Is the universe fine-tuned for intelligent life?

Is the universe fine-tuned for intelligent life?
Updated 7 August 2023 (c) 2023


Is the universe fine-tuned for intelligent life? Many researchers in a variety of disciplines, ranging from astrobiology to cosmology and physics, have wondered why the laws of physics, the masses of fundamental particles and other facts of nature appear to be “just right” to foster the formation of atoms, molecules, galaxies and stars, and, several billion years after the big bang, carbon-based organisms and ultimately intelligent observers who are cognitively and technologically equipped to ponder this question.

Recently astrophysicist Geraint Lewis (University of Sydney, Australia) and cosmologist Luke Barnes (University of Western Sydney) waded into this perplexing and controversial arena in a new book entitled A Fortunate Universe: Life in a Finely Tuned Cosmos [Lewis2016]. The book presents a comprehensive analysis of the issue, delving into nuclear physics, astrophysics, cosmology, biology and philosophy. A more detailed technical analysis is available in Luke Barnes’ 2013 paper “The fine-tuning of the universe for intelligent life” [Barnes2013]. Some other recent references for the general reader include [Barrow2008; Cossins2018; Donoghue2016; Than2018; Wolchover2013; Wolchover2014] and [Baggott2013].

Cosmic coincidences

Here are some of the “cosmic coincidences” mentioned by Lewis and Barnes [Lewis2016; Barnes2013] and by other authors:

  1. Carbon resonance and the strong force. Although the laws of physics can readily explain the abundances of hydrogen, helium, lithium and beryllium (they were formed in the first 100 seconds or so after the big bang), the synthesis of heavier elements, beginning with carbon, was a deep mystery until 1951, when astronomer Fred Hoyle hypothesized and then discovered a resonance that is just energetic enough to permit a triple-helium nuclear reaction to produce a carbon nucleus. If the strong force were slightly stronger or slightly weaker (by just 1% in either direction), then the binding energies of the nuclei would be different, and this resonance would not work. In that case, there would be no carbon or any heavier elements anywhere in the universe, and thus no carbon-based life forms to contemplate this intriguing fact. By the way, although one can imagine living organisms based on other elements, carbon is by far is the most suitable element for the construction of complex molecules, as required for any conceivable form of living or sentient beings. In any event, nuclear chemistry precludes any heavier elements (i.e., elements beyond hydrogen, helium, lithium and beryllium) if carbon cannot form.
  2. The electromagnetic-gravitational strength ratio. The ratio of the strengths of the electromagnetic and gravitational fields is roughly 1040. If gravity were slightly stronger (so that the ratio is lower), all stars would be radiative rather than convective, and planets might not form. But if gravity were somewhat weaker (so that the ratio was higher), then all stars would be convective and supernovas might not happen. Since all elements from carbon on up are synthesized in stellar explosions, we might not be here to discuss the issue.
  3. The weak force and the proton-neutron balance. Had the weak force been somewhat weaker, the amount of hydrogen in the universe would be greatly decreased, starving stars of fuel for nuclear energy and leaving the universe a cold and lifeless place.
  4. Neutrons and the proton-to-electron mass ratio. The neutron’s mass is very slightly more than the combined mass of a proton, an electron and a neutrino. As a result, neutrons that are not tied up in the nucleus of an atom spontaneously decay, with a half life of only ten minutes. If neutrons were very slightly less massive, then they could not decay without energy input and the universe would be entirely protons (i.e., hydrogen). But if their mass were lower by 1%, then all isolated protons would decay into neutrons, and no atoms other than hydrogen, helium, lithium and beryllium (which were synthesized in the big bang) could form.
  5. Anisotropy of the cosmic microwave background. For many years after the discovery of the cosmic microwave background radiation, measurements indicated that it was isotropic (constant in all directions), except for a well-understood effect resulting from our galaxy’s motion. In 1992, scientists discovered that there is a very slight anisotropy in this radiation, roughly one part in 100,000, which is just enough to permit the formation of stars and galaxies. If this anisotropy had been significantly smaller, the early universe would have been too smooth for stars and galaxies to have formed before matter dispersed. If it had been significantly greater, galaxies would have been much denser, resulting in numerous stellar collisions, so that stable, long-lived stars with planetary systems would have been extremely rare, if they existed at all. In sharp contrast, planetary systems are plentiful in our universe.
