Modern Theoretical and Observational Cosmology

Manolis Plionis & Spiros Cotsakis plionis@sapfo. astro. noa. gr skot@aegean. gr Since the dawn of human civilisation natural phenomena have been subject to observation and investigation by the humans who initially ascribed to them 'divine' powers. Gods of 'good' and 'evil' werecreatedaccording to the useful­ ness or notofsuch unexplained, atthetime, phenomena. Astheir understanding of the world developed and deepened, the divine powers, religious beliefs, su­ perstitions and mysticism gave their place to the knowledge, limited that it may be, of physical reality. However, many issues have been and still are out of grasp of human understanding. These issues have always been at the center of philosophical, theological, and more recently, scientific debate. It is to us incredible that many of the conclusions concerning the true scientific explanation of the external world, to which the ancient Greeks arrived purely on the basis of abstract thought, came so near to modem scientific ideas and also form the basis of modem science. We cannot but stand with amazement at the original thoughts of Archimedes who, among his many extraordinary achieve­ ments in mathematics and physics, calculated (cf. TheSandReckoner) the mass density of the observable universe and came up with a figure that is in complete agreement with current estimates coming from observational cosmology.