Primordial Power Spectrum features and $f_{NL}$ constraints

TL;DR

This paper investigates how relaxing the primordial power spectrum assumptions affects future constraints on local non-gaussianity parameter $f_{NL}$, highlighting potential biases and the importance of combining multiple data sources for robust results.

Contribution

It provides a model-independent analysis of the impact of primordial power spectrum features on $f_{NL}$ constraints from future large scale structure surveys.

Findings

Errors on $f_{NL}$ increase by 60% when spectrum features are ignored.

Fitting to an incorrect spectrum can bias $f_{NL}$ by about 2.5.

Adding CMB priors yields nearly unbiased $f_{NL}$ estimates with errors close to standard models.

Abstract

The simplest models of inflation predict small non-gaussianities and a featureless power spectrum. However, there exist a large number of well-motivated theoretical scenarios in which large non-gaussianties could be generated. In general, in these scenarios the primordial power spectrum will deviate from its standard power law shape. We study, in a model-independent manner, the constraints from future large scale structure surveys on the local non-gaussianity parameter $f_{\rm NL}$ when the standard power law assumption for the primordial power spectrum is relaxed. If the analyses are restricted to the large scale-dependent bias induced in the linear matter power spectrum by non-gaussianites, the errors on the $f_{\rm NL}$ parameter could be increased by $60\%$ when exploiting data from the future DESI survey, if dealing with only one possible dark matter tracer. In the same context, a nontrivial bias $|\delta f_{\rm NL}| \sim 2.5$ could be induced if future data are fitted to the wrong primordial power spectrum. Combining all the possible DESI objects slightly ameliorates the problem, as the forecasted errors on $f_{\rm NL}$ would be degraded by $40\%$ when relaxing the assumptions concerning the primordial power spectrum shape. Also the shift on the non-gaussianity parameter is reduced in this case, $|\delta f_{\rm NL}| \sim 1.6$. The addition of Cosmic Microwave Background priors ensure robust future $f_{\rm NL}$ bounds, as the forecasted errors obtained including these measurements are almost independent on the primordial power spectrum features, and $|\delta f_{\rm NL}| \sim 0.2$, close to the standard single-field slow-roll paradigm prediction.