The argument for multiple-worlds seems to be that if you take the most elegant possible formulation of the "equations of the universe", you find that some of constants in these equation have to be set to apparently arbitrary values to imply a universe like our own.
This seems to rest on an assumption that simple patterns tend to continue beyond our immediate perception, like if a person see a part of a sphere sticking out of the sand on a beach, they would tend to expect an entire sphere to buried beneath.
The two questions that appear would: is formulation really the simplest and does this kind of reasoning apply "outside the universe", given the universe is all that we use to make such judgments.
I am probably wrong, but I've always liked the (fictional, in my head) solution that black holes are these "buried spheres" and that all the problems we have with dark matter and energy are really result of black holes "draining" mass from one "world" to another. In my Sci-fi mind, the big bang is what comes out the other side of a black hole, so, when a black hole forms, it starts draining matter and energy from one universe into another.
Essentially, we'd never be able to see all the mass our theories predict, because we'd never be able to observe all the linked worlds/universes past the event horizons.
Maybe not! Some people think the universe is shaped like a torus, and that everything in the universe, all of the energy flow, all of the minute systems, also reflect this. If that were the case as well, it may make sense to say that the universe is not a closed system. This universe would be a toroidal aspect of a larger toroidal universe and so on. And perhaps in the middle of that torus, you would see energy merging to a singularity.
Obviously I'm not claiming any of this as fact. But it is interesting to think about. After a certain point there is non fact-based validity in applying philosophy to our conception of universes.
I believe it is mentioned in passing in several of the books, but if I recall correctly the Hydrogen Sonata and Excession went into more detail. He does discuss it in his notes on the culture[0]:
> We accept that the three dimensions of space we live in are curved, that space-time describes a hypersphere, just as the two dimensions of length and width on the surface of a totally smooth planet curve in a third dimension to produce a three-dimensional sphere. In the Culture stories, the idea is that - when you imagine the hypersphere which is our expanding universe - rather than thinking of a growing hollow sphere (like a inflating beach-ball, for example), think of an onion.
> An expanding onion, certainly, but an onion, nevertheless. Within our universe, our hypersphere, there are whole layers of younger, smaller hyperspheres. And we are not the very outer-most skin of that expanding onion, either; there are older, larger universes beyond ours, too. Between each universe there is something called the Energy Grid (I said this was all fake); I have no idea what this is, but it's what the Culture starships run on. And of course, if you could get through the Energy Grid, to a younger universe, and then repeat the process... now we really are talking about immortality. (This is why there are two types of hyperspace mentioned in the stories; infraspace within our hypersphere, and ultraspace without.)
> Now comes the difficult bit; switch to seven dimensions and even our four dimensional universe can be described as a circle. So forget about the onion; think of a doughnut. A doughnut with only a very tiny hole in the middle. That hole is the Cosmic Centre, the singularity, the great initiating fireball, the place the universes come from; and it didn't exist just in the instant our universe came into being; it exists all the time, and it's exploding all the time, like some Cosmic car engine, producing universes like exhaust smoke.
> As each universe comes into being, detonating and spreading and expanding, it - or rather the single circle we are using to describe it - goes gradually up the inner slope of our doughnut, like a widening ripple from a stone flung in a pond. It goes over the top of the doughnut, reaches its furthest extent on the outside edge of the doughnut, and then starts the long, contracting, collapsing journey back in towards the Cosmic Centre again, to be reborn...
> Or at least it does if it's on that doughnut; the doughnut is itself hollow, filled with smaller ones where the universes don't live so long. And there are larger ones outside it, where the universes live longer, and maybe there are universes that aren't on doughnuts at all, and never fall back in, and just dissipate out into... some form of meta-space? Where fragments of them are captured eventually by the attraction of another doughnut, and fall in towards its Cosmic Centre with the debris of lots of other dissipated universes, to be reborn as something quite different again? Who knows. (I know it's all nonsense, but you've got to admit it's impressive nonsense. And like I said at the start, none of it exists anyway, does it?)
I don't know, I guess it could be. It depends whether the multiverse has infinite "flavors" or not. If every black hole linked from one universe to another, then observably, we have many more drains, and only 1 know point we're expanding from. OTOH, maybe all black holes in this universe drain right back into the one we're expanding from and there are only 2. Finally, there could be more points of expansion than just the big bang in our universe, but they may be outside of our sphere of observation. I'll admit the one-ingress-many-egress thing bothers me a bit.
Or the connectivity between universes is sparse, so that we only have direct connections to a few universes. I guess the set of all universes might still be a closed system in this arrangement.
This is overall kind of an interesting idea, because it seems to me like it'd be more testable than some other more intangible multiverses. My (non-physicist) intuition has always been that black holes somehow are the key to many mysteries of physics, but we're still pretty far from really knowing I think.
Riffing off that a bit, I think back to this illustration of spacetime - https://simple.wikipedia.org/wiki/Space-time#/media/File:GPB... - and imagine a black hole being a place where that fabric is punched through. as if spacetime is just resting on another universe. Where things get massive enough, they punch through and the matter begins to drain away. We don't know what the rate of flow is, though.
I like this because referring back to the comment about not being able to see enough matter - we know black holes aren't massive enough to account for all the dark matter - but, what if the discrepancy of observation vs expectation gave us some idea of what the flow rate through black holes might be?
This is using the Anthropic principle to argue the point for the many worlds hypothesis because it is being argued that life is so fragile that it could not have formed without certain constants being how they were. So that there must have been a bunch of other realities that failed in creating sentient life.
The constants would apply outside the universe because the constants are invariant other all of our observations (the fine structure constant is an observed number and its value is arbitrary and if it was greater or less than it we wouldn't exist). But they aren't part of our perception they are part of our measurements.
This is using the Anthropic principle to argue the point for the many worlds hypothesis because it is being argued that life is so fragile that it could not have formed without certain constants being how they were. So that there must have been a bunch of other realities that failed in creating sentient life.
I don't think the existence or non-existence of sentient life in alternate universe is in anyway necessary for the argument.
If we assume possible universes is a set parameterized by a constant C and if we know only value X gives a universe with the properties we observe, we can deduce C = X.
If a value Y yields a universe with sentients that can't be us, we still know that isn't our universe and so we know C!=Y.
This seems to rest on an assumption that simple patterns tend to continue beyond our immediate perception, like if a person see a part of a sphere sticking out of the sand on a beach, they would tend to expect an entire sphere to buried beneath.
The two questions that appear would: is formulation really the simplest and does this kind of reasoning apply "outside the universe", given the universe is all that we use to make such judgments.