  6. The cosmological constant paradox. The cosmological constant paradox derives from the fact that when one calculates, based on known principles of quantum mechanics, the “vacuum energy density” of the universe, one obtains the incredible result that empty space “weighs” 1093 grams per cubic centimeter (since the actual average mass density of the universe is roughly 10-28 grams per cc, this is in error by 120 orders of magnitude). Physicists, who have fretted over this discrepancy for decades, have noted that calculations such as the above involve only the electromagnetic force, and so perhaps when the contributions of the other known forces are included, all terms will cancel out to exactly zero as a consequence of some heretofore unknown physical principle. These hopes were shattered with the 1998 discovery that the expansion of the universe is accelerating, which implies that the cosmological constant must be slightly positive. But this means that physicists are left to explain the startling fact that the positive and negative contributions to the cosmological constant cancel to 120-digit accuracy, yet fail to cancel beginning at the 121-st digit. Curiously, this observation is in accord with a prediction made by physicist Steven Weinberg in 1987, who argued from basic principles that the cosmological constant must be zero to within one part in roughly 10120, or else the universe either would have dispersed too fast for stars and galaxies to have formed, or would have recollapsed upon itself long ago. Numerous “solutions” have been proposed for the cosmological constant paradox (Lewis and Barnes mention eight — see pg. 163-164), but they all fail, rather miserably.
  7. Mass of the Higgs boson; the hierarchy and flavor problems. A similar coincidence has come to light recently in the wake of the 2012 discovery of the Higgs boson at the Large Hadron Collider (LHC). Higgs was found to have a mass of 126 billion electron volts (i.e., 126 Gev). However, a calculation of interactions with other known particles yields a mass of some 1019 Gev. This means that the rest mass of the Higgs boson must be almost exactly the negative of this enormous number, so that when added to 1019 gives 126 Gev, as a result of massive and unexplained cancelation. Supersymmetry (the notion that each known particle has a “superpartner” with different properties) has been proposed as a solution to this paradox, but no hint of supersymmetric particles have been seen in the latest experiments at the LHC (and it is not clear that the required cancelation would occur even if the superparticles do exist). Similar difficulties afflict a number of other particle masses and forces — some are of modest size, yet others are orders of magnitude larger. These difficulties collectively are known as the “hierarchy” and “flavor” problems.
  8. The flatness problem. General relativity allows the space-time fabric of the universe to be open (extending forever, like an infinite saddle), closed (like the surface of a sphere), or flat. The latest measurements confirm that the universe is flat to within 1%. But looking back to the first few minutes of the universe at the big bang, this means that the universe must have been flat to within one part in 1015. The cosmic inflation theory was proposed by Alan Guth and others in the 1970s to explain this and some other phenomena, but recently even some of inflation’s most devoted proponents have acknowledged that the theory is in deep trouble and will have to be either substantially revised or discarded altogether (see Inflation).
  9. The low-entropy state of the universe. The overall entropy (disorder) of the universe is, in the words of Lewis and Barnes, “freakishly lower than life requires.” After all, life requires, at most, a galaxy of highly ordered matter to create chemistry and life on a single planet. Physicist Roger Penrose has calculated the odds that the entire universe is as orderly as our galactic neighborhood to be one in 1010123, a number whose decimal representation has vastly more zeroes than the number of fundamental particles in the observable universe. Extrapolating back to the big bang only deepens this puzzle.

