Science is all about theories and Proofs. The beginning of the universe has been a topic of discussion since long and similar to any other scientific topics theories were proposed to explain it!
Why should we believe that the Universe has a beginning?
The Hubble telescope is one of the finest telescope with the highest resolution of all telescopes. It orbits the Earth like a satellite and captures images of numerous planets, stars and galaxies by studying the radiation emitted by them (using spectroscopy and other techniques ). Likewise by measuring the doppler light shift of distant galaxies it found that the galaxies are moving away from each other with a relative velocity. Also it observed that the farther away is the galaxy the faster it moves. Since the distances of galaxies is increasing as observed today we can extrapolate to a point billions of years ago. When the galaxies were closer to each other and at zero time they were confined to a single point or singularity. This time is when the universe was formed!
Why Big Bang only?
As seen above how We extrapolated the beginning to a singularity, the question now was to explain how exactly the universe was formed from a singularity?
Here many theories were proposed like the Steady state Theory, the Big bang theory, etc. and the best theory to be accepted was the Big Bang Theory! Why?
Today, when the technology is highly developed, we have proved many predctions stated by this theory. Hence, it gave a direct approval to this theory (still scientists believe that this is not the best that we can predict. That is, more development can be made in the future !)
As of now- this is the most acceptable theory.
What is the Big Bang?
As the name suggests The Big Bang is nothing like a collision of giant objects that split into smaller ones. As mentioned above, it is a start from the singularity.
In the beginning the universe was hot and dense.
The theory takes into account the Einstein’s theory of relativity ( E = mc^2 ).
At the beginning of the universe in the dense form light (photons) and mass (fermions) existed with mutual interconversion. The requirement for two photons to be converted to mass ( electron e- positron e+ pair ) is that the photons should have minimum energy greater than rest energy of e- and e+ because owing to conservation of momentum when e- and e+ are produced they will have momentum developed and hence kinetic energy developed and hence their final energy will be sum of rest energy and kinetic energy developed. So as per conservation of energy the photons should have energy greater than their rest energy.
At t=0 as the universe was hot the photon gas had enough energy to convert to fermions(fermions are the fundamentals of mass). And fermions converted back to photons by releasing energy E=mc^2.
As the Big Bang (time event) occured the universe started expanding using its own energy and thus cooling. Hence the photon gas didn’t have enough energy to collide and form a e- and e+ pair. Hence leaving an extremely small amount of fermions. These fermions combined to form protons and atoms and so on.
Yes even today the universe has almost >90% of energy and very very small amount of matter + dark matter. The Big Bang explains this.
By using calculations, scientists estimated the Big Bang occurrence to be around 13.7 billions of yrs ago.
However the question still remains is what aggravated the Big Bang?
We all know conversion of mass to energy in nuclear reactors and on stars. However we can proudly say that today we can even generate matter from light. Thus providing one more reason to accept the Big Bang theory.