By Fred Adams

Life requires a framework, and our universe began constructing atomic nuclei shortly after its inception. These nuclei captured electrons, forming atoms that grouped into galaxies, stars, and planets, ultimately creating environments suitable for life. While we often assume that the laws of physics naturally lead to such complex structures, this might not necessarily be true.

In recent decades, many researchers have posited that even slight alterations in the fundamental laws of physics could have resulted in a universe lacking complex structures. Simultaneously, cosmologists have begun to consider that our universe might just be one among a myriad of others in a vast multiverse. This collection of universes offers a tantalizing explanation for the seemingly delicate balance of physical laws that allow for life; we exist in a universe where these laws are conducive to observers, simply because we could not thrive in any other.

SETTING THE PARAMETERS: The universe could still support life even if the forces of electromagnetism and gravity were altered. The shaded area indicates the range of values that align with life. The asterisk marks our universe's actual values. Constraints include that stars must undergo nuclear fusion (below the black curve), endure long enough for complex life to evolve (below the red curve), be sufficiently hot to sustain biospheres (left of the blue curve), and not exceed the size of their host galaxies (right of the cyan curve). Credit: Fred C. Adams

Astrophysicists have discussed the idea of fine-tuning to the point where it's often accepted as fact: our universe appears exceptionally tailored for complex structures. Even those skeptical of the multiverse theory concede that fine-tuning exists but believe it must have a different explanation. Yet, the concept of fine-tuning remains unproven. The precise laws of physics necessary for the emergence of astrophysical structures—which are prerequisites for life—are still not clearly defined. Recent studies in stellar evolution, nuclear astrophysics, and structure formation suggest that the argument for fine-tuning may not be as strong as previously believed. A broad spectrum of universes could potentially harbor life, indicating that our universe might not be as unique as it seems.

The first aspect of fine-tuning pertains to the fundamental forces in stars. If the electromagnetic force were too strong, it would prevent nuclear fusion in stellar cores, causing stars to fail. Conversely, if it were too weak, nuclear reactions could become uncontrollable, resulting in catastrophic stellar explosions. If gravity were excessively strong, stars could collapse into black holes or fail to ignite altogether.

However, stars exhibit remarkable resilience. The strength of the electric force could fluctuate by almost 100 times in either direction without compromising stellar functionality. Gravity could be 100,000 times stronger or a billion times weaker and still permit star formation. The allowed strengths of gravitational and electromagnetic forces hinge upon the nuclear reaction rate, which is influenced by nuclear forces. If the reaction rate increased, stars could function across a broader range of these forces; a slower rate would restrict it.

A wide array of potential universes could foster life. Our universe is not as exceptional as it may appear.

Stars also need to meet additional requirements beyond basic operational criteria. For instance, their surface temperatures must be high enough to facilitate the chemical reactions necessary for life. In our universe, many regions around stars maintain temperatures around 300 kelvins, conducive to biological activity. In universes with stronger electromagnetic forces, stars would be cooler and less hospitable.

Moreover, stars must have long lifetimes. The emergence of complex life forms demands extensive timeframes. As life is driven by intricate chemical reactions, the biological evolution clock is governed by atomic time scales. Variations in the strength of electromagnetism influence these rates. Weaker forces lead to faster-burning stars with shorter lifespans.

Additionally, stars must be able to form from primordial gas. For galaxies and stars to coalesce from this gas, it must lose energy and cool down—again, contingent on the strength of electromagnetism. If this force is too weak, the gas would remain too diffuse to condense into galaxies. Furthermore, stars must also be smaller than their host galaxies to avoid complications in star formation, establishing another lower limit on electromagnetic force strength.

When considered collectively, the fundamental forces can vary significantly while still allowing for the creation of planets and stars that meet all necessary conditions. Thus, these forces are not nearly as finely tuned as many scientists have previously believed.

