Model-independent approach to effective sound speed in multi-field inflation

@article{Romano2020ModelindependentAT,
  title={Model-independent approach to effective sound speed in multi-field inflation},
  author={Antonio Enea Romano and Krzysztof Turzyński and Sergio Andr{\'e}s Vallejo-Pe{\~n}a},
  journal={The European Physical Journal C},
  year={2020},
  volume={82},
  pages={1-16}
}
For any physical system satisfying the Einstein’s equations, the comoving curvature perturbations satisfy an equation involving the momentum-dependent effective sound speed, valid for any system with a well defined energy-stress tensor, including multi-fields models of inflation. We derive a general model-independent formula for the effective sound speed of comoving adiabatic perturbations, valid for a generic field-space metric, without assuming any approximation to integrate out entropy… 
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