de Sitter space and Holography

1. Historical review and motivation• In 1917 the cosmological constant Λ was introduced byEinstein to get static homogeneous universe in the present ofmatter.• In 1920 Slipher’s works showed that the light from galaxieswas redshifted indicating that they are moving away from us.• In 1922 a matter dominated expanding universe withoutcosmological constant was constructed by Friedmann.• In 1923 in a letter to Weyl, Einstein says“ If there is no quasi-static world, then away withcosmological term.”• In 1927 the conclusive discovery was made by HubbleA linear expansion law relating redshift to distance whichmade Friedmann models the standard geometrical framework.3