Cosmology: Understanding the Evolution of the Universe
CHAPTER 1 EARLY UNDERSTANDING NN hroughout recorded history human kind has asked big questions about the universe How large is it Is it finite or does space go on forever How old is it or has it always been here and will it last forever Where in the universe are we the center Or does it even have a center Is the universe fundamentally chaotic or orderly If it is ordered is this order constant through time Or is it perhaps evolving decaying still being created or going through cycles of cre ation and destruction The quest to find answers to such ques tions about the origin history and future of the universe and its structure and order is called cosmology This word in fact comes from the Greek word kosmos mean ing order and the world ANCIENT IDEAS Until the last few centuries these ques tions remained the province of religion and 12
EARLY UNDERSTANDING philosophy Different cultures and traditions at different times offered a wide variety of answers As for our place in the universe a com mon view was the geocentric one that Earth is at the center This model was developed in detail by the ancient Greeks culminating with the elaborate theory of Ptolemy of Alexandria in about 150 CE The Pythagoreans 5th century BCE were responsible for one of the first Greek astro nomical theories Believing that the order of the cosmos is fundamentally mathematical they held that it is possible to discover the har monies of the universe by contemplating the regular motions of the heavens Theorizing that all the heavenly bodies including the Earth and Sun revolve around a central fire they constructed the first physical model of the solar system Subsequent Greek astron omy derived its character from a comment ascribed to Plato in the 4th century BCE who is reported to have instructed the astronomers to save the phenomenal in terms of uniform circular motion That is to say he urged them to develop predictively accurate theories using only combinations of uniform circular motion As a result Greek astronomers never regarded their geometric models as true or as being physical descriptions of the machinery 13
R Pythagoras believed to be the founder of the Pythagorean philosophical school demonstrating his Pythagorean theorem in the sand using a stick Photos com Thinkstock 14
EARLY UNDERSTANDING of the heavens They regarded them simply as tools for predicting planetary positions Eudoxus of Cnidus 4th century BCT was the first of the Greek astronomers to rise to Plato's challenge His model consisted of a complex system of 27 interconnected spheres with Earth at the center One sphere held the fixed stars while each planet occupied four spheres and the Sun and Moon had three each Using only uniform circular motions Eudoxus was able to save the rather complex planetary motions with some success The system was modified by Callippus a student of Eudoxus s who added spheres to improve the theory especially for Mercury and Venus Aristotle in formulating his cosmology adopted Eudoxus's homocentric spheres as the actual machinery of the heavens The Aristotelian cosmos was like an onion consisting of a series of some 55 spheres nested about the Earth which was fixed at the center In order to unify the system Aristotle added spheres in order to unroll the motions of a given planet so that they would not be transmitted to the next inner planet The theory of homocentric spheres failed to account for two sets of observations 1 brightness changes suggesting that planets are not always the same distance from the Earth 15
