Density fluctuations and single-mode thermal states in the FRW universe

Using single-mode squeezed and coherent thermal states formalism, we analyzed the validity of the semiclassical Einstein equation (SCEE) in the flat Friedmann–Robertson–Walker universe by analyzing the density fluctuations in terms of a massive inflaton. In a single-mode squeezed thermal states the density fluctuations are too large; consequently, the semiclassical theory of gravity (SCTG) does not hold good, for squeezing parameter greater than unity, i.e., rs> 1 ; however, the theory is valid when this related squeezing parameter is smaller than the unity, i.e., rs< < 1 and rs< 1. Also, it is noted that the SCEE is dependable on coherent thermal state formalism. The current study provides a description of the density fluctuations as a result of the quantum and thermal effects in the SCTG. © 2020, Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature.