Karim A. Malik developed the treatment of gauge-invariant cosmological curvature with David Wands, David H. Lyth and Andrew R. Liddle. Their 2000 analysis derived large-scale evolution from stress-energy conservation, exposing how a non-adiabatic pressure perturbation can change the uniform-density curvature perturbation.[ref]
A new approach to the evolution of cosmological perturbations on large scales by David Wands and others. Sections II–V: pressure source and generalized adiabatic perturbations
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