Gr.IV seminar: Marco De Cesare - "Cosmological effects on gravitational-wave propagation in bigravity"

Wednesday 25 Feb 2026, 16:00 → 17:00 Europe/Rome

GEO-6 (DIP.TO DI GEOLOGIA)

Gravitational-wave oscillations represent a distinctive signature of bimetric gravity, and more generally of theories with extra (massive) spin-2 fields in addition to the standard massless graviton. In bimetric gravity, gravitational waves are emitted in a superposition of massless and massive graviton states, which undergo mixing as they propagate through spacetime. This phenomenon displays some (partial) analogies to neutrino oscillations, and does not have an analogue in general relativity.
We present a detailed analytical study of the propagation of gravitational waves in ghost-free bimetric gravity in a late-time de Sitter epoch. In this regime, the dynamical equations can be decoupled into massless and massive graviton modes and solved exactly. We provide uniform approximations for the modes in terms of elementary functions, which are valid on all scales and for all viable mass windows.
We identify different dynamical regimes for the system, depending on the propagation properties of the massive graviton, and whether the massless and massive components of the signal can be temporally resolved or not at the detector. In each regime, we compute the gravitational-wave luminosity distance as a function of redshift and study the propagation of wave packets.
This allows for the derivation of a new observational bound for the ghost-free bimetric theory using the multi-messenger event GW170817.
Further, by an explicit computation, we highlight the limitations of the analogy with neutrino oscillations, showing that the massless and massive components of the gravitational-wave signal retain their coherence also in the regime where they can be temporally resolved---even when couplings to incoherent matter degrees of freedom are included.
Talk based on joint work with D. Brizuela and A. Soler Oficial: JCAP 01 (2026) 048 https://doi.org/10.1088/1475-7516/2026/01/048