Lecture 36 - The Evolution of the Universe

At a particular instant roughly 15 billion years ago, all the matter and energy we can observe, concentrated in a region smaller than a dime, began to expand and cool at an incredibly rapid ratei... Neutral atoms appeared in abundance only after the expansion had continued for 300,000 years and the universe was 1,000 times smaller than it is now.

The neutral atoms then began to coalesce into gas clouds, which later evolved into stars. By the time the universe had expanded to one fifth its present size, the stars had formed groups recognizable as young galaxies."" P. J. E. Peebles et al., 1994

In the early universe, various elementary particles and radiation were in thermal equilibrium, with everything at the same temperature. The universe was opaque. At an age of roughly 350,000 years, electron scattering off photons became inefficient and the universe became transparent. Matter and radiation decoupled from one another; radiation ‘cooled’ as the universe expanded, but still maintained a Black Body spectrum. This ‘relic radiation’ was discovered in 1964. The ‘cosmic background radiation’ was incredibly isotropic, as one would expect of black body radiation. This posed a major question, “How did the galaxies and clusters of galaxies form from this primordial soup”. The vital clue came in the early 1990s. Since then, there have been attempts to explain the origin of the ‘large-scale architecture’ of the universe. In this lecture, I describe our present understanding of the origin of structures in the universe.