The Big Bang Theory: Explained!

What is the biggest thing you can think of? The universe? All right. Have you ever wondered what the origin of this vast universe was? Well, if you never did, you are probably wondering now. Can you believe it when I tell you that this eternal universe started from the explosion of a single small hot seed of infinite pressure and density? Yes. That is what the Big Bang Theory has to say regarding the formation of all the celestial matter in space.

The theory

The Big Bang Theory tells us that the entire universe was once condensed in the form of a primeval atom or a dense mass which was very small in size. Then, at one moment which dates very long back, a gigantic explosion caused the universe to expand.

Analysis of the measurements of the rate of expansion revealed that this moment occurred approximately 13.8 billion years ago, which is considered to be the age of the universe. This dense mass expanded very rapidly filling the vast void of space with matter and energy.

The expanding universe started to cool down eventually, forming elements out of the smallest atoms, which in turn formed stars, and the stars formed huge galaxies. This inflation of a tiny point into an unimaginable size took place in the blink of an eye!



Every theory in science is based on certain evidence and observations and the Big Bang theory is no exception to this.

Cosmic microwave background

The discovery of Cosmic Microwave Background is landmark evidence of the Big Bang origin of the universe.

As we have seen earlier, the universe was very hot at the beginning. As it expanded, this heat had left behind a glow (electromagnetic radiations) that now fills it. This is the Cosmic Microwave Background which has been accurately measured by orbiting detectors and is very good evidence that the Big Bang theory is correct.


                                  WMAP image of cosmic microwave background radiation

Red shift of galaxies

To understand this concept, let us first learn about the ‘Doppler effect’.

Consider a train coming towards a platform on which you are standing. As it approaches you, the frequency of sound waves produced by its horn appears to increase because of relative motion between the source of sound (the horn) and the listener (you). This is termed as the Doppler Effect.

Also, when the train starts moving away from you, the frequency of sound waves appears to decrease.


The same is also applicable to light waves. When a source of light starts moving towards the observer, the frequency of waves increases apparently (or wavelength decreases). This is termed as the ‘Blue Shift’ as shorter waves of visible light are blue. When the source moves away, the frequency seems to decrease (or wavelength increases). This is termed as the ‘Red Shift’ as longer wavelengths of visible light are red.


Scientists have been observing red shift in the light reaching the earth from stars which substantiate the fact that the universe is expanding and that everything in the universe is moving away from everything else.

Abundance of primordial elements

As the universe expanded and cooled down, some elements that we see today were created. The Big Bang theory predicts how much of each element should have been made in the early universe, and what we see in very distant galaxies and old stars is just right.

Alternative theories

In fact, there are many theories explaining the process of evolution of the universe, of which The Big Bang is the most popular. Other theories include:

The Steady State Theory

Some scientists believe that the universe is essentially the same from the beginning and that it is expanding at a constant rate. Also, the density of the universe is constant. But what we learned at school is that when volume increases (which is happening here), density decreases. So the density is being maintained with the formation of new matter continuously.

This theory is based on the principle that the universe looks exactly the same from every point within it.steady_state_big_bang.png

Oscillating Universe

Also called the cyclic theory, this speculates that the universe undergoes an infinite cycle of expansions and contractions. Suppose a ‘big bang’ occurred at some point in time and the universe started expanding. This expansion continues till a maximum limit is reached and stops. Now, a ‘big crunch’ occurs which causes the universe to contract. This contraction continues till a minimum limit is reached and a ‘big bang’ occurs again. This goes on and on.


This theory was supported by many scientists in the 1920s including Albert Einstein.

There are other theories like ‘Eternal Inflation’, ‘Contracting Universe’, etc. Of all these different theories, The Big Bang Theory is widely accepted because of the strong evidence it offers. And is there any other better theory waiting to be discovered, only time shall reveal!

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