Kavli Affiliate: Peter W. Graham
| First 5 Authors: Peter W. Graham, Davide Racco, , ,
| Summary:
The QCD axion has important connections to early universe cosmology. For
example, it is often said that isocurvature limits rule out a combination of
high axion decay constant, $f_a$, and high inflationary Hubble scale, $H_I$.
High scales are theoretically motivated, so it is important to ask how robust
this constraint is. We demonstrate that this constraint is naturally evaded
when the quartic coupling of the complex $U(1)_mathrm{PQ}$-breaking field is
small. In this case, $f_a$ changes from a larger value during inflation to a
smaller value in the later universe, suppressing isocurvature perturbations.
Importantly, we show that in large parts of parameter space this solution is
not jeopardised by overproduction of the axion through parametric resonance.
The isocurvature bounds are thus dependent on UV physics. We have found that,
even for the minimal QCD axion, large parts of UV parameter space at both high
$f_a$ and high $H_I$ are in fact allowed, not ruled out by isocurvature
constraints.
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