Answer the following questions in about 150 words.

(i) Write an explanatory note on the ‘Big Bang Theory’.

(ii) List the stages in the evolution of the earth and explain each stage in brief.

(i) The Big Bang Theory was given by Edwin Hubble in 1920. This theory was also called the expanding universe hypothesis.

b. According to this theory, the universe is continuously expanding. The galaxies are moving away from each other which makes the universe appears to be growing larger as well.

c. In the beginning, all the matter which formed the universe existed in the form of a tiny ball with extremely small volume, infinite temperature, and infinite density.

d. At the big bang, this tiny ball exploded and lead to a huge expansion.

e. This event is said to be taken place 13.7 years ago.

f. Some of the energy converted into matter and rapid expansion to place within a few seconds. After the three minutes, the first atom began to form.

g. After 300000 years, the temperature fell down to 4500 k and developed the atomic matter.

h. The universe became transparent.

i. Most of the scientists are in favor of this theory. The expansion of the universe indicates the increasing space between the galaxies.

(ii) The stages in the evolution of the earth explained below:

STAGE 1:

a. The earth was in a volatile stage.

b. With the increase in density, the temperature also increased.

c. The material inside the earth started separating on the basis of its density.

d. The materials with a heavy density such as iron sunk towards the center of the earth and the materials to work with lower density begin to rise above.

e. As time passed, it cooled, solidified and condensed into a smaller size. This formed the crust.

f. With the process of differentiation, the material separated into different layers on the basis of their density.

g. Starting from the inside, the four layers of the earth aa our inner core, outer core, mantle and crust.

STAGE 2:

a. The atmosphere of the earth initially consisted of hydrogen and helium.

b. But the intense solar wind had stripped off the atmosphere of these gases.

c. During the cooling of the earth, gases and water vapour be released from the interior surface of the earth. This began the process of the formation of the current atmosphere.

d. The atmosphere consisted of water vapour, nitrogen, carbon dioxide, methane, ammonia, and very little oxygen.

e. With the cooling of the earth, the water vapour released began to get condensed. The carbon dioxide present in the atmosphere dissolved with the rain water.

f. The falling temperature led to more rainfalls.

g. The rainwater got collected into the surface of the earth and oceans were formed.

STAGE 3:

a. The origin of life on the earth was a chemical reaction.

b. Organic molecules which are complex in nature were generated and later assembled.

c. This assembly was in a way that they could duplicate themselves converting inanimate matter into living substance