Either we are remarkably fortunate or the existence of multiple universes is fact.
It is simple to imagine alternate universes that operate under slightly different laws of physics, where the emergence of intelligent life or any form of intricate systems is impossible. Given this, should we be astonished that we exist in a universe where we were able to evolve? This has been a question that physicists, including myself, have been attempting to answer for decades, but it remains challenging.
While we can confidently trace the history of the cosmos back to one second following the Big Bang, it is harder to determine what happened before that moment. The extreme conditions that prevailed in the first nanosecond are difficult to replicate since our accelerators cannot generate enough energy.
However, we anticipate that it was during this tiny fraction of a second that the fundamental features of our universe were established. The properties of the universe can be characterized by its “fundamental constants,” which are unchanging values in nature like the gravitational constant (G) or the speed of light (C). There are roughly 30 of these constants that determine the sizes, strengths, forces, and expansion of particles in the universe.
Our current theories fail to clarify the precise values that these constants should possess. Therefore, we have to measure them and incorporate their values into our equations to provide an accurate depiction of nature.
The range of values for these constants allows for the evolution of intricate systems like stars, planets, carbon, and ultimately, humans. Physicists have discovered that even a slight variation in some of these parameters, by a few percent, would result in a universe that is incapable of sustaining life.
The existence of life in our universe requires an explanation.
Some suggest that it is merely a fortuitous coincidence. However, an alternative viewpoint is that our universe is part of a multiverse that comprises domains with distinct physical laws and fundamental constant values.
The majority of these domains would be unsuitable for the sustenance of life. Nevertheless, a few should be statistically inclined to support life.
What is the scope of physical reality? We are confident that it encompasses more than what astronomers can observe, even in theory.
The observable universe is undoubtedly finite, much like the ocean, which has a horizon beyond which we cannot see. Just as we believe that the ocean does not terminate beyond our horizon, we assume that there are galaxies beyond the boundary of our observable universe that our descendants will never be able to observe due to the accelerating expansion of our universe.
Most physicists concur that there are more galaxies beyond our observation than there are within it. If they were to extend far enough, then everything that we can conceive of happening could be repeated multiple times. Beyond the horizon, it is possible that we all have avatars.
This vast and mostly unobservable domain would be the aftermath of our Big Bang and would likely be subject to the same physical laws that govern the observable parts of the universe. However, was our Big Bang the only one?
The theory of inflation, which suggests that the early universe underwent a period of exponential growth every trillionth of a trillionth of a trillionth of a second, is supported by genuine observations. It explains why the universe is so vast and uniform, except for the ripples and fluctuations that serve as the seeds for galaxy formation.
Physicists like Andrei Linde have demonstrated that, given certain plausible assumptions about the uncertain physics of that ancient era, there would be an “eternal” production of Big Bangs, each resulting in a new universe.
String theory, which seeks to unify gravity with the laws of microphysics, proposes that everything in the universe is composed of tiny vibrating strings. However, it assumes that there are more dimensions than the ones we experience.
The extent of physical reality is believed to be greater than what astronomers can observe, even in principle. The observable domain is finite due to the existence of a horizon beyond which galaxies remain unobservable. However, it is expected that galaxies exist beyond the observable Universe.
Physicists generally agree that the number of galaxies beyond our observable Universe is greater than the ones we can observe. This vast and unobservable domain could be the aftermath of “our” Big Bang and could be governed by the same physical laws that apply in the parts of the universe we can observe. But is our Big Bang the only one?
The theory of inflation supports the idea that the early Universe doubled in size every trillionth of a trillionth of a trillionth of a second, which accounts for why the Universe is large and smooth, with fluctuations and ripples that are the “seeds” for galaxy formation. Physicists have shown that under certain plausible assumptions about the uncertain physics at this ancient era, there could be an “eternal” production of Big Bangs, each giving rise to a new universe.
String theory suggests that there are extra dimensions, but they are compacted so tightly together that we don’t notice them all. Each type of compactification could create a universe with different microphysics, and thus different laws.
If physical reality is like this, then exploring “counterfactual” universes with different gravity and physics becomes a motivation to understand what range of parameters would allow complexity to emerge and which would lead to sterile or “stillborn” cosmos.
Some argue that the concept of the multiverse is problematic, as it may make it impossible to find a fundamental theory that explains the constants. However, our preferences are irrelevant to the way physical reality is, and we should remain open-minded about the possibility of a grand cosmological revolution.
The Copernican realization that the Earth was not the center of the Solar System was followed by the realization that there are zillions of planetary systems in our galaxy, and there are zillions of galaxies in our observable universe. Therefore, it is plausible that our observable domain, including our Big Bang, is a tiny part of a far larger and diverse ensemble.
Physics or metaphysics?
How can we determine the uniqueness of our Universe? The answer requires us to calculate the likelihood of various combinations of physical constants. Unfortunately, this is a complex task that is beyond our current capabilities and must await significant advancements in theory.
While we cannot be certain if there are other Big Bangs, the possibility cannot be dismissed as mere philosophy. If a theory describing the physics of the early Big Bang, which had also been verified through other means, predicted the existence of multiple Big Bangs, it would deserve serious consideration.
Critics have argued that the multiverse idea is unscientific because it cannot be directly observed. However, just as we accept the reality of the interior of black holes based on theories corroborated by observations, we can similarly accept the existence of other universes based on well-supported theories.
Although predicting the outcome is difficult, machine intelligence could conceivably comprehend some of the more complicated theories and elucidate the fundamental aspects of the standard model. Nonetheless, the profundity of physical reality could require the existence of posthuman species to understand it fully.
Thus, while we may not have definitive answers, we should not disregard the possibility of a multiverse as being unscientific.
Martin Rees, Emeritus Professor of Cosmology and Astrophysics, University of Cambridge
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