Could a black hole interior contain an entire universe?

[GR_fan]

But where do these conditions come from? From our observations of how things work in the portion of the universe we can observe and measure and manipulate. We observe that all the stuff we can see obeys these conditions. That's why physicists consider them to be reasonable constraints.

Notice that my comment was about regions that are arbitrarily far beyond our cosmological horizon. This statement is consistent with the claim that we should extrapolate known laws to regions just outside our cosmological horizon. You admit that we already have reason to think the energy conditions fail under specific exotic conditions. Well, could there exotic conditions, where our best physical theories are no longer appropriate, for regions arbitrarily far from us in spacelike directions? I'm agnostic about that question, and so I'm agnostic about what spacetime is like arbitrarily far away from us.

At any rate, I think this philosophical debate about how to think about GR's consequences for unobservable regions takes us very far outside the current thread. Personally, I like thinking about the philosophical foundations of our best physical theories, but I had the impression that this was a bit taboo for Physics Forums.

But that means we're allowing things in regions we can't see to be totally unconstrained. Which is pointless. If we're going to impose constraints on our models from observations at all, we're basically adopting the cosmological principle, that the parts of the universe we can't see are basically similar to the parts we can see. Otherwise we have no basis for constructing any model at all.

Well, from the perspective of most physicists, it's pointless to talk about regions of spacetime from which we will never be able to gather observations. I'm interested in such questions for philosophical reasons, though, as I said, I thought that was a bit beyond what's of interest on Physics Forums.

At any rate, I disagree that the Cosmological Principle is required for model construction. Although the CP appears to be approximately satisfied for the visible universe, we can think about what science would be like if that had failed. For example, up to the early 20th century, it was thought that the Milky Way was the only galaxy. Although we now know otherwise, I think we would still have had a basis for constructing scientific models had it turned out that the Milky Way was the only galaxy. But in that case, the CP would have been false and we would occupy a quite special location in the universe.

 

[PeterDonis]

Notice that my comment was about regions that are arbitrarily far beyond our cosmological horizon. This statement is consistent with the claim that we should extrapolate known laws to regions just outside our cosmological horizon.

I never said "just outside". I just said "the parts of the universe we can't see". That includes the parts arbitrarily far beyond our observable universe.

You admit that we already have reason to think the energy conditions fail under specific exotic conditions.

Yes--inside the deep interiors of collapse processes, where there are energy densities and pressures and spacetime curvatures many, many orders of magnitude larger than the ones we experience in our everyday lives, or that we observe anywhere else in the universe we can see.

could there exotic conditions, where our best physical theories are no longer appropriate, for regions arbitrarily far from us in spacelike directions?

Sure--inside the deep interiors of collapse processes there. Which, by the very cosmological principle I mentioned, we would assume to exist in those regions, since we expect those regions to be like the regions we can see, and such things exist in the regions we can see.

I think this philosophical debate about how to think about GR's consequences for unobservable regions takes us very far outside the current thread.

We are getting to the point where we can't really continue much useful discussion, yes. But we are at least outlining where the title question of this thread takes you when you think about it.

from the perspective of most physicists, it's pointless to talk about regions of spacetime from which we will never be able to gather observations.

Yes.

At any rate, I disagree that the Cosmological Principle is required for model construction.

I never said it was. I explicitly said that yes, logically and mathematically you can construct models that don't obey it. I just gave reasons why cosmologists currently use the cosmological principle as a constraint on what models they actually consider (as opposed to what models can logically and mathematically be constructed).

Although the CP appears to be approximately satisfied for the visible universe, we can think about what science would be like if that had failed.

But it hasn't. As a matter of historical speculation, yes, we can speculate about what kinds of models we might have constructed if the evidence had been different from what it is. But that is indeed getting well off topic for this thread.

 

[GR_fan]

To me this "possibility" is like the "possibility" of Russell's teapot or Sagan's invisible dragon in the garage.

I never said that any specific possibility is probable. On my view, every one of these models about what the universe is like, arbitrarily far beyond our cosmological horizon, is quite improbable.

Someone who endorses the invisible dragon is being unreasonable, as is someone who says that they don't know whether the dragon is probable or improbable. But I'm saying that. I'm saying that there are a vast range of possibilities for what spacetime could be like in regions that are arbitrarily far beyond our cosmological horizon and that, given the vast range of possibilities, every specific possibility is implausible. So, I agree that the time reverse Oppenheimer-Snyder model, and variations on it, are implausible. The claim we should be examining is the big disjunction of all the possible geometries spacetime could have arbitrarily far beyond our cosmological horizon and (perhaps) measures over the various geometries.

