Editing Cosmic inflation (section)

===Magnetic-monopole problem===
{{main| Big Bang#Magnetic monopoles}}

Stable magnetic monopoles are a problem for [[Grand Unified Theory|Grand Unified Theories]], which propose that at high temperatures (such as in the early universe), the [[electromagnetic force]], [[strong nuclear force|strong]], and [[weak nuclear force|weak]] [[nuclear force]]s are not actually fundamental forces but arise due to [[spontaneous symmetry breaking]] from a single [[gauge theory]]. These theories predict a number of heavy, stable particles that have not been observed in nature. The most notorious is the magnetic monopole, a kind of stable, heavy "charge" of magnetic field.<ref>
{{cite journal
 |last='t Hooft |first=Gerard  |author-link=Gerard 't Hooft 
 |year=1974
 |title=Magnetic monopoles in unified gauge theories
 |journal=[[Nuclear Physics B]]
 |volume=79  |issue=2  |pages=276–84
 |bibcode=1974NuPhB..79..276T  |hdl=1874/4686  |doi=10.1016/0550-3213(74)90486-6
 |url=http://dspace.library.uu.nl:8080/handle/1874/4686
|archive-url=https://web.archive.org/web/20241204020241/http://dspace.library.uu.nl:8080/handle/1874/4686
|url-status=dead
|archive-date=4 December 2024
}}

</ref><ref>
{{Cite journal
 |first=Alexander M. |last=Polyakov
 |year=1974
 |title=Particle spectrum in quantum field theory
 |journal=[[JETP Letters]]
 |volume=20 |pages=194–195
 |bibcode=1974JETPL..20..194P
}}
</ref>

Monopoles are predicted to be copiously produced following Grand Unified Theories at high temperature,<ref>
{{Cite journal
 |first1=Alan |last1=Guth  |author-link=Alan Guth
 |first2=S.   |last2=Tye
 |year=1980
 |title= Phase transitions and magnetic monopole production in the very early universe
 |journal=[[Physical Review Letters]]
 |volume=44  |issue=10  |pages=631–635; Erratum ''ibid.'' (1980) '''44''' p&nbsp;963
 |doi=10.1103/PhysRevLett.44.631
 |bibcode=1980PhRvL..44..631G  |osti=1447535
 |url=http://repository.ust.hk/ir/bitstream/1783.1-49322/1/PhysRevLett.44.631.pdf
 |url-status=live
 |archive-url=https://ghostarchive.org/archive/20221009/http://repository.ust.hk/ir/bitstream/1783.1-49322/1/PhysRevLett.44.631.pdf
 |archive-date=2022-10-09
}}
</ref><ref>
{{Cite journal
 |first1=Martin B. |last1=Einhorn
 |last2=Stein      |first2=D.L.
 |last3=Toussaint  |first3=Doug
 |year=1980
 |title=Are grand unified theories compatible with standard cosmology?
 |journal=[[Physical Review D]]
 |volume=21 |issue=12 |pages=3295–3298
 |bibcode=1980PhRvD..21.3295E |doi=10.1103/PhysRevD.21.3295
}}
</ref> and they should have persisted to the present day, to such an extent that they would become the primary constituent of the Universe.<ref>
{{Cite journal
 |first1=Ya.    |last1=Zel'dovich
 |first2=M. Yu. |last2=Khlopov
 |year=1978
 |title=On the concentration of relic monopoles in the universe
 |journal=[[Physics Letters B]]
 |volume=79  |issue=3  |pages=239–41
 |bibcode=1978PhLB...79..239Z  |doi=10.1016/0370-2693(78)90232-0
}}
</ref><ref>
{{cite journal
 |first=John |last=Preskill
 |year=1979
 |title=Cosmological production of superheavy magnetic monopoles |doi=10.1103/PhysRevLett.43.1365
 |journal=[[Physical Review Letters]]
 |volume=43 |issue=19 |pages=1365–1368
 |bibcode=1979PhRvL..43.1365P
 |url=https://authors.library.caltech.edu/6133/1/PREprl79.pdf
 |url-status=live
 |archive-url=https://ghostarchive.org/archive/20221009/https://authors.library.caltech.edu/6133/1/PREprl79.pdf
 |archive-date=2022-10-09 
}}
</ref> Not only is that not the case, but all searches for them have failed, placing stringent limits on the density of relic magnetic monopoles in the Universe.<ref>
{{Cite journal
 |last1=Yao |first1=W.-M.
 |display-authors=etal
 |collaboration=Particle Data Group
 |year=2006
 |title=Review of Particle Physics
 |journal=[[Journal of Physics G]]
 |volume=33 |issue=1 |pages=1–1232
 |arxiv=astro-ph/0601168 |bibcode=2006JPhG...33....1Y
 |doi=10.1088/0954-3899/33/1/001
 |s2cid=262936640
 |url=http://pdg.lbl.gov/
}}
</ref>

A period of inflation that occurs below the temperature where magnetic monopoles can be produced would offer a possible resolution of this problem: Monopoles would be separated from each other as the Universe around them expands, potentially lowering their observed density by many orders of magnitude. Though, as cosmologist [[Martin Rees]] has written,
: "Skeptics about exotic physics might not be hugely impressed by a theoretical argument to explain the absence of particles that are themselves only hypothetical. Preventive medicine can readily seem 100&nbsp;percent effective against a disease that doesn't exist!"<ref>
{{cite book
 |last=Rees |first=Martin  |author-link=Martin Rees
 |year=1998
 |title=Before the Beginning
 |place=New York, NY
 |publisher=Basic Books
 |page=185
 |isbn=0-201-15142-1
}}
</ref>