The most trusted theory of our universe’s origin centers on a cosmic cataclysm-the big bang. The theory describes how the Universe began and evolved since that time.

So, the Big Bang started about 13.7 billion years ago. Since then, the Universe is in a rapid expansion and still evolving.

This theory was born from the observation that other galaxies are moving away from our own at great speed. They are moving in all directions as if they had all been propelled by an ancient explosive force.

Timeline of oxygen formation and reionization (Image: NAOJ)

Is there a theory for what happened before the Big Bang?

If you want to start with theories… there are some.

Before the big bang, scientists believe that the entire vastness of the observable universe compressed into a hot, dense mass just a few millimeters across. This nearly incomprehensible state, scientists theorize, had existed for just a fraction of the first second of time.

Big bang advocators suggest that some 10 billion to 20 billion years ago, a massive explosion allowed all the universe’s known matter and energy to spring from some old and unknown type of energy.

The theory suggests that, in the trillionth of a second, after the big bang, the universe expanded with baffling speed from its pebble-size origin to astronomical extension. Expansion has apparently continued, but much more slowly, over the ensuing billions of years and it still is.

However, we simply don’t know what caused the big bang. Yet.

Origin of the theory

Georges Lemaître noted in 1927 that people can trace an expanding universe back in time to an originating single point. Since then, scientists have built on his idea of cosmic expansion. The scientific community was once divided between supporters of two different expanding universe theories. The Big Bang and the Steady State theory. However, empirical evidence provides strong support for the former.

Georges Lemaître (1894–1966), Belgian priest, astronomer and professor of physics at the Catholic University of Leuven. (Estimated date of photography: 1933.)

In 1929, from analysis of galactic redshifts, Edwin Hubble concluded that galaxies are drifting apart. This is an important observational evidence consistent with the hypothesis of an expanding universe. In 1964, the cosmic microwave background radiation was discovered. It was a crucial evidence in favor of the Big Bang model since that theory predicted the existence of background radiation throughout the universe before it was discovered. Measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating. Thus, approving the existence of the dark energy.

Scientists can use the known physical laws of nature to calculate the characteristics of the universe in detail back in time to an initial state of extreme density and temperature.