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The Primordial Soup
The Primordial Soup
After the Big Bang, the universe was a hot, dense, soupy plasma of particles. We will focus on the time after the Big Bang during which the universe has expanded and cooled to the point when nucleons, the stuff that resides in the nucleus of an atom, can finally form (yes, there was a time when the universe was so hot that even these building blocks have not yet formed, but we won't worry about that in this particular unit). However, the universe was still so hot that neutral atoms could not form: only their nuclei. When I say the universe 'cooled', the average temperature at this time was still around 100 Million Kelvin, or 180 Million degrees Fahrenheit. Because everything was so hot and dense, interactions between particles went both ways. Referring back to the picture from An Overview of the Atom, we focused on protons and electrons fusing to create a neutrino and a neutron. But this reaction can also go the other way: a neutrino and a neutron can fuse to form a proton and an electron! We describe this relationship as a formula shown on the right.
Yet, atoms form and we exist! Eventually, these interactions become one-sided as the universe expands and cools. Depending on the mass of the particles and how strongly they interact with other particles, they will eventually break away from equilibrium with the rest of the soup and evolve separately. This process, known as decoupling, is one of the processes which allow atoms to form in the early universe. Another important factor to consider is that we are made of matter, not anti-matter. To literally exist, there needs to be some level of matter - anti-matter annihilation in the early universe that gets rid of any residual anti-matter. Otherwise, anti-matter particles will collide with matter and destroy it all!
In the context of allowing atoms to form in the early universe, two things need to happen first:
Neutrinos and anti-neutrinos need to decouple. If they don't, they would destroy too many neutrons.
Electrons need to annihilate all positrons. If they don't, neutral atoms will have a hard time forming and we wouldn't be able to make a lot of neutrons.
The remainder of this unit will focus on the production of ionized Hydrogen and Helium atoms in the early universe, a process known as Big-Bang Nucleosynthesis, and how they became neutral atoms through a process called recombination.
How does Helium form? Through Big-Bang Nucleosynthesis, of course!
How does Helium form? Through Big-Bang Nucleosynthesis, of course!
Hydrogen is a really simple element, defined by a nucleus having only one proton. This means that protons on their own are just ionized Hydrogen. But how to elements heavier than Hydrogen form? We fling particles at each other and hope they stick together! Helium has two protons and two neutrons in its nucleus, so in principle if we can perfectly fling these four particles at each other, they can bind together to form a Helium nucleus. However, this process is extremely unlikely to work as the primary mechanism that forms Helium. All four particles would need to fuse at exact same point at the exact same time with the exact amount of energy needed to bind the Helium atom together and not scatter off each other like billiard balls. Fusing two particles together is then much more likely to work. We should then consider chains of two-particle interactions to make Helium. We should also consider how many interactions it would take in the chain to form Helium, as well as the interactions necessary to make the particles along the way. In summary, the most efficient way to make Helium will have the fewest two-particle collisions.
Take a look at this figure on the right. Here, we're fusing a proton and a neutron together. When they bind, they form what's called Deuterium. Since an element is defined by the number of protons in the nucleus and not the number of neutrons, this resulting particle still has the same properties as Hydrogen but now it's more massive.
You may have noticed that Deuterium is half of what we need to form Helium. Why not put two of these together to make Helium?
Return of the Binding Energy!
Return of the Binding Energy!
Two Deuterium atoms can, and do, fuse to form different particles. However, forming Helium is very improbable. Let's justify this statement with the formula above and ask: much energy is released when two Deuterium atoms collide to make:
A Tritium atom and a proton
A Helium-3 atom and a neutron
Helium and a...? (think about how excess energy is typically transferred out of a collision)
Discussion and Results
3.268 MeV
4.033 MeV
23.848 MeV as a photon
Based off measurements, process 1, 2, and 3 happen 45%, 55%, and 0.0001% of the time, respectfully. While this excess energy is released as a photon and transferred into the kinetic energy of the resulting particles, it turns out that releasing a particle is somehow more favorable than a single photon!
Getting to Helium, the most Efficient Way
Getting to Helium, the most Efficient Way
When we fuse atoms together to make Helium, we will focus on the most efficient route which uses the least amount of interactions and the least amount of energy to get there. We detail this process in the diagram to the left. We will need two Deuterium atoms to form first to get the process started. Step by step:
A proton and a neutron fuse to form a Deuterium atom and a photon.
A proton and a Deuterium atom fuse to form Helium-3 and a photon.
Helium-3 and a Deuterium atom collide and yield a proton, Helium, and a photon.
In total, this takes four collisions: two to make the necessary Deuterium atoms and two for the fusion of Helium-3 and Helium.
So What About the Other Routes to Helium?
So What About the Other Routes to Helium?
There do exist other paths to Helium. However, they require more than four collisions. Justify this for yourself using the diagram on the right.
An Expanded Particle Chart
An Expanded Particle Chart
At this point, we've introduced a lot of different particles. Here is a Quick-Reference chart on them and their properties. Take a minute to go over it and reinforce what we just learned.
We now know the relevant processes in the early universe that can create Helium. But how much Helium can we make? Let's delve deeper and approximate what this should be in the next page: How Much Helium Can We Make?
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