ABSTRACT
This chapter discusses a non-standard ingredient, not observed in ground laboratory experimental results, called cosmological constant. Though a static model was soon after ruled out by the discovery of the cosmological expansion made by Edwin Hubble in 1929, the role played by a cosmological constant within dynamical models was intensively studied by George Lemaitre. In 1917 A. Einstein proposed the first cosmological model based on his new general theory of relativity, presented in its final version just the year before, marking the birth of modern cosmology. He had to face the same problem that Newton also had almost three centuries earlier. The cosmological constant term can also be interpreted, instead of an additional term modifying Einstein's equations, as a contribution to the energy-momentum tensor from a non-standard fluid. Even though de Sitter model is dynamical, there is still no privileged time and, therefore, the de Sitter model is a perfectly time homogeneous model with a constant expansion rate.
