Forecasting Constraints on SIGW with Future Pulsar Timing Array Observations

Abstract: Pulsar Timing Arrays are playing a crucial role in the ongoing gravitational wave astronomy revolution. The current evidence for a stochastic gravitational wave background (SGWB) at nHz frequencies offers an opportunity to discover cosmological signals and threatens the observability of other subdominant GWs. We explore prospects to constrain second-order scalar-induced GWs (SIGWs) associated with enhanced curvature perturbations in the primordial universe, forecasting realistic future PTA datasets. We assess how the currently observed signal could eventually limit future capabilities to search for GW relics of primordial phenomena and associated phenomenological consequences such as primordial black hole (PBH) formation. Given the sensitivity of PBH abundance to spectral parameters, measuring it remains a challenge for realistic signals. However, future observation could still rule out nearly subsolar mass PBHs formed through standard formation scenarios in some cases. Future progress in constraining PBH models is expected to stem from theoretical advancements in PBH computations, which should help resolve the tension between different computational methods. The analysis is based on and extends the Python code $\texttt{fastPTA}$.