Editing Anthropic principle (section)

== Anthropic observations ==
{{Physical cosmology}}{{Intelligent Design}}{{Main|Fine-tuned universe}}
In 1961, [[Robert Dicke]] noted that the [[age of the universe]], as seen by living observers, cannot be random.<ref>{{Cite journal |author-link=Robert Dicke |author=Dicke, R. H.  |journal=Nature |title=Dirac's cosmology and Mach's principle |volume=192 |pages=440–441 |year=1961 |doi=10.1038/192440a0 |bibcode=1961Natur.192..440D |issue=4801 |s2cid=4196678 }}</ref> Instead, biological factors constrain the universe to be more or less in a "golden age", neither too young nor too old.<ref name="Davies-2006">{{Cite book |author-link=Paul Davies |author=Davies, P. |title=The Goldilocks enigma |publisher=Allen Lane |year=2006 |isbn=978-0-7139-9883-2 }}</ref> If the universe was one tenth as old as its present age, there would not have been sufficient time to build up appreciable levels of [[metallicity]] (levels of elements besides [[hydrogen]] and [[helium]]) especially [[carbon]], by [[nucleosynthesis]]. Small rocky planets did not yet exist. If the universe were 10 times older than it actually is, most stars would be too old to remain on the [[main sequence]] and would have turned into [[white dwarf]]s, aside from the dimmest [[red dwarf]]s, and stable planetary systems would have already come to an end. Thus, Dicke explained the coincidence between large dimensionless numbers constructed from the constants of physics and the age of the universe, a coincidence that inspired [[Dirac large numbers hypothesis|Dirac's varying-''G'' theory]].

Dicke later reasoned that the density of matter in the universe must be almost exactly the [[Friedmann equations#Density parameter|critical density]] needed to prevent the [[Big Crunch]] (the "Dicke coincidences" argument). The most recent measurements may suggest that the observed density of [[baryon]]ic matter, and some theoretical predictions of the amount of [[dark matter]], account for about 30% of this critical density, with the rest contributed by a [[cosmological constant]]. [[Steven Weinberg]]<ref>{{Cite journal |author-link=Steven Weinberg |author=Weinberg, S. |title=Anthropic bound on the cosmological constant |journal=Physical Review Letters |volume=59 |pages=2607–2610 |year=1987 |doi=10.1103/PhysRevLett.59.2607 |pmid=10035596 |issue=22 |bibcode=1987PhRvL..59.2607W }}</ref> gave an anthropic explanation for this fact: he noted that the cosmological constant has a remarkably low value, some 120 [[orders of magnitude]] smaller than the value [[particle physics]] predicts (this has been described as the "[[Cosmological constant problem|worst prediction in physics]]").<ref>{{Cite web |url=https://www.newscientist.com/blog/space/2007/02/physicists-debate-nature-of-space-time.html |title=New scientist space blog: Physicists debate the nature of space-time |date=February 20, 2007 |work=New Scientist Blogs: Space |first=Anil |last=Ananthaswamy |url-status=dead |archiveurl=https://web.archive.org/web/20070305195328/https://www.newscientist.com/blog/space/2007/02/physicists-debate-nature-of-space-time.html |archivedate=2007-03-05}}</ref> However, if the cosmological constant were only several orders of magnitude larger than its observed value, the universe would suffer catastrophic [[Cosmic inflation|inflation]], which would preclude the formation of stars, and hence life.

The observed values of the [[dimensionless physical constant]]s (such as the [[fine-structure constant]]) governing the four [[fundamental interaction]]s are balanced as if [[fine-tuned universe|fine-tuned]] to permit the formation of commonly found matter and subsequently the emergence of life.<ref>[http://math.ucr.edu/home/baez/constants.html How many fundamental constants are there? ] [http://math.ucr.edu/home/baez/README.html John Baez, mathematical physicist. U. C. Riverside], April 22, 2011</ref> A slight increase in the [[strong interaction]] (up to 50% for some authors<ref>{{Cite journal |last1=MacDonald |first1=J. |last2=Mullan |first2=D. J. |date=2009-08-12 |title=Big bang nucleosynthesis: The strong nuclear force meets the weak anthropic principle |url=https://link.aps.org/doi/10.1103/PhysRevD.80.043507 |journal=Physical Review D |language=en |volume=80 |issue=4 |page=043507 |doi=10.1103/PhysRevD.80.043507 |issn=1550-7998|arxiv=0904.1807 |bibcode=2009PhRvD..80d3507M |s2cid=119203730 }}</ref>) would bind the [[dineutron]] and the [[diproton]] and convert all hydrogen in the early universe to helium;<ref name=focus>{{Cite web |date=August 2010 |publisher=Focus |last=Couchman |first=D. |title=The strong nuclear force as an example of fine tuning for life |url=http://www.focus.org.uk/strongforce.php |access-date=15 July 2019 }}</ref> likewise, an increase in the [[weak interaction]] also would convert all hydrogen to helium. Water, as well as sufficiently long-lived stable stars, both essential for the emergence of life as it is known, would not exist.<ref name=leslie2>{{Cite book |last=Leslie |first=J. |title=Universes |year=1989 |isbn=0-203-05318-4 |url=https://books.google.com/books?id=dEeEAgAAQBAJ |publisher=Routledge <!--page=-->}}</ref> More generally, small changes in the relative strengths of the four fundamental interactions can greatly affect the universe's age, structure, and capacity for life.