• DOI:10.1140/epjc/s10052-022-10669-3
• Corpus ID: 251934911

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}
}

• A. Romano, K. Turzyński, S. Vallejo-Peña
• Published 29 May 2020
• Physics
• The European Physical Journal C

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 perturbations, but expressing the momentum-dependent effective sound speed in terms of the components of the total energy-stress tensor. As an application, we study a number of two-field models with a kinetic coupling between the fields, identifying the single curvature mode of the effective theory and showing that momentum-dependent effective sound speed fully accounts for the predictions for the power spectrum of curvature perturbations. Our results show that the momentum-dependent effective sound speed is a convenient scheme for describing all inflationary models that admit a single-field effective theory, including the effects of entropy pertubations present in multi-fields systems.