Conservative Constraints on Early Cosmology: an illustration of the Monte Python cosmological parameter inference code

TL;DR

This paper presents a conservative approach to constraining early cosmology parameters using CMB data, extending previous methods to marginalize over lensing contamination, and introduces the Monte Python inference code.

Contribution

It extends a method for robust early universe constraints, incorporates lensing marginalization, and introduces the Monte Python inference code as a user-friendly alternative.

Findings

Weaker constraints on early cosmology compared to standard LCDM.

No evidence for extra relativistic degrees of freedom.

Neutrinos became non-relativistic after photon decoupling.

Abstract

Models for the latest stages of the cosmological evolution rely on a less solid theoretical and observational ground than the description of earlier stages like BBN and recombination. As suggested in a previous work by Vonlanthen et al., it is possible to tweak the analysis of CMB data in such way to avoid making assumptions on the late evolution, and obtain robust constraints on "early cosmology parameters". We extend this method in order to marginalise the results over CMB lensing contamination, and present updated results based on recent CMB data. Our constraints on the minimal early cosmology model are weaker than in a standard LCDM analysis, but do not conflict with this model. Besides, we obtain conservative bounds on the effective neutrino number and neutrino mass, showing no hints for extra relativistic degrees of freedom, and proving in a robust way that neutrinos experienced their non-relativistic transition after the time of photon decoupling. This analysis is also an occasion to describe the main features of the new parameter inference code Monte Python, that we release together with this paper. Monte Python is a user-friendly alternative to other public codes like CosmoMC, interfaced with the Boltzmann code class.