Another potential fine-tuning concern arises with carbon production. Following the fusion of hydrogen into helium in moderately large stars, helium becomes the primary fuel. Through a complex set of reactions, helium is transformed into carbon and oxygen. Helium nuclei, known as alpha particles, are central to this process. Notably, the nucleus formed by two alpha particles—beryllium-8—is unstable in our universe.

This instability presents a bottleneck for carbon synthesis. As stars attempt to fuse helium into beryllium, the beryllium nuclei quickly decay back into their components. Consequently, stars maintain only a transient presence of beryllium in their cores. These fleeting beryllium nuclei can then react with helium to produce carbon through the triple-alpha process. However, the reaction proceeds too slowly to account for the carbon abundance observed in our universe.

Physicist Fred Hoyle proposed in 1953 that the carbon nucleus must possess a resonant state at a specific energy, akin to a bell that rings at a particular tone. This resonance significantly enhances the reaction rates for carbon production, explaining the observed abundance of carbon in our universe. This resonance has since been confirmed in laboratory experiments at the predicted energy level.

The concern is that, in alternate universes with different force strengths, this resonance energy might shift, resulting in insufficient carbon production. Any deviation beyond approximately 4 percent could jeopardize carbon synthesis. This scenario is sometimes referred to as the triple-alpha fine-tuning problem.

Fortunately, this issue has a straightforward resolution. What nuclear physics takes away, it can also provide. If nuclear forces were altered enough to nullify the carbon resonance, around half of these changes would concurrently stabilize beryllium, rendering the resonance irrelevant. In such universes, carbon would be generated through the more straightforward method of sequentially combining alpha particles. Helium could fuse into beryllium, which could then react with additional alpha particles to create carbon. Thus, the fine-tuning problem may not exist after all.

A third potential fine-tuning instance relates to the simplest nuclei composed of two particles: deuterium (one proton and one neutron), diprotons (two protons), and dineutrons (two neutrons). In our universe, only deuterium remains stable, serving as a precursor to helium production.

Had the strong nuclear force been stronger, diprotons could have been stable, allowing stars to produce energy via the most straightforward nuclear reactions. It is sometimes suggested that this could lead to stars exhausting their fuel too quickly, resulting in lifetimes insufficient to support biospheres. Conversely, if the strong force were weaker, deuterium would be unstable, eliminating the pathway to heavier elements. This scenario could lead to a universe devoid of complexity and life.

However, stars have proven to be remarkably stable systems. Their structures naturally adjust to burn fuel at a rate that supports stability against gravitational collapse. If nuclear reaction rates were higher, stars would operate at lower central temperatures but would not differ significantly. Our universe exemplifies this behavior: deuterium can fuse with protons to form helium via the strong force, with reaction probabilities vastly favoring this path. Yet, stars in our universe burn deuterium in a relatively uneventful manner, with core temperatures around 1 million kelvins, compared to the 15 million kelvins needed for typical hydrogen fusion. These deuterium-burning stars, slightly larger than our sun, remain unremarkable.

A wide variety of potential universes could support life. Our universe is not as special as it might seem.

If the strong nuclear force were reduced, stars could still operate without stable deuterium. Multiple processes can enable stars to generate energy and create heavy elements. In their early stages, stars contract gradually, increasing temperature and density until they glow with energy similar to the sun. While stars in our universe ignite nuclear fusion, alternate universes might allow stars to continue contracting and generate energy by releasing gravitational potential energy. The longest-lasting stars could shine for up to 1 billion years, potentially allowing biological evolution to unfold.

For sufficiently massive stars, this contraction can lead to catastrophic collapse, resulting in supernovae. The extreme temperatures and densities during these events would ignite various nuclear reactions, supplying the universe with heavy nuclei despite the lack of deuterium.

Once a universe produces trace amounts of heavy elements, subsequent generations of stars can harness different energy generation pathways. The carbon-nitrogen-oxygen cycle, for instance, does not require deuterium as an intermediary. Instead, carbon catalyzes helium production. This cycle operates within the sun's core and contributes a portion of its total energy output. In the absence of stable deuterium, this cycle would dominate energy production. Moreover, stars could also create helium through a triple-nucleon process, similar to the triple-alpha process for carbon. Thus, numerous channels exist for stars to provide both energy and complex nuclei in alternative universes.

