Though subsequently the data showed that the universe
is in fact expanding and Einstein rejected the
modification,
on a philosophical basis the question still remains
whether the *measured* cosmological constant is indeed zero
(remember that on philosophical grounds Aristotle rejected
heliocentrism: one must eventually back assumptions with observations).
For many years the best value for the cosmological was assumed to be
zero since no measurement gave positive indication to the contrary. Yet
even a very small pressure can be important if it permeates the whole
universe.

Recent measurements of the expansion rate of the universe (see Sect. 8.4.1) using type Ia supernovae (Sect. 8.4.1) favor an open universe with a small but non-zero cosmological constant. If these results are confirmed, Einstein's ``blunder'' will prove to be one more piece in the jigsaw of nature.