Photons, Particle Soup & Nucleosynthesis

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  • 0:41 Photons & Particle Soup
  • 3:05 Cooling Off
  • 5:21 Nucleosynthesis
  • 7:25 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will explore what happened in the fractions of a second and minutes after the Big Bang occurred. You'll learn about matter, antimatter, photons, deuterons, positrons, and a lot more!

Cosmologists and Our Universe

One of the most difficult things to understand in astronomy is what happened at the very beginning of the Big Bang. No one really knows for certain what was going on at time zero, but cosmologists have an idea as to what happened in the fractions of a second thereafter. A cosmologist, by the way, is a physicist or astronomer involved in figuring out the origin, evolution, and properties of the universe.

So, in this lesson, you'll take a direct look at what cosmologists believe happened during the very first breaths of the universe's evolution.

Photons and Particle Soup

In the first 10 millionth of a second after the Big Bang, the universe was filled with incredibly hot and dense high-energy photons. Photons are packets of electromagnetic energy traveling at the speed of light. You might be wondering what I mean when I say the photons were hot. To a cosmologist, hot means they have a spectrum equal to the blackbody radiation emitted by an object that has the same temperature.

Anyways, the photons in the early universe were actually gamma rays, which is a form of electromagnetic radiation, and consequently such photons were of very high energy because of the very short wavelength and high frequency nature of gamma rays.

As a result of this high energy, two photons could collide to become two distinct particles in a process called pair production, the creation of a particle and an antiparticle from gamma rays. One particle that's created is normal matter, and the other particle is antimatter, a kind of matter, composed of antiparticles, which will annihilate matching ordinary matter on contact.

For example, normal matter would be in the form of an electron, while its antimatter particle would be a positive electron, the antiparticle of the electron, called a positron (sometimes also called an anti-electron). Similarly, there are protons and antiprotons, neutrons and anti-neutrons, and so on down the line.

In any case, one microsecond after the Big Bang, gamma ray photons had enough energy to make protons, neutrons, electrons, and their respective antiparticles. When such a particle and its antiparticle collide, they annihilate one another and convert their mass into energy, the energy of two gamma rays. This means the very early universe, at 1 microsecond in age, was a dynamic soup of matter and radiation moving from photons to particles to photons again.

Cooling Off

As all of this was happening, the universe was actually expanding and the temperature of the radiation dropped as a result. The temperature drop caused the high energy of the photons to drop as well. This meant that the gamma rays no longer had enough energy to produce heavy particles like protons and neutrons.

Now, you'd think that as a result all the protons and neutrons would've simply combined with their antiparticles and destroyed themselves. This would mean there would be no matter and therefore no sun, no Earth, and no you. However, for some unknown reason, there was a small excess of normal particles. And thank your lucky stars, too! Because there would be no stars to thank if that wasn't the case. Antimatter is actually very rare nowadays.

While at about 0.0001 seconds, the gamma ray photons no longer had enough energy to make protons and neutrons, they still could make electrons and positrons because the latter two are way less massive than protons and neutrons. Just like you know it takes a lot of energy to produce something massive, like an ocean ship, you similarly know it takes far less energy to produce something a lot less massive, like a toy ship. Same thing here - the gamma ray photons had less energy, but still enough to make the less massive stuff, the electrons and positrons. But when the clock tolled one minute past the Big Bang, the ever-expanding universe had cooled so much that the photons no longer had enough energy to make electrons or positrons either!

As with the protons and neutrons, most of the electrons and positrons combined to form photons but, once again, a small excess (about one in a billion) of the electrons somehow survived. In the grand scheme of things, what all of this should mean to you is that all of the protons, neutrons, and electrons that make up our universe were made in the first minute after the Big Bang.


About two minutes after the Big Bang, the universe cooled down enough to allow for protons and neutrons to hook up and form deuterons, nuclei of deuterium (heavy hydrogen) consisting of a proton and a neutron. Meaning, a process of nucleosynthesis, the formation of new atomic nuclei, had begun.

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