Cosmology

Einstein was the first one to apply GR to the study of cosmology. Spatial homogeneity and isotropy leads to the Robertson–Walker spacetime with the overall scale factor a ( t ) controlling the time evolution of the universe. It gives rise to a spatial expansion in agreement with the Hubble relation, with a big bang singularity at t = 0 . Another great Einstein contribution is his showing that the GR field equation can accommodate a component of cosmic repulsion. A (large) cosmological constant is the key ingredient in the inflationary paradigm of the big bang. Its general prediction of a flat universe has been verified observationally. A cosmological constant in the present epoch explains the 1989 discovery that the cosmic expansion is accelerating. All different modes of astrophysical study (supernova redshift, microwave background anisotropy, and distribution of galaxies) have arrived at a concordant cosmic picture, called ΛCDM cosmology, having an energy content of 75% in dark energy (cosmological constant Λ), 21% in cold dark matter, and the familiar baryonic matter being the remaining 4%.