Gravitational clustering effects on anisotropic cosmic birefringence

Establish how the gravitational clustering of massive neutrinos at late times, when the cosmic temperature becomes comparable to the neutrino mass, modifies the scale dependence of the anisotropic cosmic-birefringence angular spectrum, including its implications for tomographic measurements during cosmic reionization.

Background

The paper treats neutrinos as massless throughout cosmic evolution when calculating the anisotropic cosmic-birefringence spectrum. The authors note that this approximation may fail at late times, once the cosmic temperature approaches the neutrino mass, because neutrino gravitational clustering can alter the angular scale dependence of the birefringence signal.

A detailed treatment is relevant for distinguishing the neutrino-current mechanism from other sources of cosmic birefringence. The authors specifically identify tomographic analyses, including signals generated during the cosmic reionization epoch, as a possible way to probe this late-time effect, but leave its quantitative study unresolved.

References

In this paper, we have approximated neutrinos as massless throughout the evolution of the universe. At later times, however, when the cosmic temperature becomes comparable to the neutrino mass, gravitational clustering of neutrinos may modify the scale dependence of the anisotropic birefringence angular spectrum. Such an effect could potentially be probed through tomographic approaches to cosmic birefringence, for example by exploiting the signal generated during the cosmic reionization epoch. A more detailed study of this issue is left for future work.

— Cosmic Birefringence from Large Lepton Asymmetry  (2610.09748 - Kasai et al., 7 Oct 2026) in Section 'Summary and Discussions'