Kavli Affiliate: Wayne Hu
| First 5 Authors: Keisuke Harigaya, Wayne Hu, Rayne Liu, Huangyu Xiao,
| Summary:
We show that enhancement of the axion relic abundance compared to the
standard misalignment contribution generically leads to the production of
nonzero momentum axion modes, resulting in warm dark matter behavior and
enhanced isocurvature perturbations. It leads to universal constraints on the
axion parameter space that are independent of detailed model assumptions and
cosmological history. For models enhancing relic abundance with gradient axion
modes, observations of the Lyman-$alpha$ forest impose a lower bound on the
axion decay constant, $f_a gtrsim 10^{15} {rm GeV},(10^{-18}{rm eV}/m_a)$,
from the free-streaming effect. For models relying on the delay of coherent
axion oscillations, we obtain a slightly weaker bound, $f_a gtrsim 10^{14}
{rm GeV},(10^{-18}{rm eV}/m_a)$. We make relatively conservative choices to
establish these universal bounds but also provide scaling parameters that can
be calibrated for stronger constraints in concrete models and updated as
observations improve.
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