Can axion clumps be formed in a pre-inflationary scenario?

The QCD axion and an axion-like particle (ALP) are compelling candidates of dark matter. For the QCD axion, it is known that when the Peccei-Quinn (PQ) symmetry is spontaneously broken after inflation, the large initial fluctuation can lead to axion clump formation. On the other hand, when the symmetry is already broken during inflation, it has been believed that the axion clump formation does not occur due to the small amplitude of the initial axion fluctuation. We revisit this prevailing understanding, considering both the QCD axion and an ALP. In our analysis, the clump formation was not observed for the QCD axion even if we consider an extremely fine-tuned initial condition. Meanwhile, for an ALP which allows a more general potential form, we show that the clump formation can take place through the tachyonic instability or/and the resonance instability, considering a multiple cosine potential.

Automated summary

The QCD axion and axion-like particle are compelling candidates for dark matter, with different mechanisms proposed for their clump formation. Research indicates that clump formation is not observed for the QCD axion even with extremely fine-tuned initial conditions, while it may occur for ALP through specific potential forms.