Recent content by GR_fan

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

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...

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...

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...

Here's the claim you made: 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.

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...

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...

Again, my point was never that we should think this is a realistic possibility. Instead, my point was that General Relativity includes solutions of the kind that I described. It should be a familiar point that the Einstein Field Equation allows for almost any geometry. Take any metric you'd...

Right. Agreed. Two points. First, it's not clear what sort of features can be reasonably "baked in" to the universe. It's not clear -- particularly in the relativistic context -- that we should understand the laws as evolving from state of the universe to another. But that's a tangential debate...

Right. My apologies, again, for the confusion. In the future, I'll try to do a better job at double checking what I wrote before pressing "Post reply"!

In the original Oppenheimer-Snyder model, the two regions are made to match without a thin shell, because the extrinsic curvature from both sides of the joining hypersurfaces exactly matches. But people often consider extensions of the Oppenheimer-Snyder model where the extrinsic curvature does...

Indeed! Agreed. (My only misgiving is that many of the arguments people give for thinking that various solutions of the Einstein Field Equation are "highly implausible" appear dubious to me, but I certainly don't want to give the impression that this is a mainstream idea or that, from a Bayesian...

Fair enough. I agree.

Well, yes and no. You are right that the condition enforced by the condition is important. However, with the right thin shell on the boundary between the two regions, you have a great deal of latitude in the geometries chosen for the two regions. As I mentioned previously, however, the resulting...

Sure. One example is this recent paper by Kinney, posted to ArXiv this year: https://arxiv.org/pdf/2607.28412 It was Kinney's article that I had in mind, because it's a paper I've studied closely. A slight variation -- where a square root is taken of the scale factor used by Kinney -- is...