Relativistic N-body simulations with mass-varying neutrinos

Recent cosmological data provide weak hints for smaller masses than what is required by neutrino-oscillation experiments, motivating investigations into variation of the neutrino masses with cosmic time. Here we study a model where the neutrinos couple to a quintessence field that acts as dark energy. We compute structure formation using fully dynamical and relativistic N-body and lattice simulations, with nonlinear field evolution and a weak-field metric expansion. Similar to previous studies, we find the generic appearance of neutrino lumps, but observe them to be highly dynamical and spinning. We study several models where the effective dark energy, including nonlinear effects of the scalar field on the cosmic expansion, can show phantom crossing. The relativistic physics available in our simulation framework can help distinguish such models from LCDM, through observations of neutrino and dark energy properties, and slip between the gravitational potentials due to anisotropic stress. The rich phenomenology offered by mass-varying neutrinos motivates further exploration and development of the simulation techniques.