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{{Short description|A phase transition for the whole universe}}
A '''cosmological phase transition''' is a physical process, whereby the overall state of matter changes together across the whole universe. The success of the [[Big Bang]] model led researchers to conjecture possible cosmological phase transitions taking place in the very early universe, at a time when it was much hotter and denser than today.<ref>{{cite journal |last1=Guth |first1=Alan H. |last2=Tye |first2=S.H. H. |title=Phase Transitions and Magnetic Monopole Production in the Very Early Universe |journal=Phys. Rev. Lett. |date=1980 |volume=44 |issue=10 |pages=631–635 |doi=10.1103/PhysRevLett.44.631|bibcode=1980PhRvL..44..631G |osti=1447535 }}</ref><ref name="witten-1984">{{cite journal |last1=Witten |first1=Edward |title=Cosmic Separation of Phases |journal=Phys. Rev. D |date=1984 |volume=30 |pages=272–285 |doi=10.1016/0550-3213(81)90182-6|bibcode=1981NuPhB.177..477W }}</ref>
Any cosmological phase transition may have left signals which are observable today, even if it took place in the first moments after the Big Bang, when the universe was [[cosmic microwave background|opaque to light]].<ref>{{cite journal |last1=Kibble |first1=T. W. B. |title=Some implications of a Cosmological Phase Transition |journal=Phys. Rept. |date=1980 |volume=67 |issue=1 |pages=183–199 |doi=10.1016/0370-1573(80)90091-5|bibcode=1980PhR....67..183K }}</ref>
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As the universe cooled after the hot Big Bang, such a phase transition would have released huge amounts of energy, both as heat and as the kinetic energy of growing bubbles. In a strongly first-order phase transition, the bubble walls may even grow at near the [[speed of light]].<ref>{{cite journal |last1=Moore |first1=Guy D. |last2=Prokopec |first2=Tomislav |title=Bubble wall velocity in a first order electroweak phase transition |journal=Phys. Rev. Lett. |date=1995 |volume=75 |issue=5 |pages=777–780 |doi=10.1103/PhysRevLett.75.777|pmid=10060116 |arxiv=hep-ph/9503296 |bibcode=1995PhRvL..75..777M |s2cid=17239930 }}</ref> This, in turn, would lead to the production of a [[gravitational wave background|stochastic background of gravitational waves]].<ref name="witten-1984" /><ref name="hogan-gws">{{cite journal |last1=Hogan |first1=C. J. |title=Gravitational radiation from cosmological phase transitions |journal=Mon. Not. R. Astron. Soc. |date=1986 |volume=218 |issue=4 |pages=629–636 |doi=10.1093/mnras/218.4.629 |url=https://adsabs.harvard.edu/pdf/1986MNRAS.218..629H |access-date=9 August 2023|doi-access=free }}</ref> Experiments such as [[NANOGrav]] and [[Laser Interferometer Space Antenna|LISA]] may be sensitive to this signal.<ref name="nanograv">{{cite journal |last1=NANOGrav |title=The NANOGrav 15 yr Data Set: Search for Signals of New Physics |journal=Astrophys. J. Lett. |date=2023 |volume=951 |issue=1 |pages=L11 |doi=10.3847/2041-8213/acdc91|arxiv=2306.16219 |bibcode=2023ApJ...951L..11A |doi-access=free }}</ref><ref name="lisa-pt">{{cite journal |last1=LISA Cosmology Working Group |title=Science with the space-based interferometer eLISA. II: Gravitational waves from cosmological phase transitions |journal=JCAP |date=2016 |volume=04 |issue=4 |pages=001 |doi=10.1088/1475-7516/2016/04/001|arxiv=1512.06239 |bibcode=2016JCAP...04..001C |s2cid=53333014 }}</ref>
Shown below are two snapshots from simulations of the evolution of a first-order cosmological phase transition.<ref name="weir">{{cite journal |last1=Weir |first1=David |title=Gravitational waves from a first order electroweak phase transition: a brief review |journal=Philos. Trans. R. Soc. Lond. A |date=2018 |volume=376 |issue=2114 |pages=20170126 |doi=10.1098/rsta.2017.0126|pmid=29358351 |pmc=5784032 |arxiv=1705.01783 |bibcode=2018RSPTA.37670126W |doi-access=free }}</ref> Bubbles first nucleate, then expand and collide, eventually converting the universe from one phase to another.
