Resonant gravitational waves in dynamical Chern-Simons-axion gravity

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In this paper, we consider dynamical Chern-Simons gravity with the identification of the scalar field coupled though the Pontryagin density with the axion dark matter, and we discuss the effects of the parametric resonance on gravitational waves (GWs).

• When we consider GWs in a coherently oscillating axion cloud, we confirm that significant resonant amplification of GWs occurs in a narrow frequency band, and the amplification is restricted to the late epoch after the passage of the incident waves.
• We also identify the condition that an axion cloud spontaneously emits GWs.
• Once we take into account the randomness of the spatial phase distribution of the axion oscillations, we find that the amplification is suppressed compared with the coherent case, but significant amplification of GWs can still occur.