Behold the multiverse hypothesis. Conceived by visionaries wrestling with real enigmas in physics, it stands as a structure of remarkable intricacy—a house of cards—where its stability depends upon the careful arrangement of mathematical precision and speculative creativity. Yet, for all its alleged promise and apparent elegance, it is, after all, a house of cards, vulnerable to collapsing under even the slightest pressure. To sustain this feeble edifice, one must acquiesce to an extensive series of conjectures underlying its attractive propositions: an inflation field exists, the decay of inflation fields will produce new bubble universes, the process of inflation will continue eternally into the future, the different universes will adopt varying conditions via unimaginably small vibrating strings of energy, an extra six dimensions exist and are compactified into those strings, the string landscape exists, and both inflation and string theory work in tandem. Let us not criticize it for its lack of proof, for proof will never hold sway in this discourse; rather, critique is to be found in the near absence of any supporting evidence. No telescope, however advanced, has or will pierce the veil to glimpse another universe. No particle collider has captured the elusive superstring, and almost certainly never will, given their theoretical size. Cosmic inflation remains sheer conjecture, with its simplest models predicting larger variations in cosmic background radiation and detectable gravity waves, neither of which have been observed. Moreover, the necessity of accepting six additional spatial dimensions compacted into realms too small to detect has been enough to cause many physicists to recoil from the hypothesis entirely. Brian Greene, the prominent advocate of string theory mentioned earlier, has acknowledged string theory’s shortfall with striking candor in a recent interview:
“There are absolutely valid reasons to be highly critical of string theory, and I am indeed very critical of it. It has made no experimentally testable predictions that would allow us to determine whether it is right or wrong—a process fundamental to science. I am hypercritical of the subject and do not believe string theory is correct. However, I do believe it represents our best approach for reconciling quantum mechanics and general relativity, a necessary task. Until the theory can make contact with observation and experiment, though, nobody should believe it.”1
What began as an endeavor to solve the incompatibility of quantum mechanics and general relativity, the ever-vital pursuit of physical inquiries today, has grown wings and left the field of science, landing among the philosophers. George F. R. Ellis, who collaborated with Stephen Hawking on singularity theorems, admonishingly declares, “The very nature of the scientific enterprise is at stake: multiverse proponents are proposing that we weaken the idea of scientific proof. Science is about two things: testability and explanatory power. Is it worth giving up the former to achieve the latter?”2 Peter Woit, a mathematician at Columbia University, has also sharply critiqued the multiverse, noting, “The problem with the multiverse is that it’s an empty idea, predicting nothing. It is functioning not as what we would like from science, a testable explanation, but as an untestable excuse for not being able to predict anything.”3 After more than thirty years and thousands of papers, the multiverse, particularly through the lens of string theory, has become a theory so flexible it can accommodate any outcome, rendering it explanatory null. It generates no specific predictions, no breakthroughs to anchor its claims, only a flurry of books and public fascination.
A philosophical defense of the multiverse is certainly permissible, contending that it provides the only naturalistic framework for resolving the fine-tuning of our universe. But here, our curiosity directs us toward interrogating the scientific power the multiverse holds. And so far, as per the assessment of physicists spanning across numerous ideological backgrounds and aforementioned advocates of it, it holds little to none. But let the science speak for itself.
Explanatory power is perhaps one of the least of concerns when it comes to the multiverse. Even if one grants it premises, it fails to resolve the very problem it was designed to address: the fine-tuning of the universe. By positing a unbounded collection of universes with varying physical conditions, the multiverse claims to explain why our universe is uniquely suited for life—we happen to inhabit one of the rarely generated universes where conditions are just right. However, this account merely shifts the explanatory burden to a higher order. Herein lies the rub: To generate universes—those meant to explain away the fine-tuning we observe in ours—the universe generator itself must be finely-tuned.
Suppose we programmed a 3D printer to endlessly produce random objects. My goal is for it to create a small model plane for my shelf. Assuming the mechanism has an unlimited supply of material that doesn’t require maintenance, given an endless cycle of printing new objects, one day—however many millions of years later—I shouldn’t be too surprised if it generates a model plane. I wouldn’t say, “Wow, look at the fine-tuning and precision of this plane,” because I know it came after trillions of other random prints. While the printer’s mechanics explain the plane’s existence without needing an intentional blueprint uploaded to the printer, the printer itself requires precisely tuned components and settings to function and produce such a complex model. The stepper motors must move accurately to position the print head, the extruder needs to feed filament smoothly, the heated bed must maintain the right temperature to hold the print in place, and so forth. Similarly, as cosmologist and theoretical astrophysicist Luke Barnes quips, “We would need to. . . ahem. . . fine-tune the multiverse,”4 shifting the fine-tuning problem to the generator’s own precise conditions.
