Getting rid of Many Worlds doesn’t get rid of the Multiverse. They pop up in many different ways in cosmology. Max Tegmark elaborated four levels., the simplest of which (Level I) ends up looking like a multiverse if the ordinary universe is sufficiently large.
In the field of astronomy, there’s the concept of a cosmological horizon. There are several kinds of these depending on exactly how they’re defined. That’s why they use the term “observable universe”. Whatever process kicked off the Big Bang obviously created a lot more of it than we can see, and the horizon is expanding over time as light has more time to get to us. What we can see is not all there is.
Our current understanding of physics implies the Bekenstein bound: for any finite region of space, there is a finite amount of information it can contain. Interestingly, this measure increases with surface area, not volume. (If you pack too much mass/energy in a finite volume, you get an event horizon, and if you try to pack in more, it gets bigger.) Therefore, the current cosmological horizon also contains a finite amount of information, and there are a finite number of possible initial conditions for the part of the Universe we can observe, which must eventually repeat if the Cosmos has a larger number of regions than that number, by the pigeonhole principle. We also expect this to be randomized, so any starting condition will be repeated (and many times) if the Cosmos is sufficiently large. Tegmark estimated that there must be a copy of our volume, including a copy of you about 1010115 meters away, and this also implies the existence of every physically realizable variation of you, which ends up looking like branching timelines.
I am also interested in “Level II: Universes with different physical constants”. Do you happen to know if there’s actually good ways to approach that question — ie any methodology much better than wild speculation? I would love to learn about such a method
Well, slightly better than wild speculation. We observe broken symmetries in particle physics. This suggests that the corresponding unbroken symmetries existed in the past and could have been broken differently, which would correspond to different (apparent) laws of physics, meaning particles we call “fundamental” might have different properties in different regions of the Cosmos, although this is thought to be far outside our observable universe.
The currently accepted version of the Big Bang theory describes a universe undergoing phase shifts, particularly around the inflationary epoch. This wouldn’t necessarily have happened everywhere at once. In the Eternal Inflation model, in a brief moment near the beginning of the observable universe, space used to be expanding far faster than it is now, but (due to chance) a nucleus of what we’d call “normal” spacetime with a lower energy level occurred and spread as the surrounding higher-energy state collapsed, ending the epoch.
However, the expansion of the inflating state is so rapid, that this collapse wave could never catch up to all of it, meaning the higher-energy state still exists and the wave of collapse to normal spacetime is ongoing far away. Due to chance, we can expect many other lower-energy nucleation events to have occurred (and to continue to occur) inside the inflating region, forming bubbles of different (apparent) physics, some probably corresponding to our own, but most probably not, due to the symmetries breaking in different directions.
Each of these bubbles is effectively an isolated universe, and the collection of all of them constitutes the Tegmark Level II Multiverse.
What’s the latest word on size of the whole universe? Last I heard, it was like at least 10,000x the diameter of the observable universe but maybe infinity times bigger.
I don’t know where you heard that, but the short answer is that no-one knows. There are models of space that curve back in on themselves, and thus have finite extent, even without any kind of hard boundary. But astronomical observations thus far indicate that spacetime is pretty flat, or we’d have seen distortions of scale in the distance. What we know to available observational precision, last I heard indicates that even if the universe does curve in on itself, it must be so slight that the total Universe is at least thousands of times larger (in volume) than the observable part, which is still nowhere near big enough for Tegmark Level I, but that’s a lower bound and it may well be infinite. (There are more complicated models that have topological weirdness that might allow for a finite extent with no boundary and no curvature in observable dimensions that might be smaller.)
I don’t know if it makes any meaningful difference if the Universe is infinite vs “sufficiently large”. As soon as it’s big enough to have all physically realizable initial conditions and histories, why does it matter if they happen once or a googol or infinity times? Maybe there are some counterintuitive anthropic arguments involving Boltzmann Brains. Those seem to pop up in cosmology sometimes.
