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arXiv · 2512.23838

Thermodynamic stability in an Einstein universe

E. S. Moreira Jr.·J. P. A. Paula
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Abstract

We calculate the Feynman propagator at finite temperature in an Einstein universe for a neutral massive scalar field arbitrarily coupled to the Ricci curvature. Then, the propagator is used to determine the mean square fluctuation, the internal energy, and pressure of a scalar blackbody radiation as functions of the curvature coupling parameter $\xi$. By studying thermodynamics of massless scalar fields, we show that the only value of $\xi$ consistent with stable thermodynamic equilibrium at all temperatures and for all radii of the universe is $1/6$, i.e., corresponding to the conformal coupling. Moreover, if electromagnetic and neutrino radiations are present at the regime of high temperatures and/or large radii, we show that at least one scalar field must also be present to ensure thermodynamic stability.

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E. S. Moreira Jr., J. P. A. Paula. 2025-12-29. Thermodynamic stability in an Einstein universe. https://doi.org/10.1103/zcml-wwcz

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