  Oscillating scalar field condensates induce small amplitude oscillations of
the Hubble parameter which can induce a decay of the condensate due to a
parametric resonance instability [1]. We show that this instability can lead to
the decay of the coherence of the condensate of axion-like particle (ALP)
fields during the radiation phase of standard cosmology for rather generic ALP
parameter values, with possible implications for certain experiments aiming to
search for ALP candidates. As an example, we study the application of this
instability to the QCD axion. We also study the magnitude of the induced
entropy fluctuations.
