How the universe was formed perhaps is the greatest mystery and the root of other questions – How did life begin? What is consciousness? What are dark matter, dark energy, and gravity?
As we are still looking for answers to how the universe was born and what was before that, the big bang theory might be the best-supported one that describes the origin of our universe. The theory was born of the observation that other galaxies are moving away from the Milky Way at a great speed in different directions.
In the 1920s, a Belgian priest and contemporary physicist named Georges Lemaître first suggested the big bang theory. His idea got major boost from Edwin Hubble's observations that galaxies are speeding away from us in all directions.
He theorised that the universe began from a single primordial atom. He named it the "primeval atom."
However, the first public use of the common term "Big Bang" actually came from a critic — English astronomer Fred Hoyle. On March 28, 1949, Hoyle coined the phrase during a defence of his preferred theory of an eternal universe that created matter to cancel out the dilution of expansion.
Hoyle said the notion that "all matter of the universe was created in one big bang at a particular time in the remote past" was irrational. This is how the term was invented.
What happened after the Big bang?
According to the European Space Agency, the universe's age is around 13.82 billion years, which means that the Big Bang happened 13.82 billion years ago.
At the very beginning of its existence, the universe was very compact. It was less than a million billion billionths the size of a single atom. Right after the big bang, the temperature and average energies within the universe were so high that everyday subatomic particles could not form. Even gravitation, electromagnetism, the weak and the strong nuclear force were combined and formed one fundamental force.
Later, things started cooling down, and according to Berkeley University of California, during the first three minutes of the universe, light elements were born during a process known as Big Bang nucleosynthesis.
According to the National Aeronautics and Space Administration (NASA), roughly 380,000 years after the Big Bang, matter cooled enough for atoms to form. Also, after a billion years of the big bang, more particles began to form, resulting in condensing into the stars and galaxies of our present universe.
From the very beginning of its birth, the expansion of the universe did not stop, but it gradually slowed down. However, according to NASA, a mysterious force now called dark energy began speeding up the expansion of the universe again about five or six billion years after the Big Bang, a phenomenon that continues today.
Our solar system was born a little after nine billion years of the Big Bang.
During the earliest moment, the universe did not have any structure.
According to NASA, the gravitational pull of small fluctuations in the density of matter back then gave rise to the vast web-like structure of stars and emptiness that are seen today. Dense regions pulled in more and more matter through gravity.
The more massive they became, the more matter they could pull in through gravity, forming stars, galaxies and larger structures known as clusters, superclusters, filaments, and walls, with "great walls" of thousands of galaxies reaching more than a billion light-years in length.
Less dense regions did not grow, evolving into an area of seemingly empty space called voids.
The shape of the universe depends on the struggle between the rate of its expansion and the pull of gravity.
According to NASA, the universe is not infinite but has no end. It is just like the area of a sphere.
Thus, the universe will eventually stop expanding and start collapsing in on itself, the so-called "Big Crunch."