  Quantum fluctuation of unstable modes about gravitational instantons causes
the instability of flat space at finite temperature, leading to the spontaneous
process of nucleating quantum black holes. The energy-density of quantum black
holes, depending on the initial temperature, gives the cosmological term, which
naturally accounts for the inflationary phase of Early Universe. The reheating
phase is attributed to the Hawking radiation and annihilation of these quantum
black holes. Then, the radiation energy-density dominates over the
energy-density of quantum black holes, the Universe started the Standard
cosmology phase. In this phase the energy-density of quantum black holes
depends on the reheating temperature. It asymptotically approaches to the
cosmological constant in matter domination phase, consistently with current
observations.
