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 the 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 |pages=777-780 |doi=10.1103/PhysRevLett.75.777}}</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. Roy. 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>
