Planckian axions and the weak gravity conjecture

DOI10.1007/JHEP01(2016)091zbMATH Open1388.83632arXiv1503.07853WikidataQ122911431 ScholiaQ122911431MaRDI QIDQ1638133

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Publication date: 12 June 2018

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Abstract: Several recent works have claimed that the Weak Gravity Conjecture (WGC) excludes super-Planckian displacements of axion fields, and hence large-field axion inflation, in the absence of monodromy. We argue that in theories with $Ngg1$ axions, super-Planckian axion diameters $calD$ are readily allowed by the WGC. We clarify the nontrivial relationship between the kinetic matrix $K$ --- unambiguously defined by its form in a Minkowski-reduced basis --- and the diameter of the axion fundamental domain, emphasizing that in general the diameter is not solely determined by the eigenvalues $f_1^{2}le\mathrm{...}le{f}_N^2$ of $K$: the orientations of the eigenvectors with respect to the identifications imposed by instantons must be incorporated. In particular, even if one were to impose the condition $f_N<M_{pl}$, this would imply neither $calD<M_{pl}$ nor $calD<sqrtN{M}_{pl}$. We then estimate the actions of instantons that fulfill the WGC. The leading instanton action is bounded from below by $SgecalS{M}_{pl}/f_N$, with $calS$ a fixed constant, but in the universal limit $SgtrsimcalSsqrtN{M}_{pl}/f_N$. Thus, having $f_N>M_{pl}$ does not immediately imply the existence of unsuppressed higher harmonic contributions to the potential. Finally, we argue that in effective axion-gravity theories, the zero-form version of the WGC can be satisfied by gravitational instantons that make negligible contributions to the potential.

Full work available at URL: https://arxiv.org/abs/1503.07853

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