Today mankind has a pretty good understanding of what physical processes followed the big bang, and lead to the universe we see  today. This understanding is based on the solid foundation of  direct observations of the first light to travel through a  transparent universe. We also have data gathered by experiments in nuclear and particle physics labs on Earth. From these pieces of data a picture can be drawn of how the universe evolved in the first half million years of existence.

Traditionally this story is told in chronological order, starting with the big bang. However the farther back we go, the less we know about what happened. To know is to observe. The farther back we go the more we think or theorize about what happened. The farther back we go, the more we rely on indirect observations. Experimental and observational results take precedence over theory in science. After a certain point the universe would have been denser and more energetic than anything we have ever seen. For this reason I will tell this story in reverse chronological order. Starting from phenomena we have direct knowledge of and working back to what we don't know.

More is known by direct observation right now about the Cosmic Microwave Background (CMB) than about anything that came before it. What came before it was the synthesis of nuclei heavier than common hydrogen. Earlier still was the synthesis of matter. Practically at the same time as those were occurring, the universe was going through a phase where at least one physical parameter which we can now treat a constant varied wildly from what we now know it to be. Most physicist think this parameter was the expansion rate of the universe, some think it was the speed of light. Even sooner than that was the divergence of the four fundamental forces. Then at the beginning, there are many theories on what happened, all of which concern “the big bang”. There are no direct, or indirect observations at the scale on which the big bang would have occurred. 
A brief history of time.

The Cosmic Microwave Background.

When Astronomers look into the sky the Cosmic MicrowaveBackground is the farthest back they can see. In many popular accounts this is termed the afterglow of the big bang. This is a misstatement. The Cosmic Microwave Background (CMB for short) is really the first light to escape into a transparent universe. The universe was so energetic that any light that was emitted would have the energy to knock a electron completely free of a proton thus ionizing the universe.  After the universe had expanded and cooled enough, free electrons and protons could combine to form hydrogen. The universe then became transparent to light. The boundary between these two phases is known as the surface of last scattering. It is the last interaction that the photons of the CMB would have with matter, until they interacted with our instruments.

As this first light has traveled through the universe it has become red shifted into a longer wavelengths and lower energies. From gamma rays to microwaves. These microwaves are everywhere, in every direction we look for them. They provide a direct link to the first half million years of existence.

The existence of this afterglow was predicted by a number of theoretical physicist with a range of temperatures. The closest prediction of the actual temperature of the CMB was made by Robert Dickie. He predicted that it would be less than 20 kelvin. All the other predictions that this author knows of were higher, as high as 50 k (Gamow). The first detection of this radiation was by Wilson and Penzias using a instrument known as “the big
ear”. At first they thought it was noise in their instruments. After correcting for all possible sources of noise they were left with noise of a temperature of 3 kelvin. The background of the universe is like the black body radiation from a 3 kelvin source. This temperature seemed to be uniform in every direction according to measurements of Wilson, Penzias and others. For their discovery they were awarded a Nobel Prize.

For my story the most important data was gathered by the Wilkinson Microwave Anisotropy Probe. This probe was designed to detect small variations in the intensity and wavelength of the CMB. Once this data was collected and gathered the famous picture of the CMB was compiled. In this picture what we see is slight variations in temperature of the surface of last scattering. These slight variations that WMAP observed correspond to a map of
where galaxy clusters and super clusters are in the sky. There is a correspondence from the CMB to the formation of large scale structures.

What the CMB has told us

As I have discussed earlier the CMB looks the same in every direction to within a small fraction of a degree kelvin. This smoothness of the CMB poses a number of problems. If the CMB was too smooth then no large scale structure could form. There would be no density fluctuations which could lead to the first stars
and galaxy's. The almost completely uniform temperature of the CMB means that every part of the CMB would have to have been in thermal equilibrium. The way that matter comes into thermal equilibrium is by exchange of matter, or radiant heating and cooling. For this to happen all of the area's of the CMB would have to have been in close contact. However we see area's in thermal equilibrium which without some extra physics could not
have been in casual contact. Even light could not travel between them . This leads to the so called “horizon problem” in cosmology. The various points in the CMB would not have been able to see each other, their event horizon's could not have been big enough. In terms of Special Relativity their past light cones did not overlap.

Concurrent with measurements of the CMB measurements were taken which determined that on large scales the space-time of the universe is flat. Just what does it mean when physicist speak of a universe which is flat or space-time which is curved? What we mean can be thought of in terms of straight lines.

Consider a piece of paper laid flat on a table. Draw two parallel lines on it. Those lines if they are truly parallel will never cross each other, ever. No matter how far they are extended. Now take the same piece of paper and roll it up into a cone, or a pinch one end together. In this now curved space it is possible for parallel lines to cross. This curving of space, is according to Einstein's General Relativity the reaction of space-time to the presence of matter and energy. The universe being flat everywhere we look means that the universe was very nearly of a uniform density.

All of this nice smoothness we have observed is supposed to have resulted from an explosion of sorts. In our earthly experience explosions are not uniform at all. The uniformity observed in the universe needs an explanation.

The synthesis of heavy nuceli, and the generation of matter.

