Abstract
A model of high scale inflation is presented where the radial part of the Peccei-Quinn (PQ) field with a non-minimal coupling to gravity plays the role of the inflaton, and the QCD axion is the dark matter. A quantum fluctuation of $\mathcal{O}(H/2\pi)$ in the axion field will result in a smaller angular fluctuation if the PQ field is sitting at a larger radius during inflation than in the vacuum. This changes the effective axion decay constant, $f_a$, during inflation and dramatically reduces the production of isocurvature modes. This mechanism opens up a new window in parameter space where an axion decay constant in the range $10^{12}\text{ GeV}\lesssim f_a\lesssim 10^{15}\text{ GeV}$ is compatible with observably large $r$. The exact range allowed for $f_a$ depends on the efficiency of reheating. This model also predicts a minimum possible value of $r=10^{-3}$. The new window can be explored by a measurement of $r$ possible with \textsc{Spider} and the proposed CASPEr experiment search for high $f_a$ axions.
Highlights
In the last few years there has been a lot of excitement among inflationary cosmologists
Our mechanism opens up a new window for intermediate-scale axions with 1012 GeV ≲ fa ≲ 1015 GeV to be consistent with observable primordial B-modes, as could be observed, for example, by near future experiments like SPIDER
Axion dark matter (DM) constraints for non-minimal PQ inflation model showing the new window unavailable to other axion models
Summary
In the last few years there has been a lot of excitement among inflationary cosmologists. A constraint on H is of utmost importance because it can be used to rule out different models of inflation and particle cosmology It can have profound consequences for the cosmology of axions [8,9,10,11]. If the energy scale of inflation is high this generates unacceptably large [1] axion isocurvature perturbations if the axion Peccei-Quinn [22] (PQ) scale, fa, is larger than OðHÞ It appears that only situations where the PQ symmetry is restored after inflation are compatible with an observable r. There is still room for 0.01 ≲ r ≲ 0.1 to be observable and consistent with current constraints Such a detection could be made, for example, by SPIDER [33], and the consequences for inflationary cosmology and the axion would still be just as profound [8].2.
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