Contemporary cosmology confronts a striking mystery: there exist perplexingly narrow margins of physical constants, laws of nature, and initial conditions required for life, which seem, by general consensus, mind-bogglingly improbable—that is, unless there are countless other universes, each with its own set of physical constants, laws of nature, and initial conditions, ultimately rendering such precision as observed in our universe a statistical inevitability. For instance, guessing a single correct number between one and one hundred is highly unlikely given one attempt; but with, say, eighty attempts, the likelihood of success increases significantly. Correspondingly, what has been termed the multiverse hypothesis posits that if an innumerable number of universes exist, the probability of at least one being fine-tuned for life becomes far more plausible. Determining whether this hypothesis holds any explanatory power requires us to confront it directly and elucidate it thoroughly. “Theoretical prejudice should not blind one to the evidence,”1 writes mathematician Bernard Carr, defending the multiverse; and this essay contends that such evidence is far from absent.
First and foremost, it’s true that this highly theoretical yet earnest hypothesis becomes increasingly convoluted the deeper one delves into its mechanics. Unlike the neatly packaged, unexplained A-to-B-to-C multiverse timelines portrayed in fiction, a scientific multiverse demands a rigorous approach to answer its pressing questions: What mechanism produces other universes? What determines the parameters of each one? How many universes exist? And, most crucially, how can it be evaluated and demonstrated as not just possible, but probable?
Categorizing multiverse models can be challenging—yes, there are far more than one. If this were an exhaustive exploration of competing multiverse hypotheses, it would require a historical overview and in-depth examination of every framework and intersection. However, such comprehensiveness would verbosely detract from our present goal—that is, to examine its leading construction. Thus, rather than enumerating all hypotheses chronologically, I will employ a categorical dichotomy inspired by American philosopher Robin Collins, grouping multiverse hypotheses into two categories: restricted and unrestricted models. Here, my case will advance the restricted multiverse hypothesis. However, before examining these, a brief explanation of the unrestricted model and the rationale for its not receiving primary attention is warranted.
The unrestricted multiverse hypothesis (UMU) posits that every possible universe exists—yes, every possible universe—each representing a different configuration of reality. In another universe, you exist, but you never read this essay. In yet another, you exist and are reading this essay, but this paragraph is missing. For every number imaginable, there’s the potential of another universe. In other words, an infinite number of universes, or as many models describe it, an eternal expanse of realities exist. As the infinite monkey theorem suggests, an infinite amount of time spent by a monkey randomly typing on a typewriter will yield any predetermined text.
Unsurprisingly, the UMU suffers from severe scientific and philosophical challenges. Principally, claiming that the UMU removes the need for explanation by asserting a life-permitting universe inevitably undermines the scientific foundations of both reasoning and probability assessments. On the flip side, claiming that the UMU still preserves the need for scientific explanations fails to account for the improbability of fine-tuning without introducing pick-and-choose restrictions. Another camp of the UMU adopts a non-deductive stance, avoiding probabilistic considerations altogether. One might argue, “We can’t be certain whether our universe is special or not because we can’t dismiss the possibility of other universes. So, why worry about how unlikely our universe seems?” However, this line of reasoning doesn’t hold up, as the same argument can be turned around: “Exactly! We can’t be sure. But that just means we still lack a compelling explanation for why our universe is so perfectly suited for life.” In the end, an explanation is needed, and this is evidenced by the ongoing search for a well-supported multiverse hypothesis. Genuine scientists are dissatisfied with explanations that simply conclude, “that’s just the way it is.” What’s the best current explanation? I believe the restricted multiverse model offers the best solution.
As mentioned, there are numerous variations of the multiverse hypothesis. Multiple examples appeared in the unrestricted model, and many more seem to exist under the restricted interpretation. Despite these variations, they generally aim to answer two primary questions: 1) What is the mechanism that creates new universes? and 2) What causes variations within each universe so that, through trial and error, one might eventually become suitable for life? First, let’s explore a popular hypothesis for the former.
