Editing Axion (section)

=== Cosmological implications ===
The properties of the axion, such as the axion mass, decay constant, and abundance, all have implications for cosmology.<ref name="peccei2008" />

Inflation theory suggests that if they exist, axions would be created abundantly during the [[Big Bang]].<ref>{{cite journal |last1=Redondo |first1=J. |last2=Raffelt |first2=G. |last3=Viaux Maira |first3=N. |year=2012 |title=Journey at the axion meV mass frontier |journal=[[Journal of Physics: Conference Series]] |volume=375 |issue=2 |page=022004 |doi=10.1088/1742-6596/375/1/022004 |doi-access=free |bibcode=2012JPhCS.375b2004R}}</ref> Because of a unique coupling to the [[instanton]] field of the primordial [[universe]] (the "[[misalignment mechanism]]"), an effective [[dynamical friction]] is created during the acquisition of mass, following [[cosmic inflation]]. This robs all such primordial axions of their kinetic energy.{{citation needed|date=June 2023}}

Ultralight axion (ULA) with {{nowrap|{{mvar|m}} ~ {{val|e=-22|u=eV/c2}}}} is a kind of [[scalar field dark matter]] that seems to solve the small scale problems of CDM. A single ULA with a GUT scale decay constant provides the correct relic density without fine-tuning.<ref>{{cite journal |last=Marsh |first=David J.E. |date=2016 |title=Axion cosmology |journal=Physics Reports |language=en |volume=643 |pages=1–79 |doi=10.1016/j.physrep.2016.06.005 |arxiv=1510.07633|bibcode=2016PhR...643....1M |s2cid=119264863 }}</ref>

Axions would also have stopped interaction with normal matter at a different moment after the [[Big Bang]] than other more massive dark particles.{{why|date=May 2016}} The lingering effects of this difference could perhaps be calculated and observed astronomically.{{citation needed|date=June 2020}}

If axions have low mass, thus preventing other decay modes (since there are no lighter particles to decay into), the low coupling constant thus predicts that the axion is not scattered out of its state despite its small mass so that the universe would be filled with a very cold [[Bose–Einstein condensate]] of primordial axions. Hence, axions could plausibly explain the [[dark matter]] problem of [[physical cosmology]].<ref>{{cite journal |last=Sikivie |first=P. |year=2009 |title=Dark matter axions |journal=International Journal of Modern Physics A |volume=25 |issue=203 |pages=554–563 |arxiv=0909.0949 |doi=10.1142/S0217751X10048846 |bibcode=2010IJMPA..25..554S|s2cid=1058708 }}</ref> Observational studies are underway, but they are not yet sufficiently sensitive to probe the mass regions if they are the solution to the dark matter problem with the fuzzy dark matter region starting to be probed via [[superradiance]].<ref>{{cite journal|last1=Davoudiasl |first1=Hooman |last2=Denton |first2=Peter |year=2019 |title=Ultralight Boson Dark Matter and Event Horizon Telescope Observations of M87 |journal=Physical Review Letters |volume=123 |issue=2 |pages=021102 |doi=10.1103/PhysRevLett.123.021102 |pmid=31386502 |bibcode=2019PhRvL.123b1102D |arxiv=1904.09242 |s2cid=126147949 }}</ref> High mass axions of the kind searched for by Jain and Singh (2007)<ref>{{cite journal |last1=Jain |first1=P. L. |last2=Singh |first2=G. |year=2007 |title=Search for new particles decaying into electron pairs of mass below 100&nbsp;''MeV''/c<sup>2</sup> |journal=Journal of Physics G |volume=34 |issue=1 |pages=129–138 |bibcode=2007JPhG...34..129J |doi=10.1088/0954-3899/34/1/009 |quote=possible early evidence of 7±1 and 19±1&nbsp;MeV axions of less than 10<sup>−13</sup>&nbsp;s lifetime}}</ref> would not persist in the modern universe. Moreover, if axions exist, scatterings with other particles in the thermal bath of the early universe unavoidably produce a population of hot axions.<ref>{{cite journal |last1=Salvio |first1=Alberto |last2=Strumia |first2=Alessandro |last3=Xue |first3=Wei |year=2014 |title=Thermal axion production |journal=Journal of Cosmology and Astroparticle Physics |volume=2014 |issue=1 |pages=11 |url=http://inspirehep.net/record/1262100 |bibcode=2014JCAP...01..011S |doi=10.1088/1475-7516/2014/01/011 |arxiv=1310.6982|s2cid=67775116 }}</ref>

Low mass axions could have additional structure at the galactic scale. If they continuously fall into galaxies from the intergalactic medium, they would be denser in "[[Caustic (mathematics)|caustic]]" rings, just as the stream of water in a continuously flowing fountain is thicker at its peak.<ref>{{cite tech report |first=P. |last=Sikivie |year=1997 |title=Dark matter axions and caustic rings |doi=10.2172/484584 |osti=484584 |s2cid=13840214 }}</ref> The gravitational effects of these rings on galactic structure and rotation might then be observable.<ref>{{cite web |first=P. |last=Sikivie |url=http://www.phys.ufl.edu/~sikivie/triangle/ |title=Pictures of alleged triangular structure in Milky Way }}{{self-published inline|date=January 2024}}</ref><ref name=Duffy2010>{{cite conference |doi=10.1063/1.3489563 |title=The Milky Way's Dark Matter Distribution and Consequences for Axion Detection |series=AIP Conference Proceedings |date=2010 |last1=Duffy |first1=Leanne D. |last2=Tanner |first2=David B. |last3=Van Bibber |first3=Karl A. |conference=Axions 2010 |volume=1274 |pages=85–90 |bibcode=2010AIPC.1274...85D }}</ref> Other cold dark matter theoretical candidates, such as [[Weakly interacting massive particles|WIMP]]s and [[MACHO]]s, could also form such rings, but because such candidates are [[fermion]]ic and thus experience friction or scattering among themselves, the rings would be less sharply defined.{{citation needed|date=June 2023}}

João G. Rosa and Thomas W. Kephart suggested that axion clouds formed around unstable [[primordial black hole]]s might initiate a chain of reactions that radiate electromagnetic waves, allowing their detection. When adjusting the mass of the axions to explain dark matter, the pair discovered that the value would also explain the luminosity and wavelength of [[fast radio burst]]s, being a possible origin for both phenomena.<ref>{{cite journal |last1=Rosa |first1=João G. |last2=Kephart |first2=Thomas W. |year=2018 |title=Stimulated axion decay in superradiant clouds around primordial black holes |journal=Physical Review Letters |volume=120 |issue=23 |pages=231102 |arxiv=1709.06581 |doi=10.1103/PhysRevLett.120.231102 |pmid=29932720 |bibcode=2018PhRvL.120w1102R |s2cid=49382336 }}</ref> In 2022 a similar hypothesis was used to [[#Astrophysical axion searches|constrain]] the mass of the axion from data of M87*.{{citation needed|date=June 2023}}

In 2020, it was proposed that the axion field might actually have influenced the evolution of [[Chronology of the universe|the early Universe]] by creating more imbalance between the amounts of matter and antimatter – which possibly resolves the [[baryon asymmetry]] problem.<ref>{{cite journal |last=Anonymous |date=2020-03-19 |title=Axions Could Explain Baryon Asymmetry |url=https://physics.aps.org/articles/v13/s38 |journal=Physics |language=en |volume=13 |issue=11 |pages=s38 |doi=10.1103/PhysRevLett.124.111602|pmid=32242736 |arxiv=1910.02080 }}</ref>