How many universes are available?

The sheer contemplation of how many universes might exist immediately forces a reckoning with the limits of our current physical understanding. It is not a simple counting exercise with a definitive answer like tallying the planets in our solar system; rather, the concept of a "multiverse" is a collection of distinct theoretical predictions arising from different areas of physics, cosmology, and quantum mechanics. ^[3][5][7] The resulting answer depends entirely on which model you subscribe to—is it merely an extension of our own observable spacetime, or a vast ensemble where the laws of physics themselves are variable?^[4][10]

# Taxonomy Levels

To bring some order to this potentially infinite collection of realities, physicists and cosmologists often employ classification schemes, most famously the four-level hierarchy often attributed to physicist Max Tegmark. ^[5] This structure helps distinguish between universes that share our fundamental laws but differ in initial conditions, and those that operate under completely different physical rules. ^[4][10]

# Level One

The most straightforward extension of our universe is the Level I Multiverse. ^[5] This relies on the cosmological assumption that space is spatially infinite and that matter is distributed more or less uniformly over the very large scales. ^[1][4] If space is infinite, then eventually, every possible configuration of particles within a Hubble volume—the region of space we can currently observe—must repeat itself simply due to the finite number of ways particles can be arranged. ^[4] This implies there are an infinite number of observable universes, all governed by the exact same physical constants and laws as ours, but where events unfolded differently because the initial conditions were distinct. ^[5] Think of it as an infinite set of identical decks of cards being shuffled an infinite number of times; every possible hand must appear, infinitely many times. ^[2]

# Level Two

Moving up, the Level II Multiverse emerges from theories like eternal inflation. ^[5][7] In this scenario, our universe is just one "bubble" or "pocket" that nucleated when inflation—the rapid expansion immediately following the Big Bang—stopped in our region of spacetime. ^[3][5] Inflation, however, continues elsewhere, perpetually spawning new bubble universes. ^[6] The crucial difference here, compared to Level I, is that the process that stops inflation in each bubble can be different, leading to unique outcomes for fundamental constants, dimensions, and even the effective field theories operating within that bubble. ^[4][5] These universes would possess physical laws profoundly different from ours. ^[3]

# Level Three

The third level brings us into the realm of quantum mechanics through the Many-Worlds Interpretation (MWI). ^[5] In this view, every time a quantum measurement is made—every time a probability wave "collapses"—the universe actually splits into parallel branches, one for every possible outcome. ^[4][10] If you flip a quantum coin, one universe continues with heads, and another branches off with tails. ^[2] This creates an uncountable, perhaps infinite, number of universes emerging moment by moment, existing in the same abstract physical space, but entirely inaccessible to one another. ^[5] These universes share the same physical constants as ours, as they are all branches of our original quantum history, differing only in the specific events that transpired. ^[4]

# Level Four

The most abstract concept is the Level IV Multiverse, also known as the Mathematical Universe Hypothesis. ^[5] This proposes that any mathematically consistent structure corresponds to a real physical universe. ^[4][5] If a set of equations can be written down that describes a reality, then that reality exists somewhere in the Level IV ensemble. ^[10] This implies the existence of universes governed by entirely different mathematical frameworks, which goes far beyond simply changing constants or initial conditions; it suggests entirely new forms of mathematics define existence. ^[5]

# Vast Numbers

When non-physicists ask "how many," they are often seeking a concrete figure. While Levels I and III suggest infinity, ^[4][10] other models yield enormous, but potentially finite, counts based on underlying physical mechanisms. ^[9]

One prominent source for a large but non-infinite number arises from the string theory landscape. ^[3][5] String theory, in its effort to unify quantum mechanics and general relativity, suggests that there are a vast number of ways the extra spatial dimensions (beyond our familiar three spatial and one temporal) can be "compactified," or curled up. ^[3] Each configuration of compactification corresponds to a different set of effective physical laws and constants that would manifest in a four-dimensional spacetime. ^[5] While estimates vary significantly, some physicists have calculated that the number of possible vacuum states—the unique stable configurations of physics—could be around $10^{500}$. ^[9] This figure, derived from specific theoretical frameworks, represents the number of distinct physical possibilities allowed by the mathematics of the string theory landscape, often associated with the Level II concept of bubble universes. ^[8]

It is instructive to compare the implications of this calculated number with the concept of infinity implied by Level I. If our observable universe repeats itself every $10^{10^{118}}$ meters (a massive but finite distance), then $10^{500}$ distinct universes with different laws seems, in practical terms, incredibly small when set against the backdrop of true infinity. ^[1] In essence, the difference between an actual infinity of possibilities and a number that requires 501 zeros to write down becomes a philosophical rather than a practical distinction when discussing empirical confirmation. ^[4]

How many universes could exist?

# Universe Variety

What truly sets these proposed universes apart? The distinction lies in what is different between them. ^[4]

In Level I universes, the difference is purely configurational. The fundamental constants—the speed of light ($c$), the gravitational constant ($G$), the elementary charge ($e$)—are identical across all these copies. The variation comes solely from the initial placement of matter and energy, leading to variations in history, biology, and specific arrangements of objects. ^[5]

Contrast this with Level II universes, where the differences are fundamental. Because these universes arise from different "breaks" in the inflationary field, they can settle into different vacuum states, meaning they might possess a different number of spatial dimensions, different particle masses, or different strengths for the fundamental forces. ^[3][5] A universe might exist where gravity is stronger than electromagnetism, rendering stable structures impossible, or one where the electron has a different charge, preventing the formation of stable atoms. ^[4]

The Level III quantum branches differ only by outcome. They share the same foundational laws as our universe, but the probabilistic nature of quantum events has manifested differently in each split. ^[5]

For instance, imagine a universe where the laws of physics allow for only three stable elements, while our universe allows for over a hundred. This would be a Level II difference. In contrast, a Level I universe would have the exact same three-element limitation, but the distribution of those elements across space would differ from our own four-element cosmos. ^[4]

# Empirical Limits

A major hurdle in discussing the quantity of universes is the question of verification. Since most of these proposed universes are, by definition, causally disconnected from our own—meaning no signal, light, or information can travel between them—directly testing their existence remains science fiction. ^[3]

However, some avenues offer tenuous hope. For instance, in the Level II scenario, if our universe collided with another bubble universe early in its history, it might have left a detectable "bruise" or pattern in the Cosmic Microwave Background (CMB) radiation. ^[3] While searches have been conducted for such anomalies, definitive evidence remains elusive. ^[6]

From a scientific perspective, a theory that predicts an unobservable infinity often sits uncomfortably close to untestable metaphysics. ^[7] The calculations yielding numbers like $10^{500}$ offer a measure of comfort because they arise from specific, mathematical theories (like string theory) that also attempt to describe our own universe successfully, lending them a degree of authority within theoretical physics. ^[8] Yet, without an observable signature, determining whether the actual count is $10^{500}$, $10^{1000}$, or truly infinite remains outside the current remit of empirical science. ^[5]

If we approach this from a purely logical standpoint, considering the complexity inherent in the Level IV model—where all mathematical structures are real—the set of possible universes is so vast that calculating a number becomes meaningless; it is simply the set of all possible, self-consistent physical realities. ^[5] The current scientific consensus, then, is not a number, but an acknowledgment that depending on which accepted physical theory one extrapolates to its logical extreme, the universe may not be singular. ^[3][7]