We construct an axion model for generating isocurvature fluctuations with blue spectrum, nᵢₛₒ=2--4, which is suggested by recent analyses of admixture of adiabatic and isocurvature perturbations with independent spectral indices, nad≠nᵢₛₒ. The distinctive feature of the model is that the spectrum is blue at large scales while scale invariant at small scales. This is naturally realized by the dynamics of the Peccei-Quinn scalar field.
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Kasuya et al. (2009) studied this question.
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