  Expanding the black hole thermodynamics from the horizon to achronal Cauchy
hypersurface, the general relation between the Einstein equation and
thermodynamics is established. Starting from trivial entropy that is
generalized by Bekenstein-Hawking entropy, the entropic mass of matter emerges
naturally together with Unruh temperature. The key idea is that the cause of
mass formation comes down to trivial entropy, and mass density is just the
external manifestation of mass. The full Einstein equation with the
cosmological constant is derived from the requirement that entropic mass and
proper mass are equivalent. This perspective suggests that trivial entropy that
causes mass in gravitation may be the best choice for the origin of space-time
geometry.