Paul Davies, an English physicist, wrestles with the evident fine-tuning of our universe in his 2016 book, The Goldilocks Enigma, where, like Goldilocks’ porridge, our universe is “just right.” Exploring various explanations for this phenomenon, none of which he definitively settles on, he expresses reservations about the multiverse hypothesis due to this very issue:
“The multiverse does not provide a complete account of existence because it still requires a lot of unexplained and very ‘convenient’ physics to make it work. For example, there has to be a universe-generating mechanism, quantum mechanics has to describe everything, and unified laws of some sort (such as those that arise from string/M theory) have to be simply accepted as ‘given.’. . . The problem of existence has therefore not gone away, but only been shifted up one level.”5
The multiverse, then, does not eliminate the mystery of fine-tuning; it merely relocates it to a more abstract and remote natural entity. To maintain that we exist in a life-sustaining universe because such universes are possible by some generative mechanism begs the question of why such a mechanism exists in the first place and by what laws it is sustained in its operation. Leaving no stone unturned in his meticulous critique of the multiverse, American Philosopher Robin Collins stresses this same fatal flaw and offers what some of these laws would entail:
“One major possible theistic response to the multiverse generator scenario, whether of the inflationary variety or some other type, is that the laws of the multiverse generator must be just right – fine-tuned – in order to produce life-sustaining universes. . . Thus, it seems, invoking some sort of multiverse generator as an explanation of the fine-tuning reinstates the fine-tuning up one level, to the laws governing the multiverse generator. . . In sum, even if an inflationary-superstring multiverse generator exists, it must have just the right combination of laws and fields for the production of life-permitting universes: if one of the components were missing or different, such as Einstein’s equation or the Pauli Exclusion Principle, it is unlikely that any life-permitting universes could be produced.”6
General relativity, as Collins argues, must, in advance to an diversely created universe, be instituted and effective to drive the rapid expansion of space that enables universes with sufficient matter to support life. Additionally, the Pauli Exclusion Principle ensures electrons arrange in diverse, stable atoms critical for chemistry. If these laws were absent or altered, the multiverse generator would likely churn out only desolate, uninhabitable worlds.
An additional issue complicating the multiverse hypothesis is the problem of entropy. Roger Penrose calculated that the odds of our universe starting in such a low-entropy state by chance are 1 in 10¹⁰¹²³—odds probably not worth betting on. Might inflation provide a naturalistic explanation for this challenging initial state? While it does, in theory, offer and explanation for the flatness of the cosmos and the Horizon problem, it does nothing to solve the fine-tuning problem; in fact, it makes matters worse. For inflation to begin, it too requires a rare, highly ordered patch of space—already in a low-entropy state—where the inflaton field is smooth and uniform. This need for extreme initial homogeneity is even more pronounced because inflation introduces a massive surge of energy during expansion, generating more entropy than the standard Big Bang model. In other words, inflation doesn’t explain why the universe started in such a special state; it assumes it, and exacerbates the fine-tuning problem by requiring an even lower-entropy starting point to account for the high degree of order we observe today. Philosopher of Science, Bruce Gordon reinforces this point, noting that “inflation solves the horizon problem only by exponentially increasing the fine-tuning of the already hyper-exponentially fine-tuned entropy of the Big Bang.”7 Additionally, after inflation ends, the universe undergoes a process called thermalization, in which the energy released by the decaying inflaton field is distributed among particles, facilitating the transition from an inflationary state to the hot, dense, thermal state of the Big Bang. This process, however, also increases entropy, as Penrose himself argues:
“Indeed, it is fundamentally misconceived to explain why the universe is special in any particular respect by appealing to a thermalization process. For, if the thermalization is actually doing anything (such as making temperatures in different regions more equal than they were before), then it represents a definite increasing of entropy. Thus, the universe would have had to be more special before the thermalization than after. This only serves to increase whatever difficulty we might have had previously in trying to come to terms with the initial extraordinarily special nature of the universe…. invoking arguments from thermalization, to address this particular problem [of the specialness of the universe], is worse than useless!”8
The processes invoked to construct the mechanism for this theoretical multiverse only imply that the universe must have begun in an even more finely tuned, low-entropy state, particularly to account for the order we observe following inflation and its subsequent thermalization. Rather than resolving the mystery of fine-tuning, inflation—and by extension the multiverse—inherits and amplifies it.
Ultimately, the remedy that is the multiverse hypothesis carries its initial appeal, yet the cure is worse than the disease.
- the_science_fact. (2023, May 7). Why are some people very critical of String theory? #briangreene [Short video]. YouTube. https://www.youtube.com/shorts/2sfDAqL11UI ↩︎
- Carroll, S. (2009, September 22). Philosophy and cosmology: Day three [Blog post]. Preposterous Universe. https://www.preposterousuniverse.com/blog/2009/09/22/philosophy-and-cosmology-day-three/ ↩︎
- Woit, P. (2018, January 17). Beyond falsifiability [Blog post]. Not Even Wrong. https://www.math.columbia.edu/~woit/wordpress/?p=9938 ↩︎
- Lewis, G. F., Barnes, L. A., & Schmidt, B. (2020). A fortunate universe: Life in a finely-tuned cosmos. Cambridge University Press. ↩︎
- Davies, P. C. W. (2008). The goldilocks enigma: Why is the Universe just right for life? Houghton Mifflin.
pg. 264 ↩︎ - Collins, R. (2012). The Teleological Argument. In The Blackwell Companion to Natural Theology (Blackwell Companions to Philosophy) (p. 263). essay, The Blackwell Companion. ↩︎
- Gordon, B. L. (2011). Balloons on a String: A Critique of Multiverse Cosmology. In The Nature of Nature: Examining the Role of Naturalism in Science (p. 571). essay, Intercollegiate Studies Institute. ↩︎
- Penrose, R. (2007). The road to reality: A complete guide to the laws of the universe. Vintage Books.
pg. 755 ↩︎
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