Editing Axion (section)

=== Pre-inflationary scenario ===
If both (a) and (b) are satisfied, [[Inflation (cosmology)|cosmic inflation]] selects one patch of the Universe within which the spontaneous breaking of the PQ symmetry leads to a homogeneous value of the initial value of the axion field. In this "pre-inflationary" scenario, [[topological defect]]s are inflated away and do not contribute to the axion energy density. However, other bounds that come from [[isocurvature mode]]s severely constrain this scenario, which require a relatively low-energy scale of inflation to be viable.<ref>{{cite journal |last1= Crotty |first1=P. |last2=Garcia-Bellido |first2=J. |last3=Lesgourgues |first3=J. |last4=Riazuelo |first4=A. |year=2003 |journal=Physical Review Letters |volume=91 |pages=171301 |title=Bounds on isocurvature perturbations from CMB and LSS data |issue=17 |doi=10.1103/PhysRevLett.91.171301 |pmid=14611330 |bibcode=2003PhRvL..91q1301C |arxiv=astro-ph/0306286 |s2cid=12140847 }}</ref><ref>{{cite journal |last1= Beltran |first1=Maria |last2=Garcia-Bellido |first2=Juan |last3=Lesgourgues |first3=Julien |last4=Liddle |first4=Andrew R. |last5=Slosar |first5= Anze |year=2005 |journal=Physical Review D |volume=71 |issue=6 |pages= 063532 |title=Bayesian model selection and isocurvature perturbations |bibcode=2005PhRvD..71f3532B |doi=10.1103/PhysRevD.71.063532 |arxiv=astro-ph/0501477|s2cid=2220608 }}</ref><ref>{{cite journal |last1= Beltran |first1=Maria |last2=Garcia-Bellido |first2=Juan |last3=Lesgourgues |first3=Julien |year=2007 |journal=Physical Review D |volume=75 |issue=10 |pages= 103507 |title=Isocurvature bounds on axions revisited |doi=10.1103/PhysRevD.75.103507 |bibcode=2007PhRvD..75j3507B |arxiv=hep-ph/0606107 |s2cid=119451896 }}</ref>