Mass fluctuations in non-rotating BTZ black holes

Abstract: We investigate the impact of oscillations of a black-hole mass around its average value on the three-dimensional black hole geometry. Drawing on a classical framework that conceptualizes fluctuations near an event horizon as mass variations, we introduce a model where the metric of a black hole, formed from the collapse of a massive null shell, exhibits oscillatory behavior in spherical modes. This dynamic is encapsulated by a non-rotating BTZ-Vaidya solution, characterized by the black hole mass fluctuating at a resonant frequency $\omega$ and a small amplitude parameter $\mu_0$. Using a perturbative approach, solutions to the null geodesic equation are determined up to the second order in $\mu_0$. The temporal fluctuations of the event horizon's location induce alterations in the thermodynamic variables' values. Upon calculating the time-averaged values, it is observed that the mean Hawking temperature experiences a slight decrease due to these fluctuations, while the mean entropy exhibits an increase, deviating from trends observed in four- and higher-dimensional spacetimes. Further, the study delves into the influence of these fluctuations on the trajectories of null rays near the horizon, ultimately reaching the anti-de Sitter boundary at late times. The analytical computation of the general solution for the perturbed rays up to the second order underscores the novel approach of this study in examining the effects of mass oscillations on black hole thermodynamics and geometry, contributing a unique perspective to the field.