A general proof of the conservation of the curvature perturbation

@article{Lyth2005AGP,
  title={A general proof of the conservation of the curvature perturbation},
  author={David H. Lyth and Karim A. Malik and Misao Sasaki},
  journal={Journal of Cosmology and Astroparticle Physics},
  year={2005},
  volume={2005},
  pages={004 - 004}
}
Without invoking a perturbative expansion, we define the cosmological curvature perturbation, and consider its behaviour assuming that the universe is smooth over a sufficiently large comoving scale. The equations are simple, resembling closely the first-order equations, and they lead to results which generalize those already proven in linear perturbation theory and (in part) in second-order perturbation theory. In particular, the curvature perturbation is conserved provided that the pressure… 

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