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Has fine-tuning been refuted?
David H. Bailey
Updated 26 March 2022 (c) 2022
Is the universe fine-tuned for intelligent life? In 2016, astrophysicist Geraint Lewis and cosmologist Luke Barnes, both at the University of Sydney, Australia, waded into this perplexing and controversial arena in a new book entitled A Fortunate Universe: Life in a Finely Tuned Cosmos [Lewis2016]. The core of the Lewis-Barnes book and an accompanying technical paper by Luke Barnes [Barnes2013] is their review of the "cosmic coincidences":
- Carbon resonance and the strong force. If the strong force were slightly stronger or slightly weaker (by just 1% in either direction), 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.
- 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. If neutrons were very slightly less massive, then the universe would be entirely protons (i.e., hydrogen); but if lower by 1%, then all protons would decay into neutrons, and no atoms other than hydrogen, helium, lithium and beryllium could form.
- Anisotropy of the cosmic microwave background. 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; but if significantly greater, then stable, long-lived stars with planetary systems would have been extremely rare.
- The cosmological constant paradox. The positive and negative contributions to the cosmological constant (or, more properly, the vacuum energy density) cancel to 120-digit accuracy, yet fail to cancel beginning at the 121-st digit.
- Mass of the Higgs boson. The rest mass of the Higgs boson is very much less than that required from its interactions with other particles, requiring a huge cancellation between positive and negative terms. Similar difficulties afflict a number of other particle masses and forces -- these are the "hierarchy" and "flavor" problems.
- The flatness problem. Looking back to the first few minutes of the universe at the big bang, the universe must have been flat to within one part in 1015.
- 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."
For full details see Fine-tuned on this site, the Lewis-Barnes book [Lewis2016] and Luke Barnes' 2013 paper [Barnes2013].
The multiverse and the anthropic principle
One of the more widely accepted explanations is the multiverse (see Multiverse), combined with the anthropic principle (see Anthropic-principle). The argument is that 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 in a vast multiverse (at least 10500 separate universes by one reckoning), and, besides, if our universe were not life-friendly, then we would not be here to talk about it.
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. They argue that proposing a staggeringly large number of unseen universes, all to explain the cosmic coincidences, is a flagrant violation of Occam's razor. 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]:
- The book ignores the fact that the fundamental constants of physics (speed of light, fine structure constant, etc.) are not determined by the standard model but in fact are completely independent from the standard model; and these constants appear very much fine-tuned for life.
- The book claims that point-of-view invariance, via a theorem due to Emmy Noether, allows one to deduce classical mechanics, Newton's law of gravity, Maxwell's laws of electromagnetics, Einstein's relativity, quantum mechanics, and more -- essentially most if not all of the standard model. But Stenger's mathematical reasoning is deeply fallacious here. Indeed, his conclusion cannot possibly be correct, because these individual theories are based on conflicting principles and make conflicting predictions.
- The book's calculations of the effects of varying multiple parameters are not valid.
- The book does not satisfactorily deal with the extremely low entropy of the universe, which is one of the most significant instances of fine-tuning.
- The book does not mention the considerable controversy among researchers on some aspects of big bang cosmology, especially the inflation epoch, which itself requires incredible fine-tuning to produce the universe we see today.
- The book dismisses the fine-tuning of the cosmological constant, namely that it appears fine-tuned to least one part in 10120. But the consensus of other researchers is that this is arguably the most significant and inexplicable instance of fine-tuning.
- The book does not appear to appreciate the difficulty presented by the hierarchy and flavor problems of physics, which stem from the fact that some particle masses and fundamental forces are of modest size but others are orders of magnitude larger. As 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."
- The book includes unprofessional criticisms of other researchers, one of which, amusingly enough, is self-refuting --- it claims that the authors of two papers [Tegmark1998; Tegmark2006] "do not know what to make of" results in a third paper [Tegmark2001]. But the first author of the first two papers (Max Tegmark) is also the first author of the third paper. By the way, the second author of the first paper (Martin Rees) is an extremely knowledgeable figure in the field; he most certainly is not perplexed by the third paper.
Other prominent researchers on fine-tuning
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. Here is a partial list of researchers who have written or commented on this general topic:
Shing-tung Yau and
For a bibliography in which each of the above researchers is listed as a co-author or commenter in one or more of the listed sources, see Fine-tuned bibliography. Some of the above authors comment on this topic in detail in the collections [Barrow2008] and [Carr2009]. Some recent overviews of this topic include [Cossins2018; Donoghue2016; Than2018; Wolchover2013; Wolchover2014] and [Baggott2014].
Needless to say, the above list of researchers includes many of the most distinguished figures in modern physics and cosmology, including three Nobel Prize winners (George Smoot, Gerard t'Hooft and Steven Weinberg), several other very prominent physicists and cosmologists (John Barrow, Paul Davies, David Deutsch, Brian Greene, Alan Guth, Stephen Hawking, Fred Hoyle, Roger Penrose, Joseph Polchinski, John Polkinghorne, Lisa Randall, Martin Rees, Lee Smolin, Leonard Susskind, Max Tegmark, Frank Tipler, Alexander Vilenkin, John Wheeler and Edward Witten, just to list a few), together with numerous other active, publishing researchers in the field, with publications dating from 1974 to the present day. Luke Barnes, in commenting on a similar list that includes many of the above names, pointed out that even though these researchers 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, they are unanimous in agreeing that the universe we reside in appears to be anomalously fine-tuned, and, at the least, that this feature of the universe begs an explanation [Barnes2013].
Stenger, on the other hand, attempts to claim that the universe is not fine-tuned, so that there is no need to consider the possibility of a multiverse, an anthropic principle or any other explanation. 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!
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.
For additional discussion, see