Gravitational instanton, inflation and cosmological constant
  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 density of vacuum energy-gain in such process gives the cosmological term in the Einstein equation. This naturally results in the inflationary phase of Early Universe. While the reheating phase is attributed to the Hawking radiation of these quantum black holes. In the Standard cosmology era, this cosmological term depends on the reheating temperature and asymptotically approaches to the cosmological constant in matter domination phase, consistently with current observations. 
She-Sheng Xue