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1. Observable Universe. 2. Level I Multiverse - Infinite Multiverse - Our whole flat Universe with the great amount of “Observable Universes“ - Millions, Billions, Trillions, maybe even infinite number of “Observable Universes“ in it. 3. Level II Multiverse - The Bubble Multiverse. The concept of a single, all-encompassing universe has given way to theories proposing multiple or even an infinite number of universes - a concept known as the multiverse. At the heart of this theory is the idea of bubble universes. Each “bubble“ represents a separate universe, distinct but possibly similar to our own. 4. Level II Multiverse - The Cyclical Multiverse. The cyclic universe theory is a model of cosmic evolution according to which the universe undergoes endless cycles of expansion and cooling, each beginning with a “big bang” and ending in a “big crunch”. 5. Level II Multiverse - Conformal cyclic cosmology. In CCC, the universe iterates through infinite cycles, with the future timelike infinity (i.e. the latest end of any possible timescale evaluated for any point in space) of each previous iteration being identified with the Big Bang singularity of the next. 6. Level II Multiverse - ekpyrotic universe. The ekpyrotic model proposes that the universe has no beginning or end. Instead of a bang, what happened is thought of as a “Big Bounce”, the moment at which the universe, which was slowly contracting to an incredibly, but not infinitely, small point, “bounced” to expansion. 7. Level II Multiverse - Branes in the Bulk. The central idea is that the visible, three-dimensional universe is restricted to a brane inside a higher-dimensional space, called the “bulk“ (also known as “hyperspace“). String theory can realistically predict a multiverse populated by 10^500 different universes. but Branes aren’t a part of simple String Theory, instead they shows their presence in a more advanced theory called — M-Theory. 8. Level II Multiverse - Baby Universes. baby-universe formation, a process in which a new domain of spacetime formed by quantum tunneling in a pre-existing universe can evolve by creating its own space, and eventually (causally) disconnecting from the par- ent spacetime. Sometimes expanding star becomes the birth of a new universe. The point where time ends inside a black hole becomes joined to the point where time begins in a Big Bang in a new universe. 9. Level II Multiverse - Large Extra Dimensions. Current versions of string theory require 10 dimensions total, while an even more hypothetical über-string theory known as M-theory requires 11. But when we look around the universe, we only ever see the usual three spatial dimensions plus the dimension of time. In particle physics and string theory (M-theory), the ADD model, also known as the model with large extra dimensions (LED), is a model framework that attempts to solve the hierarchy problem. (Why is the force of gravity so weak compared to the electromagnetic force and the other fundamental forces?) 10. Level III Multiverse - Many-Worlds. The many-worlds interpretation implies that there are most likely infinitely many universes. It is one of a number of multiverse hypotheses in physics and philosophy. Basically, many-worlds proposes the idea that the quantum system doesn’t actually decide. Rather, that at every junction where large everyday stuff interacts with the quantum system, the timeline of history splits and both possibilities happen on different alternate branches. 11. Level IV Multiverse - All-Encompassing Mathematical Multiverse - Multiverse of Multiverses. If there is a type-4 multiverse, that is, Max Tegmark’s Mathematical Universe Hypothesis or something similar, then the number of consistent mathematical structures is trivially going to be infinite (unless the mathematical foundation is itself inconsistent). You can’t debunk infinity being infinite.
