1. What is the multiverse?
The basic idea is multiple separate universes. The multiverse is the grand ensemble of all the separate universes hypothesized to exist.
Moreover, physicists Max Tegmark and Brian Greene have come up with classification schemes.
2. Where is the multiverse?
The multiverse is largely conjectural work based on scant evidence. It’s not as if we can observe and gather information from other parallel universes if they exist.
At best, it seems the evidence for the multiverse is indirect. If the multiverse exists, it should be able to make predictions (e.g. Steve Weinberg and Alexander Vilenkin’s predictions regarding the cosmological constant have had limited success). Inflation and M-theory might lend some limited support (more on this below). However, even if this is the case, the multiverse is more like science fiction than scientific fact: there is a kind of basis in science coupled with a huge dose of imagination.
In addition, there are philosophical objections to the multiverse. Take the Boltzmann brain problem, i.e., brains brought into existence by random quantum fluctuations but with memories identical the ones we have about our lives now; this is one of the most challenging problems for the multiverse. Take the fact that the multiverse itself needs to have specific finely-tuned initial conditions in order to generate baby universes. Take the fact that the multiverse itself would need an explanation for its existence since the multiverse causing itself would be incoherent (e.g. it would need to be argued the multiverse is necessary rather than contingent). And so on.
To put it another way: there’s a plethora of evidence to theorize about the multiverse when smoking a doobie, wacky tobaccy, left handed cigarette, some downtown brown, or married iguanas, but the evidence suddenly vanishes in a puff of smoke when one is not lit!
3. Multiple multiverses
That said, there are multiple conceptions of what the multiverse might look like. Multiple multiverses! Here are the main ones I’m aware of:
a. Possible worlds. Presumably the multiverse has an ancient pedigree or pre-“modern physics” basis in philosophical ideas about possible worlds.
Along the same lines, various religions arguably have something like a multiverse. Take Mormonism and pagan polytheism. Likewise, perhaps some conceptions of Hinduism, Buddhism, and Taoism are not inconsistent with the multiverse. Although one suspects such religions may not be inconsistent with virtually anything!
Interestingly, the multiverse could arguably be well grounded in classical theism or similar, whereas the multiverse has shaky foundations on atheism/naturalism. I believe William Lane Craig has discussed this (e.g. “Is the Multiverse Dead?“).
b. An infinitely spaced universe. I suppose the most basic conception, though not strictly speaking a multiverse if the multiverse is supposed to consist of parallel universes, is that there is a single universe infinite in space with multiple large areas more or less cordoned off from one another (perhaps by local variations of physical laws).
Stephen Hawking, along with C.B. Collins, showed this wasn’t tenable decades ago. I assume most physicists had already suspected as much (e.g. the universe is finite in space), but nevertheless thank Hawking and Collins for taking the trouble to disprove it!
c. Cyclic models. Such as John Wheeler’s oscillating universe and Roger Penrose’s conformal cyclic cosmology. Our single universe expands until (say) gravitation forces the universe to collapse back into itself, contracting into a big crunch, then another big bang, only to repeat the cycle. This continues indefinitely. From big bang to big crunch. (Perhaps at some point it might move from big bang to big crunch to Cap’n Crunch. Yes, I’m talking about an entire universe filled with Cap’n Crunch cereal! One can only hope the same universe has enough milk as well. Otherwise the problem of evil and suffering may take on a newfound edge in that universe.)
Strictly speaking, however, this isn’t a multiverse either, since the universes do not run in parallel with one another. Rather the universes are in a series.
Wheeler’s oscillating universe has been been shown to have significant issues by Penrose. It’s not widely accepted among cosmologists. However, Wheeler may have had his revenge when Penrose’s own model became an object of doubt! Penrose’s model is not widely accepted by cosmologists either, and even Penrose himself acknowledges several weaknesses. Not least of which is the lack of a Cap’n Crunch cosmological constant.
d. Many worlds. Perhaps the most popular conception of the multiverse (in part thanks to pop culture: e.g. Star Trek‘s mirror universe, Community‘s darkest timeline, Futurama, Rick and Morty) is Hugh Everett III’s many worlds interpretation of quantum mechanics. According to quantum mechanics, a system is in a state of superposition until a measurement is taken by an observer. Once a measurement is taken by an observer, then it collapses into a single state. By contrast, Everett hypothesized that once a measurement is taken by an observer, rather than a wavefunction collapse, the system or universe splits into two or more separate universes.
