  Cosmological evolution of axion field in the early universe might be
significantly affected by a thermal friction induced by the axion coupling to
thermalized hidden sector. We examine the effects of such a thermal friction on
axion dark matter density and its perturbation when the thermal friction
dominates over the Hubble friction until when the axion field begins to
oscillate around the potential minimum. We show that in the presence of sizable
thermal friction there can be an exponential decay phase of the axion field
before the oscillation phase, during which the axion energy density is
efficiently dissipated into hidden thermal bath. Consequently, the previously
excluded parameter region due to overclosing relic axion density becomes
cosmologically viable with thermal friction. In particular, a QCD axion much
lighter than $\mu$eV is viable without tuning the initial misalignment angle.
We also find that thermal friction can affect the density perturbation of axion
dark matter in various ways. For instance, it can alleviate the large-scale
isocurvature bound on axion dark matter in the pre-inflationary PQ breaking
scenario, which would make the pre-inflationary axion dark matter compatible
with high scale inflation over a wide range of model parameters. In the
post-inflationary PQ breaking scenario, thermal friction can also significantly
change the scaling behavior of axionic strings, and therefore the typical size
of the resultant axion miniclusters.
