Abstract
I proffer a survey of physical theories concerning parallel universes, which coalesce into a natural four-tiered hierarchy of multiverses, permitting progressively greater diversity. Level I: A generic prediction arising from inflation is an infinite ergodic universe, replete with Hubble volumes realising all initial conditions – inclusive of an identical copy of thyself situated circa 10¹⁰ m hence.
Level II: Within chaotic inflation, alternative thermalised regions may evince disparate physical constants, dimensionality, and particle constitution. Level III: In unitary quantum mechanics, alternative branches of the wavefunction contribute naught qualitatively novel, a matter indeed ironic, given this level's historical propensity for controversy. Level IV: Divergent mathematical structures engender disparate fundamental equations of physics. The crux of the matter lies not in whether parallel universes subsist (Level I constituting the uncontroversial cosmological concordance model), but rather the enumeration of said levels. I shall discuss how multiverse models may be falsified and contend that there exists a grave “measure problem” which must be resolved in order to furnish testable predictions at Levels II-IV.
Doth there exist another simulacrum of thee, perusing this very article, deliberating upon its dismissal ere the conclusion of this sentence, even as thou continuest to read on? A person residing upon a planet christened Earth, adorned with mist-wreathed mountains, fertile fields, and sprawling metropolises, within a solar system comprising eight other planets. The life of this person hath been, in all respects, identical to thine own – until this juncture, when thy decision to persist in reading signals a divergence betwixt thy respective existences.
Thou art, perchance, inclined to deem this notion strange and implausible, and I must concede that such is my own visceral reaction as well. Yet, it would appear that we must needs reconcile ourselves thereto, inasmuch as the most straightforward and widely accepted cosmological model of this era doth predict that such a personage verily doth exist within a Galaxy situated approximately 10¹⁰ metres hence. This assertion doth not even presuppose speculative modern physics, merely that space is infinite and rather uniformly populated with matter, as indicated by recent astronomical observations. Thine alter ego is simply a prediction stemming from the so-called concordance model of cosmology, which doth accord with all current observational evidence and is employed as the basis for the majority of calculations and simulations presented at cosmological symposia. Contrariwise, alternatives such as a fractal universe, a closed universe, and a multiply connected universe have been subjected to serious challenges by observation.
The furthest extent to which thou mayest observe is the distance that light hath been capable of traversing during the fourteen billion years since the commencement of the Big Bang expansion. The most remote visible objects are currently situated approximately 4×10²⁶ metres distant, and a sphere of this radius defines our observable universe, also denominated our Hubble volume, our horizon volume, or simply our universe. Likewise, the universe pertaining to thine aforementioned twin is a sphere of identical dimensions, centred yonder, none of which we may perceive or with which we may have any causal intercourse as yet. This constitutes the simplest (albeit far from the sole) example of parallel universes.
By this very definition of “universe,” one might anticipate the notion that our observable universe is merely a diminutive component of a larger “multiverse” to reside perpetually within the domain of metaphysics. Yet, the epistemological boundary between physics and metaphysics is demarcated by the experimental testability of a theory, and not by its eccentricity or invocation of unobservable entities. Experimental breakthroughs, empowered by technology, have thereby expanded the frontiers of physics to encompass ever more abstract (and, at the time, counterintuitive) concepts such as a rotund rotating Earth, an electromagnetic field, temporal dilation at elevated velocities, quantum superpositions, curved space, and black holes. As elucidated within this article, it is becoming increasingly evident that multiverse models grounded in modern physics can, in actuality, be empirically testable, predictive, and falsifiable. Indeed, as many as four discrete types of parallel universes have been discussed in recent scientific literature, such that the crux of the matter lies not in the mere existence of a multiverse (Level I being rather uncontroversial), but rather in its hierarchical depth.