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!

The Big Bang and the Greatest Mystery

Where we left off the fundamental forces of the universe were being defined and the universe continued to be an extremely high energy soup of constant collision. The universe at this point was in a constant state of high energy photons converting into matter-antimatter pairs only to annihilate with each other back into photons (Das & Ferbel, 2013). Antimatter has the same mass but opposite electric charge of their matter component. So you can have a quark and an antiquark, or an electron and an antielectron (also called a positron). The universe was basically a photon gas, as well as a "quark-gluon soup" of tightly packed matter and antimatter continuing this process.

 

Diagram of pair production.  γ is “gamma” or the term for a high energy photon.  The energy it contains is so high that it converts into a pair of an electron and an antielectron (positron).  At this point, the universe was so hot that photons would also convert into pairs of quarks and antiquarks.  Note, you would actually need a pair of photons, not just one, to collide with one another and produce this pair.

Image by Christian Nölleke / CC BY-SA 3.0

That would change. Sometime in the early universe (we don't exactly know when this happened) the temperature of the universe dropped to a point where this photon matter creation process would happen en masse for  one last time. This critical temperature was 2x10^12 K, leading to the final annihilation which is called Baryogenesis (baryo- meaning baryon). Mysteriously, even though a particle and antiparticle are seemingly always created together in pairs, this final annihilation left us with mostly matter and almost no antimatter (Cline, 2004). This is the great mystery of Baryogenesis, the asymmetry between matter and antimatter. It has been calculated that there must have been an extra particle for every 5 billion particle-antiparticle pair. 

Note: This baryonic asymmetry has actually been observed in our own experiments. In 2010, Fermilab managed to generate 1% more matter than antimatter. The reason why this happens is still not known. To learn more about the experiment and other modern quantum experiments, see here. 

Diagram of annihilation.  The matter and antimatter attract each other and collide, converting back into high energy photons.  Note again, there would not be a single photon in this transition, but a pair.  This should happen, but for some reason a small amount of matter is left over.

Image by Iamion / CC BY-SA 4.0

There are many ideas, though unverified, for how this could have occured. Within the  Standard Model, it is possible that when the electroweak force was splitting into the weak and electromagnetic forces symmetry breaking occured. Symmetry breaking famously occurs during phase transitions (as in the phases of matter). When an object decreases in temperature it reaches a critical point where it stops decreasing in temperature and instead transitions to a new phase of matter. Objects of lower temperature phases are less symmetrical than objects of higher energy phases. So as the universe cooled and the symmetry of the electroweak force broke, then it would lead to the change in how matter and antimatter are created and eventually annihilated. This could have led to the matter-antimatter asymmetry that we observe today. 

Note: The idea of symmetry is very important in physics. Basically, symmetry describes any situation in which certain features remain unchanged during a sort of transformation, such as a phase change. Sometimes there is a symmetry breaking, meaning these features change. This is what could have happened after the Electroweak Epoch, changing the features of the creation and annihilation of matter and antimatter, possibly allowing for  the matter asymmetry we observe today. For more information on symmetry in physics, see here. 

A second idea involves this occurring after the Grand Unified Epoch. It states that in order for this baryogenesis asymmetry to occur, there must be a not only a violation (breaking) of the baryon number (a mathematical constant) and violation of charge-symmetry (C Symmetry) and parity-symmetry (P Symmetry), but also thermal equilibrium (Today’s Science, 2015). This means there would be no flow of heat from one part of space to  another, that all parts of space had equal amounts of heat. This is how heat acts, it wants to flow from an area of greater heat to an area of lower heat so that both areas have  the same amount of heat. In thermal equilibrium the flow of heat does not occur because this state is already reached. This too, has not been verified. 

A third, much more daring idea is that there are equal amounts of matter and antimatter, yet they somehow separated without annihilating each other. They would create their own regions, meaning supposedly there are antimatter stars and even antimatter galaxies somewhere out there (Thomsen, 1973). However, this has never been observationally verified.

At this point all matter for the universe has been produced. In this truly Hadron Epoch, quark production had ceased. The remaining quarks would form into various types of hadrons, of which there are baryons and mesons. A baryon is made up of 3 quarks, and they include protons and neutrons, whilst a meson is made up of 2 quarks. These hadrons, and especially the baryons, would become the foundation of nuclear chemistry. 

The following Lepton Epoch (t=1 to t=180 or 3 minutes) was very similar. For leptons, a type of fermion just like the quarks but with much smaller masses, the required amount of energy needed for constant photon conversion into lepton and antilepton pairs  was much lower. So the universe had to cool for even longer before this too would end. The leptons include the muon, tau, and of course the famous electron. Leptons interact with all the fundamental interactions except for the strong force. So they did not form into heavier hadrons like the quarks did. These leptons have not only corresponding antimatter versions but also neutrino, meaning it does not interact with the electromagnetic force. So there is such a thing as a neutrino electron, though the vast majority of electrons are charged. During the Electroweak Epoch, when electromagnetism and the weak force acted the same, neutrinos were able to interact with the other fermions. This is neutrino decoupling, and it occurred at around this time, as evident of the Cosmic Neutrino Background. 

Note: The Cosmic Neutrino Background (CNB), has very recently been directly observed for the first time.  For more information on this discovery, see here. 

The famous diagram of the Standard Model outlining all the elementary particles of the universe.  Note, this diagram is dated and the Higgs boson has been confirmed.

Image by Andres Rojas / CC BY-SA 4

At this point the temperature of the universe was so low that photons no longer had the energy to constantly convert into various quarks and leptons like it used to. These photons made up the majority of the universe and would continue to exist as the catalyst of the electromagnetic force. The quarks and leptons would now be able to exist without constant annihilation, and antimatter would become the ultimate rarity. As the universe continued to cool and space continued to expand, more familiar processes could take hold. That will be next week.  For now, the Standard Model is complete.

References:

Das, A., & Ferbel, T. (2013). Introduction to nuclear and particle physics (2nd ed.). World Scientific. 

Cline, J. (2004). The Origin of Matter. American Scientist, 92(2), 148.  https://doi.org/10.1511/2004.46.926  

“Why More Matter Than Antimatter?” Today's Science, Infobase, 2015. Science Online, online.infobase.com/Auth/Index?aid=103612&itemid=WE40&articleId=1009338. Accessed 13 Nov. 2020.

Thomsen, D. E. (1973). Does the Universe Need Antimatter? Science News, 103(13), 211. https://doi.org/10.2307/3958135