Theories of the Universe
1. Ptolemy (2 century AD) : He gave a geocentric (centered at the Earth) theory of the universe. According to the theory, planets move in small circles called epicycles each having a centre which moves in a larger circle called an eccentric or a defferent.
2. Copernicus (1543) : His theory was the first scientifically based heliocentric (centred at the Sun) theory of the universe. He also explained that the apparent motion of the stars is because of the Earth's motion.
3. Static Newtonian Universe : This theory states that the universe is infinite (in space and time), static and in steady state. Every particle in the universe attracts every other particle with a gravitational force. The model is unstable.
4. Einstein's universe : The theory introduces a cosmological constant to stabilize the static non-expanding universe. However, this model of the universe is still unstable.
5. Oscillating universe : The universe, in this theory, is said to be in perpetual oscillation of big bang and big crunch. This model was favoured by Friedmann, a famous Soviet cosmologist who discovered that general relativity supports an expanding universe.
6. Big Bang model : This is the most widely accepted model of the universe. The universe started with a big bang and has since then, continued to expand. Time and space originated at the big bang. The Hubble's constant quantifies the rate of expansion. There are still many observed facts that cannot be explained by the big bang model.