Alice Garoffolo (Leiden)

## Cosmological distances in scalar-tensor theory of gravity

Cosmological distances provide a powerful tool to test the evolution of the Universe since they depend directly on its dynamics. However, the distances we measure are inferred via the detection of a signal, whether an electromagnetic one or a gravitational wave, and hence they also depend on its propagation history and the theory of gravity considered. For instance in scalar-tensor theories aimed at modeling dark energy, gravitational waves can suffer extra damping, among other effects, while the photon dynamics is left unaffected. In this talk I will describe how gravitational waves luminosity and angular diameter distances change, though preserving the distance-duality relation, in the subset of scalar-tensor theories where they propagate at the speed of light. I will also show how it is possible to detect dark energy fluctuations by exploiting the difference between electromagnetic and gravitational signals. Finally, I will discuss the detectability of the scalar wave in light of screening mechanism, feature of all viable scalar-tensor theories which suppresses the fifth force carried by the scalar field in high density regions where General Relativity has passed every test performed.