Quantum cosmic fluids in the early universe

We present a new framework to study the early universe by replacing the classical perfect fluid background with a quantum fluid. The widely accepted idea that the universe is fundamentally quantum in nature, favours the argument that the energy-momentum tensor for the matter source, in the Einstein field equations, should show inherent quantum behaviour. Our approach tries to implement this perspective by considering the early universe as a quantum microcanonical ensemble of energy states, for a non-interacting scalar field. Building upon its partition function, a detailed calculation for deriving an expression for a propagator in this ensemble has been performed. Furthermore, to model the accelerated expanding universe, we re-derive the expression for the propagator in the conformally flat Friedmann–Lemaître–Robertson–Walker (FLRW) universe under the de Sitter space approximations. The motivation is to set a stage to perform calculations with regard to quantum backgrounds in physical cosmology.