By Gary Felder, Smith College
A hundred thousandth of a second after the big bang, the universe contained protons, antiprotons, neutrons, antineutrons, electrons, and positrons. However, after the formation of protons and neutrons, the next big change wasn’t about particles combining, instead particles were being destroyed. Why?
Antiparticles
Most particles have corresponding antiparticles with the same mass but opposite charge. For example, electrons have negative charge, and their antiparticles, called positrons, have positive charge. Collectively, these antiparticles are called antimatter.
We don’t see naturally occurring antimatter around us. That’s because when a particle collides with its own antiparticle, the two can annihilate and release their energy, usually in the form of electromagnetic radiation.
In the reverse process, if you concentrate high-energy radiation in a small enough region, the energy of that radiation can create a particle-antiparticle pair, such as an electron and positron or a quark and antiquark.
Matter and Antimatter
We produce antimatter all the time in laboratories, and even in hospitals. A PET scan, which stands for Positron Emission Tomography, uses antimatter to scan for activity in different regions of the brain. But once we produce antimatter, it only stays around for a fraction of a second before finding its matter counterpart and annihilating.
When the universe was initially filled with a dense collection of elementary particles, those particles included almost identical amounts of matter and antimatter. At the same time that quarks combined into protons and neutrons, antiquarks combined into antiprotons and antineutrons.
So, a hundred thousandth of a second after the big bang, the universe contained protons, antiprotons, neutrons, antineutrons, electrons, and positrons.
Of course, the matter and antimatter were constantly annihilating, creating high-energy radiation. But, at the same time, that intense radiation was constantly creating new matter and antimatter.
The annihilation and creation processes were in equilibrium, meaning the rate at which matter and antimatter was destroyed was equal to the rate at which it was created, so the amounts of matter and antimatter remained constant.
This article comes directly from content in the video series The Big Bang and Beyond: Exploring the Early Universe. Watch it now, on Wondrium.
Equilibrium Gets Broken
However, everything in the early universe was subject to the effects of expansion and cooling. As the temperature decreased, the radiation became less intense, and creation of matter-antimatter particle pairs became less likely. The equilibrium was broken, and annihilations began to outpace creation events.
This happened first with the protons and neutrons because they are heavier and thus harder to produce than electrons and positrons. After roughly a tenth of a second, all of the antiprotons and antineutrons were gone.
The positrons and electrons remained in equilibrium—with equal rates of creation and destruction—for most of the first second.
By one second, however, annihilation had begun to overtake creation even for the positrons and electrons, and by three to 10 seconds later, the positrons were gone. From that moment on, the observable universe has had matter, but essentially no antimatter.
The Matter in the Universe Today
After all the annihilation was done, why was there still matter? Because, before the annihilation, the early universe had slightly more matter than antimatter. For every billion antiquarks and positrons, there were a billion and one quarks and electrons.
By a few seconds after the big bang, all the antiquarks and positrons were gone, and their energy had been converted to radiation. Most of the original matter was gone, too, annihilated in the collisions that eliminated all the antimatter.
But there were still those one-in-a-billion particles of matter left over, with nothing to annihilate them. All the matter in the universe today—the stars, galaxies, planets, and so on—is made of the one part in a billion of matter particles left over after the antimatter was gone.
We don’t know why the early universe had a bit more matter than antimatter, but it’s a very good thing for us that it did.
Electromagnetic Radiation and the Ratio of Matter to Antimatter
By this point, the antimatter was gone, and a little matter remained. But there was also something else in the universe: electromagnetic radiation, which includes light, x-rays, radio waves, and a variety of other forms.
After matter and antimatter annihilated, there was a lot of this radiation because that’s where all the energy from those annihilations went. In the 10-second-old universe, radiation had more than a billion times as much energy as the matter.
In fact, the reason we know the ratio of matter to antimatter prior to annihilation is that we can observe the radiation that got produced. We think of light and other forms of electromagnetic radiation as continuous, but when we measure it with very sensitive instruments, we find that a beam of light is made of discrete particles of energy that are called photons.
When a matter and antimatter particle annihilate, they produce two photons. In the universe today, there are over 2 billion photons for every electron. That tells us that for each electron in the universe today, we originally had a billion and one electrons and a billion positrons, and when they annihilated, we were left with one electron and 2 billion photons.
Common Questions about the story of Matter and Antimatter
Most particles have corresponding antiparticles with the same mass but opposite charge. For example, electrons have negative charge, and their antiparticles, called positrons, have positive charge. Collectively, these antiparticles are called antimatter.
A hundred thousandth of a second after the big bang, the universe contained protons, antiprotons, neutrons, antineutrons, electrons, and positrons.
Matter was still there because, before the annihilation, the early universe had slightly more matter than antimatter. For every billion antiquarks and positrons, there were a billion and one quarks and electrons.
