Dark Radiation in extended cosmological scenarios

TL;DR

The paper analyzes the cosmological implications of a dark radiation component, parameterized by $N_{ m eff}$ and its perturbation properties $c_{ m eff}^2$ and $c_{ m vis}^2$, and investigates how these properties correlate with the dark energy equation of state and the running of the scalar spectral index. By modifying CAMB and performing MCMC on current data (with variations including SPT and SNIa), the authors show that current hints of excess relativistic species can be driven by non-standard perturbation parameters, and they reveal strong degeneracies with $n_s$ and $w$. They then forecast Planck and COrE performance, demonstrating that non-standard dark radiation can masquerade as a running spectral index or evolving dark energy, unless high-$ ext{ℓ}$ information and proper perturbation modeling are used. Overall, the work emphasizes the need to jointly constrain $N_{ m eff}$, $c_{ m eff}^2$, and $c_{ m vis}^2$ to avoid biased inferences about inflation and dark energy.

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.

Figures (6)

• Figure 1: The top, middle and bottom panels depict the $68\%$ and $95\%$ CL bounds in the $c_{\rm eff}^2-n_s$, $c_{\rm vis}^2-n_s$ and $c_{\rm vis}^2-c_{\rm eff}^2$ planes, respectively. The blue, red and green regions refer to the Baseline, BaselineSPT and BaselineSPT-SNIa models, respectively.
• Figure 2: The top and bottom panels depict the $68\%$ and $95\%$ CL bounds in the $N_{\rm {eff}}-w$ and $c_{\rm vis}^2-w$ planes, respectively. The blue, red and green regions refer to the Baseline, BaselineSPT and BaselineSPT-SNIa models, respectively.
• Figure 3: The top and the bottom panels depict the 68% and 95% CL bounds in the $N_{\rm {eff}}-n_s$ and $c_{\rm eff}^2-N_{\rm {eff}}$ planes, respectively. The blue, red and green regions refer to the Baseline, BaselineSPT and BaselineSPT-SNIa models respectively.
• Figure 4: 68% and 95% CL allowed regions in the $n_s-n_{\rm run}$ plane from MCMC fits of Planck (blue regions) and COrE (red regions) CMB mock data.
• Figure 5: Same as Fig. \ref{['fig:ns_nrun']} but in the $w-n_s$ plane.