For full details, see [Lewis2016] and [Barnes2013], or the other references mentioned above, including [Barrow2008; Cossins2018; Donoghue2016; Than2018; Wolchover2013; Wolchover2014] and [Baggott2013].

The multiverse and the anthropic principle

Numerous explanations have been proposed over the years to explain these difficulties. As mentioned above, the inflationary cosmology pioneered by Alan Guth, mentioned above, was proposed as a solution to the flatness problem. But as noted before, many researchers see serious difficulties in the theory; it may have to be either substantially revised or discarded. Laura Mersini-Houghton has advanced a theory that suggests that some aspects of universes like ours may be probabilistically more likely, lessening the paradox of fine tuning [Mersini-Houghton2022]. Boyle and Turok argue that gravitational entropy favors universes like ours that are flat and isotropic [Boyle2022].

One of the more widely accepted explanations is the multiverse (see Multiverse), combined with the anthropic principle (see Anthropic-principle). The theory of inflation, mentioned above, suggests that our universe is merely one pocket that separated from many others in the very early universe (see Inflation). Similarly, string theory suggests that our universe is merely one speck in an enormous landscape of possible universes, by one count 10500 in number, each corresponding to a different Calabi-Yau manifold.

Thus, the thinking goes, we should not be surprised that we find ourselves in a universe that has somehow beaten the one-in-10120 odds to be life-friendly (to pick just the cosmological constant paradox), because it had to happen somewhere, and, besides, if our universe were not life-friendly, then we would not be here to talk about it. In other words, these researchers propose that the multiverse (or the “cosmic landscape”) actually exists in some sense, but acknowledge that the vast, vast majority of these universes are utterly sterile — either very short-lived or else completely devoid of atoms or other structures, much less sentient living organisms like us contemplating the meaning of their existence.

However, other prominent researchers, including Lee Smolin, Joseph Ellis and Joseph Silk [Smolin2015; Ellis2014], to name just three, remain extremely uncomfortable with hypothesizing a vast multiverse and invoking the anthropic principle. For one thing, it sounds too much like a tautology with no real substance. More importantly, proposing a staggeringly large number of unseen universes, all to explain the cosmic coincidences, is a flagrant violation of Occam’s razor, “Entities must not be multiplied beyond necessity.”

Jim Baggott, for instance, rather bluntly writes [Baggott2013]:

I reject the weak anthropic principle because it is simply empty of scientific content. It adds absolutely nothing to the debate. And yet it is used by some contemporary theorists to provide a rather facile logic, a veneer to deflect the fact that multiverse theories themselves are not scientific. Anthropic reasoning is the last refuge of theorists desperate to find a way to justify and defend their positions.

But one way or another, the paradox of cosmic fine-tuning remains unanswered. We just don’t know.

Stenger’s The Fallacy of Fine Tuning

In contrast to the consensus mentioned above about the puzzling nature of these paradoxes, in 2011 Victor Stenger published The Fallacy of Fine Tuning [Stenger2011]. In this book, Stenger argues that some symmetry laws and other basic principles are sufficient to derive all the basic laws of the universe; in fact, they forbid the universe to be any different than it is. Thus there is no fine-tuning, and no assumption of a multiverse, the anthropic principle or anything else is required to explain our universe. Stenger’s book has attracted significant public attention — for instance, it was cited in Steven Pinker’s 2018 book Enlightenment Now: The Case for Reason, Science, Humanism, and Progress [Pinker2018, pg. 423-424].

Lewis and Barnes, among others, have countered that Stenger is very deeply mistaken here. Among Stenger’s errors are the following [Barnes2013]: (a) the book ignores the fact that the fundamental constants of physics are not determined by the standard model, and these constants appear very much fine-tuned for life; (b) the book claims that point-of-view invariance allows one to deduce most if not all of classical and modern physics, but this conclusion cannot possibly be correct, because these individual theories are based on conflicting principles and make conflicting predictions; (c) the book does not satisfactorily deal with the extremely low entropy of the universe; (d) the book dismisses the fine-tuning of the cosmological constant, which is arguably the most significant and inexplicable instance of fine-tuning; and (e) the book does not appear to appreciate the difficulty presented by the hierarchy and flavor problems of physics. With regards to the last item (e), Barnes writes, “Stenger is either not aware of the hierarchy and flavor problems, or else he has solved some of the most pressing problems in particle physics and not bothered to pass this information on to his colleagues.”

As noted above, Lewis and Barnes are hardly alone in observing that the universe appears fine-tuned for life and, at the least, that this question deserves further analysis. In fact, many of the most distinguished figures in modern physics and cosmology (including at least three Nobel Prize winners) agree that the universe we reside in appears anomalously fine-tuned, and that this feature begs further analysis. This consensus holds even though they practice a number of different technical specialties, come from a wide range of philosophical and religious backgrounds (mostly not particularly religious) and differ, often rather strenuously, on possible explanations. A bibliography listing over 100 of these references is: Fine-tuned bibliography. Some useful collections include [Barrow2008] and [Carr2009]. Other recent overviews for the general reader include [Cossins2018; Donoghue2016; Than2018; Wolchover2013; Wolchover2014] and [Baggott2013].


Lewis and Barnes, as well as virtually all of the numerous authors mentioned above (see Fine-tuned bibliography), do not offer any firm answers to these issues. They agree, though, that our knowledge of the basic underlying mathematical laws governing the universe is incomplete. If examination of these paradoxes eventually leads to a deeper understanding of these laws, it will have been well worthwhile.

We might add here that while some writers from the religious world have claimed that the apparent fine-tuning of universal law constitutes “proof” that our universe was designed by a Supreme Being, many others recommend caution. For example, the search for “design” in the creation of the universe is reminiscent of the search for “design” in the evolution of life on Earth. And long experience makes it clear that claims that one can “prove” the existence of a Supreme Being via arguments based on apparent design or other inexplicable phenomena in the natural world are likely to disappoint in the long run. It is clearly better to leave such matters to peer-reviewed scientific research, where they belong.

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