Neutrinogenic CMB spectral distortions

TL;DR

The paper addresses the degeneracy in interpreting $N_eff$ by showing how CMB spectral distortions can reveal the underlying energy-injection source when the injection is neutrinogenic. It develops a semi-analytic framework connecting $ΔN_eff$ to the CMB $μ$ distortion and demonstrates this synergy with a long-lived particle decay example, showing that $ΔN_eff$ of order 0.01–0.1 can coincide with a detectable $μ$ distortion around $10^{-8}$. The work provides analytic pathways to relate neutrino-energy leakage to photon heating, indicating that joint CMB anisotropy and absolute spectroscopy can probe hidden sectors that couple predominantly to neutrinos. These insights can guide next-generation CMB experiments (absolute spectroscopy and anisotropy) to test scenarios with hidden neutrino-coupled sectors and long-lived particles, extending sensitivity beyond current $N_eff$ bounds.

Abstract

Extra radiation injection after neutrino decoupling in the early Universe contributes to the effective number of neutrino species that can be constrained by the cosmic microwave background (CMB). However, any effective neutrino number itself cannot uniquely determine the underlying source. We argue that the degeneracy can be relaxed by CMB spectral distortions, which are caused by energy exchange between the extra radiation and photons. We consider neutrinogenic CMB spectral distortions, where extra energy is released in the form of neutrinos but still creates the CMB spectral distortions via electroweak interactions. The synergy between the effective neutrino number and CMB spectral distortions provides a complementary probe of hidden sectors that dominantly couple to neutrinos, opening up parameter space that can be targeted by joint CMB anisotropy and spectral distortion experiments.

Figures (3)

• Figure 1: The electromagnetic energy transfer rates from coannihilation (co) and pair annihilation (pair) at $T=0.5$ keV with an initial yield $Y_X=10^{-8}$. Three lifetime examples $\tau_X=10^4, 10^7, 10^{10}$ s are chosen such that the long-lived particle decays before, around, and near the end of the $\mu$ distortion formation.
• Figure 2: The correlated predictions of $\Delta N_{\rm eff}$ and the CMB $\mu$ distortion from pair annihilation of injected neutrinos $\nu_{\rm inj}+\bar{\nu}_{\rm inj}\to e^++e^-$. The arrow in the left (right) panel denotes the increase of lifetime (mass). The current $2\sigma$ observational bounds from DESI DESI:2024mwx: $\Delta N_{\rm eff}<0.395$, and Planck Planck:2018vyg: $\Delta N_{\rm eff}<0.285$ are shown, together with the detection sensitivity at $2\sigma$ significance level $\Delta N_{\rm eff}=0.1$ from upcoming Simons Observatory (SO) experiment SimonsObservatory:2018koc. We take $\mu=10^{-8}$ as the forecast detection limit of FOSSIL for reference, which is a factor of 2 smaller than the largest $\mu$ distortion predicted in the standard $\Lambda$CDM model Chluba:2012gqChluba:2013wsa.
• Figure 3: The correlated predictions of $\Delta N_{\rm eff}$ and the CMB $\mu$ distortion from pair annihilation and coannihilation of injected neutrinos $\nu_{\rm inj}+\bar{\nu}_{\rm inj}\to e^++e^-, \nu_{\rm inj}+\bar{\nu}_{\rm bg} (\bar{\nu}_{\rm inj}+\nu_{\rm bg}) \to e^++e^-$. See the caption of Fig. \ref{['fig:Neffmu-pair']} for more details.