Description
The upcoming generation of wide and deep galaxy surveys, such as SPHEREx and Euclid, will provide a unique opportunity to probe the ultra-large scales of structure formation, where primordial non-Gaussianity, often parameterized by fNL, is expected to leave its most detectable imprint. At the same time, these surveys inevitably enter regimes where relativistic and wide-angle effects become significant, requiring careful modeling to extract unbiased cosmological information. We address these challenges by applying the total-angular-momentum (TAM) formalism to describe redshift-space clustering beyond the flat-sky approximation. The standard Fourier-mode description, which characterizes distortions by the angle between a mode and a line of sight, becomes ill-defined over wide fields, whereas the TAM basis naturally separates radial and angular contributions and incorporates the relevant relativistic effects. Within this framework, we provide a new parameterization of wide-angle predictions that can be compared directly with survey data. We then introduce PowerFull, a modification of the Julia-based 2-F AST package, which enables efficient computation of the full relativistic angular power spectrum. Finally, using Fisher information matrixbased analyses, we show that neglecting these effects biases parameter estimation, and that achieving O(1) precision on fNL with surveys like SPHEREx requires a consistent relativistic and wide-angle treatment of galaxy clustering.