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Big Bang Theory

by Ron Kurtus (updated 18 January 2022)

The Big Bang theory is the major explanation of how the universe began. Observations show that the universe is expanding, and the galaxies seem to be moving out from a specific point. By interpolating backward, it is estimated that all of the galaxies in the universe started with an explosion some 13.7 billion years ago. An opposing theory states that the Universe is oscillating. Also, there are other questions concerning the beginning of the universe.

Questions you may have include:

This lesson will answer those questions.

Galaxies moving away

The whole idea of the Big Bang theory came when astronomers noticed that distant nebulas (or more correctly, nebulae) and galaxies were all moving away from us. It was also seen that they were moving away at speeds proportional to their distance from us. Since it is assumed that our galaxy is also moving in some direction, it was calculated that all of the galaxies in the universe were moving away from some given point in space.

Estimating speed

The way astronomers were able to estimate the speed of the galaxies is by what is called the "red-shift" of their light. The red-shift is a form of Doppler effect with light, such that when an object is moving away from you at high speeds, the light shifts towards lower frequencies.

Calculating point

Knowing the velocities of the galaxies and their distances from this given point in space, the astronomers were able to calculate backwards to the time when all these galaxies were at a single point. Thus it was assumed that some sort of explosion blew all of the material apart.

Explains formation of stars

The turbulence of this explosion of pure matter would later result in the spinning galaxies as the matter collected together into stars and other objects. In fact, scientists have even made a scenario of what happened within fractions of a second after the explosion. This was extended to explain how various elements were formed and then the stars, planets and galaxies in the millions and billions of years afterwards.

The theory also predicts the existence of a cosmic background radiation, which is radiation or light left over from the explosion. This radiation was discovered in 1964.

Opposing theories

Although it is accepted that the universe is expanding at an accelerated rate, it is not completely accepted that it started from a large explosion. One theory is that the universe is oscillating, such that it will expand for awhile and then contract and then start expanding again.

Some questions

A problem with the Big Bang theory concerns whether the universe will continue to expand forever. Will the galaxies spread further and further apart until infinity?

Another problem is that the belief that all motion is relative. If there was a big bang, that means there is a distinct reference point for all motion. One interesting thought is that if all matter was compressed to a single point, were space and time also compressed?

Finally, there is the question concerning what existed before this Big Bang? These theories—including that of the Big Bang—can never be proven for sure.


The Big Bang theory explains how the universe began. The universe is expanding, moving out from a specific point. By interpolating backward, it is estimated that all of the galaxies in the universe started at one point. The theory states a great explosion was the start of the universe. There are opposing theories and questions concerning the beginning of the universe.

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Resources and references

Ron Kurtus' Credentials


Big Bang Theory: An Overview - All About Science

The Big Bang Theory - Wikipedia

Astronomy Resources


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