Getting rid of Many Worlds doesn’t get rid of the Multiverse. They pop up in many different ways in cosmology. Max Tegmark elaborated four levels., the simplest of which (Level I) ends up looking like a multiverse if the ordinary universe is sufficiently large.
In the field of astronomy, there’s the concept of a cosmological horizon. There are several kinds of these depending on exactly how they’re defined. That’s why they use the term “observable universe”. Whatever process kicked off the Big Bang obviously created a lot more of it than we can see, and the horizon is expanding over time as light has more time to get to us. What we can see is not all there is.
Our current understanding of physics implies the Bekenstein bound: for any finite region of space, there is a finite amount of information it can contain. Interestingly, this measure increases with surface area, not volume. (If you pack too much mass/energy in a finite volume, you get an event horizon, and if you try to pack in more, it gets bigger.) Therefore, the current cosmological horizon also contains a finite amount of information, and there are a finite number of possible initial conditions for the part of the Universe we can observe, which must eventually repeat if the Cosmos has a larger number of regions than that number, by the pigeonhole principle. We also expect this to be randomized, so any starting condition will be repeated (and many times) if the Cosmos is sufficiently large. Tegmark estimated that there must be a copy of our volume, including a copy of you about 1010115 meters away, and this also implies the existence of every physically realizable variation of you, which ends up looking like branching timelines.
I am also interested in “Level II: Universes with different physical constants”. Do you happen to know if there’s actually good ways to approach that question — ie any methodology much better than wild speculation? I would love to learn about such a method
Well, slightly better than wild speculation. We observe broken symmetries in particle physics. This suggests that the corresponding unbroken symmetries existed in the past and could have been broken differently, which would correspond to different (apparent) laws of physics, meaning particles we call “fundamental” might have different properties in different regions of the Cosmos, although this is thought to be far outside our observable universe.
The currently accepted version of the Big Bang theory describes a universe undergoing phase shifts, particularly around the inflationary epoch. This wouldn’t necessarily have happened everywhere at once. In the Eternal Inflation model, in a brief moment near the beginning of the observable universe, space used to be expanding far faster than it is now, but (due to chance) a nucleus of what we’d call “normal” spacetime with a lower energy level occurred and spread as the surrounding higher-energy state collapsed, ending the epoch.
However, the expansion of the inflating state is so rapid, that this collapse wave could never catch up to all of it, meaning the higher-energy state still exists and the wave of collapse to normal spacetime is ongoing far away. Due to chance, we can expect many other lower-energy nucleation events to have occurred (and to continue to occur) inside the inflating region, forming bubbles of different (apparent) physics, some probably corresponding to our own, but most probably not, due to the symmetries breaking in different directions.
Each of these bubbles is effectively an isolated universe, and the collection of all of them constitutes the Tegmark Level II Multiverse.
Your explanation is much appreciated! It’s probably time that I properly go and understand the broken symmetries stuff.
What’s the latest word on size of the whole universe? Last I heard, it was like at least 10,000x the diameter of the observable universe but maybe infinity times bigger.
I don’t know where you heard that, but the short answer is that no-one knows. There are models of space that curve back in on themselves, and thus have finite extent, even without any kind of hard boundary. But astronomical observations thus far indicate that spacetime is pretty flat, or we’d have seen distortions of scale in the distance. What we know to available observational precision, last I heard indicates that even if the universe does curve in on itself, it must be so slight that the total Universe is at least thousands of times larger (in volume) than the observable part, which is still nowhere near big enough for Tegmark Level I, but that’s a lower bound and it may well be infinite. (There are more complicated models that have topological weirdness that might allow for a finite extent with no boundary and no curvature in observable dimensions that might be smaller.)
I don’t know if it makes any meaningful difference if the Universe is infinite vs “sufficiently large”. As soon as it’s big enough to have all physically realizable initial conditions and histories, why does it matter if they happen once or a googol or infinity times? Maybe there are some counterintuitive anthropic arguments involving Boltzmann Brains. Those seem to pop up in cosmology sometimes.