By Gary Felder, Smith College
A hundred thousandth of a second after the big bang, the universe was too cold to produce unstable particles, and almost all of the unstable particles had decayed. What remained were the few stable particles—meaning that, when left alone, they last indefinitely without decaying into anything else. And Quarks finally combined into protons and neutrons.
The Higgs Boson
Our current theory of particle physics is called the standard model of particle physics. The standard model includes about 20 fundamental particles. However, most of those particles are unstable; so once produced, they decay, meaning they turn into other kinds of particles, in a tiny fraction of a second.
One of the fundamental particles in the standard model is the Higgs boson. It made headlines in 2012, when one was produced and detected for the first time at the Large Hadron Collider (or LHC) in Switzerland.
Particle physicists produced a Higgs boson by smashing two protons together really hard. But the Higgs boson they produced only lasted a billionth of a trillionth of a second, after which that Higgs boson decayed into a shower of other particles.
In the early universe, super high-energy collisions were happening constantly. So new Higgs bosons were continually being created and then decaying.
However, the Higgs boson is just an example. The same thing was occurring with many types of particles. For a fraction of a second in the early universe, all of the elementary particles in the standard model were continually being produced and destroyed.
This article comes directly from content in the video series The Big Bang and Beyond: Exploring the Early Universe. Watch it now, on Wondrium.
The Electroweak Phase Transition
A major transition occurred a hundred billionth of a second after the big bang. Before that point, one could not classify one interaction as being due to the electromagnetic force, and a different interaction as being due to the weak force. Those two forces acted identically, as a single electroweak force.
But after that moment, when the temperature dropped below a quadrillion degrees, that one force started acting like two separate forces that behave in different ways.
One of those is the electromagnetic force, and the other is the weak force. That change from one force to two apparently different ones is what we call the electroweak phase transition.
So, at one hundred billionth of a second, the universe underwent the electroweak phase transition, splitting the electroweak force into two.
Particles and Forms of Energy
The next major transition occurred after a hundred thousandth of a second. At that point, the universe primarily consisted of the same particles and other forms of energy it does today: quarks, electrons, neutrinos, electromagnetic radiation, dark matter, dark energy, and antimatter.
The biggest difference between the particles then and now is that today all of the quarks are bound up in protons and neutrons. But at this early time, they were all flying about freely. To explain why that changed, we need to understand something about the forces between quarks.
Quarks
Quarks interact with each other via the strong force, which binds them tightly together into heavier particles such as protons and neutrons. Within a proton or neutron, the quarks each exert roughly 10 tons of force on each other. This means that it is extremely hard to pull quarks away from each other.
Quarks come in three different charges. By analogy with the primary colors of light, which can be combined to make all possible colors, we call the quark charges red, green, and blue. The strong force makes different ‘colors’ attract, and like colors repel.
Of course, quarks don’t really have colors in the usual sense; color is just a term used to describe which quarks attract or repel each other.
Attractive and Repulsive Forces
A proton or neutron is made of three quarks, one of each color. So, each proton is held together by the strong force that binds its quarks to each other. However, if you look at two different protons, the attractive force of the different colors cancels the repulsive force of the like colors, so the strong force between two protons mostly cancels out.
When the universe was above a trillion degrees, the quarks were all moving with too much energy to stick together.
For the first hundred thousandth of a second, quarks were constantly colliding so forcefully that they bounced off each other, despite the tons of force pulling the differently colored ones together. Even if two quarks that happened to have less energy than the average were able to stick to each other, another quark would immediately hit them so hard that they would be knocked apart again.
In the very rapidly expanding early universe, though, the density and temperature dropped quickly. That means that over time the quarks kept moving, on average, more and more slowly.
Protons and Neutrons
At a hundred thousandth of a second, when the temperature dropped below a trillion degrees, the quarks were no longer moving fast enough to resist the forces pulling them together. Each quark almost instantly found quarks of the two other colors, combining into ‘color-balanced’ triads of red-green-blue. Those triads were protons and neutrons, which is the form most quarks in the universe have been in ever since.
Quarks combining into protons and neutrons was the universe’s first example of the process wherein as the universe cooled, particles combined into ever-larger structures.
Common Questions about the Formation of Stable Particles
One of the fundamental particles in the standard model is the Higgs boson. It made headlines in 2012, when one was produced and detected for the first time at the Large Hadron Collider (or LHC) in Switzerland.
When the universe reached a hundred thousandth of a second, it primarily consisted of the same particles and other forms of energy it does today: quarks, electrons, neutrinos, electromagnetic radiation, dark matter, dark energy, and antimatter.
Quarks don’t have colors in the usual sense. Color is just a term used to describe which quarks attract or repel each other.
