Numerical thermodynamic studies of classical gravitational collapse in 3+1 and 4+1 dimensions

We study a thermodynamic potential during the classical gravitational collapse of a 4D (3+1) massless scalar field to a Schwarzschild black hole in isotropic coordinates. We track numerically the function $F(t)=-dI/dt=-L$, where $I$ is the total action of matter plus gravitation, $L$ the total Lagrangian and $t$ is the time measured by a stationary clock at infinity. At late stages in the collapse, this function can be identified with the free energy $F=E-TS$ of the black hole where $E$ is the ADM mass, $T$ the Hawking temperature and $S$ the entropy. From standard black hole thermodynamics, the free energy of a 4D Schwarzschild black hole is equal to $E/2$. Our numerical simulations show that at late stages of the collapse the function $-L$ approaches a constant to within 5% of the value of $E/2$. We also present numerical results for the thermodynamics of 5D collapse where the free energy in this case is $E/3$. In both 4D and 5D, our numerical simulations show that at late stages of the collapse, the metric fields are nonstationary in a thin region just behind the event horizon (and are basically static everywhere else). The entropy stems mostly from the nonstationary interior region where there is a significant dip (negative contribution) to the free energy.Comment: 35 pages, 14 figures, to appear in Phys. Rev.

Extremal black holes, gravitational entropy and nonstationary metric fields

We show that extremal black holes have zero entropy by pointing out a simple fact: they are time-independent throughout the spacetime and correspond to a single classical microstate. We show that non-extremal black holes, including the Schwarzschild black hole, contain a region hidden behind the event horizon where all their Killing vectors are spacelike. This region is nonstationary and the time $t$ labels a continuous set of classical microstates, the phase space $[\,h_{ab}(t), P^{ab}(t)\,]$, where $h_{ab}$ is a three-metric induced on a spacelike hypersurface $\Sigma_t$ and $P^{ab}$ is its momentum conjugate. We determine explicitly the phase space in the interior region of the Schwarzschild black hole. We identify its entropy as a measure of an outside observer's ignorance of the classical microstates in the interior since the parameter $t$ which labels the states lies anywhere between 0 and 2M. We provide numerical evidence from recent simulations of gravitational collapse in isotropic coordinates that the entropy of the Schwarzschild black hole stems from the region inside and near the event horizon where the metric fields are nonstationary; the rest of the spacetime, which is static, makes no contribution. Extremal black holes have an event horizon but in contrast to non-extremal black holes, their extended spacetimes do not possess a bifurcate Killing horizon. This is consistent with the fact that extremal black holes are time-independent and therefore have no distinct time-reverse.Comment: 12 pages, 2 figures. To appear in Class. and Quant. Gravity. Based on an essay selected for honorable mention in the 2010 gravity research foundation essay competitio