Free Streaming Length of Axion-Like Particle After Oscillon/ I-ball
  Decays
  Axion-like particles (ALPs) are pseudoscalar bosons predicted by string
theory. The ALPs have a shallower potential than a quadratic one, which induces
the instability and can form the solitonic object called oscillon/I-ball.
Although the lifetime of oscillons can be very long for some type of
potentials, they finally decay until the present. We perform the numerical
lattice simulations to investigate the decay process of oscillons and evaluate
the averaged momentum of ALPs emitted from the oscillon decay. It is found
that, if oscillons decay in the early universe, the free-streaming length of
ALPs becomes too long to explain the small-scale observations of the matter
power spectrum. We show that oscillons with long lifetimes can change the
density fluctuations on small scales, which leads to stringent constraints on
the ALP mass and the oscillon lifetime.
