Editing Fine-tuned universe (section)

== Motivation ==
Physicist [[Paul Davies]] said: "There is now broad agreement among physicists and cosmologists that the Universe is in several respects 'fine-tuned' for life. But 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".<ref>Smith, W. S., Smith, J. S., & Verducci, D., eds., ''Eco-Phenomenology: Life, Human Life, Post-Human Life in the Harmony of the Cosmos'' (Berlin/Heidelberg: Springer, 2018), [https://books.google.com/books?id=4-piDwAAQBAJ&pg=PA131&redir_esc=y#v=onepage&q&f=false pp. 131–32].</ref> He also said that {{" '}}[[Anthropic principle|anthropic]]' reasoning fails to distinguish between minimally [[Biophilia hypothesis|biophilic]] universes, in which life is permitted, but only marginally possible, and optimally biophilic universes, in which life flourishes because [[biogenesis]] occurs frequently".<ref name=Davies200>{{cite journal |author1=Davies |title=How bio-friendly is the universe |date=2003 |volume=2 |issue=115 |journal=Int. J. Astrobiol. |arxiv=astro-ph/0403050 |doi=10.1017/S1473550403001514 |page=115|bibcode = 2003IJAsB...2..115D |s2cid=13282341 }}</ref> Among scientists who find the evidence persuasive, a variety of [[Naturalism (philosophy)|natural explanations]] have been proposed, such as the existence of [[multiverse|multiple universes]] introducing a [[survivorship bias]] under the [[anthropic principle]].<ref name=stanford_encylopedia/>

The premise of the fine-tuned universe assertion is that a small change in several of the physical constants would make the universe radically different. [[Stephen Hawking]] observed: "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".<ref name=":2">[[Stephen Hawking]], 1988. ''A Brief History of Time,'' Bantam Books, {{ISBN|0-553-05340-X}}, pp. 7, 125.</ref>

For example, if 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, [[diproton]]s would be stable; according to Davies, hydrogen would [[hydrogen fusion|fuse]] into them instead of [[deuterium]] and [[helium]].<ref name=Davies1993>Paul Davies, 1993. ''The Accidental Universe'', Cambridge University Press, [https://books.google.com/books?id=s2s4AAAAIAAJ&lpg=PP1&hl=cs&pg=PA70&redir_esc=y#v=onepage&q&f=false pp. 70–71]</ref> This would drastically alter the physics of [[star]]s, and presumably preclude the existence of life similar to what we observe on Earth. The diproton's existence would short-circuit the slow fusion of hydrogen into deuterium. Hydrogen would fuse so easily that it is likely that all the universe's hydrogen would be consumed in the first few minutes after the [[Big Bang]].<ref name=Davies1993/> This "diproton argument" is disputed by other physicists, who calculate that as long as the increase in strength is less than 50%, stellar fusion could occur despite the existence of stable diprotons.<ref name =macdonald>{{cite journal | last1 = MacDonald | first1 = J. | last2 = Mullan | first2 = D. J. | year = 2009 | title = Big Bang nucleosynthesis: The strong nuclear force meets the weak anthropic principle | journal = [[Physical Review D]] | volume = 80 | issue = 4| page = 043507 |quote=Contrary to a common argument that a small increase in the strength of the strong force would lead to destruction of all hydrogen in the Big Bang due to binding of the diproton and the dineutron with a catastrophic impact on life as we know it, we show that provided the increase in strong force coupling constant is less than about 50% substantial amounts of hydrogen remain. | doi=10.1103/physrevd.80.043507| arxiv = 0904.1807 | bibcode = 2009PhRvD..80d3507M | s2cid = 119203730 }}</ref>

The precise formulation of the idea is made difficult by the fact that it is not yet known how many independent physical constants there are. The [[standard model of particle physics]] has 25 freely adjustable parameters and [[general relativity]] has one more, the [[cosmological constant]], which is [[Accelerating expansion of the universe|known to be nonzero]] but profoundly small in value. Because physicists have not developed an empirically successful theory of [[quantum gravity]], there is no known way to combine quantum mechanics, on which the standard model depends, and general relativity.<ref name="The Mystery of the Cosmological Con">
{{cite journal
 | last1 = Abbott | first1 = Larry
 | authorlink = Larry Abbott
 | date = May 1988
 | title = The Mystery of the Cosmological Constant | journal = [[Scientific American]]
 | volume=258
 | issue = 5
 | pages=106–13
 | doi=10.1038/scientificamerican0588-106
 | bibcode = 1988SciAm.258e.106A
}}</ref>

Without knowledge of this more complete theory suspected to underlie the standard model, it is impossible to definitively count the number of truly independent physical constants. In some candidate theories, the number of independent physical constants may be as small as one. 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".<ref name="The Mystery of the Cosmological Con"/>