Cosmological perturbations of extreme axion in the radiation era

Subhorizon perturbations under the extreme initial condition of the axion model are investigated, where initial axion angles start near the potential maximum and the mass oscillation is substantially delayed. This work focuses on a few new features found in the extreme axion model but absent in the free-particle model. A particularly novel new feature is the spectral excess relative to the cold dark matter model in some wave number range, where the excess may be so large that landscapes of high-redshift universe beyond z = 10 can be significantly altered. For axions of particle mass 10(-22) eV, this range of wave number corresponds to first galaxies of few times 10(9)-10(10) M-circle dot. We demonstrate that subhorizon perturbations are accurately described by Mathieu's equation and subject to parametric instability, which explains this novel feature. Actually the axion model is not a special one; perturbations in a wide range of scalar field models can share the similar characteristic.