Dark Energy

Abstract

Soon after Einstein published his general theory of relativity, in order to derive a static solution out of his equations, he modified the equations by adding  Λ, the cosmological constant term [1]. This extra term could be used to explain the observations of that time that were indicating a non-evolving universe. Although Einstein may not have been aware of this fact originally, this cosmological constant can be interpreted as the vacuum energy density [2], which generates a repulsive force that can balance the attractive gravitational forces due to matter and hence grant a static, although extremely unstable, universe. The cosmological term seemed unnecessary when Hubble observed the cosmic expansion of the universe [3], and Friedmann [4] and Lemaitre [5] developed a model that could well explain the new data. Therefore, Einstein and de Sitter [6] accepted a spatially flat, matter dominated, homogeneous, isotropic, and expanding universe as the cosmological model where the matter density (ρ _m ) is equal to the critical density (ρ _c ), $\Omega _m \equiv \frac {\rho _m}{\rho _c} = 1$, and there is no room for other types of energy. In the 1990s, two independent groups of cosmologists [7, 8] reported direct evidence of cosmic expansion with a positive rate from studies of supernova explosions, although other studies of the age of the universe together with cosmic microwave background (CMB) observations [9] were already indicating the shortcomings of the Einstein–de Sitter model.