The fine-tuned universe is the proposition that the conditions that allow life in the Universe can only occur when certain universal fundamental physical constants lie within a very narrow range, so that if any of several fundamental constants were only slightly different the universe would be unlikely to be conducive to the establishment and development of matter, astronomical structures, elemental diversity, or life as it is presently understood.[1] The existence and extent of fine-tuning in the universe is a matter of dispute in the scientific community.
Physicist Paul Davies has stated that "There is now broad agreement among physicists and cosmologists that the universe is in several respects ‘fine-tuned' for life".[2] However he continues "...the conclusion is not so much that the universe is fine-tuned for life; rather it is fine-tuned for the building blocks and environments that life requires".[2] He also states that "... 'anthropic' reasoning fails to distinguish between minimally biophilic universes, in which life is permitted but only marginally possible, and optimally biophilic universes, in which life flourishes because biogenesis occurs frequently ..."[2] Among scientists who find the evidence persuasive, a variety of natural explanations have been proposed, e.g., the anthropic principle along with multiple universes. The proposition is also discussed among philosophers, theologians, creationists and intelligent design proponents.
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In 1913, the chemist Lawrence Joseph Henderson (1878–1942) wrote The Fitness of the Environment, one of the first books to explore concepts of fine tuning in the universe. Henderson discusses the importance of water and the environment with respect to living things, pointing out that life depends entirely on the very specific environmental conditions on Earth, especially with regard to the prevalence and properties of water.[3]
In 1961, the physicist Robert H. Dicke claimed that certain forces in physics, such as gravity and electromagnetism, must be perfectly fine-tuned for life to exist anywhere in the universe.[4][5] Fred Hoyle also argued for a fine-tuned universe in his 1984 book Intelligent Universe. He compares "the chance of obtaining even a single functioning protein by chance combination of amino acids to a solar system full of blind men solving Rubik's Cube simultaneously".[6]
John Gribbin and Martin Rees wrote a detailed history and defence of the fine-tuning argument in their book Cosmic Coincidences (1989). According to Gribbin and Rees, carbon-based life was not haphazardly arrived at, but the deliberate end of a universe "tailor made for man."[7]
The premise of the fine-tuned universe assertion is that a small change in several of the dimensionless fundamental physical constants would make the universe radically different. As Stephen Hawking has noted, "The laws of science, as we know them at present, contain many fundamental numbers, like the size of the electric charge of the electron and the ratio of the masses of the proton and the electron. ... The remarkable fact is that the values of these numbers seem to have been very finely adjusted to make possible the development of life."[8]
If, for example, the strong nuclear force were 2% stronger than it is (i.e., if the coupling constant representing its strength were 2% larger), while the other constants were left unchanged, diprotons would be stable and hydrogen would fuse into them instead of deuterium and helium.[9] This would drastically alter the physics of stars, and presumably preclude the existence of life similar to what we observe on Earth. The existence of the di-proton would short circuit the slow fusion of hydrogen into deuterium. Hydrogen would fuse so easily that it is likely that all of the universe's hydrogen would be consumed in the first few minutes after the big bang. [9] However, some of the fundamental constants describe the properties of the unstable strange, charmed, bottom and top quarks and mu and tau leptons which seem to play little part in the universe or the structure of matter.
