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Semiclassical Einstein equations are used to describe the interaction of the back-reaction of the classical gravitational field with quantum matter fields in semiclassical gravity. We in our previous studies have made use of the semiclassical approximation to demonstrate the phenomenon of particle production, commonly called as preheating/reheating of the universe, which occurs after the inflationary epoch during the oscillatory phase of two-mode quantised massive scalar field of the chaotic inflationary model. We have previously used the language of two-mode coherent and squeezed quantum optical states formalisms to represent the massive scalar field; therefore, it would be useful to examine whether the field states exhibits classical or nonclassical nature in the cosmological context. In the present article, we will examine the nonclassical nature of two-mode quantum optical states in the cosmological context. We have made use of the criterion suggested by Lee in quantum optics, for the existence of nonclassical effects in two-mode states and calculated the equivalent Lee's D 12 (2) parameter with the associated cosmological parameters, to examine the nonclassical nature of the states after inflation during the oscillatory phase of the scalar field. [ABSTRACT FROM AUTHOR]

Semiclassical Einstein equations are used to describe the interaction of the back-reaction of the classical gravitational field with quantum matter fields in semiclassical gravity. We in our previous studies have made use of the semiclassical approximation to demonstrate the phenomenon of particle production, often called preheating/reheating of the universe, which occurs after the inflationary epoch during the oscillatory phase of two-mode quantized scalar field of chaotic inflationary model. During this oscillatory phase, back-reaction effects from the created particles, on account of the quantum nature of the states considered, could be significant and one might be concerned about the validity of the semiclassical approximation in these two-mode quantum optical states. The validity of the semiclassical approximation in these states is examined and it is presented how the magnitude of states parameter draws limit on the applicability and reliability of semiclassical theory of gravity. It is argued that semiclassical theory to gravity is a good approximation for states which are closer to coherent states i.e., with coherent parameters greater than unity and with squeezed parameter much smaller than unity. [ABSTRACT FROM AUTHOR]

We used coherent thermal and squeezed thermal vacuum states to represent the massive inflaton field in the semi-classical gravity; therefore, it would be useful to examine whether the field exhibits classical or nonclassical nature in the cosmological context. In the present article, we examine the non-classical nature of thermal quantum optical states in the cosmological context using Mandel's Q parameter. [ABSTRACT FROM AUTHOR]

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., r s > 1 ; however, the theory is valid when this related squeezing parameter is smaller than the unity, i.e., r s < < 1 and r s < 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. [ABSTRACT FROM AUTHOR]

By using single-mode squeezed thermal states prescription, we study the particle production due to thermal black hole. We analyze that thermal squeezing for a black hole can also be a possible mechanism in order to compute the variation of entropy and mass parameter. We also find a relation of Hawking's temperature with thermal squeezing parameter. [ABSTRACT FROM AUTHOR]