Editing Cosmic inflation (section)

==Motivations==
Inflation tries to resolve [[Big Bang#Problems|several problems]] in [[Big Bang]] cosmology that were discovered in the 1970s.<ref>Much of the historical context is explained in {{harvp|Peebles|1993|at=ch&nbsp;15–17}}.</ref> Inflation was first proposed by Alan Guth in 1979 while investigating the problem of why no [[magnetic monopoles]] are seen today; he found that a positive-energy [[false vacuum]] would, according to [[general relativity]], generate an exponential expansion of space. It was quickly realised that such an expansion would resolve many other long-standing problems. These problems arise from the observation that to look like it does ''today'', the Universe would have to have started from very [[fine tuned universe|finely tuned]], or "special" initial conditions at the Big Bang. Inflation attempts to resolve these problems by providing a dynamical mechanism that drives the Universe to this special state, thus making a universe like ours much more likely in the context of the Big Bang theory.

===Horizon problem===
{{Main |Horizon problem}}
The [[horizon problem]] is the problem of determining why the universe appears statistically homogeneous and isotropic in accordance with the [[cosmological principle]].<ref>{{cite journal |title=The isotropy of the universe |doi=10.1088/0264-9381/15/2/008 |first1=Charles W. |last1=Misner |date=1968 |journal=[[Astrophysical Journal]] |volume=151 |issue=2 |pages=431 |last2=Coley |first2=A A |last3=Ellis |first3=G F R |last4=Hancock |first4=M |bibcode=1998CQGra..15..331W |s2cid=250853141 }}</ref><ref name="mtw">{{Cite book |last=Misner |first=Charles |author2=Thorne, Kip S. |author3=Wheeler, John Archibald |name-list-style=amp |title=Gravitation |url=https://archive.org/details/gravitation00whee |url-access=limited |location=San Francisco |publisher=W. H. Freeman |date=1973 |isbn=978-0-7167-0344-0 |pages=[https://archive.org/details/gravitation00whee/page/n521 489]–490, 525–526}}</ref><ref name="weinberg">{{Cite book |first=Steven |last=Weinberg |title=Gravitation and Cosmology |publisher=John Wiley |date=1971 |isbn=978-0-471-92567-5 |pages=[https://archive.org/details/gravitationcosmo00stev_0/page/740 740, 815] |url=https://archive.org/details/gravitationcosmo00stev_0/page/740 }}</ref> For example, molecules in a canister of gas are distributed homogeneously and isotropically because they are in thermal equilibrium: gas throughout the canister has had enough time to interact to dissipate inhomogeneities and anisotropies. The situation is quite different in the big bang model without inflation, because gravitational expansion does not give the early universe enough time to equilibrate. In a big bang with only the [[matter]] and [[radiation]] known in the Standard Model, two widely separated regions of the observable universe cannot have equilibrated because they move apart from each other faster than the [[speed of light]] and thus have never come into [[causal contact]]. In the early Universe, it was not possible to send a light signal between the two regions. Because they have had no interaction, it is difficult to explain why they have the same temperature (are thermally equilibrated). Historically, proposed solutions included the ''Phoenix universe'' of Georges Lemaître,<ref>{{Cite journal |last=Lemaître |first=Georges |title=The expanding universe |journal=Annales de la Société Scientifique de Bruxelles |volume=47A |pages=49 |date=1933}}, English in ''Gen. Rel. Grav.'' '''29''':641–680, 1997.</ref> the related [[oscillatory universe]] of [[Richard Chase Tolman]],<ref>{{Cite book |author=R. C. Tolman |title= Relativity, Thermodynamics, and Cosmology |location=Oxford |publisher=Clarendon Press |date=1934 |isbn=978-0-486-65383-9 |lccn=34032023}} Reissued (1987) New York: Dover {{ISBN|0-486-65383-8}}.</ref> and the [[Mixmaster universe]] of [[Charles Misner]]. Lemaître and Tolman proposed that a universe undergoing a number of cycles of contraction and expansion could come into thermal equilibrium. Their models failed, however, because of the buildup of [[entropy]] over several cycles. Misner made the (ultimately incorrect) conjecture that the Mixmaster mechanism, which made the Universe ''more'' chaotic, could lead to statistical homogeneity and isotropy.<ref name="mtw" /><ref>{{Cite journal |title=Mixmaster universe |doi=10.1088/1751-8113/41/15/155201 |first1=Charles W. |last1=Misner |date=1969 |journal=[[Physical Review Letters]] |volume=22 |issue=15 |pages=1071–74 |last2=Leach |first2=P G L |bibcode=2008JPhA...41o5201A |s2cid=119588491 }}</ref>

===Flatness problem===
{{Main|Flatness problem}}
The [[flatness problem]] is sometimes called one of the [[Robert H. Dicke|Dicke]] coincidences (along with the [[cosmological constant problem]]).<ref>
{{cite book
 |last=Dicke |first=Robert H.
 |title=Gravitation and the Universe
 |location=Philadelphia, PA
 |publisher=American Philosophical Society
 |date=1970
}}
</ref><ref>
{{cite conference
 |last1=Dicke   |first1=Robert H.
 |last2=Peebles |first2=P.J.E.
 |year=1979
 |title=The big bang cosmology – enigmas and nostrums
 |editor=Hawking, S.W.
 |editor2=Israel, W.
 |book-title=General Relativity: An Einstein centenary survey
 |publisher=Cambridge University Press
}}
</ref> It became known in the 1960s that the density of matter in the Universe was comparable to the [[Critical density (cosmology)|critical density]] necessary for a flat universe (that is, a universe whose large-scale [[geometry]] is the usual [[Euclidean geometry]], rather than a [[non-Euclidean geometry|non-Euclidean]] [[hyperbolic geometry|hyperbolic]] or [[spherical geometry]]).<ref name="Lightman1993">
{{cite book
 |first=Alan P. |last=Lightman
 |title=Ancient Light: Our Changing View of the Universe
 |date=1 January 1993
 |publisher=Harvard University Press
 |isbn=978-0-674-03363-4
}}
</ref>{{rp|style=ama|p= 61}}

Therefore, regardless of the [[shape of the universe]], the contribution of spatial curvature to the expansion of the Universe could not be much greater than the contribution of matter. But as the Universe expands, the curvature [[redshift]]s away more slowly than matter and radiation. Extrapolated into the past, this presents a [[Fine-tuning (physics)|fine-tuning]] problem because the contribution of curvature to the Universe must be exponentially small (sixteen orders of magnitude less than the density of radiation at [[Big Bang nucleosynthesis]], for example). Observations of the cosmic microwave background have demonstrated that the Universe is flat to within a few percent.<ref>
{{cite web |title=WMAP – Content of the Universe |url=http://map.gsfc.nasa.gov/universe/uni_matter.html |website=nasa.gov}}
</ref>

===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>