Induced gravitational waves from multi-sound speed resonances during cosmological inflation

Abstract We explore the possibility of multi-parametric resonances from time varying sound speed during cosmological inflation. In particular, we fix our set-up to simpler case beyond a single oscillation model already explored in literature: two sinusoidal harmonics around constant equal one. find that, within perturbative regime, except for some certain extreme corners parameter space, primordial density spectrum is characterized by groups amplified peaks centered critical oscillatory frequencies speed. As general result, show that energy secondary induced GWs inflationary era has major broad peak, whereas one radiation dominated phase consists one/two principle peak-like configuration(s) relatively small/large ratio frequencies. The GW relic stochastic backgrounds carry gravitational memory parametric signals double can be tested complementary channels Pulsar-timing radio-astronomy, space and terrestrial interferometers.