Enhancement of small-scale induced gravitational waves from the soliton/oscillon domination

We investigate density fluctuations and scalar-induced gravitational waves (GWs) arising from the production of long-lived solitons and oscillons, which can dominate the early Universe and drive reheating prior to the standard radiation-dominated era. Curvature perturbations are generated not only by the Poisson distribution of these solitons/oscillons but are also amplified during the early matter-dominated phase when the solitons/oscillons dominate. Scalar-induced GWs with characteristic energy spectra emerge from these amplified curvature perturbations, particularly at the sudden transition from matter domination to radiation domination. We analyze this scenario and its GW predictions in detail, focusing on the impact of the initial energy fraction, the lifetime, and the average separation of these solitons/oscillons. This provides a wealth of potential observational targets for various GW detectors. Furthermore, for inflation and preheating models that produce oscillons with large separations, we derive new constraints on these models based on the upper bounds on the effective number of relativistic degrees of freedom, as anticipated from future cosmic microwave background and large-scale structure observations.