The Multiverse Hypothesis Part 1: Inflation
Inflation remains the leading cosmological hypothesis postulating how multiple universes, or a “multiverse,” could come into existence. Not the kind of inflation that devalues your dollar, but inflation in its natural sense—like a balloon expanding. In 1980, cosmologist Alan Guth proposed the concept of inflation to explain a pair of anomalous observations about the universe: thorough analysis of the Cosmic Microwave Background Radiation (CMBR) discovered in the 1960s revealed that the universe is exceptionally flat and homogeneous throughout. Flatness and homogeneity are two major challenges to the standard Big Bang model—the key issues that Guth set out to resolve. But what exactly are these concepts, and why are they problematic?
Cosmologists offer three possible shapes to the universe: flat, closed (positive), and open (negative). To understand, it’s best to consider a triangle drawn on the shape of each potential universe shape. A flat universe can be likened to a piece of paper. When you draw a triangle on it, all angles will add up to 180 degrees. Now take a closed (ball-shaped) universe. Draw a triangle on it and all of the angles will add up to greater than 180 degrees, hence it also being known as having “positive curvature.” Lastly, an open universe (saddle-shaped) with a triangle drawn on it will add up to less than 180 degrees, earning the name “negative curvature.” When scientists examined the CMBR, a measurement of the residual heat from the Big Bang, basically no matter where they triangulated points in the universe, it always came up to almost exactly 180 degrees. (It must be noted just because the universe is perceived as flat, that doesn’t mean it’s actually flat. For example, our planet appears to be flat but eventually if you keep walking on you will return to the same point. Likewise, a flat universe doesn’t necessarily indicate that the far opposite ends of the universe don’t touch.) The problem is introduced when you consider the sheer size of the universe’s timescale. A universe 13.8 billion years old should not be so flat. Consider rolling a bowling ball. If you were to try and roll it directly down the middle, any slight deviation from middle line would cause the ball to progressively deviate off track as it continues toward the pins. Now say the bowling lane was from New York City to London. Assuming you had the strength to roll it across the Atlantic Ocean, to ensure that the ball remains on lane, the ball would have to remain extremely close to the middle line. A similar logic applies to the universe. The problem is that the universe appears to be too finely balanced, or “flat,” given its immense age and size. The universe’s curvature should have become more pronounced over billions of years, but instead, it remains remarkably flat.
The second, less convoluted anomaly that the CMBR unveiled is known as the Horizon Problem. No matter where you look, point out the two farthest parts of the universe on opposite ends and they have been measured to have the approximately the same temperature and composition. This is far from what should be expected in a random, undirected Big Bang. To understand, an analogy will help. The horizon problem can be likened to a massive bomb explosion, where shrapnel is scattered across a vast distance. You’d expect the shrapnel to be scattered randomly, with different velocities, directions, and temperatures. But instead, imagine finding that the shrapnel is surprisingly coordinated, moving at the same velocity, direction, and temperature.
Guth proposed that shortly after the initial moment of the Big Bang—within a fraction of a second—space underwent a period of rapid and exponential expansion. This brief but intense period, known as inflation, smoothed out any irregularities in the universe, explaining why it appears so homogeneous and flat. In essence, inflation “fast-forwarded” the universe’s expansion via an energy source called the “inflation field,” allowing space to grow to a vast size and mature quickly, appearing more homogeneous and flat than would be expected if it explained at a constant rate.
Even though notions of a multiverse had been floating around since the 1950s, cosmic inflation was a catalyst for a more focused hypothesis with a potential working mechanism at play. Over the next few years following Guth’s publication, multiple physicists, principally Russian-American physicist Andrei Linde, would map out a multiverse hypothesis with inflation as is mechanism.
