Dark Radiation in extended cosmological scenarios

TL;DR

This paper investigates the properties of dark radiation in extended cosmological models, highlighting how its clustering parameters can be mistaken for other cosmological effects like dark energy evolution or spectral index running.

Contribution

It analyzes the correlations between dark radiation properties and key cosmological parameters, emphasizing the importance of viscosity parameters in interpreting cosmological data.

Findings

Dark radiation viscosity parameters can mimic evolving dark energy.

Higher Neff values suggest additional relativistic components beyond standard neutrinos.

Misinterpretation of dark radiation properties can lead to incorrect cosmological inferences.

Abstract

Recent cosmological data have provided evidence for a "dark" relativistic background at high statistical significance. Parameterized in terms of the number of relativistic degrees of freedom Neff, however, the current data seems to indicate a higher value than the one expected in the standard scenario based on three active neutrinos. This dark radiation component can be characterized not only by its abundance but also by its clustering properties, as its effective sound speed and its viscosity parameter. It is therefore crucial to study the correlations among the dark radiation properties and key cosmological parameters, as the dark energy equation of state or the running of the scalar spectral index, with current and future CMB data. We find that dark radiation with viscosity parameters different from their standard values may be misinterpreted as an evolving dark energy component or as a running spectral index in the power spectrum of primordial fluctuations.