Abstract: Unlike other theories of bimetric gravity, bigravity theories can be interpreted as describing two classical universes interacting only through gravity. In this framework I will present the “generalized Gordon ansatz”, an ansatz which greatly simplifies calculations in cosmological scenarios (and other situations of physical interest), and can be used to extract conclusions about the interplay of both gravitational sectors when considering the fulfillment of the null energy condition (NEC) by the effective stress energy tensors. In fact, I will show that in general situations the effective stress energy tensors associated to both gravitational sectors can only simultaneously satisfy the NEC when it is saturated. Nevertheless, this anti-correlation is only present for the pure gravitational degrees of freedom, and the occurrence of some extremality events (as bounces and singularities) in one universe would generically imply that they also take place in the other universe.
Place: Physics Dept. Meeting Room, 2th floor, IST