|Landscape in Carina Nebula [Courtesy NASA]
||Sistine Chapel #3 [courtesy Wikimedia]
Is the universe fine-tuned for intelligent life?
David H. Bailey
2 July 2017 (c) 2017
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 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.
For several decades, researchers have puzzled over deeply perplexing indications, many of them highly mathematical in nature, that the universe seems inexplicably well-tuned to facilitate the evolution of complex molecular structures and sentient creatures.
Some of these "cosmic coincidences" include the following (these and numerous others are presented and discussed in detail by Lewis and Barnes):
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 stronger, primordial neutrons produced in the first few seconds after the big bang would have decayed faster, and there would be little if any carbon or heavier elements in our universe. On the other hand, if the weak force had 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.
The electromagnetic-gravitational strength ratio. In 1974, Brandon Carter noted an interesting relationship between the ratio of the strengths of the electromagnetic and gravitational fields, which is roughly 1040, and the properties of stars. If gravity were slightly stronger (so that the ratio is lower), all stars would be radiative rather than convective, and planets would not form. But if gravity were somewhat weaker (so that the ratio was higher), then all stars would be convective and supernovas would not happen. Since most elements beyond carbon are synthesized in stellar explosions, we would not be here to discuss the issue.
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 10 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.
Uniformity 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).
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.
For additional discussion, see Cosmic-coincidences.
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 there 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; Ellis2017, to name just three) remain extremely uncomfortable with hypothesizing a 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." Isn't there a better explanation than this?!
Lewis and Barnes explore these issues in substantial detail. In this regard, they follow a long line of very eminent researchers who have puzzled over these same problems in published books and articles (some dating back to the 1970s): John Barrow, Bernard Carr, Sean Carroll, Paul Davies, David Deutsch, Brian Greene, Alan Guth, Edward Harrison, Stephen Hawking, Andre Linde, Roger Penrose, John Polkinghorne, Martin Rees, Lee Smolin, Leonard Susskind, Max Tegmark, Frank Tipler, Alexander Vilenkin, Steven Weinberg and Frank Wilczek, among others.
Another useful reference is Luke Barnes's 2011 paper [Barnes2011], which summarizes many of these issues.
In the end, the Lewis-Barnes book [Lewis2016] does not offer any firm answers -- only more questions. The one thing that is certain, though, is 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 all been worthwhile.