If I said that any one of the models was plausible, I wouldn't be an agnostic!

Incidentally, the dragon example is dissimilar from what I'm saying in other respects. The dragon is a poor fit with our background knowledge, makes for an overly complex hypothesis, etc. I haven't made a claim that suffers from these sorts of issues.

 

[PeterDonis]

On my view, every one of these models about what the universe is like, arbitrarily far beyond our cosmological horizon, is quite improbable.

If you are including our actual best current model, which says thsat the rest of the universe looks like the part we can see, I strongly disagree. That model is adopted by cosmologists precisely because they consider it to be highly probable.

 

[PeterDonis]

If I said that any one of the models was plausible, I wouldn't be an agnostic!

Yes, and I am saying that most cosmologists are not agnostics on this issue. Most cosmologists do not consider all possible models of what the part of the universe we don't see could be like to be equally implausible or improbable. They assign a much higher probability weight to our actual best current model, which obeys the cosmological principle, than to all the other models which don't.

Of course nothing requires you to accept that position just because most cosmologists do. But I think it should be made clear, not just for you and me but for the OP and other readers of this thread, what the current mainstream position within cosmology, for better or worse, is.

 

[GR_fan]

I just gave reasons why cosmologists currently use the cosmological principle as a constraint on what models they actually consider (as opposed to what models can logically and mathematically be constructed).

Here's the claim you made:

If we're going to impose constraints on our models from observations at all, we're basically adopting the cosmological principle, that the parts of the universe we can't see are basically similar to the parts we can see. Otherwise we have no basis for constructing any model at all.

The point I was making with my alternative history is that, even if we reject the Cosmological Principle, we could still have a basis for constructing models.

 

[GR_fan]

Yes, and I am saying that most cosmologists are not agnostics on this issue. Most cosmologists do not consider all possible models of what the part of the universe we don't see could be like to be equally implausible or improbable. They assign a much higher probability weight to our actual best current model, which obeys the cosmological principle, than to all the other models which don't.

I don't think this is correct. A fairly large number of cosmologists endorse a multiverse. In the multiverse context, the Cosmological Principle is approximately true of the visible universe, but fails outside it.

Furthermore, consider that there's a live debate about how to make predictions in a multiverse context -- that is, the measure problem. This problem asks us to construct a measure over the universes within the multiverse, so that we can say what sorts of features are probable or typical among them. There's no agreement on how to resolve this problem, but plenty of proposals.

The issue I'm pointing to is broader than the measure problem. Instead of constructing a measure over isotropic/homogeneous baby universes, we need to construct a measure over spacetimes generally. You might call that the "extended measure problem". Since the ordinary measure problem is currently unresolved, the situation is even worse with respect to the extended measure problem.

 

[PeterDonis]

A fairly large number of cosmologists endorse a multiverse. In the multiverse context, the Cosmological Principle is approximately true of the visible universe, but fails outside it.

Only in the sense that the multiverse is "everywhere the same" in a different way than our particular bubble universe inside it--it's everywhere composed of bubbles popping out of eternal inflation, or something like that; some way in which our individual universe is one of an infinite number of "bubbles" in something else, and the something else is "everywhere the same" as regards how bubbles pop out of it, the average "density of bubbles", etc.

there's a live debate about how to make predictions in a multiverse context -- that is, the measure problem. This problem asks us to construct a measure over the universes within the multiverse, so that we can say what sorts of features are probable or typical among them. There's no agreement on how to resolve this problem, but plenty of proposals.

Yes, but all of them have the common feature that the multiverse is "typical" everywhere in some way. There's just no agreement on what particular way that is.

Instead of constructing a measure over isotropic/homogeneous baby universes, we need to construct a measure over spacetimes generally.

Not generally. There are still constraints on what spacetimes get included. There's no general agreement in the multiverse context on what specific constraints should be imposed, but nobody is arguing for a multiverse that literally includes all possible spacetimes whatever, with no constraints at all, and struggles to come up with a measure over that set.

 

[PeterDonis]

The point I was making with my alternative history is that, even if we reject the Cosmological Principle, we could still have a basis for constructing models.

Yes, some basis in observations, which on your hypothetical would have been different than they actually were.

