Properties of the false vacuum as a quantum unstable state

We analyze properties of unstable vacuum states from the standpoint of quantum theory. Some suggestions can be found in the literature that some false (unstable) vacuum states can survive up to times when their survival probability takes a nonexponential form. At asymptotically large times, the survival probability as a function of the time $t$ has an inverse power-law form. We show that in this time region, the energy of false vacuum states tends to the energy of the true vacuum state as $1/t^2$ as $t\to\infty$. This means that the energy density in the unstable vacuum state and hence also the cosmological constant $\Lambda=\Lambda(t)$ should have analogous properties. The conclusion is that $\Lambda$ in a universe with an unstable vacuum should have the form of a sum of the "bare" cosmological constant and a term of the type $1/t^2$: $\Lambda(t)\equiv \Lambda_{\text{bare}}+d/t^2$ (where $\Lambda_{\text{bare}}$ is the cosmological constant for a universe with the true vacuum).