Gravitational wave oscillations in Multi-Proca dark energy models

Gravitational wave oscillations arise from the exchange of energy between the metric perturbations and additional tensor modes. This phenomenon can occur even when the extra degrees of freedom consist of a triplet of massive Abelian vector fields, as in Multi-Proca dark energy models. In this work, we study gravitational wave oscillations in this class of models minimally coupled to gravity with a general potential, allowing also for a kinetic coupling between the vector field and dark matter that can, in principle, enhance the modulation of gravitational wave amplitudes. After consistently solving the background dynamics, requiring the model parameters to reproduce a phase of late-time accelerated expansion, we assess the accuracy of commonly used analytical approximations and quantify the impact of gravitational wave amplitude modulation for current detectors (LIGO--Virgo) and future missions such as LISA. Although oscillations are present in these scenarios, we find that the effective mass scale (the mixing mass) governing the phenomenon is $m_g \sim \mu_A$, where $\mu_A$ is the (time-dependent) effective mass of the vector dark-energy field. Detectability of gravitational wave oscillations, however, requires $m_g \gg H_0$, which is in tension with the ultra-light masses typically needed to drive accelerated expansion $\mu_A \sim H_0 \sim 10^{-33}\,\mathrm{eV}$. Therefore, if gravitational wave oscillations were to be detected at the corresponding frequencies, they could not be attributed to these classes of dark-energy models.