<p>I'm reading Carroll and Ostlie's &quot;An Introduction to Modern Astrophysics&quot;. In the BBN section, they describe that the universe contains a mixture of photons, electron-positron pairs, and electron and muon neutrinos and their antiparticles. They also mention that there was a small number of protons and neutrons and that protons and neutrons were constantly being transformed into each other from various reactions. From my understanding (not C &amp; O necessarily), the Heisenberg Uncertainty Principle permits matter to spontaneously appear because there is some uncertainty as to how much mass there is in a vacuum; however, these particles must appear in &quot;pairs&quot; (ie. positron and an electron). If we assume inflation is correct, it is thought that these particles would become diluted after this process. So I guess I'm wondering how are these particles predicted in general (during the radiation era)? Is the energy released during inflation thought to have created these particles or something else?</p>

CC BY-SA 4.0
2021-04-20T12:37:09.883Z
I don’t have the expertise to write a full answer, but basically the energy to create the massive particles is coming from the photons.  “Pair production” can produce a particle and its antiparticle from a single photon *if* that photon has more than the rest-mass energy of the two particles. This creation process competes with processes that destroy particles, like annihilation of matter-antimatter pairs, and photodissociation of any nuclei that form.  The net effect of all these processes determines the matter-photon balance at any time.
Eric Jensen
CC BY-SA 4.0
2021-04-20T14:54:55.277Z
Thanks. I was reading something about the particles being created from reheating after inflation. Does it have something to do with that?
Astroturf
<cosmology><big-bang-theory><cosmological-inflation><nucleosynthesis><particles>
How do we know or predict which particles were present before Big Bang Nucleosynthesis occurred?
Astroturf
# How do we know or predict which particles were present before Big Bang Nucleosynthesis occurred?

(Asked by: username_0 on 2021-04-19)

I'm reading Carroll and Ostlie's "An Introduction to Modern Astrophysics". In the BBN section, they describe that the universe contains a mixture of photons, electron-positron pairs, and electron and muon neutrinos and their antiparticles. They also mention that there was a small number of protons and neutrons and that protons and neutrons were constantly being transformed into each other from various reactions. From my understanding (not C & O necessarily), the Heisenberg Uncertainty Principle permits matter to spontaneously appear because there is some uncertainty as to how much mass there is in a vacuum; however, these particles must appear in "pairs" (ie. positron and an electron). If we assume inflation is correct, it is thought that these particles would become diluted after this process. So I guess I'm wondering how are these particles predicted in general (during the radiation era)? Is the energy released during inflation thought to have created these particles or something else?

**Tags:** cosmology, big-bang-theory, cosmological-inflation, nucleosynthesis, particles

> **Comments:**
> **username_1**: I don’t have the expertise to write a full answer, but basically the energy to create the massive particles is coming from the photons.  “Pair production” can produce a particle and its antiparticle from a single photon *if* that photon has more than the rest-mass energy of the two particles. This creation process competes with processes that destroy particles, like annihilation of matter-antimatter pairs, and photodissociation of any nuclei that form.  The net effect of all these processes determines the matter-photon balance at any time.  
> **username_0**: Thanks. I was reading something about the particles being created from reheating after inflation. Does it have something to do with that?  

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