Forecasting Primordial Non-Gaussianities with UNIONS and Planck

This study forecasts the ability to measure local Primordial Non-Gaussianities (PNGs) by cross-correlating high-redshift tracer samples from the UNIONS survey with the Cosmic Microwave Background (CMB) lensing potential measured by Planck. The research highlights the potential of using Lyman-Break Galaxies (LBGs) selected from UNIONS data as tracers of large-scale structure. The authors perform a MCMC-based forecast to estimate the uncertainties on fNLloc, a parameter characterizing the amplitude of local PNGs, and on a galaxy bias parameter. The results indicate that an idealized photometric sample of LBGs can achieve a precision of σ(fNLloc)=34, which can be further improved to σ(fNLloc)=20 with spectroscopic follow-up using DESI-II. The study also investigates a more realistic LBG selection using early UNIONS data, finding similar precision achievable after DESI spectroscopic follow-up. However, the authors emphasize the importance of accurately calibrating the redshift distribution of the LBG sample, as uncertainties in the clustering-redshift distribution can significantly degrade the fNLloc constraining power. The findings suggest that the combination of UNIONS data and Planck CMB lensing, particularly with future spectroscopic follow-up, offers a promising avenue for probing the physics of inflation and constraining models of the early universe.

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