We study the impact of the cosmological parameters uncertainties on the
measurements of primordial non-Gaussianity through the large-scale non-Gaussian
halo bias effect. While this is not expected to be an issue for the standard
LCDM model, it may not be the case for more general models that modify the
large-scale shape of the power spectrum. We consider the so-called local
non-Gaussianity model and forecasts from planned surveys, alone and combined
with a Planck CMB prior. In particular, we consider EUCLID- and LSST-like
surveys and forecast the correlations among fNL and the running of the
spectral index αs, the dark energy equation of state w, the effective
sound speed of dark energy perturbations cs2, the total mass of massive
neutrinos Mν=∑mν, and the number of extra relativistic degrees of
freedom Nνrel. Neglecting CMB information on fNL and scales k>0.03h/Mpc, we find that, if Nνrel is assumed to be known, the
uncertainty on cosmological parameters increases the error on fNL by
10 to 30% depending on the survey. Thus the fNL constraint is
remarkable robust to cosmological model uncertainties. On the other hand, if
Nνrel is simultaneously constrained from the data, the fNL
error increases by ∼80. Finally, future surveys which provide a large
sample of galaxies or galaxy clusters over a volume comparable to the Hubble
volume can measure primordial non-Gaussianity of the local form with a
marginalized 1--σ error of the order ΔfNL∼2−5, after
combination with CMB priors for the remaining cosmological parameters. These
results are competitive with CMB bispectrum constraints achievable with an
ideal CMB experiment.Comment: 17 pages, 1 figure added, typos corrected, comments added, matches
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