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{{Short description|A phase transition for the whole universe}}
A '''cosmological phase transition''' is
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.
== Character ==
The [[Standard Model]] of particle physics, parameterized by values measured in laboratories, can be used to predict the nature of cosmic phase transitions.<ref name=Manzudar-2019>{{Cite journal |last=Mazumdar |first=Anupam |last2=White |first2=Graham |date=2019-06-25 |title=Review of cosmic phase transitions: their significance and experimental signatures |journal=Reports on Progress in Physics |volume=82 |issue=7 |pages=076901 |doi=10.1088/1361-6633/ab1f55 |issn=0034-4885|arxiv=1811.01948 }}</ref> A system in the ground state at a high temperature changes as the temperature drops due to expansion of the universe. A new ground state may become favorable and a transition between the states is a phase transition.<ref name=Manzudar-2019/>{{rp|9}}
A phase transition can be related to a difference in symmetry between the two states. For example liquid is isotropic but solid water, [[ice]], has directions with different properties. The two states have different energy: ice has less energy than liquid water.
A system like an iron bar being cooled below its [[Curie temperature]] can have two states at the same lower energy with electron magnetic moments aligned in opposite directions. Above the Curie temperature the bar is not magnetic corresponding to isotropic moments; below its magnetic properties have two different values corresponding to inversion symmetry. The process is called [[spontaneous symmetry breaking]].<ref name="Chow-2008">{{Cite book |last=Chow |first=Tai L. |url=https://www.worldcat.org/title/166358163 |title=Gravity, black holes, and the very early universe: an introduction to general relativity and cosmology |date=2008 |publisher=Springer |isbn=978-0-387-73629-7 |___location=New York |oclc=166358163}}</ref>{{rp|178}}
=== Transition order ===
Phase transitions can be categorised by their [[Phase Transition#Classifications|order]]. Transitions which are first order proceed via [[False_vacuum_decay#Bubble_nucleation|bubble nucleation]] and release [[latent heat]] as the bubbles expand.
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=
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Second order transitions are continuous rather than abrupt and are less likely to leave observable imprints cosmic structures.<ref name=Manzudar-2019/>
The [[Standard Model]] of particle physics contains three [[fundamental force]]s, the [[electromagnetic force]], the [[weak force]] and the [[strong force]]. Shortly after the Big Bang, the extremely high temperatures may have modified the character of these forces. While these three forces act differently today, it has been conjectured that they may have been unified in the high temperatures of the early universe.<ref name="georgi-glashow">{{cite journal |last1=Georgi |first1=H. |last2=Glashow |first2=S. L. |title=Unity of All Elementary Forces |journal=Phys. Rev. Lett. |date=1974 |volume=32 |pages=438–441 |doi=10.1103/PhysRevLett.32.438}}</ref><ref name="weinberg-gauge">{{cite journal |last1=Weinberg |first1=Steven |title=Gauge and Global Symmetries at High Temperature |journal=Phys. Rev. D |date=1974 |volume=9 |issue=12 |pages=3357–3378|doi=10.1103/PhysRevD.9.3357 |bibcode=1974PhRvD...9.3357W }}</ref>
===
{{for|particle physics|QCD matter#Phase_diagram}}
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 took place at a temperature of approximately 155 [[MeV]], and is 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 }}</ref>▼
[[File:QCD phase diagram.png|thumb|300px|right|Conjectured form of the [[QCD matter#Phase_diagram| phase diagram of QCD matter]], with temperature on the vertical axis and quark [[chemical potential]] on the horizontal axis, both in mega-[[electron volt]]s.<ref name='RMP'>{{cite journal|author1=Alford, Mark G.|author2=Schmitt, Andreas|author3=Rajagopal, Krishna|author4=Schäfer, Thomas|title=Color superconductivity in dense quark matter|arxiv=0709.4635 |journal=Reviews of Modern Physics |volume=80|issue=4 |pages=1455–1515 |year=2008|doi=10.1103/RevModPhys.80.1455|bibcode=2008RvMP...80.1455A|s2cid=14117263}}</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=266–270 |doi=10.1016/j.ppnp.2011.01.017|arxiv=1012.3342|bibcode=2011PrPNP..66..266B |s2cid=118745752 }}</ref><ref name="Schwarz2009">{{cite journal |last1=Schwarz |first1=Dominik J. |last2=Stuke |first2=Maik |title=Lepton asymmetry and the cosmic QCD transition |journal=JCAP |date=2009 |volume=2009 |issue=11 |pages=025 |doi=10.1088/1475-7516/2009/11/025|arxiv=0906.3434|bibcode=2009JCAP...11..025S |s2cid=250761613 }}</ref><ref name="Cao2023">{{cite journal |last1=Cao |first1=Gaoging |title=First-order QCD transition in a primordial magnetic field |journal=Phys. Rev. D |date=2023 |volume=107 |issue=1 |pages=014021 |doi=10.1103/PhysRevD.107.014021|arxiv=2210.09794
|bibcode=2023PhRvD.107a4021C |s2cid=252967896 }}</ref>
===Electroweak phase transition===
The electroweak phase transition marks the moment when the [[Higgs mechanism]] breaks the <math>SU(2)\otimes U(1)</math> symmetry of the Standard model.<ref name=Peacock-1998/>{{rp|305}}
Lattice studies of the electroweak model have found the transition to be a smooth crossover, taking place at a temperature of {{nobr| 159.5 ± 1.5 [[GeV]].}}<ref name=donofrio-rummukainen>
{{cite journal
}}
</ref>
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>
▲===Phase transitions beyond the Standard Model===
{{cite journal
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" />▼
|first1=James |last1=Cline
|first2=Kimmo |last2=Kainulainen
|year=2013
|title=Electroweak baryogenesis and dark matter from a singlet Higgs
|journal=[[Journal of Cosmology and Astroparticle Physics]]
|volume=01 |issue=1 |page=012
|doi=10.1088/1475-7516/2013/01/012
|arxiv=1210.4196 |s2cid=250739526
|bibcode=2013JCAP...01..012C
}}
</ref> The [[electroweak baryogenesis]] model may explain the [[baryon asymmetry]] in the universe, the observation that the amount of matter vastly exceeds the amount of antimatter.<ref name=Manzudar-2019/>
==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" /> A GUT transition that breaks this hypothetical unified state into the Standard model's <math>SU(3)\otimes SU(2)\otimes U(1)</math> symmetry may be responsible for the observed excess of matter over antimatter.<ref name=Peacock-1998/>{{rp|305}} 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>
== Observational consequences ==
Among the ways that cosmological phase transitions can have measurable consequences are the production of primordial [[gravitational waves]] and the prediction of the baryon asymmetry. Adequate confirmation has not yet been achieved.<ref name=Manzudar-2019/>
==See also==
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==References==
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{{Big Bang timeline}}
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