**1.1.2. The Legacy of the Ancient Greeks**

Bearing in mind that a rich diversity of cosmological models were proposed by many different peoples and cultures throughout history, let us focus on the one which has undoubtedly exerted the greatest influence on our modern cosmological theory - that of the ancient Greeks. The advent of Pythagorean geometry coupled with a bias towards perfection in the Universe, ultimately led to the Aristotelian (circa 350 BC) system of crystalline spheres, with the Earth (Athens?) at the center. A key feature of this cosmological model is the important concept that the Universe is finite. The earlier Universe of mysticism, wonder and discovery had now evolved to a logical, apparently simple geometric universe which could now be drawn on a small piece of paper!. Figure 1-1 shows this simple two-sphere Universe.

The physical foundation of this Universe was defined in terms of four primal forces - Earth, Air, Fire and Water. All that is observed or created in this Universe could be understood as the interaction between these primal forces. This was a highly deterministic universe where the role of chance was explicitly dismissed. However, roughly 100 years prior to Aristotle, Democritus raised a dissenting view. In his Cosmology, nothing existed except for "Atoms and the Void". All else, he argued was "Opinion and Illusion". Furthermore, in this Cosmological model, the Universe essentially ran by itself and was, arguably, a random occurrence, resulting from the collisions of atoms.

Interestingly, much of this framework crafted by the early Greeks is still retained in our modern cosmology. In particular, 1) we still use geometry as the defining characteristic of our cosmological model, 2) we still attribute all observations to the interaction of four forces, in this case the Strong and Weak nuclear forces together with Gravity and Electromagnetism and 3) the role of chance, as described via Quantum Mechanics plays is a key feature of the very early Universe. These three ingredients define an evolving Universe which, like the Greeks understood, becomes available to us through observation and measurement.