Inasmuch as I am neither mathematician nor physicist, I cannot speak to those equations mentioned. But, on my own view, the notion of a multiverse as provable or not, is no more useful (the pragmatist---or pessimist---emerges) than whether or not there is something we call infinity. I have previously offered my thoughts on the infinity 'problem', so it would be redundant to restate those here. George Ellis' book, as described, is wholly and mindfully conceived. Physics, I am fairly certain, underlies everything in the universe we know, including life itself. If such is true in any possible unknown universes, or in fact, a posited multiverse, that would make those contingencies consistent with our own, meaning the laws of physics, known AND unknown, are consistent with one another(?) But wait: we cannot know this. So, it is all pie-in-the-sky. So to speak. This is just one aspect of philosophy which makes it fun. No robots here. They don't have fun.

HGN and Prof Ellis both illustrate the joy of doing philosophy. Though doing it is completely useless (except for "drawing the line" when it is immoral not to draw a line), it is so much fun! Especially the idea, which I have long espoused in these corners, that science is only one way to understanding...matter. (The dualism of matter, and consciousness: the realm of philosophy, poetry, and comedy.) My mantra:

"What is the answer?"
"The answer is another question:
Why do you think you need an answer?"

I so appreciated Ellis' description of philosophical "reasoning" to pursue questions. Not necessarily to answer them, but to use methods and thinking that cannot be called science or scientific to explore with.

If one believes in prophecy, then it stands to reason that our "future" has already occurred (somewhere) and we are merely making choices that have already been made.
But where has our future occurred? In a universe exactly like our own? Are these so called multi verses in black holes hence our inability to detect them?
But here lies the main question; which universe is real and which is the "simulation"?

I have asserted that which we view as eternity and that we call infinity are essentially the same: they both portray a void; an unknowable nothingness. I have also said we ought not worry about infinity because no one can ever get there; there being no THERE to get to. Eternity, however, does mean something to many people---they are the 'long-lifers' who believe they are going to be around forever. I wish them good speed and happy landings and hope they are not disappointed. At least not in any cognitive or cognizant way... (It is good to be back)

Another take (not mine) on eternity; infinity; truth; and whether or not any of it, as Sellars has said, 'hangs together'.
Denis Diderot, 1713-1784, allegedly said the following: Skepticism is the first step toward truth. What has never been called into question has never been proven. One can demand of me that I seek truth, but not that I must find it. Mssr. Diderot was not generally known as a philosopher. He possessed a brilliant, incisive mind and was chief architect of the famous encyclopedie (I cannot add the ACCENT GRAVE) of the Enlightenment era. Diderot appeared to be saying something like: go find your own truth; I'll go find mine. But don't foist upon me ineffable bullshit! Things do, mostly, hang together, and so Sellars said it as well as anyone who comes to mind. Eternity; infinity; truth; justice---these are terms we use loosely when finding ourselves in a tight spot. Rhetorically, they are useful, but they are also relative, dependent upon which wind is blowing and whether we are in a time of enlightenment or a time of depravity. Diderot was a skeptical pragmatist. We can learn much from his sort. Attorneys and judges have known this for, well, an eternity (ha, ha, ha.)