<gallery mode="packed" heights="140">
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===Strong force phase transition===
Today the strong force binds together [[quarks]] into [[protons]] and [[neutrons]], in a phenomenon known as [[color confinement]]. However, at sufficiently high temperatures, protons and neutrons disassociate into free quarks. The strong force phase transition marks the end of the [[quark epoch]]. Studies of this transition based on [[lattice QCD]] have demonstrated that it would have taken place at a temperature of approximately 155 [[MeV]], and would have been a smooth crossover transition.<ref name="aoki-qcd">{{cite journal |last1=Aoki |first1=Y. |last2=Endrodi |first2=G. |last3=Fodor |first3=Z. |last4=Katz |first4=S. D. |last5=Szabo |first5=K. K. |title=The order of the quantum chromodynamics transition predicted by the standard model of particle physics |journal=Nature |date=2006 |volume=443 |issue=7112 |pages=675–678 |doi=10.1038/nature05120|pmid=17035999 |arxiv=hep-lat/0611014 |bibcode=2006Natur.443..675A |s2cid=261693972 }}</ref>
This conclusion assumes the simplest scenario at the time of the transition, and first- or second-order transitions are possible in the presence of a quark, baryon or neutrino [[chemical potential]], or strong magnetic fields.<ref name="Boeckel2011">{{cite journal |last1=Boeckel |first1=Tillman |last2=Schettler |first2=Simon |last3=Schaffner-Bielich |first3=Jurgen |title=The Cosmological QCD Phase Transition Revisited |journal=Prog. Part. Nucl. Phys. |date=2011 |volume=66 |issue=2 |pages=
|bibcode=2023PhRvD.107a4021C |s2cid=252967896 }}</ref>
The different possible phase transition types are summarised by the [[QCD matter#Phase diagram|strong force phase diagram]].
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</ref>
Just as for the strong force, the conclusion that the transition is a crossover assumes the minimal scenario, and is modified by the presence of additional fields or particles. Particle physics models which account for [[dark matter]] or which lead to successful [[baryogenesis]] may predict a strongly first-order electroweak phase transition.<ref name="Cline2013">{{cite journal |last1=Cline |first1=James |last2=Kainulainen |first2=Kimmo |title=Electroweak baryogenesis and dark matter from a singlet Higgs |journal=JCAP |date=2013 |volume=01 |issue=1 |pages=012 |doi=10.1088/1475-7516/2013/01/012|arxiv=1210.4196|bibcode=2013JCAP...01..012C |s2cid=250739526 }}</ref>
===Phase transitions beyond the Standard Model===
If the three forces of the Standard Model are unified in a [[Grand Unified Theory]], then there would have been a cosmological phase transition at even higher temperatures, corresponding to the moment when the forces first separated out.<ref name="georgi-glashow" /><ref name="weinberg-gauge" /> Cosmological phase transitions may also have taken place in a dark or [[hidden sector]], amongst particles and fields that are only very weakly coupled to visible matter.
<ref name="Schwaller2015">{{cite journal |last1=Schwaller |first1=Pedro |title=Gravitational waves from a dark phase transition |journal=Phys. Rev. Lett. |date=2015 |volume=115 |issue=18 |pages=181101 |doi=10.1103/PhysRevLett.115.181101|pmid=26565451 |arxiv=1504.07263 |bibcode=2015PhRvL.115r1101S |doi-access=free }}</ref>
==See also==
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