The logical implication of Everett’s many worlds is every possible outcome of every event exists in its own universe. Not only can anything happen, but anything does happen. Schrödinger’s cat is truly both alive and dead, and the Einstein–Podolsky–Rosen paradox is purportedly resolved. However, Everett’s interpretation introduces new paradoxes. It has its own thorny scientific and philosophical issues. For example, if the atomic or subatomic particles in an observer’s brain are in a superposition state, then shouldn’t the observer simultaneously see both or multiple states rather than only one state? In any case, acceptance or rejection of many worlds depends on one’s preferred interpretation of quantum mechanics. Different interpretations are hotly debated among physicists, philosophers, and other relevant scholars. And what Richard Feynman once said in The Character of Physical Law hasn’t changed:
There was a time when the newspapers said that only twelve men understood the theory of relativity. I don’t believe there ever was such a time. There might have been a time when only one man did, because he was the only guy who caught on, before he wrote his paper. But after people read the paper, a lot of people kind of understood the theory of relativity in some way or other, but more than twelve. On the other hand, I think I can safely say that nobody understands quantum mechanics.
e. Eternal inflation. Alan Guth famously postulated and argued for inflation (along with other physics luminaries including Andreas Albrecht, Andrei Linde, and Paul Steinhardt). Inflation is widely accepted by most cosmologists today, though there are rival theories (e.g. string gas cosmology). The general idea is that the universe rapidly expanded after the big bang, then expansion decelerated. Inflation isn’t perfect (e.g. its mechanism is unknown), but it seems to fix significant problems in standard big bang cosmology, the horizon problem, why the shape of our universe seems to be flat, and so on.
There are literally scores of inflation models, but perhaps the one that’s most relevant to the multiverse is the eternal inflation model. The eternal inflation model argues bubble universes (and bubble universes within bubble universes ad infinitum!) result from different inflation rates with different parts of the universe ending their inflationary periods at different times. When a bubble universe’s inflationary period ends, a new universe with a potentially new set of laws comes into existence. In theory, some inflationary periods could be infinite.
Scientifically speaking, eternal inflation is arguably the most “plausible” (as far as that goes) conception of the multiverse. That’s largely due to inflation’s empirical support (e.g. WMAP) and its explanatory power. However, an important distinction needs to be made. Just because inflation is well-supported does not necessarily imply an inflation model of the multiverse is well-supported too. That would be to inflate the evidence out of proportion. #dadjokes
f. M-theory. Physicists are still on a quest for a theory of everything (TOE). A TOE would unite quantum mechanics with general relativity. A TOE would be able to describe the fundamental forces that operate from the very small to the very large.
At present the leading TOE contender is string theory, though there are rival TOEs (e.g. loop quantum gravity). The basic idea behind string theory is particles (e.g. electrons, quarks) are not fundamentally particles at all. Instead, particles are fundamentally the result of a vibrating “string” of energy. A different pattern of a vibrating string results in a different particle.
M-theory is an attempt to unite multiple versions of string theory into a single theory. M-theory’s main idea is our universe exists on a single vibrating superstring known as a membrane (“brane”).
If M-theory is combined with an inflationary theory like eternal inflation, then it supposedly lends support for the multiverse. Eternal inflation posits inflation creates multiple bubble universes, each of which instantiates the various possibilities of M-theory.
M-theory is favored by the likes of Edward Witten (father of M-theory), Leonard Susskind (father of string theory), and Stephen Hawking (father of jumping onto bandwagons theory, though he may have jumped onto the whole “father of jumping onto bandwagons theory” thing too, so perhaps it all devolves into an infinite regress).
However, M-theory has a lot of issues. Perhaps the most obvious problem is M-theory’s fortunes rise or fall with string theory, which is hugely contestable (e.g. Lee Smolin’s Not Even Wrong). For example, the equations involved in string theory are at best approximations and hence their solutions are at best approximations too. Many physicists argue against and even reject M-theory and string theory in general (e.g. Paul Steinhardt at Princeton).
g. Virtual reality simulations. As Paul Davies and others have pointed out, once we allow the possibility of a multiverse, then there’s little to disallow the possibility that we may live in a virtual reality simulation. The same or similar arguments, if they apply, could apply to both.
On the one hand, the gazillions of virtual reality simulations idea could be a reductio ad absurdum of the multiverse idea. On the other hand, scholars are increasingly taking the virtual reality simulation idea seriously (e.g. Nick Bostrom, David Chalmers). I’ve talked a little bit about this in my post “Oculus Grift“.
A related or perhaps sub idea is holographic universes.