Assumptions of the primordial spectrum and cosmological parameter estimation

TL;DR

This paper investigates how assumptions about the primordial power spectrum shape influence cosmological parameter estimates, highlighting potential biases and advocating for joint estimation approaches to avoid prejudiced conclusions.

Contribution

It demonstrates that fixed forms of the primordial power spectrum can bias parameter estimation and emphasizes the need for simultaneous estimation of parameters and spectrum shape.

Findings

Assumed PPS forms create distinct likelihood basins affecting parameter estimates.

Allowing free-form PPS can significantly improve likelihoods outside these basins.

Current estimates may be biased due to fixed assumptions about the primordial spectrum.

Abstract

The observables of the perturbed universe, CMB anisotropy and large structures, depend on a set of cosmological parameters, as well as, the assumed nature of primordial perturbations. In particular, the shape of the primordial power spectrum (PPS) is, at best, a well motivated assumption. It is known that the assumed functional form of the PPS in cosmological parameter estimation can affect the best fit parameters and their relative confidence limits. In this paper, we demonstrate that a specific assumed form actually drives the best fit parameters into distinct basins of likelihood in the space of cosmological parameters where the likelihood resists improvement via modifications to the PPS. The regions where considerably better likelihoods are obtained allowing free form PPS lie outside these basins. In the absence of a preferred model of inflation, this raises a concern that current cosmological parameters estimates are strongly prejudiced by the assumed form of PPS. Our results strongly motivate approaches toward simultaneous estimation of the cosmological parameters and the shape of the primordial spectrum from upcoming cosmological data. It is equally important for theorists to keep an open mind towards early universe scenarios that produce features in the PPS.