Cosmological Constraints on Interacting Light Particles

TL;DR

The paper investigates cosmological constraints on dark radiation that can be either free-streaming or interacting at recombination. It introduces the DR parametrization in terms of $N_{ m tot}$ and $f_{ m fs}$ to replace the degeneracy between $N_{ m eff}$ and $N_{ m fld}$, and performs likelihood analyses with Planck 2015 plus BAO, $H_0$, and LSS data, including mock future datasets. The authors find $\,\Delta N_{ m tot}<0.39$ (2$\sigma$) and a best-fit around $N_{ m tot}\approx3.06$, with hints that a fraction of DR could be interacting, potentially alleviating tensions in $H_0$ and $\sigma_8$. They forecast substantial improvements for Advanced ACTPol, CMB-S4, and Euclid in pinning down DR properties and separating free-streaming from interacting DR, with implications for beyond-Standard-Model neutrino physics and dark sectors.

Abstract

Cosmological observations are becoming increasingly sensitive to the effects of light particles in the form of dark radiation (DR) at the time of recombination. The conventional observable of effective neutrino number, $N_{\rm eff}$, is insufficient for probing generic, interacting models of DR. In this work, we perform likelihood analyses which allow both free-streaming effective neutrinos (parametrized by $N_{\rm eff}$) and interacting effective neutrinos (parametrized by $N_{\rm fld}$). We motivate an alternative parametrization of DR in terms of $N_{\rm tot}$ (total effective number of neutrinos) and $f_{\rm fs}$ (the fraction of effective neutrinos which are free-streaming), which is less degenerate than using $N_{\rm eff}$ and $N_{\rm fld}$. Using the Planck 2015 likelihoods in conjunction with measurements of baryon acoustic oscillations (BAO), we find constraints on the total amount of beyond the Standard Model effective neutrinos (both free-streaming and interacting) of $ΔN_{\rm tot} < 0.39$ at 2$σ$. In addition, we consider the possibility that this scenario alleviates the tensions between early-time and late-time cosmological observations, in particular the measurements of $σ_8$ (the amplitude of matter power fluctuations at 8$h^{-1}$ Mpc), finding a mild preference for interactions among light species. We further forecast the sensitivities of a variety of future experiments, including Advanced ACTPol (a representative CMB Stage-III experiment), CMB Stage-IV, and the Euclid satellite. This study is relevant for probing non-standard neutrino physics as well as a wide variety of new particle physics models beyond the Standard Model that involve dark radiation.

Figures (7)

• Figure 1: Here we show the 1d posteriors for the above scans. The colors of the posterior indicate which scan they belong to; in the same order as above, Planck T, Planck P, Planck P+BAO, Planck P+BAO+$H_0$, and Planck P+BAO+$H_0$+LSS.
• Figure 2: Here we show two different 2d posteriors for three of the five scans ( Planck T, Planck P+BAO, and Planck P+BAO+$H_0$+LSS). The solid lines are 1$\sigma$ contours, and the dot-dashed lines are 2$\sigma$ contours. The posteriors in the top figure exhibit degeneracy between $N_{\rm eff}$ and $N_{\rm fld}$, motivating the parametrization in terms of $N_{\rm tot}$ and $f_{\rm fs}$ in the bottom figure, and demonstrating that the strongest constraints arise on the sum $N_{\rm tot}$.
• Figure 3: Here we show different 2d posteriors for three of the five scans ( Planck P+BAO, Planck P+BAO+$H_0$, and Planck P+BAO+$H_0$+LSS). The solid lines are 1$\sigma$ contours, and the dot-dashed lines are 2$\sigma$ contours. The posteriors demonstrate how free-streaming DR affects late-time cosmological observables differently than interacting DR does. In the left figures, we explore the effect of $N_{\rm eff}$; in the right figures, $N_{\rm fld}$. In the top figures, we explore the effect on $H_0$; in the bottom figures, $\sigma_8$.
• Figure 4: Here we show two different 2d posteriors for two of the three future constraints ( Planck P+S3 and Planck P+S4) alongside one of the present-day constraints ( Planck P). The solid lines are 1$\sigma$ contours, and the dot-dashed lines are 2$\sigma$ contours. The left figure shows the conventional parametrization in terms of $N_{\rm eff}$ and $N_{\rm fld}$, whereas the right figure shows our alternative parametrization in terms of $N_{\rm tot}$ and $f_{\rm fs}$.
• Figure 5: Here we show two different 2d posteriors for one present-day and two mock scans ( Planck P, Planck P+S3, and Planck P+S4). The solid lines are 1$\sigma$ contours, and the dot-dashed lines are 2$\sigma$ contours. The posteriors demonstrate how free-streaming DR will affect late-time cosmological observables differently than interacting DR will. In the left figures, we explore the effect of $N_{\rm eff}$; in the right figures, $N_{\rm fld}$. In the top figures, we explore the effect on $H_0$; in the bottom figures, $\sigma_8$.