Cosmology: Traveling Back and Forth in Einstein’s Vehicle of GR

The most remarkable aspect of our description of the Universe, is that energy considerations at its very early stages in its evolutionary process, led to postulate the existence of a period in which the Universe expanded enormously, referred to as inflation, which shaped the standard modern model of cosmology. The latter model is commonly referred to as the Inflationary Big Bang theory. We first provide a general, but rather comprehensive, treatment of the standard modern model, in its entirety, using, in the process, the knowledge acquired by the reader on GR developed in the previous chapter, starting from Einstein’s 1917 model in which he introduced a cosmological constant, emphasizing, in the process, historical aspects. In subsequent sections, additional pertinent details are given such as on dark matter and dark energy, evolution of the Universe, its age and the extension of its visible part, as well as on the cosmic microwave background radiation and power spectrum. Finally, inflation is developed in some detail in the concluding section, shaping the commonly accepted modern theory of cosmology.