Editing Cosmic inflation (section)

===Eternal inflation===
{{Main|Eternal inflation}}

In many models, the inflationary phase of the Universe's expansion lasts forever in at least some regions of the Universe. This occurs because inflating regions expand very rapidly, reproducing themselves. Unless the rate of decay to the non-inflating phase is sufficiently fast, new inflating regions are produced more rapidly than non-inflating regions. In such models, most of the volume of the Universe is continuously inflating at any given time.

All models of eternal inflation produce an infinite, hypothetical multiverse, typically a fractal. The multiverse theory has created significant dissension in the scientific community about the viability of the inflationary model.

[[Paul Steinhardt]], one of the original architects of the inflationary model, introduced the first example of eternal inflation in 1983.<ref name="steinhardt" >{{cite book |editor1-last=Gibbons |editor1-first= Gary W. |editor1-link= Gary Gibbons |editor2-first=Stephen W. |editor2-last=Hawking |editor3-last=Siklos |editor3-first= S.T.C. |title="Natural Inflation," in The Very Early Universe |publisher=Cambridge University Press |pages= 251–66 |year=1983 |isbn=978-0-521-31677-4}}</ref> He showed that the inflation could proceed forever by producing bubbles of non-inflating space filled with hot matter and radiation surrounded by empty space that continues to inflate. The bubbles could not grow fast enough to keep up with the inflation. Later that same year, [[Alexander Vilenkin]] showed that eternal inflation is generic.<ref name="vilenkin">{{cite journal|last1=Vilenkin|first1=Alexander|year=1983|title=Birth of Inflationary Universes|journal=[[Physical Review D]]|volume=27|issue=12|pages=2848–2855|bibcode=1983PhRvD..27.2848V|doi=10.1103/PhysRevD.27.2848}}</ref>

Although new inflation is classically rolling down the potential, quantum fluctuations can sometimes lift it to previous levels. These regions in which the inflaton fluctuates upwards, expand much faster than regions in which the inflaton has a lower potential energy, and tend to dominate in terms of physical volume. It has been shown that any inflationary theory with an unbounded potential is eternal. There are well-known theorems that this steady state cannot continue forever into the past. Inflationary spacetime, which is similar to de Sitter space, is incomplete without a contracting region. However, unlike de Sitter space, fluctuations in a contracting inflationary space collapse to form a gravitational singularity, a point where densities become infinite. Therefore, it is necessary to have a theory for the Universe's initial conditions.

In eternal inflation, regions with inflation have an exponentially growing volume, while regions that are not inflating do not. This suggests that the volume of the inflating part of the Universe in the global picture is always unimaginably larger than the part that has stopped inflating, even though inflation eventually ends as seen by any single pre-inflationary observer. Scientists disagree about how to assign a probability distribution to this hypothetical anthropic landscape. If the probability of different regions is counted by volume, one should expect that inflation will never end or applying boundary conditions that a local observer exists to observe it, that inflation will end as late as possible.

Some physicists believe this paradox can be resolved by weighting observers by their pre-inflationary volume. Others believe that there is no resolution to the paradox and that the [[multiverse]] is a critical flaw in the inflationary paradigm. Paul Steinhardt, who first introduced the eternal inflationary model,<ref name="steinhardt"/> later became one of its most vocal critics for this reason.<ref name="Inflation Debate">
{{cite magazine
 |author=Steinhardt, Paul J.
 |date=April 2011
 |title=Inflation Debate: Is the theory at the heart of modern cosmology deeply flawed?
 |url=http://www.physics.princeton.edu/~steinh/0411036.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.physics.princeton.edu/~steinh/0411036.pdf |archive-date=2022-10-09 |url-status=live
 |magazine=[[Scientific American]]
 |volume=304 |issue=4 |pages=36–43
 |bibcode=2011SciAm.304d..36S
 |doi=10.1038/scientificamerican0411-36 |pmid=21495480
}}
</ref><ref>{{cite book |last=Steinhardt |first=Paul J. |chapter-url=http://www.physics.princeton.edu/~steinh/vaasrev.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.physics.princeton.edu/~steinh/vaasrev.pdf |archive-date=2022-10-09 |url-status=live |editor-last=Vaas |editor-first=Rudy |publisher=Springer |date=2011 |chapter=The Cyclic Theory of the Universe |title=Beyond the Big Bang: Competing Scenarios For An Eternal Universe |series=The Frontiers Collectuion |type=Unpublished manuscript}}{{Better source needed|reason=Was this book ever published? Can we find something else which was?|date=April 2022}}
</ref><ref>{{cite journal |last1=Ijjas |first1=Anna |last2=Steinhardt |first2=Paul J. |last3=Loeb |first3=Abraham |title=Pop Goes the Universe |journal=Scientific American |date=17 January 2017 |volume=316 |issue=2 |pages=32–39 |doi=10.1038/scientificamerican0217-32 |jstor=26047449 |pmid=28118351 |bibcode=2017SciAm.316b..32I |url=https://www.cfa.harvard.edu/~loeb/sciam3.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.cfa.harvard.edu/~loeb/sciam3.pdf |archive-date=2022-10-09 |url-status=live}}</ref>