The universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space, although this usage may differ with the context. The term universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world, or nature. Observations of earlier stages in the development of the universe, which can be seen at great distances, suggest that the universe has been governed by the same physical laws and constants throughout most of its extent and history.
Three Theories of How the Universe Began:
Astronomers have three rival theories about how the universe come into being. Of the three, the simplest the Big Bang Theory is the current favorite.
The Bing Bang Theory:
According to this theory, the universe had its origin in a giant explosion about 18,000 million years ago. The matter flung out from the explosion condensed into lumps called galaxies, which are still rushing outwards. As the Universe grows old, the matter in it thins out. The expansion continues indefinitely. The Big Bang theory received its strongest confirmation when ‘cosmic background radiation ‘(the glow left over from the explosion itself) was discovered in 1964 by Amo Penzias and Robert Wilson, who won the Nobel Prize for this discovery. In 2003, NASA’s Wilkinson Microwave Antisotropy Probe made measurements of the temperature of this ‘cosmic microwave background’ radiation to within millionth of a degree. From these measurements scientists were able to deduce that our universe is 13.7 billion years old and that first generation stars began to form a mere 200 million years after the Big Bang.
The Oscillating Universe Theory: This theory, a variation of the Big Bang Theory, suggests that expansion of the Universe will eventually slow down and stop, followed by contraction of the galaxies into another Big Bang. The Universe, therefore, continues in endless cycles of expansion and contraction; the laws of nature may differ in each cycle.
The steady State Theory: An alternative view of the Big Bang Theory, this theory says that Universe never originated at any one instant, nor will it ever die. According to this theory, as the universe expands new matter is created to fill the space left. Therefore, the appearance of the Universe remains constant with time.