The precise formulation of the idea is made difficult by the fact that physicists do not yet know how many independent physical constants there are. The current standard model of particle physics has 25 freely adjustable parameters (there is an additional parameter for gravitation, the cosmological constant). However, because the standard model is not mathematically self-consistent under certain conditions (e.g., at very high energies, at which both quantum mechanics and general relativity are relevant), physicists believe that it is underlaid by some other theory, such as a grand unified theory, string theory, or loop quantum gravity. In some candidate theories, the actual number of independent physical constants may be as small as 1. For example, the cosmological constant may be a fundamental constant, but attempts have also been made to calculate it from other constants, and according to the author of one such calculation, "the small value of the cosmological constant is telling us that a remarkably precise and totally unexpected relation exists among all the parameters of the Standard Model of particle physics, the bare cosmological constant and unknown physics."[10]
Martin Rees[11] formulates the fine-tuning of the universe in terms of the following six dimensionless constants:
Computer simulations suggest that not all of the purportedly "fine-tuned" parameters may be as fine-tuned as has been claimed. Victor Stenger has simulated different universes in which four fundamental parameters are varied. He found that long-lived stars could exist over a wide parameter range, and concluded that "... a wide variation of constants of physics leads to universes that are long-lived enough for life to evolve, although human life need not exist in such universes".[12]
Fred Adams has done a similar study to Stenger, investigating the structure of stars in universes with different values of the gravitational constant G, the fine-structure constant α, and a nuclear reaction rate parameter C. His study suggests that roughly 25% of this parameter space allows stars to exist.[13] Harnik, Kribs and Perez have argued for the viability of a universe with no weak interaction at all. However, they noted that their analysis does not extend to the supposed fine tuning of the cosmological constant, and concluded that "the fine-tuning problems associated with the electroweak breaking scale and the cosmological constant appear to be qualitatively different from the perspective of obtaining a habitable universe."[14]
The validity of fine tuning examples is sometimes questioned on the grounds that such reasoning is subjective anthropomorphism applied to natural physical constants. Critics also suggest that the fine-tuned universe assertion and the anthropic principle are essentially tautologies.[15] The fine-tuned universe argument has also been criticized as an argument by lack of imagination because it assumes no other forms of life, sometimes referred to as carbon chauvinism. Conceptually, alternative biochemistry or other forms of life are possible.[16] In addition, critics argue that humans are adapted to the universe through the process of evolution, rather than the universe being adapted to humans (see puddle thinking). They also see it as an example of the logical flaw of hubris or anthropocentrism in its assertion that humans are the purpose of the universe.[17]
There are fine tuning arguments that are naturalistic.[18] As modern cosmology developed, various hypotheses have been proposed. One is an oscillatory universe or a multiverse where fundamental physical constants are postulated to resolve themselves to random values in different iterations of reality.[19] Under this hypothesis, separate parts of reality would have wildly different characteristics. In such scenarios the issue of fine-tuning does not arise at all, as only those "universes" with constants hospitable to life (such as what we observe) would develop life capable of contemplating the question of the origin of fine-tuning.
Based upon the Anthropic principle, physicist Robert H. Dicke proposed the "Dicke coincidence" argument that the structure (age, physical constants, etc.) of the universe as seen by living observers is not random, but is constrained by biological factors that require it to be roughly a "golden age".[20]
The Multiverse hypothesis assumes the existence of many universes with different physical constants, some of which are hospitable to intelligent life (see multiverse: anthropic principle). Because we are intelligent beings, we are by definition in a hospitable one. Mathematician Michael Ikeda and astronomer William H. Jefferys have argued that the anthropic principle resolves the entire issue of fine-tuning,[21][22] as does philosopher of science Elliott Sober.[23] Philosopher and theologian Richard Swinburne reaches the opposite conclusion using Bayesian probability.[24]
This approach has led to considerable research into the anthropic principle and has been of particular interest to particle physicists because theories of everything do apparently generate large numbers of universes in which the physical constants vary widely. As of yet, there is no evidence for the existence of a multiverse, but some versions of the theory do make predictions which some researchers studying M-theory and gravity leaks hope to see some evidence of soon.[25] Some multiverse theories are not falsifiable, thus scientists may be reluctant to call any multiverse theory "scientific". UNC-Chapel Hill professor Laura Mersini-Houghton claims that the WMAP cold spot may provide testable empirical evidence for a parallel universe.
Variants on this approach include Lee Smolin's notion of cosmological natural selection, the Ekpyrotic universe, and the Bubble universe theory.
Critics of the multiverse-related explanations argue that there is no evidence that other universes exist.
The bubble universe model by physicist Andrei Linde, postulates that our universe is one of many that grew from a multiverse consisting of vacuum that had not yet decayed to its ground state.
According to this scenario, by means of a random quantum fluctuation the universe "tunneled" from pure vacuum ("nothing") to what is called a false vacuum, a region of space that contains no matter or radiation but is not quite "nothing." The space inside this bubble of false vacuum was curved, or warped. A small amount of energy was contained in that curvature, somewhat like the energy stored in a strung bow. This ostensible violation of energy conservation is allowed by the Heisenberg uncertainty principle for sufficiently small time intervals. The bubble then inflated exponentially and the universe grew by many orders of magnitude in a tiny fraction of a second. (For a not-too-technical discussion, see Stenger 1990[26]). As the bubble expanded, its curvature energy was converted into matter and radiation, inflation stopped, and the more linear big bang expansion we now experience commenced. The universe cooled and its structure spontaneously froze out, as formless water vapor freezes into snowflakes whose unique patterns arise from a combination of symmetry and randomness.—Victor J. Stenger, The Anthropic Coincidences[27]
In standard inflation, inflationary expansion occurred while the universe was in a false vacuum state, halting when the universe decayed to a true vacuum state. The bubble universe model proposes that different parts of this inflationary universe (termed a Multiverse) decayed at different times, with decaying regions corresponding to universes not in causal contact with each other. It further supposes that each bubble universe may have different physical constants.
