Abstract
The Weak Gravity Conjecture, if valid, rules out simple models of Natural Inflation by restricting their axion decay constant to be sub-Planckian. We revisit stringy attempts to realise Natural Inflation, with a single open string axionic inflaton from D-branes in a warped throat. We show that warping allows the requisite super-Planckian axion decay constant to be achieved consistently with the Weak Gravity Conjecture. However, there is a tension between large axion decay constant and high string scale, where the requisite high string scale is difficult to achieve in all attempts to realise large field inflation using perturbative string theory. We comment on the Generalized Weak Gravity Conjecture in the light of our results.
Highlights
Cosmological inflation stands strong as the leading mechanism to provide the seeds that gave rise to the large structure we observe today in the Universe
The latest results from Planck [1] are in perfect agreement with the simplest inflationary models, driven by the dynamics of a single scalar field rolling down a very flat potential
Planck suppressed corrections to the slow-roll inflaton potential typically become large when the inflaton varies over super-Planckian scales
Summary
This conjecture lacks convincing motivation from black hole physics, the same phenomenon was observed in [9] in several diverse string theoretic setups If valid, it would essentially rule out single field models of inflation with super-Planckian axion decay constants, as instanton corrections would always introduce higher harmonics to the inflation potential: en MP l f einθ (4). Any nonperturbative instanton effects are by construction exponentially suppressed, and would not rule out single field, large field, slow-roll inflation These results do not lead to promising models of large field inflation and observable primordial gravitational waves. This is because in explicit constructions, there is a tension between obtaining large decay constant and high string scale.
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