Atronomy

ASTRONOMY
Unit I our picture of the universe
The lesson ‘Our picture of the universe’ has been adapted from Stephen Hawking’s ‘A Brief history of time’. It is the most popular book about ‘Cosmology’. More than 10 million copies have been sold. Even a film has been made on that book. stephen Hawking is one of the well known living scientists. At the age of twenty, he was diagnosed with a terrible disease ‘ALS’ (Amyo trophic lateral sclerosis).He was expected to live two years but with his strong will he completed his research and still he is alive .This lesson makes us think about some long-standing questions like whether the universe had the beginning and if so when? What is the nature of the time? And many more which doesn’t have an exact answer.

Our ancestors believed that the earth was a flat plate supported on the back of a gaint tortoise standing on an infinite tower of tortoises. In 340bc the greek philosopher Aristotle, in his book ‘on the heavens’, put forward two arguments for believing that the earth was a round sphere rather than a flat plate. First, he realized that eclipses of the moon were caused by the earth coming between the sun and the moon. The earth’s shadow on the moon was always round which would be true only if the earth was spherical. Second, the Greeks knew from their travels that the North Star appeared lower in the sky when viewed in the south than it did in more northerly regions. Aristotle thought that the earth was stationary and the sun, the moon, the plants and the stars moved in circular orbits about the earth. This idea was elaborated by Ptolemy in the second century AD into a complete cosmological model. The earth stood at the centre, surrounded by eight spheres that carried the moon, the sun, the stars, and the five planets known at that time: mercury, venus, mars Jupiter and Saturn.
In 1514 Nicholas copernicus proposed that the sun was stationary at the centre and the planets moved in circular orbits around the sun. In 1609 galileo found that several small satellites rotate around the planet Jupiter. This implied that the planets need not orbit directly around the earth as Aristotle and Ptolemy had thought. At the same time Johannes kepler had modified Copernicus theory and suggested that the planets moved not in circles but in ellipses. But he could not prove that the planets were made to orbit the sun by magnetic forces. In 1687, sir Issac Newton in his book ‘philosophiae naturalis principia mathematica’, postulated a law of vniversal gravitation according to which each body in the universe is attracted towards every other body by a force depending on their mass and distance between them. Also it is due to this gravitational force the earth and the planets follow elliptical paths around the sun.

According to Newton’s law of gravity the stars should attract each other and they may fall in at some points. But in an infinite space, if infinite stars are distributed uniformly, this would not happen because there is no central point for them to fall. When we consider a finite situation the stars fall in on each other. If we add additional stars outside this region, the extra stars would make no difference at all and would fall in just as fast. Also it is realized that it is impossible to have an infinite static model of the universe in which gravity is always attractive. Newton‘s theory of gravity is modified by making the gravitational force repulsive at very large distances. It allows an infinite distribution of stars to remain in equilibrium as the attractive forces are balanced by the repulsive forces.
In 1929 Edwin Hubble observed that the distant galaxies were moving rapidly from us. This implied that the universe is expanding. This means that at earlier times objects would have been closer together. Hubble’s observations suggested that there was a time, called the big bang when the universe was infinitesimally small and infinitely dense. It may be supposed that time had a beginning at the big bang. In an unchanging universe a beginning in time is something that has to be imposed by some being outside the universe. One could imagine that god created the universe at the instant of the big Bang.

Any physical theory is always provisional in the sense that is only a hypothesis. The eventual goal of science is to provide a single theory that describes the whole universe. If everything in the universe depends on everything else in a fundamental way. It might be impossible to get close to a full solution by investigating parts of the problem in isolation even though these partial theories holds good in describing and predicting certain class of observations. Today scientists describe the universe in terms of two basic partial theories namely the general theory of relativity and Quantum mechanics. The general theory of relativity describes the force of gravity and the large- scale structure of the universe. Quantum mechanics deals with phenomena on extremely small scales such as a millionth of a millionth of an inch. Unfortunately, these two theories are known to be inconsistent with each other i.e. they both cannot be correct. One of the major endeavours in physics today is the search for a new theory that will incorporate both of them. It seems difficult to justify on practical grounds. But ever since the dawn of civilization, people’s desire for knowledge of the underlying order in the world is justified. We still yearn to know why we are here and where we came from. Our goal is to describe the universe we live in with the help of a complete unified theory.