Cosmological Signatures of Interacting Neutrinos

TL;DR

This work investigates cosmological signatures of non-standard neutrino interactions with a light boson that can keep neutrinos in thermal contact, forming a tightly coupled ν–φ fluid. The authors develop two limiting frameworks—massless/constant total energy density (Model A) and neutrino annihilation to scalars (Model B)—and derive the corresponding perturbation evolution, including tight-coupling equations. They confront predictions with CMB and large-scale structure data, along with Lyman-α and polarization measurements, using MCMC to derive constraints. The results show that current data do not rule out interacting-neutrino scenarios; CMB polarization and Lyman-α data tighten constraints on the number of free-streaming neutrinos and on neutrino masses, while annihilation can significantly modify mass bounds, particularly in the neutrinoless universe case. Overall, cosmology remains a viable laboratory for probing neutrino interactions beyond the Standard Model.

Abstract

We investigate signatures of neutrino scattering in the Cosmic Microwave Background (CMB) and matter power spectra, and the extent to which present cosmological data can distinguish between a free streaming or tightly coupled fluid of neutrinos. If neutrinos have strong non-standard interactions, for example, through the coupling of neutrinos to a light boson, they may be kept in equilibrium until late times. We show how the power spectra for these models differ from more conventional neutrino scenarios, and use CMB and large scale structure data to constrain these models. CMB polarization data improves the constraints on the number of massless neutrinos, while the Lyman--$α$ power spectrum improves the limits on the neutrino mass. Neutrino mass limits depend strongly on whether some or all of the neutrino species interact and annihilate. The present data can accommodate a number of tightly-coupled relativistic degrees of freedom, and none of the interacting-neutrino scenarios considered are ruled out by current data -- although considerations regarding the age of the Universe disfavor a model with three annihilating neutrinos with very large neutrino masses.

Figures (14)

• Figure 1: The interactions that keep the neutrinos and the scalar coupled. If the scalar is heavier than $m_\nu$, the process $\nu \leftrightarrow \nu \phi$ is replaced by $\phi \leftrightarrow \nu \nu$.
• Figure 2: CMB and matter power spectra as a function of the fraction of interacting neutrinos, with $N^{\rm eff}_\nu \equiv N^{\rm SM}_\nu+N^{\rm int}_\nu=3$. The power spectra are normalized (to an arbitrary value) at large scale.
• Figure 3: Upper: The CMB power spectra due to the monopole, velocity, and ISW terms of the source function for the standard $N^{\rm int}_\nu=0$ case (For a pedagogical description of these terms see, e.g., Refs. Seljak:1996isDodelson:2003ft). Lower: The contribution of each source term to $\Delta C_l$, the difference between a model with $N^{\rm int}_\nu=3$ and $N^{\rm int}_\nu=0$ with $N^{\rm eff}_\nu=3$ held fixed. The ISW-monopole cross term (nonzero due to the differing evolution of the gravitational potentials) contributes significantly to the difference in the first peak despite the identical background evolution of the two models.
• Figure 4: The CMB and matter power spectra as a function of the number of standard model neutrinos, with $N^{\rm int}_\nu=0$. The power spectra are normalized (to an arbitrary value) at large scale.
• Figure 5: The evolution of the effective number of interacting neutrinos $N^{\rm int}_\nu$ with $m_{\nu}=0.1$ eV (dashed curves), $m_{\nu}=1$ eV (dotted curves) and $m_{\nu}=10$ eV (solid curves) for 3 interacting neutrinos (top/red), 2 interacting neutrinos (middle/green), and 1 interacting neutrino (bottom/blue). Notice that the $N^{\rm int}_\nu$ initially includes an extra $4/7$ to account for the new scalar degree of freedom and that the effective number of interacting neutrinos after annihilation is greater than prior to annihilation.