What even more recently occured to me, is that we have a nice literary analogy to quantum monism in Dt. 10:14, "Behold, to the Lord, your God, belong the heavens and the heavens of [beyond] the heavens, the earth, and all that is on it." (Source: https://www.chabad.org/library/bible_cdo/aid/9974 .) I stress the word "analogy" here - I do not (obviously) suggest the authors of the Torah were quantum physicists, but I would argue there is a certain spirit that is similar, that is to say, the authors of the Torah, were they alive today, would probably like quantum monism.
Let me explain. Quantum monism essentially is a updated rephrasing of an idea that has been around since the 1950's, which is that the wave function of quantum particles is primal, or fundamental. What does this mean? This means that if (for example) I have an electron that can be in two states, say, "up", or "down", and I measure this electron to be - say - "up", there is a "parallel reality" somewhere (in a higher dimensional space) where observers will measure that same electron to be "down", that is to say, all possible paths of particles are realized somewhere, in what is often (mis) labelled a set of "parallel universes". Why do I say this is mislabelled? Because these are Not "separate" universes, but rather are different dimensions in a deeper underlying reality. (This underlying reality can be given by Hilbert space that has an infinit number of dimensions, each dimension corresponding to a particle "possible configuration" of matter, but of course Hilbert space is just a model, and perhaps a final model is still yet to be elided.)
How I relate this by analogy to the above mentioned verse in Torah is that was positing that there was an underlying reality - in the case of the context of that passage, this reality being g-d, that unified all of all existence, so while there might be "heavens of the heavens" (i.e. in the context of the time, parts of the world even beyond or past the stars visible to the naked eye which were the "heavens", so here the author(s) are prescient enough to know that there could well be parts of the world not visible to the naked eye which was very forward-thinking for its time, given that this was written perhaps up to 3,000 years ago) - even those parts of the world that could not be seen were in a sense "tied together" with the more familiar parts of the world by belonging to g-d (for the writers of this passage), or, in modern parlance we might say, all aspects of the "world" - the universe, has some underlying reality or set of laws that govern it.
So, this notion that while there are different aspects of the universe that there is notwithstanding this something unifying everything, is old, and predates quantum physics. The Hindus had the Brahmin, Spinoza had his "Substance", the 1800's physicists had the luminferous ether. In different ages and cultures, the smart people of the time hit upon this basic truth, that however diverse the universe might be, there is something "underneath" that binds it together. So - it is not perhaps precisely right to say that there is a quantum "multiverse" - there is just one universe, but one within which all quantum possibilities are realized.
To go into my own "spin" on things here, and here I draw from reading Stephen Hawking's work, this is - for what it is worth - my own sort of understanding of this notion of a "quantum monism". I also draw from if you will my personal "bias" of being a relativist (in the sense of drawing my own thinking from Einstein's Relativity, a totally different sense of the word "relativist" than the sense of the word that arises in discussions of ethics). But, here goes.
In my view, all possible metrics - solutions - of General Relativity exist, and the reality we observe, and, as importantly, can predict, is basically the average of these solutions. Hawking's No Boundary Proposal restricts the "possible metrics" to compact ones, but that is more a minor, technical point. For the purposes of this discussion, we can think of "all of reality" is the summation of some subset, perhaps all, of the solutions of General Relativity, which have some - as yet not fully understood - way of being "averaged together" to form the world we live in. This can be modelled mathematically as a type of mathematical object called "the wave function of the universe", from which predictions can be derived for the universe we live in. The fact that we live in the universe alters how we do this, that is to say, we weight the contributive metrics which allow for observers more heavily than those that do not in order to make reasonable predictions from this "wave function" for the world we actually experience.
To use an - always imperfect - sort of analogy, the universe can be treated like a quantum partitcle that goes from one state to another through time. This abstract "particle" starts in a state - or a metric - that is small and dense, and goes at length into another metric that is expanding and large. So the "trajectory" of how the universe evolves through time can be modelled by treating "the universe" as if it were some sort of abstract (not real, but by way of analogy) "particle" that "travels" from one metric (or solution) to another, and this "trajectory" is simply the "average" path that "it" can take by "summing over" all possible metrics given by General Relativity (or the subset of those metrics that we have decided to include in our average).
