Cosmological perturbations with multiple fluids and fields (Q2784142)

The authors of this interesting paper investigate the evolution of perturbed cosmological spacetime with multiple fluids and fields in Einstein gravity. The equations are presented in gauge-ready forms. Moreover, they are presented in various forms using the curvature (\(\Phi \) or \(\varphi_{\chi }\)) and isocurvature (\(S_{(ij)}\) or \(\delta\phi _{(ij)}\)) perturbation variables in the general background with \(K\) and \(\Lambda \). \(K\) is the normalized spatial curvature and \(\Lambda \) is the multiplier in the gravitational field equation. The conditions for conserved curvature and isocurvature perturbations in the large-scale limit are clarified. Evolution of curvature perturbations in many different gauge conditions is analyzed extensively. In the multi-field system, the authors present a general solution to the linear order in slow-roll parameters.