Nonlinear resonant oscillation of gravitational potential induced by ultralight axion in $f(R)$ gravity

Understand

We study the ultralight axion dark matter with mass around $10^{-22}$ eV in $f(R)$ gravity which might resolve the dark energy problem.

• In particular, we focus on the fact that the pressure of the axion field oscillating in time produces oscillations of gravitational potentials.
• We show that the oscillation of the gravitational potential is sensitive to the model of gravity.
• Remarkably, we find that the detectability of the oscillation through the gravitational wave detectors can be significantly enhanced due to the nonlinear resonance between the ultralight axion and the scalaron.