Albert Einstein biggest blunder

This is a theory proposed by Albert Einstein to prove his later failed assertion that the universe is not changing.
That is, a stationary universe.Or,
The cosmological constant ?

However, soon after Einstein states this static theory, observations by Edwin Hubble shows that the universe seem to be expanding, this was the same with a cosmological solution to the original general-relativity equations that had been discovered by mathematician Friedman. Einstein later referred to his failure to determine the expansion of the universe from theory, before it was observed by the cosmological red shift, as the "biggest blunder" of his life.
It is now thought that including the cosmological constant to Einstein's equations cannot lead to a stationary universe at equilibrium as the equilibrium is unstable, if the universe expands minute, it will release vacuum energy, which will lead to more expansion.
In as much, a universe which contracts slightly will not stop contracting. However, the cosmological constant persisted been a subject of theoretical and empirical interest.
On the surface the demise of cosmological data in the past years strongly suggests that our world has a positive cosmological constant.
The explanation of this minute but important value is a hard theoretical challenge on its own.