Multiverse inflation posits that our universe is only a pocket of one larger, rapidly expanding universe. Rather than our universe undergoing a rapid period of inflation and then slowing down, multiverse inflation posits that all of space is actually in a constant state of inflation and that our universe is just a pocket or bubble that slow down. Over time, other pockets of the space will also slow down and this process will occur forever. And these other pockets that slow down are considered other universes. Not because they are in a different dimension, but because they are expanding away from us at the speed of light so we could never gain access to them.
Perhaps we fall back onto our trusty illustrations. It’s difficult to illustrate this idea. A quick Google search of the multiverse brings up images of bubbles sprouting from other bubbles, each one representing a newly formed universe. While this is a useful metaphor, it can be misleading. The inflationary model suggests that everything is still part of a single, expanding space rather than separate, detached entities. For that reason, I will stray from the bubble illustration and instead present a different analogy to convey the core idea—acknowledging, of course, that all analogies have their limits. Imagine a fleet of cars starting from a single point, accelerating outward in all directions at 200 mph. At that speed, the drivers can focus on nothing but the road—there’s no time for distractions like checking a phone, eating, or talking. Every once in a while, however, a car slows down, shifting into neutral and cruising along at a much lower speed. Now, the driver has the freedom to take a bite of food, glance at their phone, or hold a conversation. Meanwhile, the majority of cars continue racing ahead at full speed, never slowing down. This parallels eternal inflation in several key ways. The cars speeding at 200 mph represent the inflating space, which continues expanding at an exponential rate. This space never stops inflating as a whole—it remains the dominant “background” of reality. However, in certain regions (like the cars that slow down), inflation ends locally. These regions “drop out” of inflation and transition into a slower, more stable state, where particles can form and a universe can emerge. These are the bubble universes, much like how a driver who slows down can suddenly engage in new activities. But just as not all cars slow down, not all regions of space exit inflation. Most of space continues expanding at full speed, meaning the overall process of inflation never stops—it remains eternal, constantly producing new regions where inflation ends and new universes are born. Meanwhile, the different “bubble universes” remain causally isolated from one another, just as the slowed-down cars have no direct connection to the ones still speeding away.
While inflation provides a mechanism for generating multiple universes, it does not explain why each of these universes might have different physical laws and constants. This is where the multiverse hypothesis extends its reach. If enough pocket universes emerge, then by sheer probability, at least one of them should have the precise conditions necessary to support life—we just happen to be in that one. However, this explanation raises a deeper question: what causes the variation in physical laws and constants once inflation halts in a given region? After all, if each universe emerges from the same inflationary process, why wouldn’t they all share identical physical properties? To answer this, physicists have turned to superstring theory, or more broadly, M-theory, as a framework for explaining why different universes might obey different sets of physical laws.
The Multiverse Hypothesis Part 2: String Theory
Commencing in the 16th century, physics operated under three ceilings: objects had to be visible to the naked eye, move no faster than one percent of the speed of light, and be subject only to the gravitational forces present on Earth, thus characterizing the era of classical physics. Many principles of classical physics, nonetheless, remain highly relevant today—the tangible, applicable aspects of physics that underpin engineering, architecture, aerospace, and related fields. But by the 20th century, these ceilings were raised to unforeseen heights. In a paradigm shift to modern physics, physicists could now study particles moving at the speed of light, the contingencies of gravitational forces, and most pertinent, objects at a very very small scale—the quantum scale. It is at this infinitesimally small scale that many physicists believe the answer to the variations of a multiverse lies. The leading theory behind this, one that has gained significant public attention over the past decades despite its abstruseness, is none other than string theory. Its explanation is inherently technical, but I’ve worked to present it as clear and concise as possible while still preserving its key concepts, using illustrations and visual aids to ensure clarity.
