The First Instants
The Planck Era
According to the Heisenberg's uncertainty principle, it's physically impossible to know what happens in a time period inferior to 10-43 second (the time of Planck), equivalent to the time spent by light to cross 10-32 mm. Therefore, nothing is known about what happened during the first 10-43 second after the Big Bang. The Universe occupied a space 1020 times smaller than an atomic nucleus (10-32 mm) and, according to the beliefs of most scientists, it was at an instant close to that moment that gravitation was set apart from the other three fundamental forces of the Universe (electromagnetism, weak and strong nuclear forces). Previously all of these forces had been together in one single force, eventually described by the Superstring Theory.
At the 10-35 second occurs the second most important event in the Universe's history: according to the Alan Guth's Inflation Theory, in each 5 x 10-35 second from then on, the Universe doubled its size, having the expansion only been slowed down when a 10-32 second period had elapsed since the Big Bang. At that time it's estimated that the Universe was about 1050 times bigger than it was prior to the inflation period, having the nowadays observable Universe reached an extension of about 10 cm and the Universe as a whole reached an extension that was 1030 times bigger that value (if the hypothesis that it is closed was correct) or infinite (in the case it's open).
Causes of Inflation
The inflation, according to Alan Guth, would have been provoked by a super-cooling state (as it happens when the water is cooled below 0 ēC (273 ēK) but, for some instants, it doesn't freeze), in which the Universe reached temperatures low enough for the union between the strong nuclear force and the electroweak force (electromagnetic and weak nuclear forces united) to become unstable. The excess of energy that this union contained was set free under the form of an insufflation force that caused the expansion of the Universe. The end of the inflation would have been determined by the complete separation between the two mentioned forces. Huge amounts of matter (endowed with positive energy) would have been created during the inflationary ages. That newly existent positive energy would have been counterbalanced by a corresponding increase of the negative gravitational energy. The total quantity of the Universe's energy (balance between the positive and negative energy) would have, thus, been kept at a zero level.
More recently, the physicist Andrei Linde has made attempts to deny the existence of a super-cooling period, having tried to explain the inflation as a result of an initial energetic field that acted as a spatial insufflation force. This energetic field acted unevenly in several regions and weakened as the Universe expanded. This model explains more successfully the soft temperature fluctuations observed in the cosmic background radiation, which would have resulted from the amplification of very small regions that even before had slightly different densities.
The Explaining Power of Inflation
The inflation is called to explain the uniformity of the Universe in the big scales: it elucidates why are so similar parts of the Universe that otherwise could have never been in touch with each other. Thus, the inflation tells us that in the beginning all the regions were actually in touch with each other.
On the other hand, the inflation has also the power to explain the reason that makes the Universe so flat, or in other words, why does it expand at a moderate velocity, close to the threshold that separates the Universes that collapse from those that expand eternally.
The 1st Second
The scenery of the Universe immediatelly after the inflation may have been characterized by amazingly high temperatures (1027 K, just like immediatelly before the inflation), resulting from the re-heating provoked by the creation of particles during the inflationary period. Quarks and anti-quarks, electrons and positrons would mutually annihilate, generating fabulous quantities of energy. Initially, the energy was enough to compensate this destruction, through the generation of new fermions (quarks, electrons) and anti-fermions (anti-quarks, positrons).
The Great Annihilation
Afterwards, the cooling prevented any further recycling of energy into quarks and anti-quarks, paving the way of their disappearance. The electrons and the positrons, on the other hand, continued to be produced until 1 second after the Big Bang.
The laws of physics determine that during this primordial phase, there should be more positrons being converted into quarks than electrons being converted into anti-quarks. This fact ended up driving to a slight numeral unbalance favourable to quarks. When, at the end, the cooling of the Universe determined the end of such convertions, it was that unbalance that allowed the survival of a tiny fraction of the initial quantity of quarks. An overwhelming majority of quarks and the totality of anti-quarks were had then been annihilated in the mutual colisions that transformed them in radiation.
The neutrinos and anti-neutrinos very hardly interact with anything and, for this reason, it's extremely difficult dor them to mutually annihilate. Therefore, they shall constitute a very significant portion of the fermionic remainder that survived until our time.
The Division of the Electroweak Force
Barions and Mesons
At the 10-9 second after the Big Bang, the quarks ended up uniting themselves in barions of 3 quarks (protons, neutrons) and mesons of 2 quarks.
In the beginning, the protons and the neutrons existed in equal quantities. The mutual convertion between barions resulted from collisions between neutrons and positrons generating protons and anti-neutrinos, and from collisions between protons and electrons generating neutrons and neutrinos.
Physicists like Stephen Hawking defend that given the higher quantity of energy required by the convertion of the protons into neutrons, at the end there should be 6 protons for each surviving neutron, having all the neutrons that were kept free decayed into protons (and electrons and neutrinos) during the nuclear fusion era, described next.
Simulation of the Big Bang: a proton and an anti-proton annihilate themselves at the central intersection (A Brief History of Time, Stephen Hawking)
From the Nuclear Fusion to Transparence
The 1st Nuclear Fusion
When 1 second had elapsed over the Big Bang, the the registered temperature in the Universe would be 1010 (10 million) degrees and the protons and neutrons got connected into deuterium nuclei (1 proton and 1 neutron), tritium (1 proton and 2 neutrons), helium (2 protons and 2 neutrons) and some lithium 7 (3 protons and 4 neutrons), berilium 7 (4 protons and 3 neutrons) and helium 3 (2 protons and 1 neutron). This initial nuclear synthesis was terminated when some minutes had ocurrred after the Big Bang, resulting from it the up-to-date proportion of elements in the Universe (about 75% of hydrogen and 25% of helium).
The Gravitation Comes to the Throne
After 10 000 years over the Big Bang, the temperature fell enough for the energy of the Universe to be dominated by matter and not by radiation (light). From then on the gravity became the ruling force, driving to the concentration of the matter into aglomerations that, millions of years later, provoked the formation of galaxies and galactic clusters.
The 1st Atoms and the Release of Light
During the 100 000 to 300 000 following years, the photons possessed enough energy to break the atoms, separing the electrons from the nuclei. After this period, called radiation era, the Universe reached the temperature of 3000 degrees and the photons lost their power to destroy the atoms. The electrons were definitely caught by the nuclei and the photons began to travel freely across the Universe.
It was from this time that were made the observations of the initial state of the Universe, effectuated first by Arno Penzias and Robert Wilson in 1965 (detection of the microwave background), then by COBE, in 1992 (detection of slight fluctuations in the temperature of the primitive Universe) and, recently, by WMAP (detection of those fluctualtions in a sharper detail).
Temperature fluctuations 100 000 years after the Big Bang, detected by COBE in 1992
The temperatures fluctuations detected by COBE in a shaper detail, as they were seen by WMAP