Through the science of nuclear physics we know some of what must have occurred in the era just before the CMB. This era is known as the era of big bang nucleosynthesis (BBN). Nucleosynthesis is the creation of nuclei heavier than hydrogen. Mostly Helium along with some Lithium and Beryllium. The era of nucleosynthesis ended when the universe became too diffuse to support nuclear fusion reactions. During this period the universe was densely packed with protons, neutrons, and electrons a healthy number of neutrino's and according to many models dark matter. The neutrino's and dark matter did not do much other than by their gravity. The protons, and neutrons would combine to form the nuclei of heavy isotopes of hydrogen. These are known as Deuterium and Tritium. A relatively small amount of Lithium and Beryllium was also synthesized at this point. True atoms did not last long in such an energetic universe. As I wrote of above the photons in the universe at that stage were so energetic that they would strip any nucleus of it's electrons.

Before the creation of heavier nuclei the universe was for a time a seething collection of protons, neutrons, electrons, neutrino's, and anti particles. These anti particles of antimatter and the particles of matter would have combined to release flashes of gamma rays. To get the universe we have today there had to be a relatively small amount of matter which was not annihilated. Practically all of the matter we are made of came into existence at that point.

Just before this could occur all of these forms of matter had to come into existence. In standard quantum field theory it is predicted that empty space is filled with pairs of matter and antimatter particles which are constantly created an annihilated. This is so right now, everywhere all around us. These very processes have been harnessed to get anti protons, and positrons, for particle physics experiments. This is known as pair creation.
Matter and anti matter are created in pairs of particles of opposite charges in order to conserve the net charge of the universe. For the universe we live in to exist there has to have been a violation in the total number of particles, or a violation in the conservation of charge.

The Inflationary Epoch.

Concurrent with the above described processes the universe achieved the thermal equilibrium, isotropy, and flatness which we have observed in the cosmic microwave background.

One solution to these problems is to propose that the universe expanded faster in the past than it is in the present. The universe grew at least 60 e-folds in the space of a few minutes. At the beginning of the inflationary epoch the universe was small enough that it was possible for the universe to attain thermal equilibrium.

This expansion was driven by a scalar field which filled the cosmos at the time. This scalar field was related to a function denoted a(t) in most literature. The function a(t) appears in a solution to Einsteins Field Equations known as the Friedman-Robertson-Walker-Lemaître metric. A metric is a function which gives the distance between two points in a space-time. The increase in a(t) causes the value of the distance between points in space to grow with time. This results in universal expansion. a(t) is in turn driven by the hypothetical scalar field. As the field drives inflation it decays into photons which reheats the universe.

At the same time the inflation of the universe would smooth out most of the inhomogeneity of the universe while at the same time increasing the size of any regions of greater density. Regions which were small area's of greater density when inflation began would become large scale clouds of hydrogen and helium. From
which clusters of galaxy's and all they contain could eventually form.

The inflation of the universe also plays a role in era of Big Bang Nucleo synthesis. The inflation rate of the universe has to be just right in order to explain the relative abundances of Hydrogen, Helium, and Beryllium in the universe. If the inflation is too slow the universe would remain dense enough to synthesize more heavy elements than we see. If it was too fast we would not see nearly as much Helium and Beryllium in relation to hydrogen.

This is the paradigm, subscribed to by a majority of cosmologist. It fits the data we have gathered and agreed upon so far. The big bang nucleosynthesis data provides a very tight constraint that any cosmology must be able to fit. Inflationary cosmology fits this data, as well as the CMB data very well.

A minority of cosmologist think that instead of inflation the speed of light was much higher at this time. This is based on (controversial and disputed) observations of possible time variance of the fine structure constant ( ). The fine structure constant is a dimensionless number composed from the electron charge (e), Planck's constant (   ) and the speed of light c. For to   vary one of those three constants must vary. If a short period of a larger speed of light is allowed it would account for the same data as inflation as well as a varying   . Most cosmologist are not yet convinced that   in fact varies and at least until they are this will be a minority viewpoint.

The Quantum Era.

The quantum era goes from the beginning of the inflationary epoch and the creation of the first particles of matter back to what is known popularly as the big bang. There are a number of theories which concern the events of the quantum era. I have chosen this name because at this point in time the universe was so dense that the quantum nature of gravity was unignoreable. The physics of the time was not classical at all. All interactions occurred in a
highly energetic environment just after the big bang. Just before the generation of matter in the form of subatomic
particles, the fundamental forces of nature would have split apart. The force of gravity would have split off soonest as it is so weak compared to the others. Electromagnetism and the weak nuclear force, would split from the strong nuclear force shortly there after. According to some cosmologist the divergence of these four forces from the original super force is the big bang, there is no earlier phase. Based on General relativity the universe is thought to have been compressed into one point called a quantum singularity.

In the quantum cosmologies of two well studied theories there exist a minimum unit of distance in space-time. The universe could have been no smaller than this distance, and no younger than this age. For example in a theory known as Loop Quantum Cosmology there is no singularity at the beginning of the universe.

This brings us to the beginning of the universe, and the end of what anyone can claim to know happened . There are a number of theories. Some propose a big crunch which lead to the big bang. Others propose that two older universes collided. Nobody knows.