Preheating with extra dimensions
  We investigate preheating in a higher-dimensional generalized Kaluza-Klein theory with a quadratic inflaton potential $V(\phi)=\frac12 m^2\phi^2$ including metric perturbations explicitly. The system we consider is the multi-field model where there exists a dilaton field $\sigma$ which corresponds to the scale of compactifications and another scalar field $\chi$ coupled to inflaton with the interaction $\frac12 g^2\phi^2\chi^2+\tilde{g}^2\phi^3\chi$. In the case of $\tilde{g}=0$, we find that the perturbation of dilaton does not undergo parametric amplification while the $\chi$ field fluctuation can be enhanced in the usual manner by parametric resonance. In the presence of the $\tilde{g}^2\phi^3\chi$ coupling, the dilaton fluctuation in sub-Hubble scales is modestly amplified by the growth of metric perturbations for the large coupling $\tilde{g}$. In super-Hubble scales, the enhancement of the dilaton fluctuation as well as metric perturbations is weak, taking into account the backreaction effect of created $\chi$ particles. We argue that not only is it possible to predict the ordinary inflationary spectrum in large scales but extra dimensions can be held static during preheating in our scenario. 
S. Tsujikawa (Waseda University)