# Agenda

# Friday 6 April 2018

### Towards a classification of healthy scalar-tensor theories

#### Karim Noui (Tours)

Once again, Einstein was right! The recent detection of gravitational waves is exceptional, but it confirms what Einstein predicted 100 years ago... However, there are reasons to think that general relativity may not be the "final" theory of space-time: it could be modified at very "large" scale (to account for dark energy) or at very "short" scale (to account for quantum gravity).

Scalar-tensor theories form a very large class of modified theories of gravity where, in addition to the usual two tensorial modes, a scalar mode propagates. Such theories have been studied a lot these last years to model dark energy or some aspects of quantum gravity for instance. In particular, there has been a huge activity around scalar-tensor theories with higher-order derivatives of the scalar field in the action, which appear to have very interesting physical properties.

In this seminar, I will show how we managed to classify healthy higher-order scalar-tensor theories in a systematic way. Then, I will also illustrate physical applications of these theories in cosmology and in quantum gravity. Finally, in the context of cosmology, I will show how the recent observation of the gravitational wave event GW170817 and of its electromagnetic counterpart GRB 170817A, which shows that the speed of gravitational waves is the same as

the speed of light (within deviations of order 10^*-15*), strongly constrains these theories.