Origin and Evolution of Universe and Earth

Universe:

• The universe is a huge cluster of galaxies. Galaxies contain stars and clouds of gas and dust. Considering the size of the universe, the earth is indeed a speak i.e. The universe is vast.  
• Stellar distances are measured in light-years. When we look at stars in a clear night sky we are, in a way, looking back in time. What we see today is an object whose emitted light started its journey millions of years back and from trillions of kilometers away and reaching our eyes now. However, when we see objects in our immediate surroundings, we see them instantly and hence in the present time. Therefore, when we see stars, we apparently are peeping into the past. 
• The Big Bang theory attempts to explain to us the origin of the universe.  

Big Bang Theory - Proposed by Abbe Lemaitre

• According to it, the universe originated about 20 billion years ago due to a thermonuclear explosion of a dense entity. This single huge explosion which is unimaginable in physical terms is called the big bang.
• The universe expanded and hence, the temperature came down. 
• The gaseous clouds which were formed by the big bang condensed under gravitation and converted into many flat discs-like structures called nebula made up of atoms and small particles. Solar nebula was one of them, which formed our solar system. 
• The very hot central part of the solar nebula became still hotter and converted into the sun. 
• Later, due to the condensation of atoms and dust particles moving around the sun other planets were formed. 
• In the solar system of the Milky Way galaxy, the earth was supposed to have been formed about 4.5 billion years back. 
• There was no atmosphere on early earth. It was formed later.