Evidence for extra radiation? Profile likelihood versus Bayesian posterior

TL;DR

This paper compares Bayesian and likelihood profile methods in analyzing cosmological data, confirming a slight preference for extra radiation beyond the standard model, and clarifies the robustness of this evidence.

Contribution

It demonstrates that the preference for extra radiation is consistent across different statistical methods, addressing potential biases from marginalisation effects.

Findings

Bayesian posterior prefers higher N_eff values.

Difference between methods diminishes with external data.

Preference for extra radiation is robust and not an artifact.

Abstract

A number of recent analyses of cosmological data have reported hints for the presence of extra radiation beyond the standard model expectation. In order to test the robustness of these claims under different methods of constructing parameter constraints, we perform a Bayesian posterior-based and a likelihood profile-based analysis of current data. We confirm the presence of a slight discrepancy between posterior- and profile-based constraints, with the marginalised posterior preferring higher values of the effective number of neutrino species N_eff. This can be traced back to a volume effect occurring during the marginalisation process, and we demonstrate that the effect is related to the fact that cosmic microwave background (CMB) data constrain N_eff only indirectly via the redshift of matter-radiation equality. Once present CMB data are combined with external information about, e.g., the Hubble parameter, the difference between the methods becomes small compared to the uncertainty of N_eff. We conclude that the preference of precision cosmological data for excess radiation is "real" and not an artifact of a specific choice of credible/confidence interval construction.