Editing Cosmic inflation (section)

==Criticisms==
Since its introduction by Alan Guth in 1980, the inflationary paradigm has become widely accepted. Nevertheless, many physicists, mathematicians, and philosophers of science have voiced criticisms, claiming untestable predictions and a lack of serious empirical support.<ref name=Steinhardt2011/> In 1999, John Earman and Jesús Mosterín published a thorough critical review of inflationary cosmology, concluding,
: "we do not think that there are, as yet, good grounds for admitting any of the models of inflation into the standard core of cosmology."<ref name="Earman-Mosterín">{{cite journal |last1=Earman |first1= John |first2=Jesús |last2=Mosterín |date=March 1999 |title=A Critical Look at Inflationary Cosmology |journal= Philosophy of Science |volume=66 |issue= 1 |pages= 1–49 |jstor=188736 |doi=10.1086/392675|s2cid= 120393154 }}</ref>

As pointed out by [[Roger Penrose]] from 1986 on, in order to work, inflation requires extremely specific initial conditions of its own, so that the problem (or pseudo-problem) of initial conditions is not solved:
: "There is something fundamentally misconceived about trying to explain the uniformity of the early universe as resulting from a thermalization process. ... For, if the thermalization is actually doing anything ... then it represents a definite increasing of the entropy. Thus, the universe would have been even more special before the thermalization than after."<ref>
{{cite book
 |last=Penrose |first=Roger |author-link=Roger Penrose
 |year=2004
 |title=The Road to Reality: A Complete Guide to the Laws of the Universe
 |place=London, UK
 |publisher=Vintage Books
 |page=755}}<br/>
{{cite journal
 |last=Penrose |first=Roger |author-link=Roger Penrose
 |year=1989
 |title=Difficulties with Inflationary Cosmology
 |journal=[[Annals of the New York Academy of Sciences]]
 |volume=271 |pages=249–264
 |bibcode=1989NYASA.571..249P
 |doi=10.1111/j.1749-6632.1989.tb50513.x |s2cid=122383812
}}
</ref>

The problem of specific or "fine-tuned" initial conditions would not have been solved; it would have gotten worse. At a conference in 2015, Penrose said that
: "inflation isn't falsifiable, it's falsified. ... [[BICEP and Keck Array|BICEP]] did a wonderful service by bringing all the inflation-ists out of their shell, and giving them a black eye."<ref name=Hložek>
{{cite conference
 |last=Hložek |first=Renée
 |date=10–12 June 2015
 |title=CMB@50 day three
 |conference=Cosmic Microwave Background @50
 |place=Princeton, NJ
 |url=http://physics.princeton.edu/cmb50/home.shtml
 |access-date=15 July 2015  |url-status=dead
 |archive-url=https://web.archive.org/web/20171219102718/http://physics.princeton.edu/cmb50/home.shtml
 |archive-date=19 December 2017
}} — collated remarks from the third day of the conference.
</ref>

A recurrent criticism of inflation is that the invoked inflaton field does not correspond to any known physical field, and that its [[potential energy]] curve seems to be an ad hoc contrivance to accommodate almost any data obtainable. [[Paul Steinhardt]], one of the founding fathers of inflationary cosmology, calls 'bad inflation' a period of accelerated expansion whose outcome conflicts with observations, and 'good inflation' one compatible with them:
: "Not only is bad inflation more likely than good inflation, but no inflation is more likely than either ... Roger Penrose considered all the possible configurations of the inflaton and gravitational fields. Some of these configurations lead to inflation ... Other configurations lead to a uniform, flat universe directly – without inflation. Obtaining a flat universe is unlikely overall. Penrose's shocking conclusion, though, was that obtaining a flat universe without inflation is much more likely than with inflation – by a factor of [[googolplex|10 to the googol]]{{efn|
A googol is {{10^|100}}, hence Steinhardt<ref name=Steinhardt2011/> is claiming the probability ratio is {{10^|{{10^|100}} }}.
}} power!"<ref name=Steinhardt2011>
{{cite magazine
 |last=Steinhardt |first=Paul J.  |author-link=Paul Steinhardt
 |year=2011
 |title=The inflation debate: Is the theory at the heart of modern cosmology deeply flawed?
 |magazine=[[Scientific American]]
 |volume=304 |issue=4 |pages=18–25
 |doi=10.1038/scientificamerican0411-36
 |pmid=21495480 |bibcode=2011SciAm.304d..36S
}}
</ref><ref name=SteinhardtTurok2007>
{{cite book
 |last1=Steinhardt |first1=Paul J. |author1-link=Paul Steinhardt
 |last2=Turok      |first2=Neil    |author2-link=Neil Turok
 |date=2007
 |title=Endless Universe: Beyond the Big Bang
 |publisher=Broadway Books
 |isbn=978-0-7679-1501-4
}}
</ref>
Together with Anna Ijjas and [[Abraham Loeb]], he wrote articles claiming that the inflationary paradigm is in trouble in view of the data from the [[Planck (spacecraft)|Planck satellite]].<ref name=Steinhardt2013>
{{cite journal
 |first1=Anna    |last1=Ijjas
 |first2=Paul J. |last2=Steinhardt  |author2-link=Paul Steinhardt
 |first3=Abraham |last3=Loeb        |author3-link=Abraham Loeb
 |year=2013
 |title=Inflationary paradigm in trouble after Planck 2013
 |journal=[[Physics Letters B]]
 |volume=723 |issue=4–5 |pages=261–266
 |arxiv=1304.2785 |bibcode=2013PhLB..723..261I
 |doi=10.1016/j.physletb.2013.05.023 |s2cid=14875751
}}
</ref><ref>
{{cite journal
 |first1=Anna    |last1=Ijjas 
 |first2=Paul J. |last2=Steinhardt  |author2-link=Paul Steinhardt
 |first3=Abraham |last3=Loeb        |author3-link=Abraham Loeb
 |year=2014
 |title=Inflationary schism after Planck 2013
 |journal=[[Physics Letters B]]
 |volume=736  |pages=142–146
 |arxiv=1402.6980  |bibcode=2014PhLB..736..142I
 |doi=10.1016/j.physletb.2014.07.012  |s2cid=119096427
}}
</ref>

Counter-arguments were presented by [[Alan Guth]], [[David Kaiser (physicist)|David Kaiser]], and [[Yasunori Nomura]]<ref name=GKN>
{{cite journal
 |last1=Guth   |first1=Alan H.  |author1-link=Alan Guth
 |last2=Kaiser |first2=David I. |author2-link=David Kaiser (physicist) 
 |last3=Nomura |first3=Yasunori
 |year=2014
 |title=Inflationary paradigm after Planck 2013
 |journal=[[Physics Letters B]]
 |volume=733 |pages=112–119
 |arxiv=1312.7619 |bibcode=2014PhLB..733..112G
 |doi=10.1016/j.physletb.2014.03.020 |s2cid=16669993
}}
</ref> and by Linde,<ref>
{{cite conference
 |last=Linde |first=Andrei |author-link=Andrei Linde
 |date=8 July – 2 August 2013
 |title=Inflationary cosmology after Planck 2013
 |at=session&nbsp;C
 |location=Oxford, UK
 |conference=Post-Planck Cosmology: École de physique des Houches
 |publication-date=2015
 |publisher=Ecole d'été de physique théorique / Oxford University Press
 |isbn=978-0-19-872885-6
}}
</ref> saying that
: "cosmic inflation is on a stronger footing than ever before".<ref name=GKN/>