The Universe or the Cosmos, as perceived today, consists of millions of galaxies. A galaxy is a huge congregation of stars which are held together by the forces of gravity. Most of the galaxies appear to be scattered in the space in a random manner, but there are many others which remain clustered into groups. Our own galaxy, called the Milky Way o r Akash Ganga, which appears as a river of bright light flowing through the sky, belongs to a cluster of some 24 galaxies called the ‘local group’. The Milky Way is made up of over a hundred billion sparkling stars, which, though quite distant from one another, seem from the Earth as having been placed close together. The two other nearest galaxies are the Large Magellanic Cloud and the Small Magellanic Cloud, named after the famous Portuguese navigator, Ferdinand Magellan (1480- 1521), who discovered them.
The Universe is infinite, both in time and space. Its age was formerly believed to be between 10-15 billion years. However, in 1999, a NASA’s Hubble Space Telescope Project team determined the age of cosmos to be 12 billion years (plus or minus 10 percent). In June 2001, NASA launched the MAP (Microwave Anisotropy Probe) to study the cosmic, microwave background radiation in greater detail according to which the exact age of the universe is 13.7 billion years after the theoretical Big Bang.The human perception of the Universe has, however, been different at different times over the long span of history of civilisation. The innate human inquisitiveness and tireless pursuit of knowledge have brought about revolutionary changes about our ideas of the Universe. The Moon and the stars are no longer looked upon as heavenly bodies or the abodes of gods. Solar and lunar eclipses are no more dreaded as foretellers of natural calamities. Man’s conquest of the Moon has now blown off many a myth of the religious testaments.
It was around 6th century BC that men started enquiring into the mysteries of the Universe in an endeavour to rationally analyse the earthly and the heavenly
phenomena. They posed to themselves several questions : What is the Universe ? Why do things change ? Why do things move ? What is life ? and so on. These questions were of far-reaching significance to the development of modern science.
Ancient Greek astronomers and mathemati-cians came up with the view that the Earth was a perfect motionless sphere, surrounded by seven other crystalline spheres—the Sun, the Moon, and the five known planets, vi%., Mercury, Venus, Mars, Saturn and Jupiter, which revolved round the Earth on seven inner spheres. The stars were permanently fixed to the outer sphere that marked the edge of the Universe.
Ptolemy, a second century Greco- Egyptian astronomer, synthesised the various data gathered by the early Greek astronomers and in his book, Almagest, presented his system of astronomy based on a Geocentric (Earth-centred) Universe. He maintained that the Earth was the centre of the Universe, and the Sun and other heavenly bodies revolved round the Earth. This view of the Universe remained firmly entrenched in the minds of the people right up to the middle of the 16th century. Most menin the Middle Ages strongly adhered to the Ptolemaic system as they felt that they did, indeed, live in a physically limited, rigidly structured Universe centred round a motionless Earth. The Greeks had also estimated the visible Universe to be about 125 million miles in diameter.
The generally accepted view of Geocentric Universe received its first real jolt with the publication of the monumental work by Copernicus (1473-1543) De Revoktimihus0rbmm C&tkstinm (On the Revolution of Celestial Bodies). The main points of the Copemican system are: (i) the Sun and the stars are motionless; (ii) the Sun lies at the centre of the Universe and the stars at its circumference; (iii) the Earth rotates on its axis taking 24 hours to complete one rotation; and (iv) the Earth and the planets revolve round the Sun; whereas the Moon revolves round the Earth.
This system of Universe, as propounded by Copernicus, was more consistent than that of Ptolemy. But its major flaw was that while it changed the centre of the Universe from the Earth to the Sun, it did not enlarge the limits of the Universe, as the Universe still remained equated with the Solar System.
Later, Italian astronomer Galileo Galilei (1564-1642), too, with his newly- invented telescope demonstrated the validity of the Copernican system through his studies of the phases of the Venus and the moons of Jupiter that the Earth did revolve round the Sun. He discovered many new stars and proved that sensory appearances could be deceptive and that it is our own limitations of perception and reason that place boundaries around the Universe. To be punished for telling the truth was not uncommon in the 16 th century, and those who dared to do so, had to face the wrath of the Church. Indeed, Galileo had to pay the penalty for telling the truth.
English scientist Isaac Newton (1642- 1727) demonstrated that forces of gravitation linked all material bodies in an immense Universe and showed that these bodies moved in accordance with strict mathematical laws. God was still the Creator, but he exercised a thorough mastery over mathematics and engineering.
The perception of the Universe was farther widened in the 19 th century when the British astronomer, Sir William Herschel (1738-1822), came out with his observation that the Universe was not limited to the Solar System, but is much vaster than that. The Solar System, according to Herschel, was only a small part of a much bigger star system, called the Galaxy. The Galaxy consisted of millions of stars scattered in the sky in a unique pattern of a band of light called the Milky Way.
Thus, gone are the days of a finite two-sphere geocentric system of
Universe in which the Earth occupied the key position. The Earth is just a planet of the Solar System and there are millions or billions of such systems existing in the skies, some of which have been discovered, while many others remain unobserved. When the great German scientist, Albert Einstein (1879-1955) developed his general theory of relativity, he visualised that the Universe would either expand or else collapse, but in 1920s, Edwin Hubble discovered that the Universe was actually expanding, everything moving away from everything else.