Probing decoupling in dark sectors with the cosmic microwave background

Abstract

The acoustic peaks in the angular power spectrum of cosmic microwave background (CMB) temperature and polarization anisotropies play an important role as a probe of the nature of new relativistic particles contributing to the radiation density in the early universe, parametrized by ∆N ef f . The amplitude and phase of the acoustic oscillations provide information about whether the extra species are free-streaming particles, like neutrinos, or tightly-coupled, like the photons, during eras probed by the CMB. On the other hand, some extensions of the Standard Model produce new relativistic particles that decouple from their own non-gravitational interactions after neutrinos, but prior to photons. We study the signature of new relativistic species that decouple during this intermediate epoch. We argue that the decoupling species will cause a scale-dependent change in the amplitude and phase shift of the acoustic oscillations, different from the usual constant shifts on small scales. For intermediate decoupling times, the phase and amplitude shifts depend not only on ∆N ef f but the redshift z dec,X at which the new species decoupled. For ∆N ef f > 0.334, a Stage IV CMB experiment could determine N ef f at the percent level and z dec,X at the ∼ 10% level. For smaller values, ∆N ef f ∼ 0.1, constraints on z dec,X weaken but remain ∼ 20 − 50% for z dec,X ∼ O(10 3 − 10 4 ). As an application, we study the contributions to ∆N ef f and determine the z dec,X values for simple implementations of the so-called N naturalness model.

Neutrino puzzle: Anomalies, interactions, and cosmological tensions

“…In the standard cosmological paradigm, free-streaming neutrinos travel supersonically through the photonbaryon plasma at early times, hence gravitationally pulling photon-baryon wavefronts slightly ahead of where they would be in the absence of neutrinos [42,128,129]. As a result, the free-streaming neutrinos imprint a net phase shift in the CMB power spectra towards larger scales (smaller ), as well as a slight suppression of its amplitude.…”

“…Free-streaming neutrinos thus lead to a physical size of the photon sound horizon at last scattering r * that is slightly larger than it would otherwise be. This phase shift is thought to be a robust signature of the presence of free-streaming radiation in the early Universe [42,130,131].…”

Twin Higgs portal dark matter

“…where g * S is the total number of relativistic degrees of freedom in the bath at temperature T D , and g is the number of degrees of freedom of the target fermion. To estimate T D , we note that roughly 80% of the annihilations for a species leaving the bath happen in the interval 1 6 m < T < m [134]. This means that to a good approximation we can construct continuous expressions for the effective Yukawa couplings y i A and y j B by linearly interpolating between the coupling of the lightest active particle at T = m and the next lightest at T = 1 6 m, keeping the expression constant outside of these intermediary regions.…”

Cosmological constraints with self-interacting sterile neutrinos

“…[51,52] it is argued that this new interaction can reduce N eff to 2.7 via a late production of sterile neutrinos. And with a large coupling it can reduce the free-steaming of neutrinos, which can induce changes in the amplitude and phase of the acoustic peaks in the CMB spectrum [54], and avoid the constraint on Σm ν from the measurement of large scale structure. Still, the study in Ref.…”