But suppose, for example, that observations had told us that our galaxy was an "island" in an otherwise empty universe. That's still a model about what things are like "everywhere". I don't think the mainstream model in such a case would have been "our galaxy is an island in an otherwise empty universe as far as we can see, but outside our observable universe there's a domain wall and on the other side is something completely different". We would still be extending the best model we can come up with of what we can see, out beyond to where we can't see, and assuming nothing else intrudes.

 

[GR_fan]

There's no general agreement in the multiverse context on what specific constraints should be imposed, but nobody is arguing for a multiverse that literally includes all possible spacetimes whatever, with no constraints at all, and struggles to come up with a measure over that set.

Agreed! But that's not the point I was making. Importantly, I never argued that we should consider a multiverse that somehow includes all possible spacetimes.

Remember that I described what I called the extended measure problem. The standard measure problem includes fairly heavy constraints on what sort of baby universes are considered. Consider, now, the possibility that the multiverse is part of some even larger structure. What sort of measure should we include to reach conclusions in that context? I submit that I simply have no idea.

Anyway, I'm not sure what point you are making. Even if we suppose that my view is non-standard, I'm not bothered by that. I'm not telling anyone that they must agree with me; I'm merely offering my own opinion, and I've been clear about that throughout. If you think my opinion is silly or that my opinion breaks with some sort of consensus, I can live with that.

Since this is a physics forum, and not a philosophy forum, I think we should stick to issues that can be settled either by mathematics or by experiment.

 

[PeterDonis]

I never argued that we should consider a multiverse that somehow includes all possible spacetimes.

You did say "a measure over spacetimes generally". But your clarification is helpful, yes.

The standard measure problem includes fairly heavy constraints on what sort of baby universes are considered. Consider, now, the possibility that the multiverse is part of some even larger structure. What sort of measure should we include to reach conclusions in that context?

Hm, ok. I'm not familiar with speculations about a "multi-multiverse". But if there are papers on the subject, they are better suited to discussion in the Beyond the Standard Models forum. (Indeed, we're likely at that point with the multiverse topic overall.)

 

[GR_fan]

Hm, ok. I'm not familiar with speculations about a "multi-multiverse".

I worry that I wasn't clear and may have misled you here. The idea of a "multi-multiverse" (as you described it) is my own idea. I mention it in a few papers, currently undergoing peer review, but it's not an established idea discussed by others (as far as I know). Nonetheless, I think it's fair to say that it's a possibility allowed by the Einstein Field Equation, and that the mathematics involved is entirely standard.

But if there are papers on the subject, they are better suited to discussion in the Beyond the Standard Models forum. (Indeed, we're likely at that point with the multiverse topic overall.)

I'm new to Physics Forums. Is that the place where more speculative ideas are discussed? I gather that, according to PF rules, speculative ideas can be discussed as long as they appear in some peer reviewed articles. Perhaps when/if I've published something on this, I'll make a post over there, and we can discuss it!

 

[renormalize]

I gather that, according to PF rules, speculative ideas can be discussed as long as they appear in some peer reviewed articles. Perhaps when/if I've published something on this, I'll make a post over there, and we can discuss it!

Even after publication, Physics Forums restricts most posters from discussing their own work here. Read this mentor response from another recent thread:

I’d be more than happy to share my own experimental results right here in this thread—just want to ensure that it aligns with the PF guidelines and that the moderators and colleagues are comfortable with that.

Only if they have been published in a peer-reviewed journal that meets our requirements (see the Rules under INFO at the top of the page), and only after they have been published for long enough for other researchers to have reviewed the results and do not have significant objections. Even then, we generally only allow our vetted Science Advisors to discuss their published work here.

 

[PeterDonis]

The idea of a "multi-multiverse" (as you described it) is my own idea. I mention it in a few papers, currently undergoing peer review, but it's not an established idea discussed by others (as far as I know).

Then unfortunately it would be off topic here until it's been published and there has been some time for the relevant scientific community to assess it.

I'm new to Physics Forums. Is that the place where more speculative ideas are discussed?

Generally speaking, no. We do have the Beyond the Standard Models forum for discussion of models that go beyond the current mainstream models, but even there things are limited; just having a published paper is not enough. PF's primary purpose is to help people understand what is currently mainstream, not to facilitate research.

 

[GR_fan]

We do have the Beyond the Standard Models forum for discussion of models that go beyond the current mainstream models, but even there things are limited; just having a published paper is not enough. PF's primary purpose is to help people understand what is currently mainstream, not to facilitate research.

Fair enough! I'm comfortable not discussing my own research, and just discussing results that are already mainstream. :)