Editing Triple-alpha process (section)

==Improbability and fine-tuning==
{{Main article|Fine-tuned universe}}

Carbon is a necessary component of all known life. <sup>12</sup>C, a stable isotope of carbon, is abundantly produced in stars due to three factors:

# The decay lifetime of a [[Beryllium-8|<sup>8</sup>Be]] nucleus is four orders of magnitude larger than the time for two <sup>4</sup>He nuclei (alpha particles) to scatter.<ref name="uzan 2003">{{cite journal|last1=Uzan|first1=Jean-Philippe|title=The fundamental constants and their variation: observational and theoretical status|journal=Reviews of Modern Physics|date=April 2003|volume=75|issue=2|pages=403–455|doi=10.1103/RevModPhys.75.403|arxiv = hep-ph/0205340 |bibcode = 2003RvMP...75..403U |s2cid=118684485 }}</ref>
# An excited state of the <sup>12</sup>C nucleus exists a little (0.3193&nbsp;MeV) above the energy level of <sup>8</sup>Be + <sup>4</sup>He. This is necessary because the ground state of <sup>12</sup>C is 7.3367&nbsp;MeV below the energy of <sup>8</sup>Be + <sup>4</sup>He; a <sup>8</sup>Be nucleus and a <sup>4</sup>He nucleus cannot reasonably fuse directly into a ground-state <sup>12</sup>C nucleus. However, <sup>8</sup>Be and <sup>4</sup>He use the [[kinetic energy]] of their collision to fuse into the excited <sup>12</sup>C (kinetic energy supplies the additional 0.3193&nbsp;MeV necessary to reach the excited state), which can then transition to its stable ground state. According to one calculation, the energy level of this excited state must be between about 7.3&nbsp;MeV and 7.9&nbsp;MeV to produce sufficient carbon for life to exist, and must be further "fine-tuned" to between 7.596&nbsp;MeV and 7.716&nbsp;MeV in order to produce the abundant level of <sup>12</sup>C observed in nature.<ref>{{cite journal|last1=Livio|first1=M.|last2=Hollowell|first2=D.|last3=Weiss|first3=A.|last4=Truran|first4=J. W.|title=The anthropic significance of the existence of an excited state of <sup>12</sup>C|journal=Nature|date=27 July 1989|volume=340|issue=6231|pages=281–284|doi=10.1038/340281a0|bibcode = 1989Natur.340..281L |s2cid=4273737 }}</ref> The Hoyle state has been measured to be about 7.65&nbsp;MeV above the ground state of <sup>12</sup>C.<ref>{{cite journal |last1=Freer |first1=M. |last2=Fynbo |first2=H. O. U. |title=The Hoyle state in <sup>12</sup>C |url=https://core.ac.uk/download/pdf/185481311.pdf |archive-url=https://web.archive.org/web/20220718214344/https://core.ac.uk/download/pdf/185481311.pdf |archive-date=2022-07-18 |url-status=live |journal=Progress in Particle and Nuclear Physics |date=2014 |volume=78 |pages=1–23 |doi=10.1016/j.ppnp.2014.06.001|bibcode=2014PrPNP..78....1F |s2cid=55187000 }}</ref>
# In the reaction <sup>12</sup>C + <sup>4</sup>He →  <sup>16</sup>O, there is an excited state of oxygen which, if it were slightly higher, would provide a resonance and speed up the reaction. In that case, insufficient carbon would exist in nature; almost all of it would have converted to oxygen.<ref name="uzan 2003"/>

Some scholars argue the 7.656 MeV Hoyle resonance, in particular, is unlikely to be the product of mere chance. [[Fred Hoyle]] argued in 1982 that the Hoyle resonance was evidence of a "superintellect";<ref name=Kragh/> [[Leonard Susskind]] in ''[[The Cosmic Landscape]]'' rejects Hoyle's [[intelligent design]] argument.<ref>{{cite journal|last1=Peacock|first1=John|title=A Universe Tuned for Life|journal=American Scientist|volume=94|issue=2|pages=168–170|jstor=27858743|year=2006|doi=10.1511/2006.58.168}}</ref> Instead, some scientists believe that different universes, portions of a vast "[[multiverse]]", have different fundamental constants:<ref>{{cite news|title=Stars burning strangely make life in the multiverse more likely|url=https://www.newscientist.com/article/2104223-stars-burning-strangely-make-life-in-the-multiverse-more-likely/|access-date=15 January 2017|work=[[New Scientist]]|date=1 September 2016}}</ref> according to this controversial [[Fine-tuned universe|fine-tuning]] hypothesis, life can only evolve in the minority of universes where the fundamental constants happen to be fine-tuned to support the existence of life. Other scientists reject the hypothesis of the multiverse on account of the lack of independent evidence.<ref>{{cite journal | last1 = Barnes | first1 = Luke A | year = 2012 | title = The fine-tuning of the universe for intelligent life | journal = Publications of the Astronomical Society of Australia | volume = 29 | issue = 4| pages = 529–564 | doi = 10.1071/as12015 | arxiv = 1112.4647 | bibcode = 2012PASA...29..529B | doi-access = free }}</ref>