Stephen Hawking, along with Thomas Hertog of CERN, proposed that the universe's initial conditions consisted of a superposition of many possible initial conditions, only a small fraction of which contributed to the conditions we see today.[28] According to their theory, it is inevitable that we find our universe's "fine-tuned" physical constants, as the current universe "selects" only those past histories that led to the present conditions. In this way, top-down cosmology provides an anthropic explanation for why we find ourselves in a universe that allows matter and life, without invoking the existence of the multiverse.
One hypothesis is that the Universe may have been designed by extra-universal aliens. Some believe this would solve the problem of how a designer or design team capable of fine-tuning the Universe could come to exist. Cosmologist Alan Guth believes humans will in time be able to generate new universes. By implication previous intelligent entities may have generated our universe.[29] This idea leads to the possibility that the extraterrestrial designer/designers are themselves the product of an evolutionary process in their own universe, which must therefore itself be able to sustain life.
The Simulation hypothesis promoted by Nick Bostrom and others suggests that our universe may be a computer simulation by aliens.[30]
The Biocosm hypothesis and the Meduso-anthropic principle both suggest that natural selection has made the universe biophilic. The universe enables intelligence because intelligent entities later create new biophilic universes. This is different from the suggestion above that aliens from a universe which is less finely tuned than ours made our universe finely tuned.
The Designer Universe theory of John Gribbin suggests that the universe could have been made deliberately by a member or members of a technologically advanced civilization in another part of the multiverse and that this advanced civilization may have been responsible for causing the big bang.[31]
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As with theistic evolution, some individual scientists, theologians, and philosophers as well as certain religious groups argue that providence or creation are responsible for fine-tuning.
Christian philosopher Alvin Plantinga argues that random chance, applied to a single and sole universe, only raises the question as to why this universe could be so "lucky" as to have precise conditions that support life at least at some place (the Earth) and time (within millions of years of the present).
One reaction to these apparent enormous coincidences is to see them as substantiating the theistic claim that the universe has been created by a personal God and as offering the material for a properly restrained theistic argument—hence the fine-tuning argument. It's as if there are a large number of dials that have to be tuned to within extremely narrow limits for life to be possible in our universe. It is extremely unlikely that this should happen by chance, but much more likely that this should happen if there is such a person as God.—Alvin Plantinga, The Dawkins Confusion; Naturalism ad absurdum[32]
This fine-tuning of the universe is cited[33] by theologian and philosopher William Lane Craig as an evidence for the existence of God or some form of intelligence capable of manipulating (or designing) the basic physics that governs the universe. Craig argues, however, "that the postulate of a divine Designer does not settle for us the religious question."
Proponents of Intelligent design argue that certain features of the universe and of living things are best explained by an intelligent cause, not an undirected process such as natural selection. The fine-tuned universe argument is a central premise or presented as a given in many of the published works of prominent intelligent design proponents, such as William A. Dembski and Michael Behe.
Most religions have some kind of account of the creation of the universe, although they generally differ in detail from the ones listed above. Some of these may be compatible with known scientific facts (Old Earth Creationism, Theistic Evolution, Progressive Creationism). Others are either incompatible with, or indifferent to, scientific understandings (Young Earth Creationism). For example scientist-theologians such as John Polkinghorne emphasize the implications of Anthropic Fine-Tuning within an orthodox Christian framework whilst fully accepting the scientific findings about Evolution and the age of the Universe. This is also the position of the Roman Catholic Church and of most Anglican theologians.[34] The Jewish physicist Gerald Schroeder argues that the apparent discrepancy between the "days" in Genesis and the billions of years in a scientific understanding are due to the differences in frames of reference. Other scientists with similar views are physicist Freeman Dyson and astronomer Owen Gingerich.
Victor Stenger argues that "... The fine-tuning argument and other recent intelligent design arguments are modern versions of God of the gaps reasoning, where a God is deemed necessary whenever science has not fully explained some phenomenon".[12]
The argument from imperfection suggests that if the universe were designed to be fine-tuned for life, it should be the best one possible and that evidence suggests that it is not.[35] In fact, most of the universe is highly hostile to life.
Additionally Stenger argues, "We have no reason to believe that our kind of carbon-based life is all that is possible. Furthermore, modern cosmology indicates that multiple universes may exist with different constants and laws of physics. So, it is not surprising that we live in the one suited for us. The universe is not fine-tuned to life; life is fine-tuned to the universe."[36]
... imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn't it? In fact it fits me staggeringly well, must have been made to have me in it!' This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, it's still frantically hanging on to the notion that everything's going to be all right, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise. I think this may be something we need to be on the watch out for.
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