Standard cosmology and Eintein ’ s cosmo - logical constant

In this work we review briefly the origin and history of the cosmological constant and its recent reincarnation in the form of the dark energy component of the universe. We also comment on the fundamental problems associated to its existence and magnitude which require and urgent solution for the sake of the internal consistency of theoretical physics. 1 Standard cosmology and Eintein’s cosmological constant In 1915 Einstein introduced his famous field equations for gravity in the context of General Relativity, namely: Rμν − 1 2 Rgμν = 8πGTμν , (1) where Rμν is the Ricci tensor and R is the scalar curvature corresponding to the space-time metric gμν of signature (+,−,−,−), G is the Newton constant and Tμν is the matter energy-momentum tensor which for a perfect fluid has the general form: Tμν = (ρ+ p)uμuν − pgμν , (2) with ρ and p being the energy density and pressure respectively and uμ is the four-velocity. From the fluid proper frame this velocity is by definition ∗Contribution to the proceedings of ”Space Astronomy: The UV window to the Universe”, El Escorial, Spain, 2007