Editing Flatness problem (section)

===Anthropic principle===
{{main|Anthropic principle}}
One solution to the problem is to invoke the [[anthropic principle]], which states that humans should take into account the conditions necessary for them to exist when speculating about causes of the universe's properties. If two types of universe seem equally likely but only one is suitable for the evolution of [[Sapience|intelligent life]], the anthropic principle suggests that finding ourselves in that universe is no surprise: if the other universe had existed instead, there would be no observers to notice the fact.

The principle can be applied to solve the flatness problem in two somewhat different ways. The first (an application of the 'strong anthropic principle') was suggested by [[C. B. Collins]] and [[Stephen Hawking]],<ref name="Collins Hawking">{{cite journal |bibcode=1973ApJ...180..317C |title=Why is the Universe Isotropic? |last=Collins |first=C. B. |author2=Hawking, S. |journal=Astrophysical Journal |pages=317–334 |volume=180 |date=1973 |doi=10.1086/151965 |doi-access=free }}</ref> who in 1973 considered the existence of an [[multiple universes|infinite number of universes]] such that every possible combination of initial properties was held by some universe. In such a situation, they argued, only those universes with exactly the correct density for forming galaxies and stars would give rise to intelligent observers such as humans: therefore, the fact that we observe Ω to be so close to 1 would be "simply a reflection of our own existence".<ref name="Collins Hawking" />

An alternative approach, which makes use of the 'weak anthropic principle', is to suppose that the universe is infinite in size, but with the density varying in different places (i.e. an [[Homogeneity (physics)|inhomogeneous]] universe). Thus some regions will be over-dense {{nowrap|(Ω > 1)}} and some under-dense  {{nowrap|(Ω < 1)}}. These regions may be extremely far apart - perhaps so far that light has not had time to travel from one to another during the [[age of the universe]] (that is, they lie outside one another's [[Observable universe#Horizons|cosmological horizon]]s). Therefore, each region would behave essentially as a separate universe: if we happened to live in a large patch of almost-critical density we would have no way of knowing of the existence of far-off under- or over-dense patches since no light or other signal has reached us from them. An appeal to the anthropic principle can then be made, arguing that intelligent life would only arise in those patches with Ω very close to 1, and that therefore our living in such a patch is unsurprising.<ref>{{cite book |last=Barrow |first=John D. |author2=Tipler, Frank J. |title=The Anthropic Cosmological Principle |date=1986 |publisher=Clarendon Press |location=Oxford |isbn=978-0-19-851949-2 |page=[https://archive.org/details/anthropiccosmolo00barr_0/page/411 411] |url-access=registration |url=https://archive.org/details/anthropiccosmolo00barr_0/page/411 }}</ref>

This latter argument makes use of a version of the anthropic principle which is 'weaker' in the sense that it requires no speculation on multiple universes, or on the probabilities of various different universes existing instead of the current one. It requires only a single universe which is infinite - or merely large enough that many disconnected patches can form - and that the density varies in different regions (which is certainly the case on smaller scales, giving rise to [[galactic cluster]]s and [[void (astronomy)|voids]]).

However, the anthropic principle has been [[Anthropic principle#Reception and controversies|criticised]] by many scientists.<ref name="Anthropic Explanations">{{cite web | url=http://philsci-archive.pitt.edu/archive/00001658/ | last = Mosterín | first = Jesús | title = Anthropic Explanations in Cosmology | date = 2003 | access-date = 2008-08-01 }}</ref> For example, in 1979 [[Bernard Carr]] and [[Martin Rees]] argued that the principle "is entirely post hoc: it has not yet been used to predict any feature of the Universe."<ref name="Anthropic Explanations" /><ref>{{cite journal|last = Carr |first = Bernard J. |author2=Rees, Martin |title = The anthropic principle and the structure of the physical world |date=April 1979 |journal=Nature |volume=278|issue = 5705 |pages=605–612 |bibcode=1979Natur.278..605C|doi = 10.1038/278605a0|s2cid = 4363262 }}</ref> Others have taken objection to its philosophical basis, with [[Ernan McMullin]] writing in 1994 that "the weak Anthropic principle is trivial ... and the strong Anthropic principle is indefensible." Since many physicists and philosophers of science do not consider the principle to be compatible with the [[scientific method]],<ref name="Anthropic Explanations" /> another explanation for the flatness problem was needed.