First, the basics. Everything is composed of units called atoms. However, as you may recall from high school science, atoms themselves can be further divided. At their core lies a nucleus containing varying numbers of protons and neutrons, while electrons orbit the outer region. Advances in modern physics have revealed that even these three primary subunits of atoms can be broken down into even smaller components. Particle physics, the branch of science that studies subatomic particles (those smaller than atoms), has expanded rapidly in the mid-20th century, introducing a whole new collection of particles. Instead of just protons, neutrons, and electrons as rudimentary science teaches us, there exists bosons, gluons, photons, and more (all of whose names and properties are irrelevant for our current focus). In modern physics, the organization of these particles is known as the Standard Model, serving as the most precise mathematical framework for currently understanding the quantum world. In short, the Standard Model is to particle physics as the periodic table is to chemistry. Yet unlike the periodic table, the Standard Model maintains a significant hole: it fails to incorporate gravity. While we can observe and measure the effects of gravity at the macro level, at micro scales, attempting to integrate a “gravity particle” ultimately leads to inexorable infinities and mathematical absurdities. This very conundrum remains one of the biggest enigmas in contemporary physics. Regardless, physicists have sought after a more comprehensive alternative to the Standard Model, and so enters string theory, a leading candidate to seal the fissures and rectify the aforementioned conflicts in unraveling the physical universe—a prospect for what some hope to be a “theory-of-everything.”
Within the framework of the Standard Model, particles like electrons, quarks, and other subatomic particles are considered point-like objects, having no dimensions, String Theory flips the script on what we thought particles were. Instead of being tiny dots, they’re actually tiny, vibrating strings. These strings can be open or closed and vary in tension. American physicist Brian Greene, one of the first scientists to introduce string theory to the public at an introductory level in his 1999 book The Elegant Universe, draws connections between string theory and music, writing: “Just as the different vibrational patterns of a violin string give rise to different musical notes, the different vibrational patterns of a fundamental string give rise to different masses and force charges.”2 Sound, after all, is simply waves of vibration, where the traits of each wave—amplitude, wavelength, and frequency—correspond to the sound we hear. Likewise, the specific structure and vibration of a string determines whether it manifests as an electron, an up quark, a down quark, or any other fundamental particle. The best part, string theory naturally accounts for gravity through a hypothetical particle coined the graviton. In other words, when presuming that all particles are made up of smaller strings, the math works out—incorporating gravity does not undercut the model as it does in the standard model.
String theory’s development into a serious competitor to the Standard Model was neither quick nor straightforward. Emerging gradually between the late 1960s and the mid-1990s, it progressed through periods of both optimism and difficulty as physicists explored its mathematical possibilities. One persistent feature of the theory, however, is that it appears to require a universe with additional spatial dimensions beyond the four we ordinarily observe. Contemporary physics generally models our universe as having three dimensions of space and one of time, which raises the question of how these extra dimensions might be accommodated, if they exist at all. In response, string theorists have proposed several ways of reconciling these additional dimensions with observable reality, though none have been empirically confirmed. Among the more commonly discussed approaches is compactification. How about an illustration. Imagine standing a hundred yards away from a garden hose. From this distance, if asked to sketch what you see, you might draw a simple, one-dimensional line. However, as you approach, a new dimension reveals itself. The “line” gains depth, becoming an apparent cylinder. An extra dimension seemed to be “compacted” into the hose and it was our relative perspective that restricted us from viewing it. Similarly, rather than these strings being 1 dimensional, extra dimensions could be compactified in such a way that they remain undetectable at our macroscopic scales.
