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. We find that for the QCD axion, the clump formation does not
occur even if we consider an extremely fine-tuned initial condition. Meanwhile,
it turns out that for an ALP which allows a more general potential form, the
clump formation can take place through the tachyonic instability or/and the
resonance instability, considering a multiple cosine potential.
