Question

Name the theory which attempts to explain 'origin of universe'.

Answer 1

The universe is a very vast and expanded region of space. It is possibly infinite in volume. Billions of galaxies are present in it which are clustered together to form this huge universe. The galaxies are themselves very massive and contain many stars, planets and rocky bodies bound together with the gravitational forces and the clouds of gas and dust.

Complete answer:
Origin of the universe was the most important phenomenon in the origin of life. The Big Bang Theory attempts to explain the origin of the universe.
This theory proposes that the universe is very old and its origin took place almost 20 billion (20 x 10) years ago.
It explains the condition that a single huge explosion (i.e., Big Bang) took place in space which was so powerful that it is unimaginable in physical terms.
The gaseous packets or atmosphere was absent on the early earth. Due to volcanic eruptions, the molten mass was released which slowly covered the surface of earth.
From this molten mass, various gases released into the surroundings like methane $(\mathop {CH}\nolimits_4 )$, ammonia $(\mathop {NH}\nolimits_3 )$, carbon dioxide $(\mathop {CO}\nolimits_2 )$, water vapour etc.
The ultraviolet (UV) radiations present in the sunlight acted upon water and broke up water into hydrogen and oxygen and lighter $\mathop H\nolimits_2$, escaped.
Oxygen readily combined with other gases like $(\mathop {NH}\nolimits_3 )$, and $(\mathop {CH}\nolimits_4 )$, to form again $\mathop H\nolimits_2 \mathop O\nolimits_{}$, $(\mathop {CO}\nolimits_2 )$, and other gases. Thus, the free and nascent oxygen (like today's environment) was not present on the early earth.
When respiration takes place without the presence of oxygen it is said to be anaerobic respiration so in early times earth has no oxygen it is reducing in nature thus organisms perform anaerobic respiration.
The early atmosphere was reducing unlike today's oxidising atmosphere. It was only present in the form of compounds like $\mathop H\nolimits_2 \mathop O\nolimits_{}$, $(\mathop {CO}\nolimits_2 )$ etc. Oxygen also participated in the formation of ozone $\mathop O\nolimits_3$ molecules due to action of UV rays on it.
These oxygen molecules clustered together to form the ozone layer which acts as a shield to save our present atmosphere from harmful UV rays.
The water vapour condensed to form water drops which fell as rain and slowly filled the depressions present on the surface of earth which is supposed to be the origin of large oceans seen today.
So, these changes lead to the formation of the universe.

Note:
Life took approx. 500 million $\left( {\mathop {500 \times 10}\nolimits^6 } \right)$ years to appear or we can say that the life appeared 500 million years after the formation of earth or almost 4 billion (4 × 10 years back from now (as earth formed 4.5 billion years ago)