Competitive Examination Notes :- 002

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Big bang
Physists believes that about 13.7 billion years ago there was a vast explosion in the emptiness of space. This explosion, usually referred to as big bang, marked the moment of the birth of universe. Before big bang there was nothing thet could be considered as physical material, not even atom. Bigbang theory seeks to explain what happened at or soon after the big bang of universe. Scientists ca now model the universe 10-43 seconds after the big bang. 

Further, big bang theory does not explain what was before big bang. In the first ten thousandths of a second, of this explosion there developed unimaginably high temperature and density that helped to create some of the first building blocks from which atoms formed. These pieces of matter today are known as Quarks and Mesons.
The Big bang model is based on two assumptions. The first is Albert Einstein's General Theory of Relativity. It correctly describes the gravitational interaction of all matter. The second assumption, called the Cosmological Principles, states that an observer's view of the universe depends neither on the direction in which he looks nor his location. This principle applies only to the large-scale properties of the universe, but it does imply that the universe has no edge, so that the bigbang origin occurred not at a particular point in space but rather throughout space at the same time. These two assumptions make it possible to calculate the history of the cosmos after a certain epoch called the Planck Time. Scientists have yet to determine what prevailed before Planck Time.

Universe cooled as it expanded. After one second, protons were formed. In the following few minutes combinations of protons and neutrons formed.
In the first few minutes after the big bang with temperatre of at least 1 million degree, nuclear reactions started to create the basic structure of the gases hydrogen and helium. From about 380,000 to 1 millions years after the big bang universe cooled to about 3,000 degree centigrade, protons and electrons continued to make hydrogen atoms. Radiated energy of light and heat stemmed from the original expansion. Long process of formation of first stars started. At first new atoms joined a thin cloud of hot gases. The radiation that kept the atoms moving eventually weakened as millions of years passed. Atoms began to cluster together.

Most obvious proof of big bang theory is the fact that everythhing in the universe is moving away from everything else. Galaxies comes in different shapes and forms. Most of them spiral like the Milky Way and most of the galaxies contains thousands of millions of stars.
Next :- Milky Way
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