Reflections on the hyperbolic plane

Abstract

The most general solution to the Einstein equations in $4=3+1$ dimensions in the asymptotical limit close to the cosmological singularity under the BKL (Belinski-Khalatnikov-Lifshitz) hypothesis, for which space gradients are neglected and time derivatives only are considered, can be visualized by the behavior of a billiard ball in a triangular domain on the Upper Poincaré Half Plane (UPHP). The behavior of the billiard system (named 'big billiard') can be schematized by dividing the succesions of trajectories according to Poincaré return map on the sides of the billiard table, according to the paradigms implemented by the BKL investigation and by the CB-LKSKS (Chernoff- Barrow- Lifshitz- Khalatnikov- Sinai- Khanin- Shchur) one. Different maps are obtained, according to different symmetry-quotienting mechanisms used to analyze the dynamics according to the symmetries of the billiard domain and to the features of the geodesics on the UPHP. In the inhomogenous case, new structures have been uncovered, such that, in this framework, the billiard table (named 'small billiard') consists of 1/6 of the previous one. The connections between the symmetry-quotienting mechanisms are further investigated on the UPHP. The relation between the complete billiard and the small billiard are also further explained according to the role of Weyl reflections. The quantum properties of the system are sketched as well, and the physical interpretation of the wavefunction is further developped. In particular, a physical interpretation for the symmetry-quotienting maps is proposed, according to the Fourier decomposition of the energy levels of the wavefunction of the universe, as far as the meaning of epochs and eras is concerned from a quantum point of view.