Papers › Primordial non-Gaussianity and non-Gaussian Covariance
Primordial non-Gaussianity and non-Gaussian Covariance
Thomas Flöss, Matteo Biagetti, P. Daniel Meerburg
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In the pursuit of primordial non-Gaussianities, we hope to access smaller scales across larger comoving volumes. At low redshift, the search for primordial non-Gaussianities is hindered by gravitational collapse, to which we often associate a scale k_(NL). Beyond these scales, it will be hard to reconstruct the modes sensitive to the primordial distribution. When forecasting future constraints on the amplitude of primordial non-Gaussianity, f_(NL), off-diagonal components are usually neglected in the covariance because these are small compared to the diagonal. We show that the induced non-Gaussian off-diagonal components in the covariance degrade forecast constraints on primordial non-Gaussianity, even when all modes are well within what is usually considered the linear regime. As a testing ground, we examine the effects of these off-diagonal components on the constraining power of the matter bispectrum on f_(NL) as a function of kₘₐₓ and redshift, confirming our results against N-body simulations out to redshift z=10. We then consider these effects on the hydrogen bispectrum as observed from a PUMA-like 21-cm intensity mapping survey at redshifts 2<z<6 and show that not including off-diagonal covariance over-predicts the constraining power on f_(NL) by up to a factor of $5$. For future surveys targeting even higher redshifts, such as Cosmic Dawn and the Dark Ages, which are considered ultimate surveys for primordial non-Gaussianity, we predict that non-Gaussian covariance would severely limit prospects to constrain f_(NL) from the bispectrum.
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