Chapter 1. Our Picture Of The Universe

It was in 340 BC when Aristotle first came with his idea that earth is actually spherical in shape. Before that people used to think that earth is plane and due to this they even scared to travel in seas so they don't fall out from the point where the plane ends.

In 2 AD Ptolemy proposed that earth is the centre point of the universe.

Copernicus gave another concept in 1514. He was the first to claim that not earth but the sun is the centre and the planets like earth revolve around it.

This Copernican theory was proved by Galileo in 1609.

During those years only, Kepler proposed that planets move in an elliptical path around the sun.

Then it was time for Newton who proved Kepler's elliptical orbit theory. He also gave his laws of motion and the law of universal gravitation. His theories were published in his book 'Philosophiae Naturalis Principia Mathematica' in 1687. In 1691 he also proved that there is no central point for the stars to fall.

One interesting point to note is that before 20th century, no one even thought on the fact whether our universe is expanding or contracting.

Immanuel Kant, in 1781, came with his 'Critique of Pure Reason' in which he gave his thesis as well as antithesis. The thesis claimed that there is no beginning and there is infinite time before any event. This actually was absurd. The antithesis claimed that if the universe had a beginning and infinite time before it, then why there is a particular time for beginning.

In 1823 a German scientist Heinrich Olbers gave his theory of an infinite static universe.

Then in 1929, Edwin Hubble proved that galaxies are moving away from each other, i.e., the universe is expanding. This made everyone think about the starting of the universe when it was infinitesimally small and infinitely dense. That was the Big Bang.

Presently, two basic partial theories exist:
(1) General theory of relativity, which deals with the force of gravity and the large-scale structure of universe.
(2) Quantum mechanics, which deals with the phenomena on small scales.
Both these theories, when combined, gives the 'Quantum Theory of Gravity'.