  We investigate a generic source of stochastic gravitational wave background
due to the parametric resonance of oscillating scalar fields in the early
Universe. By systematically analyzing benchmark models through lattice
simulations and considering a wide range of parameters, we demonstrate that
such a scenario can lead to detectable signals in gravitational wave detectors
over a broad frequency range and potentially address the recent findings by
pulsar timing array experiments. Furthermore, these models naturally yield
ultralight dark matter candidates or dark radiation detectable by cosmic
microwave background observatories.
