Editing Cosmic inflation (section)

===Initial conditions===
Some physicists have tried to avoid the initial conditions problem by proposing models for an eternally inflating universe with no origin.<ref>{{Cite journal|last1=Carroll|first1=Sean M.|last2=Chen|first2=Jennifer|date=2005|title=Does inflation provide natural initial conditions for the universe?|journal=[[General Relativity and Gravitation]]|volume=37|issue=10|pages=1671–4|arxiv=gr-qc/0505037|bibcode=2005GReGr..37.1671C|doi=10.1007/s10714-005-0148-2|s2cid=120566514}}</ref><ref>{{cite journal|last1=Aguirre|first1=Anthony|last2=Gratton|first2=Steven|date=2003|title=Inflation without a beginning: A null boundary proposal|journal=[[Physical Review D]]|volume=67|issue=8|page=083515|arxiv=gr-qc/0301042|bibcode=2003PhRvD..67h3515A|doi=10.1103/PhysRevD.67.083515|s2cid=37260723}}</ref><ref>{{cite journal|last1=Aguirre|first1=Anthony|last2=Gratton|first2=Steven|date=2002|title=Steady-State Eternal Inflation|url=https://cds.cern.ch/record/526126|journal=[[Physical Review D]]|volume=65|issue=8|page=083507|arxiv=astro-ph/0111191|bibcode=2002PhRvD..65h3507A|doi=10.1103/PhysRevD.65.083507|s2cid=118974302}}</ref> These models propose that while the Universe, on the largest scales, expands exponentially it was, is and always will be, spatially infinite and has existed, and will exist, forever.

Other proposals attempt to describe the ex nihilo creation of the Universe based on [[quantum cosmology]] and the following inflation. Vilenkin put forth one such scenario.<ref name="vilenkin" /> [[Hartle–Hawking state|Hartle and Hawking]] offered the [[no-boundary proposal]] for the initial creation of the Universe in which inflation comes about naturally.<ref>{{cite journal|last1=Hartle|first1=J.|last2=Hawking|first2=S.|date=1983|title=Wave function of the universe|journal=[[Physical Review D]]|volume=28|issue=12|pages=2960–2975|bibcode=1983PhRvD..28.2960H|doi=10.1103/PhysRevD.28.2960|s2cid=121947045 }}; See also Hawking (1998).</ref><ref name="PHYS-20180502">{{cite web |author=Staff ([[University of Cambridge]]) |title=Taming the multiverse—Stephen Hawking's final theory about the big bang |url=https://phys.org/news/2018-05-multiversestephen-hawking-theory-big.html |date=2 May 2018 |work=[[Phys.org]] |access-date=2 May 2018 }}</ref><ref name="JHEP-20180420">{{cite journal |last1=Hawking |first1=Stephen |last2=Hertog |first2=Thomas |author-link1=Stephen Hawking |author-link2=Thomas Hertog |title=A smooth exit from eternal inflation? |arxiv=1707.07702 |date=20 April 2018 |journal=[[Journal of High Energy Physics]] |volume=2018 |issue=4 |pages=147 |doi=10.1007/JHEP04(2018)147|bibcode=2018JHEP...04..147H |s2cid=13745992 }}</ref>

Guth described the inflationary universe as the "ultimate free lunch":<ref>Hawking (1998), p. 129.</ref><ref>[[wikiquote:Alan Guth|Wikiquote]].</ref> new universes, similar to our own, are continually produced in a vast inflating background. Gravitational interactions, in this case, circumvent (but do not violate) the [[first law of thermodynamics]] ([[energy conservation]]) and the [[second law of thermodynamics]] ([[entropy]] and the [[arrow of time]] problem). However, while there is consensus that this solves the initial conditions problem, some have disputed this, as it is much more likely that the Universe came about by a [[quantum fluctuation]]. [[Don Page (physicist)|Don Page]] was an outspoken critic of inflation because of this anomaly.<ref>{{cite journal|last1=Page|first1=Don N.|date=1983|title=Inflation does not explain time asymmetry|journal=[[Nature (journal)|Nature]]|volume=304|issue=5921|pages=39–41|bibcode=1983Natur.304...39P|doi=10.1038/304039a0|s2cid=4315730}}; see also [[Roger Penrose]]'s book [[The Road to Reality: A Complete Guide to the Laws of the Universe]].</ref> He stressed that the thermodynamic [[arrow of time]] necessitates low [[entropy]] initial conditions, which would be highly unlikely. According to them, rather than solving this problem, the inflation theory aggravates it – the reheating at the end of the inflation era increases entropy, making it necessary for the initial state of the Universe to be even more orderly than in other Big Bang theories with no inflation phase.

Hawking and Page later found ambiguous results when they attempted to compute the probability of inflation in the Hartle–Hawking initial state.<ref>{{Cite journal |last1=Hawking |first1=Stephen W. |last2=Page |first2=Don N. |date=1988 |title=How probable is inflation? |journal=[[Nuclear Physics B]] |volume=298 |issue=4 |pages=789–809 |bibcode=1988NuPhB.298..789H |doi=10.1016/0550-3213(88)90008-9}}</ref> Other authors have argued that, since inflation is eternal, the probability doesn't matter as long as it is not precisely zero: once it starts, inflation perpetuates itself and quickly dominates the Universe.<ref name=Steinhardt2011/><ref name="SteinhardtTurok2007"/>{{rp |223–225}} However, Albrecht and Lorenzo Sorbo argued that the probability of an inflationary cosmos, consistent with today's observations, emerging by a random fluctuation from some pre-existent state is much higher than that of a non-inflationary cosmos. This is because the "seed" amount of non-gravitational energy required for the inflationary cosmos is so much less than that for a non-inflationary alternative, which outweighs any entropic considerations.<ref>{{Cite journal|last1=Albrecht|first1=Andreas|last2=Sorbo|first2=Lorenzo|date=2004|title=Can the universe afford inflation?|journal=[[Physical Review D]]|volume=70|issue=6|pages=063528|arxiv=hep-th/0405270|bibcode=2004PhRvD..70f3528A|doi=10.1103/PhysRevD.70.063528|s2cid=119465499}}</ref>

Another problem that has occasionally been mentioned is the trans-Planckian problem or trans-Planckian effects.<ref>{{Cite journal|last1=Martin|first1=Jerome|last2=Brandenberger|first2=Robert|date=2001|title=The trans-Planckian problem of inflationary cosmology|journal=[[Physical Review D]]|volume=63|issue=12|pages=123501|arxiv=hep-th/0005209|bibcode=2001PhRvD..63l3501M|doi=10.1103/PhysRevD.63.123501|s2cid=119329384}}</ref> Since the energy scale of inflation and the Planck scale are relatively close, some of the quantum fluctuations that have made up the structure in our universe were smaller than the Planck length before inflation. Therefore, there ought to be corrections from Planck-scale physics, in particular the unknown quantum theory of gravity. Some disagreement remains about the magnitude of this effect: about whether it is just on the threshold of detectability or completely undetectable.<ref>{{Cite journal|last1=Martin|first1=Jerome|last2=Ringeval|first2=Christophe|date=2004|title=Superimposed Oscillations in the WMAP Data?|journal=[[Physical Review D]]|volume=69|issue=8|pages=083515|arxiv=astro-ph/0310382|bibcode=2004PhRvD..69h3515M|doi=10.1103/PhysRevD.69.083515|s2cid=118889842}}</ref>