Axion production and CMB spectral distortion in cosmological tangled
  magnetic field
  Axion production due to photon-axion mixing in tangled magnetic field(s)
prior to recombination epoch and magnetic field damping can generate cosmic
microwave background (CMB) spectral distortions. In particular, contribution of
both processes to CMB $\mu$ distortion in the case of resonant photon-axion
mixing is studied. Assuming that magnetic field power spectrum is approximated
by a power law $P_B(k)\propto k^n$ with spectral index $n$, it is shown that
for magnetic field cut-off scales $172.5$ pc $\leq \lambda_B\leq 4\times 10^3$
pc, axion contribution to CMB $\mu$ distortion is subdominant in comparison
with magnetic field damping in the cosmological plasma. Using COBE upper limit
on $\mu$ and for magnetic field scale $\lambda_B\simeq 415$ pc, weaker limit in
comparison with other studies on the magnetic field strength ($B_0\leq
8.5\times 10^{-8}$ G) up to a factor 10 for the DFSZ axion model and axion mass
$m_a\geq 2.6\times 10^{-6}$ eV is found. A forecast for the expected
sensitivity of PIXIE/PRISM on $\mu$ is also presented.
