The Big Bang and the Story Finished

t = 10 seconds to t = 20 minutes

From 10 seconds to 20 minutes after the Big Bang, a process of primordial nucleosynthesis occurred. Nucleosynthesis is the process in which the baryons (protons and neutrons) combine with each other to form atomic nuclei. The first ever element was hydrogen, with its atomic number of 1. The atomic number of an element signifies how many protons it has in its nucleus. The atomic number is the single determining factor for what type of element an atom is. However, the amount of neutrons in an atom can change and still be the same element. For example, the element hydrogen can have no neutrons, 1 neutron, or 2. Neutrons and protons are held together by the strong nuclear force. 

At this point in the universe's history the temperature was so high that the protons had so much energy, that they could overcome  their electromagnetic repulsions (they are positively charged and hence repulse) and form heavier and heavier atomic nuclei. However, it was too high. Earlier in the universe's history, the energy of each photon was so high that it would collide with any  atomic nuclei heavier than hydrogen, forcing them to decay into lighter forms. This was known as the "deuterium bottleneck", named after the hydrogen isotope of deuterium (with 1 neutron instead of none). As the temperature of the universe cooled the photons didn't have enough energy to cause these violent events, and heavier elements could continue to form. 

Note: During this period, the ratio between baryons and photons was being determined,  This ratio played a key rule in nuclear reactions, specifically the transformation of deuterium into helium-4 (an isotope of helium with 2 neutrons). For more information on how this ratio affected nuclear reactions, and other ratios like the proton-neutron ratio, see here.

The deuterium and tritium collide to form helium-4, with an extra neutron breaking loose.

Image by Kirill Borisenko / CC BY-SA 4.0

These protons came about from a process of radioactive decay.  Many were originally neutrons which, through the weak interaction, decayed into an electron, a neutrino, and a proton.  This specific decay involving the loss of an electron is known as beta decay.  Yes, protons are very slightly less massive than neutrons.  Once the protons via their immense energies were forced close enough to each other, the strong nuclear force was able to interact between the 2 protons and overcome the  electromagnetic repulsions. This led to the creation of the next element with its atomic number of 2: helium.  This was specifically helium-4, meaning it had 2 protons and 2 neutrons.  It was created out of the fusion reaction of deuterium and tritium, isotopes of hydrogen, colliding into each other.  This type of collision could only be possible in these immense temperatures such as the early universe, as the electromagnetic repulsions were just that strong. 

Diagram of the progression of primordial nucleosynthesis. Neutrons don’t have no charge,their charge is just balanced between positive and negative.  So they can decay an electron, and effectively become a proton. In this early stage of the universe because it was all plasma,a single proton was effectively hydrogen.  Add a neutron, and you have deuterium.  Before this period in the universe the temperature was so high that the photons (y) would have so much energy, that they would rip up the deuterium.  At this point the deuterium could become tritium or helium-3, which themselves can form helium-4. This process continued into the heaviest elements of lithium-7 and beryllium-7.  However, these quickly decayed back into helium-4.

Image by Pamputt / CC BY-SA 4.0

Altogether, after this incredibly short period of primordial nucleosynthesis, the universe's mass was about 75% hydrogen, 24% helium, and trace amounts of deuterium, tritium, and even lithium and beryllium. Deuterium and tritium are isotopes of hydrogen having, instead of only 1 neutron, rather 2 and 3 neutrons respectively. Lithium, however, is an entirely new element with an atomic number of 3. Byrrelium has an atomic number of 4. As the universe continued to expand the temperature continued to drop and nucleosynthesis on such a wide scale was no longer possible.

 

t = 20 minutes to t = 388,000 years

What followed was the matter dominated Epoch. In this special time, the temperature of the universe was cool enough for matter to finally exist in a long term sense, however the universe was still so small that everywhere you looked was mostly matter. The universe was entirely a plasma, the 4th modern state of matter. A plasma is when the temperature is so hot that the atomic nuclei float alone, without any electron orbitals (note that the quark-gluon plasma from even earlier was so hot that not even atomic nuclei existed). About 380,000 years after the Big Bang, the temperature of the universe was finally cool enough for this plasma state to fall into the gas state of matter. 

This event was known as decoupling, when electrons could finally resist the incredible amount of energy and through their electromagnetic force attract toward the positively charged protons of nuclei and form orbitals around them. I use the term orbital, but it was nothing like the path a planet would take to orbit the sun. It was not a circle, but a fog of probability, the general area of the whizzing proton. The first orbital of an atom is called the s orbital, which can  have a maximum of 2 electrons. After that electrons would gather around another s orbital, followed by a p orbital which can hold 6 electrons. 

The Cosmic Microwave Background as seen by telescopes.  For more information on the amazing discovery of the cosmic microwave background, see here.

Image by NASA / WMAP Science Team

Another thing happened in this moment of decoupling. During the last 380,000 years, all the way back to the initial annihilation of antimatter, a massive array of photons were created. However, due to the immense density of the matter dominated era, this could not be appreciated. They were stuck in an endless loop of ping pong between tightly packed atomic nuclei. Now, 380,000 years later, the universe was large enough that these photons could finally gain their freedom. With that, the famous Cosmic Microwave Background which made this incredible journey for us to understand possible. And with that, our chapter of the Big Bang and the early universe is finally over (until the next big discovery!).  We’ve traveled far back in the infinite past, before the beginning of time.  We explored the mysteries of the very beginnings, and discovered the very origins of our reality.  Now it is time to look at our much more familiar recent past and even the far future.  Thank you for reading along.

P.S.: I will be taking a break to plan out the next set of posts which will see the formation of more concrete and familiar astronomical bodies.  There is simply so much though that I have to take time to plan it out.  I will also be spending this time to revise my older posts so stay tuned for that on the discord server (you can join in the about page).  See you then!