The diagram below outlines the major Eras of the Universe according to the Big Bang Theory.
PLANCK ERA: this is the time just before the Planck time (1/10^43 seconds). In this era, random energy fluctuations were so large that we cannot explain the physics at these high energies. Energy and mass are equivalent and so energy fluctuations cause changes in space and time. These fluctuations arise naturally out of the Heisenberg Uncertainty Principle. These early rapid fluctuations are large enough to destroy our theories. Explaining the Planck Era is one of the goals of String Theory (the theory of everything). So far, we do not know what happens during this time.
GUT ERA: GUT stands for grand unified theories. This is a theory that unites three of the four known forces. The four forces are gravity, electromagnetism, the weak force, and the strong force. GUT combine the strong force with the electroweak force (the combination of weak and electromagnetic force). We have learned that the forces are seperate but under high temperatures they come together. So the GUT era is when gravity and GUT force controlled the Universe.
During the GUT Era is when we think INFLATION happened: that point were the Universe underwent a dramatic expansion. When the strong force froze out of the the GUT force (like ice crystals forming on water), that may have caused an enormous energy release. Inflation is an important aspect of the Big Bang since it explains many problems: the structure problem, the smoothness problem, and the fact that we are at the critical density.
ELECTROWEAK ERA: this era is when the electromagnetic and weak forces were still united. Conditions of this era were actually achieved in a particle accelerator in 1983 (which one a Nobel prize), and it was predicted early (which gave rise to a Noble prize in 1979 to Weinberg). Thus, we have direct evidence of the electroweak era from experiments. We have not been able to go much beyond these energies.
PARTICLE ERA: during this era the Universe was just the right temperature for particles to be created and destroyed continuously. What happens is the photons have the right energy to come together and annihilate each other to form matter and antimatter. And then the matter and antimatter smash together and form gamma-rays. This continues until the Universe cools enough such that the particles do not have enough energy anymore to annihilate each other, and we are then stuck with whatever was left at that point.
At the end of the particle era, there were slightly more protons than antiprotons. For every billion antiprotons there were a billion and one protons. Another way to look at it is one billion protons were annihilated with one billion antiprotons to make a billion photons, which would result in leaving one proton in the end. It is this slight excess that makes up most of the matter in the Universe.
NUCLEOSYNTHESIS ERA: during this era, protons and neutrons would come together for a short while before interactions broke them up again. Essentially, the Universe was a big continuous fusion reaction. This era ended when the Universe was 3 minutes old. The diagram below shows which elements the Universe could form (this is mainly based on our understanding of nuclear reactions). It was at the end of this era that set the chemical composition of the Universe.
ERA of NUCLEI: during this time, protons and neutrons were together in nuclei. Electrons would form an atom with this nucleus but would soon be ionized as a photon hit it. Thus, neither an atom could form nor could a photon travel very far. At the end of this era the Universe was cool enough that photons did not instantly destroy atoms and very quickly the electrons found the nuclei, and the photons were free to travel around. This is what makes the cosmic background radiation and it happened at a temperature of 3000 Kelvin (about the temperature on the surface of red giant stars).
ERA of ATOMS: this era is marked by the first structures beginning to form and it blends in with the ERA of GALAXIES. This is about the time when astronomers take over from physicists in terms of trying to explain the Universe.
The era of atoms lasts for a long time and is sometimes referred to as the DARK AGES: that point were we have essentially no information on what was going on. Galaxies had not formed yet and so we don’t see bright objects. However, gamma-ray bursts may allow us to penetrate into this era. It is during this era that maybe black holes played a key role in terms of galaxy formation. Also, dark matter is crucial to understand as well.
Our most distant object (a galaxy) is at a redshift of 6.4, or when the Universe was about 1 billion years old. There have been some speculative claims of more distant objects. The cosmic microwave background is at a redshift around 1100, so we still have quite a ways to go to meet with that.
There are a few missions planned to observe the dark ages. Because it is at such a high redshift, the important features are shifted out of the normal wavelengths that we like to observe in (i.e., the optical). Thus, we need to use infrared obsrevations to gain any info about this era. Also, the since it is the era of atoms, the Universe maybe filled with gas and so it may be difficult to see anything in the optical region. Europe has a mission called Herschel Space Observatory , which was launched in 2009. The US is planning a follow-up to HST called NGST. Both of these mission concentrate in the infrared since the optical region is so heavily obscurred.
One important question is what was the first structure to form after the era of atoms. There are arguments that suggest both small and large galaxies. Some say that black holes would be the first and then they seeded galaxy formation (this would logically lead to a connection between galaxies and black holes so it is enticing). We don’t know the answers yet.
One important point is that since everything that we learn about the Universe comes from light (photons), if there are no photons there is no information. Thus, before the end of the era of nuclei, we have no information since the photons were trapped. We will never see this era of the Universe with photons, but maybe gravity waves (?).