Density fluctuations and single-mode thermal states in the FRW universe Renu Dhayal, Meghna Rathore, Vijay Kumar Kambila and K. K. Venkataratnam Eur. Phys. J. Plus, 135 4 (2020) 360 Published online: 15 April 2020 DOI: 10.1140/epjp/s13360-020-00362-3