Physicists use the phrase "parallel universes" to discuss diverse concepts, and it can sometimes get a little confusing. For example, some physicists believe strongly in the idea of a multiverse for cosmological purposes, but don't actually believe in the Many Worlds Interpretation (MWI) of quantum physics.
There are two basic breakdowns of parallel universes that I've found to be helpful. The first was presented in 2003 by Max Tegmark and the second was presented by Brian Greene in his book The Hidden Reality.
In 2003, MIT physicist Max Tegmark explored the idea of parallel universes in a way which I find useful to categorize them. (You can find the Tegmark's 2003 paper here.) In fact, I used his system as the organizing principle for the chapter on parallel universes in my book on string theory. Tegmark breaks the different types of parallel universes allowed by physics into four different levels:
- Level 1: Regions Beyond Cosmic Horizon - The universe is essentially infinitely big and contains matter at roughly the same distribution as we see it throughout the universe. Matter can combine in only so many different configurations. Given an infinite amount of space, it stands to reason there exists another portion of the universe in which an exact duplicate of our world - and, in fact, our entire visible universe - exists.
- Level 2: Other Post-Inflation Bubbles - Separate universes spring up as bubbles of spacetime undergoing its own form of expansion, under the rules dictated by inflation theory. The laws of physics in these universes could be very different from our own.
- Level 3: The Many Worlds of Quantum Physics - According to this approach to quantum physics, events unfold in every single possible way, just in different universes. Science fiction "alternate history" stories utilize this sort of a parallel universe model, so it's the most well known outside of physics.
- Level 4: Other Mathematical Structures - This type of parallel universes is sort of a catch-all for other mathematical structures which we can conceive of, but which we don't observe as physical realities in our universe. The Level 4 parallel universes are ones which are governed by different equations from those that govern our universe. Unlike Level 2 universes, it's not just different manifestations of the same fundamental rules, but entirely different sets of rules.
Brian Greene's system of classifications from his 2011 book, The Hidden Reality, is a more granular approach than Tegmark's. Below are Greene's classes of parallel universes, but I've also added the Tegmark Level that I think they fall under.
- Quilted Multiverse (Level 1) - Space is infinite, therefore somewhere there are regions of space that will exactly mimic our own region of space. There is another world "out there" somewhere in which everything is unfolding exactly as it unfolds on Earth.
- Inflationary Multiverse (Level 1 & 2) - Inflationary theory in cosmology predicts an expansive universe filled with "bubble universes," of which our universe is just one.
- Brane Multiverse (Level 2) - String theory leaves open the possibility that our universe is on just one 3-dimensional brane, while other branes of other number of dimensions could have whole other universes on them.
- Cyclic Multiverse (Level 1) - One possible result from string theory is that branes could collide with each other, resulting in universe-spawning big bangs that not only created our universe, but possibly other ones.
- Landscape Multiverse (Level 1 & 4) - String theory leaves open a lot of different fundamental properties of the universe which, combined with the inflationary multiverse, means there could be many bubble universes out there which have fundamentally different physical laws than the universe we inhabit.
- Quantum Multiverse (Level 3) - This is essentially the Many Worlds Intepretation (MWI) of quantum mechanics: anything that can happen does ... in some universe.
- Holographic Multiverse (Level 4) - According to the holographic principle, there is a physically-equivalent parallel universe that would exist on a distant bounding surface (the edge of the universe), in which everything about our universe is precisely mirrored.
- Simulated Multiverse (Level 4) - Technology will possibly advance to the point where computers could simulate each and every detail of the universe, thus creating a simulated multiverse whose reality is nearly as complex as our own.
- Ultimate Multiverse (Level 4) - In the most extreme version of looking at parallel universes, every single theory which could possibly exist would have to exist in some form somewhere.