Induced Gravitational Waves from Multi-Sound Speed Resonances during Cosmological Inflation

TL;DR

This paper investigates how multi-sound speed resonances during inflation can generate distinctive gravitational wave signatures, revealing potential observable effects in stochastic GW backgrounds from the early universe.

Contribution

It introduces a model with two sinusoidal sound speed oscillations during inflation, analyzing their impact on primordial spectra and GW signals, extending beyond single oscillation scenarios.

Findings

Primordial density spectrum shows two groups of amplified peaks.

Energy spectrum of secondary GWs has a broad peak from inflation and one/two peaks from radiation era.

GW signals from these resonances are testable with current and future GW detectors.

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