|GOODS South WFC3 ERS Details 3 [Courtesy NASA]
Is the universe fine-tuned for intelligent life?
David H. Bailey
Updated 31 March 2019 (c) 2019
Is the universe fine-tuned for intelligent life? In 2016, 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. It is entertainingly written, yet does not compromise in detail.
Lewis and Barnes do not present fine-tuning with the intent of promoting a theistic interpretation of creation. They do include a section on philosophy and theology in their book, but they conclude that a theological assumption leads only to an inconclusive result. Their principal interest in writing this book is to discuss fine-tuning in some technical detail and to explore possible scientific explanations [Lewis2016]. A deeper technical analysis is available in Luke Barnes' 2013 paper "The fine-tuning of the universe for intelligent life" [Barnes2013].
The core of the Lewis-Barnes book and the Barnes paper is their review of the "cosmic coincidences." Some of these include the following [Lewis2016; Barnes2013]:
- 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.
- 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.
- 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.
- 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.
- The cosmological constant paradox. The cosmological constant paradox (see Cosmological-constant) 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.
- Mass of the Higgs boson. A similar coincidence has come to light recently in the wake of the 2012 discovery of the Higgs boson at the Large Hadron Collider. 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.
- 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).
- 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.
Some recent overviews of this topic include [Cossins2018; Donoghue2016; Than2018; Wolchover2013] and [Baggott2014]. For full details see the Lewis-Barnes book [Lewis2016] and Luke Barnes' 2013 paper [Barnes2013]. Some additional discussion is available at Anthropic principle,
The multiverse and the anthropic principle
Numerous explanations have been proposed over the years to explain these difficulties. 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, many 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."
But one way or another, the paradox of cosmic fine-tuning remains unanswered.
Stenger's The Fallacy of Fine Tuning
The Lewis-Barnes book [Lewis2016] and, more especially, Barnes' 2013 paper [Barnes2013], were originally written, in part, as a response to Victor Stenger's 2011 book The Fallacy of Fine Tuning [Stenger2011]. In this book, Stenger argues that 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. Several other prominent writers, including for example Harvard social scientist Steven Pinker [Pinker2018, pg. 423-424], have cited Stenger's work.
Lewis and Barnes argue that Stenger's thesis is very deeply mistaken. 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."
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. 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 often differ vociferously in their interpretation of fine-tuning (see Fine-tuned refuted for a partial list of these scientists, with some references).
Stenger, on the other hand, attempts to claim that the universe is not fine-tuned. Obviously the best minds in the fields of physics and cosmology overwhelmingly disagree with Stenger. It is most unfortunate that his book has attracted the attention it has. For additional details, see Fine-tuned refuted.
In the end, the Lewis-Barnes book [Lewis2016] and Barnes' paper [Barnes2013] do not offer any firm answers -- only more perplexing questions. They do emphasize, though, that our knowledge of the basic underlying mathematical laws governing the universe is incomplete. If examination of these paradoxes eventually leads to a greater understanding of these laws, it will have been well worthwhile.
While some religious-minded writers conclude 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 has taught us that claims that one can "prove" God via arguments based on apparent design or other inexplicable phenomena in the natural world are likely to disappoint in the long run. Furthermore, invoking a Creator or Designer every time unexplained phenomena arise is a "thinking stopper," burying the grand questions of science and religion in the inaccessible, inscrutable mind of some transcendent being.
One good case in point is big bang cosmology: the recent emergence of the "multiverse" cosmology (see
Multiverse) has led some theologians, who once were fond of the big bang cosmology, to reconsider what their theology means in the context of the multiverse. As religious philosopher John Haught notes [Haught1995, pg. 109]:
And although it may seem for the moment that big bang physics is smoothing over some of the friction between science and religion, we know that science will continue to change. And if the big bang theory is eventually discarded as premature or inaccurate, then on what ground will those theologians stand who now see it as a vindication of theism?
For additional discussion, see