Speaker
Description
CMB limits on isocurvature are often taken to exclude high-scale inflation with large QCD axion decay constants in pre-inflationary PQ models. We show this can be avoided via PQ field-space geometry.
With a nonlinear sigma-model kinetic term, the effective decay constant is enhanced during inflation, suppressing isocurvature without extra couplings or PQ breaking. For a hyperbolic field-space geometry, the suppression of isocurvature is exponentially sensitive, and the same geometry also yields a time-dependent axion mass that controls the tilt and running, yielding a blue-tilted spectrum.
This allows $H_{\rm inf}\sim 10^{13}$ GeV with $f_{\rm a}\sim 10^{14-16}$ GeV, reopening previously excluded parameter space. We present ‘observable’ benchmarks and a semi-analytic template that relates the scale-dependence of isocurvature to the geometric lever arm, crucially providing a direct phenomenological probe on PQ field-space geometry.