Séminaire de Physique ThéoriqueDegenerate Higher-Order Scalar-Tensor theories
Karim Noui (LMPT/Tours)
Thursday 07 September 2017 14:00 - Tours - Salle 1180 (Bât E2)
There have been numerous attempts to modify or extend general relativity, with either the motivation to account for dark energy (and sometimes dark matter) or, more modestly, to construct benchmark models that are useful to test general relativity quantitatively. Scalar- tensor theories have often played a prominent role in these attempts and, lately, special attention has been devoted to scalar-tensor theories whose Lagrangians contain second-order derivatives of a scalar field. Lagrangians of this type, which contain “accelerations”, are generically plagued by an instability due to the presence, in addition to the usual scalar mode and tensor modes, of an extra scalar degree of freedom (unless the higher order terms can be treated as perturbative terms in the sense of low energy effective theories). Until recently, it was believed that only theories that yield second-order Euler-Lagrange equations were free of this dangerous extra degree of freedom. In the last couple of years, it has been realized that there in fact exists a much larger class of theories that satisfy this property.