Leonard Susskind, American physicist and one of the pioneering fathers of string theory, developed the proposal that the theory naturally leads to a multiverse, or “megaverse” as he’d prefer to call it. Susskind published an estimated 10500 possible ways to compactify these extra dimensions, a staggering number that proposes not a single, unique universe but instead describes an enormous “landscape” of possible universes—each governed by its own set of physical laws. This, Susskind dubbed, is the “string landscape.” Amid the 10500 possible “combinations” of particles, forces, and energies that could govern a universe, Susskind suggests that at least one might be suitable for life. How might he know this? Well, because we are alive to observe one, finding ourselves in a region of the “megaverse” that is anthropically fortunate—a pocket in the landscape of possible compactifications where the physical laws happen to support life. Susskind rightfully acknowledges, “The apparent coincidences cry out for an explanation,” but ultimately concludes, “There is no magic, no supernatural designer: just the laws of very large numbers.”3 In his 2005 essay “Living in the Multiverse,” delivered as the opening talk at the Cambridge symposium Expectations of a Final Theory, the late Nobel laureate Steven Weinberg argued that the enormous multiplicity of the string landscape fundamentally changes how we should think about fundamental physical laws. He wrote: “The larger the number of possible values of physical parameters provided by the string landscape, the more string theory legitimates anthropic reasoning as a new basis for physical theories: Any scientists who study nature must live in a part of the landscape where physical parameters take values suitable for the appearance of life and its evolution into scientists.”4
Conclusion
Here, we can begin to unify the two parts of the multiverse hypothesis, and we’ll do so through a parable. Imagine a banker who discovers that his vault has been robbed. This seems impossible—after all, the vault was secured with a ten-digit combination lock. With 10 billion possible combinations, the odds of randomly guessing the correct code are astronomically low. Because the lock wasn’t broken but rather properly opened, the banker concludes that the only reasonable explanation is that the thief must have known the code in advance. But what really happened? The thief used a device capable of systematically trying every possible combination until the lock opened. This scenario provides a useful analogy for understanding how the multiverse hypothesis works to address the apparent fine-tuning of our universe. Some argue that the precise conditions required for life are so improbable that they must be the result of deliberate design—a conclusion similar to the banker’s assumption that the robber couldn’t have cracked the code by mere chance. But perhaps that’s not the full picture. If cosmic inflation allows different regions of space to break off and evolve independently (Part 1), then each region could develop its own unique set of physical laws as string theory suggests (Part 2). In this context, our universe might simply be one among countless others, each with its own variation of quantum compactification—diluting fine-tuning’s explanatory power and qualifying it more as a matter of probability across a cosmic landscape. As Linde summarizes in his 2017 retrospective on the multiverse, the peculiar precision of our universe suggests that “the only presently available plausible explanation of these and many other surprising experimental results has been found within the general framework of the theory of the multiverse.”5
The purpose of engaging the detailed physics underlying the multiverse hypothesis is not merely to detail its claims, for description alone confers no genuine explanatory power. Rather, the significance of the multiverse lies in the fact that it emerges as a consequence of attempts to address real and persistent anomalies within contemporary physics. Inflation, for instance, was not introduced to generate multiple universes, but to resolve the flatness and horizon problems revealed by measurements of the cosmic microwave background; yet certain formulations of inflation appear to lead naturally to a cosmological landscape populated by many causally disconnected regions. In this respect, the multiverse is less a speculative add-on than a byproduct of physics pushing its existing tools to their limits. Ultimately, eternal inflation coupled with the string landscape furnishes the best presently viable scientific resolution to the fine-tuning puzzle. Our universe is not improbably special, but predictably habitable within a vast cosmos of alternatives.
- Carr, B. (2008). Defending the multiverse. Astronomy & Geophysics, 49(2), 2.36–2.37. https://doi.org/10.1111/j.1468-4004.2008.49229_2.x ↩︎
- Greene, B. (1999). The elegant universe: Superstrings, hidden dimensions, and the quest for the ultimate theory (p. 68). W. W. Norton & Company. ↩︎
- Susskind, L. (2005). The cosmic landscape: String theory and the illusion of intelligent design (p. 353). Little, Brown and Company. ↩︎
- Weinberg, S. (2007). Living in the multiverse (p. 3). In B. Carr (Ed.), Universe or multiverse? Cambridge University Press. ↩︎
- Linde, A. (2017). A brief history of the multiverse (p. 10). Reports on Progress in Physics, 80(2), 022001. ↩︎
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