Abstract
We develop a gauge-invariant formalism for the study of density perturbations in aFriedmann–Robertson–Walker universe with multiple interacting fluids and/or scalar fields. We show howN scalar fields maybe described by N kinetic fluids (with maximally stiff equation of state) interacting with a non-dynamicalpotential (with vacuum equation of state). We split generic perturbations into adiabaticand entropic parts, and give the coupled first-order evolution equations on all scales,including energy and momentum exchange. We identify the non-adiabatic effects on largescales, and define adiabatic initial conditions in the presence of multiple fluids and fields.
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