Decoupling local primordial non-Gaussianity from relativistic effects in the galaxy bispectrum

Upcoming galaxy surveys aim to map the Universe with unprecedented precision, depth and sky coverage. The galaxy bispectrum is a prime source of information as it allows us to probe primordial non-Gaussianity (PNG), a key factor in differentiating various models of inflation. On the scales where local PNG is strongest, Doppler and other relativistic effects become important and need to be included. Unlike for the single-tracer power spectrum, the leading order imaginary Doppler term does not cancel out in the bispectrum, leaving a smoking gun imaginary dipole signal. We investigate the detectability and importance of relativistic and local PNG contributions in the galaxy bispectrum. We compute the signal-to-noise ratio for the detection of lightcone projection effects in the bispectrum. Furthermore, we perform information matrix forecasts on the local PNG parameter, fNL, and on the parametrised amplitudes of the relativistic corrections. Finally, we quantify the bias on the measurement of fNL that arises from neglecting relativistic effects. Our results show that detections of both first- and second-order relativistic effects are promising with forthcoming spectroscopic survey specifications — and are largely unaffected by the uncertainty in fNL. Conversely, we show for the first time that neglecting relativistic corrections in the galaxy bispectrum can lead to a shift >1.5σ(fNL) on the detected value of fNL, highlighting the importance of including relativistic effects in our modelling.