History of the Universe

The Big Bang Theory

The Big Bang Theory is the leading explanation about how the universe began. At its simplest, it says the universe as we know it started with a small singularity, then inflated over the next 13.8 billion years to the cosmos that we know today.

Georges Lemaitre had proposed a model for origin of the universe which became the Big Bang theory. The Big Bang theory explains the origin of universe, with the key idea that the universe is expanding. In the past, the universe was denser and hotter. All of space was contained in a single point, which is considered the inception of the universe. After the initial expansion, the universe freezes that allowed the creation of subatomic particles, consisting of protons, neutrons, and electrons. The majority of atoms formed were hydrogen, along with helium and traces of lithium. Huge clouds of these primal elements later fused through gravity to form stars and galaxies.

The massive elements were synthesized either within stars or during supernova. Hence, the Big Bang theory does not explain the initial condition of the universe, but instead it describes the general evolution of the universe.

The Big Bang theory is based on Albert Einstein’s theory of general relativity. This also takes into consideration the homogeneity and isotropy of space. It was Edwin Hubble (1929), who discovered that the distances to remote galaxies were firmly correlated with their redshifts. According to Hubble, all observable areas of the universe are receding from each other.

Once there were two theories for explaining the expansion of universe, the Big Bang and the Steady State theory. But in 1964, with the discovery of the cosmic microwave background radiation, the Big Bang theory was confirmed.

In 1992, the launching of the Cosmic Background Explorer (COBE) satellite showed that 99.7% of the energy of the universe was released within the first year of its evolution. This confirmed the Big Bang theory, which proposes that the universe originated from a single violent explosion (which gives the name big bang) of a very minute amount of matter of high density and temperature.


First things first, let's explain why this timeline has such an odd shape. We mentioned before that based on its expansion, scientists approximate the universe to be about 15 billion years old. However, we can't see the whole universe, and we don't know how big it is. We can only see as far as light can travel, and right now we cannot "see" galaxies that are more than 15 billion light-years away because they have not yet entered our horizon.

Have you ever watched the sunset at the beach, and noticed how the sun appears to just fall below the surface of the water? You know, however, that the reason the sun appears to drop is because as the Earth rotates, the sun drops lower in the sky as it becomes further and further from our line of sight. Finally, the sun sets on the "horizon" which is the farthest distance we can see due to the curvature of the Earth. We use this same term to refer to the universe because as far as 15 billion light years is all we can see--our horizon.