Dr Norihiro TANAHASHI (DAMTP, University of Cambridge)
We study cosmological perturbations in bimetric theory focusing on the effect of coupling of matter fields to metrics. Firstly we review the results for the case that a matter field couples to the physical metric and the other one couples to the second metric, in which a stable cosmology can be realized once physically sensible conditions are satisfied. Next we study the case that a matter field couples to a composite metric made out of the two metrics, in which a matter couples to the two metrics simultaneously without introducing the Boulware-Deser ghost. We find that some cosmological solutions which is stable in the former case is destabilized in the latter case, while there is a class of solutions which is unstable in the former case but could be made viable in the latter case. We argue possible usages of this new class of stable solutions.