What are the most popular theories on the expansion of the universe?
THEORIES ON EVOLUTION OF UNIVERSE
- Big bang theory
The big bang theory was proposed by Le Maitre and Gamow. The idea of expanding the universe leads to the idea that before in time all the galaxies were closer to each other as compared to their current position. This is the fact applied in the big bang. According to big bang theory whole of this universe was concentrated in a highly dense and hot singularity having a temperature of about 1012 K. About 19.8 billion years ago a huge explosion took place and the singularity (a single point of highly dense and extremely hot fireball) burst out and was thrown out with extremely high speed in all directions forming stars and galaxies. All these are galaxies and stars are still moving away from each other.
2. Steady-state theory
This theory was proposed by Hermann Bondi, Thomas Gold, and Fred Hoyle. This theory is based on the idea that the universe has always existed. New galaxies are continuously being formed which fills the gap created by other galaxies crossing the boundary of the observable universe. Hence the matter present in the observable universe always remains constant. This theory is supported by Hubble's principle but not by CMBR. So, scientists are not giving much attention to it.
3. Pulsating theory
This theory is based upon the idea that the universe is supposed to be expanding and contracting periodically taking billions of years. At present time the universe is expanding. According to this theory after a certain time, the universe stops expanding and starts contracting up to a certain size. After contracting up to a certain size universe will again expand. This process repeats itself.