Write an explanatory note on the 'Big Bang Theory'.
The Big Bang Theory is the most popular argument regarding the origin of the universe. It is also called the expanding universe hypothesis. Edwin Hubble, in 1920, provided evidence that the universe is expanding. As time passes, galaxies move further and further apart from each other.
In the beginning, all matter forming the universe existed in one place in the form of a "tiny ball" (singular atom) with an unimaginably small volume, infinite temperature and infinite density. At the Big Bang, this "tiny ball" exploded violently, leading to a huge expansion. It is now generally accepted that this event took place 13.7 billion years before the present.
The expansion continues even to the present day. As the universe grew, some energy was converted into matter. There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion slowed down. Within the first three minutes from the Big Bang event, the first atoms began to form.
Within 300,000 years from the Big Bang, temperature dropped to 4,500 K (Kelvin) and gave rise to atomic matter. The universe became transparent. The expansion of universe means increase in space between the galaxies. An alternative was Hoyle's concept of steady state, but with greater evidence about the expanding universe, the scientific community presently favours the expanding universe argument.
Explanation
This answer covers all key aspects of the Big Bang Theory from the NCERT textbook: its definition as expanding universe hypothesis, Hubble's evidence, the initial conditions (tiny ball with infinite temperature and density), the timeline of events (13.7 billion years ago, rapid expansion, atom formation within 3 minutes, atomic matter within 300,000 years), and comparison with Hoyle's steady state theory. The answer follows the 5-mark format with 4-5 paragraphs and uses bold for key terms.