Big Bang theory:
The most important current theory for the origin of the universe is the Big Bang theory.According to this theory, the universe started with'a huge explosion. It was not an explosion like the ones with which we are familiar, which start from a definite centre and spread out. It was an explosion which occurred everywhere in space at the same time. It filled all space from the beginning, with every particle of matter rushing apart from every other particle. This was not a burst of matter into space but rather an explosion of space itself. Every particle of matter rushed away from every other particle. It is so far impossible to 'picture' the fi~st moment of 'creation' of the universe. One-hundredth of a second after the creation of the universe is the earliest time about which scientists can speak with any confidence. At this instant, the temperature of the universe was about a hundred billion degrees centigrade. This-is much hotter than in the centre of even the hottest star. At such temperatures none of the components of ordinary matter, atoms, molecules, or even nuclei of atoms, could have held together. Instead, the matter rushing apart in the explosion consisted of various types of elementary particles. The particles most abundant in the early universe were the electrons, positrons and neutrinos. There were also some protons and neutrons. The rest of the universe was filled with energy. It was a kind of a cosmic soup. As the explosion continued, matter and energy rushed apart, the universe expanded and the temperatures dropped, reaching 30 billion (3x1010) degrees centigrade after about one-tenth of a second; 10 billion degrees after about one second; and 3 billion degrees after about fourteen seconds. At the end of the first three minutes, the universe became cool enough (about 1 billion 0C) for the protons and neutrons to begin to form into simple nuclei. The first to be formed was the nucleus of heavy hydrogen which was made up of one proton and one neutron. There were also helium nuclei made of two protons and two neutrons. It was still too hot for atoms to hold together, they were ripped apart as soon as they were created. This matter continued to rush apart, becoming steadily cooler and less dense.
Many thousands of years later, it became cool enough for electrons to join with nuclei to form atoms of hydrogen and helium. Soon, the resulting gas began to form clumps under the influence of gravitation. These clumps ultimately condensed to form the galaxies and stars of the present-day universe, almost 5 billion years after the Big Bang. There is another theory about the origin of the universe known as the steady state theory. This theory holds that the universe has always been just about the same as it is now. As it expands, new matter is created continuously to fill up the gaps between the galaxies. Thus, the problem of the origin and early moments of the universe is banished: there was no early universe. However, the Big Bang theory is the most favoured by the astronomers and astrophysicists. Why is it so? This is due to the evidence based on observations which lend support to the 'Big Bang' universe.