A fourth example of fine-tuning involves the formation of galaxies and large-scale structures. These structures originated from small density fluctuations in the early universe. As the universe cooled, gravity intensified these fluctuations, leading to the formation of galaxies and clusters. The initial amplitude of these fluctuations, denoted as Q, was around 0.00001, indicating an incredibly smooth primeval universe.

If Q had been smaller, the fluctuations would have taken longer to develop into cosmic structures, resulting in less dense galaxies. Insufficient density would hinder gas cooling, preventing condensation into galactic disks or star formation. Low-density galaxies cannot sustain life. A prolonged delay could even halt galaxy formation altogether. Following about 4 billion years ago, the universe's expansion accelerated, pulling matter apart faster than it could coalesce—often attributed to dark energy. If Q were too small, galaxies might not have formed before this acceleration began, leading to a universe devoid of complexity and life. Thus, Q must not be smaller than a factor of 10.

Conversely, if Q had been larger, galaxies would have formed earlier and with greater density, which could jeopardize habitability. Stars would be closer together, leading to more frequent interactions that could disrupt planetary orbits. Additionally, closer stars would illuminate the night sky, potentially brightening it to daytime levels and harming any suitable planets.

GALACTIC WHAT-IF: A galaxy that formed in a universe with significant initial density fluctuations could be more hospitable than our Milky Way. Its central region would be too bright and hot for life, with unstable planetary orbits. However, the outer region might resemble our solar neighborhood, and the background starlight could be comparable to sunlight, making all planets potentially habitable. Credit: Fred C. Adams

In this scenario, the fine-tuning argument lacks strong constraints. While central galaxy regions could produce intense radiation, rendering planets uninhabitable, the outskirts would likely maintain a low enough density to sustain life. A significant portion of galactic space remains viable even when Q is thousands of times larger than in our universe. In some instances, a galaxy might even be more hospitable. Across much of such a galaxy, the night sky could shine as brightly as daylight on Earth. Planets could derive their energy from a collective of background stars, rather than solely from their sun, allowing for diverse orbits. In an alternate universe with larger density fluctuations, even distant planets could enjoy sunlight comparable to that of Miami.

In conclusion, the parameters defining our universe could have varied widely while still permitting the existence of functioning stars and potentially habitable planets. Gravity could be 1,000 times stronger or 1 billion times weaker, yet stars would remain stable as long-lived nuclear reactors. The electromagnetic force could also vary by factors of 100, and nuclear reaction rates could fluctuate over numerous orders of magnitude. Alternative stellar physics could yield the heavy elements essential for planets and life. Clearly, the parameters governing stellar structure and evolution are not as finely tuned as many have believed.

Given the apparent lack of fine-tuning in our universe, can we assert that it is the most conducive for life? Current understanding suggests otherwise. One can easily imagine a universe that is more favorable to life and possibly more logical. A universe with stronger initial density fluctuations would yield denser galaxies, capable of supporting more habitable planets than our own. A universe with stable beryllium would facilitate straightforward carbon production, avoiding the complexities of the triple-alpha process. Although these matters are still under investigation, it is clear that diverse pathways for the emergence of complexity and life exist in various universes, some potentially more advantageous than ours. Consequently, astrophysicists should reevaluate the implications of the multiverse, particularly regarding the degree of fine-tuning in our universe.

Fred Adams is a physics professor at the University of Michigan in Ann Arbor. He is the recipient of the Helen B. Warner Prize from the American Astronomical Society, the National Science Foundation Young Investigator Award, and multiple teaching awards from the University of Michigan. He co-authored The Five Ages of the Universe: Inside the Physics of Eternity and Origins of Existence: How Life Emerged in the Universe.

Originally published on Nautilus Cosmos in January 2017.