So, long story short, the totality of reality is - again - something in which anything that can happen does happen in different if you will parallel timelines, which - just to complicate matters - may interact in some way (not now fully understood) so observers in any given "possible timeline" will always see there own "timeline" as an "average" of timelines in a similar way to the fact that any observer in an expanding universe will always see all other galaxies receding away from them relative to their position, so it "seems" as though they are at the "center of the universe" but that is simply an illusion given by being in an expanding universe - similarly - any observers in a given "timeline" (or "solution" or "metric" depending on your preferred terminology) will see their "timeline" as being the "average of all possible timelines", because the "average" is going to be different for different observers in different "timelines". "Clear as mud", I know, but I am hamstrung here by the fact that beyond not being a professional physicist myself, even were I, the details of all this are yet to be worked out (because you need a theory of quantum gravity - a quantum description of General Relativity in order to be able to do so, and this theory is "pending" and has been for about a century).
There is a simple take-away I think worth pointing out, and that is to say, that over the years (going, as I have argued, even all the way back to the time of the Torah) there has been converging streams of thought taking different forms over the years that while reality may be diverse and be greater than what we can ever even in principle observe, that, nevertheless, it is governed by the same underlying rules, and displays at the end of the day a deep unity, however complex it may seem to be. Call this deep unity what you like - for the writers of Torah, "G-d", for the Hindu writers of the Vedas, "Brahmin", for Spinoza, "Substance", for 19th century scientists "Ether", for today's scientists, "the wave function of the universe" or a "quantum object" but your end result is the same idea, a profound and beautiful if never wholly understood deep reality that binds everything together.
I don't want to make this too long, nor do I want to get into deeper philosphical waters, but I will make very brief mention of the word "pantheism" which originally came about as almost an insult for people who didn't like Spinoza, but has re-emerged as a philosophical notion in its own right, and that is, essentially, this idea that the concept of divinity is bound up in some deep way with nature, and that these two concepts may in fact be the same, but at some far deeper level than the simplistic notions of nature given by most of the natural philosophers, so, not so much saying that "g-d is nature" but more saying "g-d is nature in the sense of nature as the total collection of those phenomena that are experienced in both the forms of matter and in the forms of mind, and is something that is deeper than both matter and mind as normally understood, and is something that can never be wholly apprehended by any particular model given by science, but is something that we are all a part of because it has a very profound unity out of which all things spring" (my words or take on it anwyay). So though the concept of "pantheist" began as basically an insult and even now remains inchaote, I think it worth pursing, and I think constitutes a sort of "third way" between the "hard core materialism" of traditional "atheism" and the "hard core mentalism" of traditional "theism". So I like to more or less consider myself a Spinoza-style "pantheist" while ever mindful that the notion demands more elucidation that it has yet been given, and hence I am here trying in a small way to do so.
I think there is one very simple and very crucial component of what I here call pantheism, the idea of an underlying reality that is the unity of the perhaps infinite number of spacetimes available from Relativity all woven together into some sort of unified whole, some sort of single "quantum object", and this simple observation I think separates what I am calling pantheism from traditional (Lucretius-style, "it is all atoms flitting about in the void") atheism, and that is that we are inside this "quantum object." You can never get outside of "reality" to observe it "objectively." Because you are in it. The fact that you are in it changes what predictions you can make about it (for instance, in Hawking's No Boundary Proposal you make different predictions than you would make if you ignored the presence of observers). So, actually, it is not "atoms flitting about in the void", it is also "observers made up of atoms flitting about in the void influencing themselves, other atoms and the void itself," so to speak. This is actually a very awesome conclusion - we are not "along for the ride" in a cold, heartless universe. We are co-creators with the universe. That "quantum object" I spoke of would not be the same without you, without me, without that gnat that crawled across my computer screen a few minutes ago. Even the word, "object," here, misleads somewhat, because reality is never static - it evolves through time - a better analogy than "quantum object" might be that of a river, where each "alternative timeline" is a "ripple" or eddy, which together build up to a larger whole, but one which continually changes and generates new realities and modes of existence, moment by moment. Yes, perhaps, "quantum river" is a better term.
We are all in this together. That is the real take-away here, what smart people from the writers of the Torah to the writers of the Vedas to Albert Einstein to Stephen Hawking all understood. The universe may have infinite regions and even "alternative timelines" that we can never even in principle observe, but it is all at the end of the day one unified whole, a whole to which we each individually contribute, a whole which just would not be the same without each one of us. Now I think that is pretty cool. :)
Appendix I: Two good introductory videos on Hawking's No Boundary Proposal (which is one possible approach to defining a "wave function of the universe") by science educator Athena Brensberger (https://astroathens.com ):
Appendix II: Brief Discussion of Pantheism by Wesleyan University Professor of Religion Mary Jane Rubenstein (https://mrubenstein.faculty.wesleyan.edu ) (or specifically discussing the need to develop the concept of pantheism more fully):