The cosmological constant

  When Einstein first studied the universe at large using the General Theory of Relativity he discovered that his equations predicted a universe which was either expanding or contracting, and this was contradicted with the best astronomical observations at the time. He then modified his equations to satisfy the observations. This modification corresponds to the assumption that the whole universe is permeated with a constant pressure (which in his case balanced the expansion yielding a steady universe). this universal pressure is called the cosmological constant

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.