Entropy Modifications from Stochastic Metric Fluctuations

Abstract: Deviations from the area law of the horizon entropy, in the cosmological setup, are known to lead to modified Friedmann equations governing the evolution of the universe. In this work, we propose that such modifications need not be introduced phenomenologically but can emerge dynamically from stochastic fluctuations of the spacetime metric. We consider a Friedmann-Robertson-Walker (FRW) universe perturbed by a conformal, time-dependent noise factor, whose ensemble average vanishes, leaving the mean background geometry unchanged. By averaging the Einstein equations to second order in the fluctuation amplitude, we derive a modified Friedmann equation that includes an effective correction term. This correction is shown to be equivalent to the general expression obtained from an arbitrary deformation of the entropy-area relation. By specifying the statistical properties, particularly the variance of the conformal noise, we successfully reproduce the Friedmann equation corrections associated with several well-known generalized entropy frameworks, including Rényi, (dual) Kaniadakis, Barrow, logarithmic, and MOND inspired hypergeometric entropies. Our results suggest that deviations from the area law can be interpreted as the macroscopic, coarse-grained imprint of unresolved, microscopic stochastic degrees of freedom in spacetime.