5.3. The History of
I wouldn't venture bets on the final status of . It is nonetheless interesting to recall its history. was of course introduced by Einstein in 1917 to permit a static unbounded universe. After 1929, the cosmic expansion rendered Einstein's motivation irrelevant. However, by that time de Sitter had already proposed his expanding -dominated model. In the 1930s, Eddington and Lemaitre proposed that the universe had expanded (under the action of the -induced repulsion) from an initial Einstein state. fell from favour after the 1930s: relativists disliked it as a ``field'' acting on everything but acted on by nothing. A brief resurgence in the late 1960s was triggered by a (now discredited) claim for a pile-up in the redshifts of quasars at a value of z slightly below 2. (The CMB had already convinced most people that the universe emerged from a dense state, rather than from an Einstein static model, but it could have gone through a coasting or loitering phase where the expansion almost halted. A large range of affine distance would then correspond to a small range of redshifts, thereby accounting for a ``pile up'' at a particular redshift. It was also noted that this model offered more opportunity for small-amplitude perturbations to grow.)
The ``modern'' interest in stems from its interpretation as an vacuum energy. This leads to the reverse problem: Why is ~ 120 powers of 10 smaller than its ``natural'' value, even though the effective vacuum density must have been very high in order to drive inflation? The interest has of course been hugely boosted recently, through the claims that the Hubble diagram for Type 1A supernovae indicates an acceleration.
(If is fully resurrected, it will be a great ``coup'' for de Sitter. His model, dating from the 1920s, not only describes the dynamics throughout the huge number of ``e-foldings'' during inflation, but also describes future aeons of our cosmos with increasing accuracy. Only for the 50-odd decades of logarithmic time between the end of inflation and the present does it need modification!).