The Primordial Universe

We enter the `Primordial Universe` when the universe itself is about 1 second old. It is about the size of our solar system and has a temperature of about 100 billion degrees Kelvin.


Nucleosynthesis: 1-180 seconds

Analysis of the evolution of the universe indicates that the ratio of baryons, i.e. protons and neutrons, has effectively remained constant throughout the life of the universe. However, there are about a billion photons for every baryon. Initially, in the early super-hot universe, these photons had so much energy that this energy exceeded the binding energy of an electron to its nucleus. Basically, this means that no atomic structure could exist at this stage. At these high temperatures, protons and neutrons essentially exist in equal numbers, defined by the following two-way transitions:

[1]      1

Where [n] is a neuron, [p] is a proton, [e+/e-] are electron/positron pairs and [v+/v-] are neutrino/anti-neutrino pairs. Energy of 0.8MeV is released within the (n->p) transition, but requires this energy input to proceed in the other direction. While the temperature of the universe was high, there was sufficient energy to maintain the two-way transition, but as the universe cools, another `phase transition` in the evolution of the universe takes place. At this point, the transitions converge to:

[2]      2

The half-life of a neutron is about 615 seconds and without some further reactions, all neutrons would decay to protons and we would be left with a universe containing only pure hydrogen. However, ~1 second after the Big Bang, the temperature has cooled to about 100 billion degrees, which allows the process known as `nucleosynthesis` to begin in which simple atomic nuclei can begin to form, i.e. protons and neutrons bind via the strong nuclear force. As this process proceeds, the proton-neutron phase transition, outlined above, continues. The production of nuclei that form the basis of the light-elements is taking place via a complex chain reaction, i.e.

[3]      3

Through this process, the neutron retains its stability, which ultimately allows heavier elements to be formed. However, initially this process only produces hydrogen and helium-4 in any significant quantity. Helium-4 contains 2 neutrons and 2 protons, while hydrogen contains only 1 proton. The net result of this process is based on a 7:1 ratio of protons to neutrons, e.g. 700 protons and 100 neutrons would create 50 helium-4 nuclei, leaving 600 protons to form the basis of 600 hydrogen nuclei. Given that protons and neutrons are nearly equivalent in mass, we get the following mass ratio of hydrogen to helium-4:

Element   Mass   Total % Mass
Hydrogen   = (1p*600)   600 75%
Helium-4   = (2p*50) + (2n*50) 200 25%

Decoupling & Recombination: 300,000 years:

The universe has expanded to some 600,000 light-years in diameter and cooled towards 3000 degrees Kelvin. However, in the +350,000 years since nucleosynthesis, the universe has essentially remained in the state of hot plasma consisting of atomic nuclei of hydrogen and helium. Above a temperature of 3000K, the photons, which outnumber matter particles in the order of 1 billion to 1, have enough energy to knock any electron out of its ground state atomic orbit. As such, the universe would have been opaque, as any photon would have almost immediately collided with an electron and scattered. However, as the universe cools below 3000K, the photons no longer have enough energy to ionise the atomic nuclei and electrons begin to bind to form the light atomic elements of hydrogen and helium. As electrons `recombine` to form atoms, photons and matter are `decoupled` and do not directly interact; this makes the universe transparent to light.

There are several important implications that result from this event. As we look out into the universe, we are effectively looking back in time. If we could look far enough, in any direction, we would come to a spherical surface, when the universe was approximately 350,000 years old. However, we would not be able to see pass this surface, as prior to this time, the universe was opaque. As the first photons left this surface they collectively created what is now called the `cosmic microwave background radiation`. Although this radiation originally had a temperature of 3000K, the subsequent redshift [z=1089] of their frequency has reduced the energy [E=hf] such that it is now measured as only being 2.7K above absolute zero.

Radiation to Matter: 100 million Years:

After the hectic pace of the quantum phase transitions, the universe entered what is sometimes referred to as the primordial universe. It defines the few hundred million years in which the universe undergoes another phase transition from radiation to matter dominance. During this time, electrons settle into their atomic orbits of hydrogen and helium, but as matter emerges, so the force of gravitational slowly starts to condense clouds of gas into the primordial structures of the universe.

The formation of structure is usually described in terms of smaller structures forming before larger ones, although there are other proposals that suggest the aggregation of larger structures in which smaller structures then emerge. However, it is generally agreed, by the supporters of this paradigm, that one of the first structures to form were quasars, which is an early type of galaxy, which forms through gravitational collapse. Later, within this process, the very first stars will start to form and via the process of fusion, heavier elements are created from the original light elements of hydrogen and helium.