Resonant gravitational waves in dynamical Chern-Simons-axion gravity

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. We also examine whether or not the amplification of GWs is possible in the present Universe, taking into account the history of the Universe. We find that resonant amplification is difficult to be tested from GW observations in the standard scenario of the axion DM model, in which the axion is the dominant component of DM. However, there is some parameter window in which the resonant amplification of GWs might be observed, if the axion is subdominant component of DM, and the axion cloud formation is delayed until the Hubble rate becomes much smaller than the axion mass.

Automated summary

This paper investigates the effects of dynamical Chern-Simons gravity with the coupling of a scalar field through the Pontryagin density with axion dark matter on gravitational waves, discussing resonant amplification and confirming its occurrence under certain conditions. Amplification of gravitational waves is shown to occur significantly when considering a coherently oscillating axion cloud, with variations in behavior when taking into account random spatial phase distributions of axion oscillations. The possibility of observing resonant amplification of gravitational waves in the present Universe is explored, highlighting potential scenarios where detection may be feasible.