Taming the Axion in the Early Universe

One big challenge for pre-inflation axion models is the so-called isocurvature problem: quantum fluctuations of the light axion field during inflation would leave imprints that conflict with observations — unless inflation happened at an unusually low energy scale.

In a recent work with Priyesh Chakraborty, Junyi Cheng and Matthew Reece, A Step in Flux to Suppress Axion Isocurvature, we propose a new mechanism that helps the axion stay quiet during inflation. The idea is to give the axion a special kind of “topological mass” that’s big during inflation but disappears later — like flipping a cosmic switch. This mass comes from a quantized flux in extra dimension that gets discharged through a phase transition through nucleation of bubbles (yes, like tiny bubbles inside boiled water). This setup not only solves the isocurvature problem in a natural way, but might also connect to deeper ideas in string theory.

Read the paper here: arXiv:2507.12519 or JHEP link