our universe is an inside-out black hole, is it possible to tell experimentally if we are inside a white hole?

Question:

  1. Is it experimentally verifiable if we are a inside a white hole that never fully formed?
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  • $\begingroup$ As you haven't included a "cosmology" tag, I feel justified in not posting this remark as an answer, but the most convincing model that I have been able to find, for a multiverse comprised by local universes that are each a white hole forming the surface of a black one, is Nikodem Poplawski's torsion-based one, described in numerous 2010-2021 papers whose preprints can be found (by his name) on Cornell University's "Arxiv" site. It's based on Einstein-Cartan Theory, which is reportedly more complex than GR, although ECT is said to reduce to GR in vacuum. $\endgroup$
    – Edouard
    Nov 9 '21 at 1:05
  • $\begingroup$ @safesphere thank you, though, I am asking from inside (like if we would be inside it). $\endgroup$ Nov 9 '21 at 16:52
  • $\begingroup$ We could be inside an Oppenheimer-Snyder white hole (most other types don't have a uniform space inside). To start proving this, you'd need to fly to, find, and cross the horizon, which is many billions light years away and recedes at three times the speed of light. While this may or may not give your the proof (I hope you get a proper answer), you wouldn't be able to notify us of your findings. Currently there is no experimental proof that the cosmological horizon exists. See this question for details: Experimental Proof of Space Expansion $\endgroup$
    – safesphere
    Nov 10 '21 at 3:55

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