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What is the multiverse? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.
Answer by Martin Silvertant, amateur astronomer, researcher, writer, on Quora:
The multiverse is the hypothetical set of finite and infinite possible universes which form a larger construct which our universe is part of.
In the late 1920s Edwin Hubble found Cepheids in the Andromeda galaxy, of which the distances could be deduced, and thus there was evidence that the stars that were thought to reside in this nebula were actually an entire galaxy beyond our own, as were many other sources in the night sky. At that point our universe grew from the Milky Way to billions of other galaxies. In a sense the multiverse is a continuation on that idea—the idea that there is more reality beyond the extent we can see.
Notions of multiverses can be found in a myriad of scientific ideas. Below are some of these scientific ideas on which the multiverse hypotheses are based (each increasingly speculative):
- Metric expansion of space — The universe is thought to be governed by cosmic inflation. The universe went through a period of exponential expansion from 10-36 to 10-32 seconds after the Big Bang, called the inflationary period. After this period the universe continued to expand, but at a less rapid rate. However, it’s accelerating in its expansion and will continue to do so. As the universe keeps expanding, the observable universe will keep getting smaller as more objects go beyond the so-called Hubble sphere. This is the sphere of observation—beyond which galaxies recede from the observer faster than light—for a given observer in the universe. As the observable universe is estimated to be 93 billion light-years in diameter, observers who are more than 46.5 billion light-years beyond this diameter will never be able to get any information from our observable universe, nor we from them. For all intents and purposes those island Hubble spheres are separate universes. Even if our universe is infinite, the ergodic hypothesis states that it must must contain Hubble volumes much like our own. This is a level I multiverse.
- Eternal inflation — In chaotic inflation theory, inflation could be going on forever, called eternal inflation. With different inflationary regions, some regions expand faster than others, and so you get distinct regions of space. As such, so-called pocket universes emerge. This is a level II multiverse.
- Many-worlds interpretation — Everett’s many-worlds interpretation of quantum mechanics indicates that every quantum possibility inherent to the quantum wave function becomes a real possibility in some reality. As such, a construct of parallel universes emerges. This is a level III multiverse.
- Ultimate Ensemble — The mathematical universe hypothesis. This theory posits that all computable mathematical structures must have a reality somewhere. This is a level IV multiverse.
The categorization above was constructed by Max Tegmark, but there are other ideas on which multiverse hypotheses could rely, and so perhaps the categorization should be expanded. For example, if different Hubble volumes in a given universe could be considered to be a level 1 multiverse, then perhaps any cyclic models should precede it. As such, I would consider cyclic models to be a level 0 multiverse. Not a real multiverse, but the multiverse concept may be fragmented and spread over time rather than space. However here is how the four multiverse levels are defined:
- Level I: An extension of our universe
- Level II: Universes with different physical constants
- Level III: Many-worlds interpretation of quantum mechanics
- Level IV: Ultimate ensemble
I would consider a cyclic model to be an extension of our universe, though that’s not how I would define Hubble spheres, which are extensions of certain regions of the universe. I suppose it depends on what you mean by ‘universe’, though I would feel more comfortable with an extra category, either before level I or after level IV. I will insist on level 0 for now and come up with an extended categorization at a later point. Do note also that the levels are increasingly speculative, which means the cyclic model should be placed around level III. Perhaps it’s not possible to come up with a consistent categorization.
- Cyclic model — The cyclic model, or oscillating model, entails any of several cosmological models in which the universe is temporally extended beyond the lifetime of each individual universe, and follows infinite or indefinite self-sustaining cycles. Each such universe would start with a Big Bang and end in a Big Crunch, which brings about the necessary conditions for a new Big Bang to take place. The process of expansion, collapse and expansion again is called the Big Bounce.
Brian Greene has further extended these “classical interpretations” with the following considerations of multiverse constructs:
- Quilted multiverse [1] — The quilted, or patchwork, universe is an extension on the idea of different Hubble volumes. Assuming the universe is infinite, there will be infinite Hubble volumes, but the type of matter is finite, and so their possible configurations are finite as well. Given infinite space but finite configurations, there is bound to be repetition. Here, different Hubble volumes can be seen as universe-patches, where many patches (infinitely many if the universe is infinite) will be repetitions.
- Inflationary multiverse [2] — Eternal inflation yields an enormous network of bubble universes, which our universe is part of. See the description on eternal inflation and a level II multiverse.
- Brane multiverse [3] — The central idea of brane cosmology is that the visible, three-dimensional universe is restricted to a brane inside a higher-dimensional space (called the ‘bulk’ or ‘hyperspace’), potentially populated by other branes/other universes.
- Cyclic multiverse [4] — The cyclic multiverse has multiple branes (each a universe) that collide and thus cause multiple Big Bangs—yielding universes that are parallel in time.
- Landscape multiverse [5] — The string theory landscape states that by combining inflationary cosmology and string theory, the many different shapes for string theory's extra dimensions cause quantum fluctuations to drop the shapes to a lower energy-level, thus creating different pockets with different sets of laws from the surrounding space—thus giving rise to many different bubble universes.
- Quantum multiverse [6] — The quantum multiverse creates a new universe when a diversion in events occurs, as in the many-worlds interpretation of quantum mechanics (see the many-worlds interpretation and the level III multiverse).
- Holographic multiverse [7] — Derived from the holographic principle (a principle of string theories and a supposed property of quantum gravity) that states that the description of a volume of space can be thought of as encoded on a lower-dimensional boundary to the region. In other words, the surface holds the information that defines the object. According to the holographic principle, there should be a physically equivalent parallel universe that exists in mirror image on a distant bounding surface—the edge of the universe, and thus you have a holographic multiverse.
- Simulated multiverse [8] — The simulated reality posits that our universe is itself simulated by an external universe. This is not to be confused with the notion that all of reality is simulated, because if that is the case then there is no distinction between simulation and reality in the first place.
- Ultimate multiverse [9] — The ultimate theory, saying the principle of fecundity (the capacity of abundant production) asserts that every possible universe is a real universe. This is a level IV multiverse.
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Footnotes
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