Chapter 2 Concordance Model of Cosmology — Λ CDM At the beginning of the 20 th century, the field of Cosmology was devoid of General Relativity, proof of the existence of other galaxies and that the Uni- verse was expanding, evidence of the Cosmic Microwave Background, here- after referred to as the CMB, as well as the Big Bang Theory, or indeed any theory of the genesis of the Universe that was scientifically based. By the end of the 20 th century however, a consistent and hugely successful model of the entire history of the Universe (save for the very first moments after the creation of spacetime, if indeed even such a creation occurred) was firmly established. Several hundred years ago, the Polish astronomer Nicolaus Copernicus proposed an alternative to the Ptolemaic view, which stated that the Earth was at the centre of the Universe. 1 Copernicus stated that it was the Sun $^{1}$The Heliocentric system was originally proposed by Aristarchus of Samos in the 3rd century BC, whom Copernicus was aware of and cited. 3