Initial data construction for Cauchy-characteristic evolution

Cauchy-characteristic evolution (CCE) is a powerful numerical tool for computing gravitational waveforms at future null infinity by solving the Einstein equations with inner boundary conditions provided by numerical relativity Cauchy simulations in the strong-field regime. A key challenge in the CCE formulation lies in specifying the initial data, which must be consistent with the spacetime's radiation content but is not directly supplied by the Cauchy simulation. Improper or inconsistent initial data can lead to unphysical, long-lived "junk" radiation contaminating asymptotic quantities and compromising waveform accuracy during the inspiral phase. In this work, we investigate the sensitivity of SpECTRE's CCE implementation to different initial data constructions, focusing on the impact of coordinate choices and the accuracy of matching between the initial data and the inner boundary conditions. Our analysis reveals a strong correlation between initial coordinate specifications and the presence of junk radiation in the extracted waveforms. This subtle yet significant effect highlights the importance of careful gauge selection in CCE implementations. We propose a diagnostic framework to identify and mitigate these artifacts, aiming to improve the robustness and fidelity of waveform extraction in practical applications.