Stability and speed tests on Scalar–Vector–Tensor theories

Scalar–Tensor theories are extensively used to build interesting cosmological solutions for the early and late universe. However, they face two major challenges: most predict a speed of gravitational waves (GW) different from the speed of light – which is in tension with the GW event GW170817– and they are subject to No-Go theorems that question their stability.
In this talk, we show a set of Scalar-Vector couplings for generalisations of Horndeski theory, such as GLPV, which enforce the propagation of vector modes at the same speed as GW. This offers a viable path to reconcile these theories with the constraints from the event GW170817.
Regarding the stability issues, we first present a No-Go theorem that holds in a subset of Horndeski theories with torsion. Then, we show that in the most general Horndeski–Cartan theories such a no-go theorem cannot be formulated.