Wrongly considered a primordial explosion, the Big Bang was not a starting point. It is better understood as a story that tells how a unique world emerged in a palpable and temporal universe where the notions of space, time and matter did not exist.
When Edward Hubble proved in 1929 that the universe was in expansion, the consequences for cosmology reached far and wide. Because if the universe was in expansion, this indicated that it was once much hotter and much more dense. Mathematically, retracing back through time is possible, but only up to a certain limit. Reaching time zero is effectively impossible, because Einstein’s equations do not allow us to do so. On the other hand, we can mathematically calculate back to what we call the “Planck barrier”, which is one ten millionth of a billionth of a billionth of a billionth of a billionth of a second after the Big Bang. Thus are written the very first lines of our history, and we will probably never know what came before it….. Or who knows, maybe one day?
Starting from this currently impenetrable barrier, a rapid sequence of events gave rise to ‘the initial singularity’. Very quickly, the four forces of nature (gravitational force, strong and weak nuclear forces and electromagnetic force) separated away from each other and the cosmos entered into a very short period of expansion, during which the size of the universe increased exponentially. The universe quasi-instantaneously ‘exploded’ within the time frame of a hundred thousandth of a billionth of a billionth of a billionth of a second. It became a burning broth of elementary particles comprised of matter and anti-matter, born of pure energy composed of quarks and anti-quarks, electrons and positrons, neutrinos and antineutrinos which were annihilated as soon as they met.
One hundred thousandth of a second after the start of the Big Bang, nuclear force made the quarks assemble in threes, forming various combinations of the protons and neutrons that in turn would later form the nuclei of atoms. It was during this first second that the matter of which we are made up took precedence over antimatter, for a reason that is still under debate. Although, in the beginning, matter and antimatter were produced in equal quantities, the two clearly did not behave in the same way.
The first 1 second was finally over. The universe continued to dilate and slowly cool down, going from an initial temperature estimated at around a billion billion billion degrees to a temperature of around one billion degrees. It was this global “cooling down” that went on to make strong nuclear forces combine protons and neutrons. After three minutes, the atom nuclei that were a little more complex than the simple nucleus of the hydrogen atom (a nucleus with one single proton) started to form.
Once these first three minutes were over, nucleosynthesis naturally followed for a quarter of an hour, after which the universe had to await thermonuclear fusion machines -the first stars -to enrich our array of elements. All of this is only known via calculations, because none of these primordial phases will ever be observable by our instruments, as precise as they may be. Our universe in fact remained dark until it was 380,000 years old, at which point it was lit up. This moment has previously been captured, and has been referred to as “the fossil radiation” from the Big Bang.
Source: Questions clés sciences – Mystérieux Big Bang