Pair Production in Empty Space

[Arman777]

I am not sure this question has been asked here before but I am curious about it. From the Modern Physics Course, I learned that we need a nucleus to create an electron and positron pair (with a photon). And the reason is stated as to conserve linear momentum. If this is the case then how the electrons or quarks created in the early universe?

Is it because in the near-nucleus case we have 1 photon that is creating 2-pairs and in the early universe we can have 2 photons so that there's no need a nucleus or there's something else?

 

[Orodruin]

And the reason is stated as to conserve linear momentum.

Truth with modification. The reason is conservation of squared total 4-momentum, i.e., com energy.

If this is the case then how the electrons or quarks created in the early universe?

This has nothing to do with pair production. If it did there would be equal amounts of quarks and antiquarks, etc. Instead, you need to create a baryon asymmetry. We don't know how that happened, but there are some theories.

In the early universe, photons and fermions were at thermal equilibrium. Mainly due to 2-to-2 scatters. There was almost as much antimatter as matter. As the universe cooled down, the symmetric component annihilated away, leaving just the asymmetric component as surplus matter.

 

[mfb]

Is it because in the near-nucleus case we have 1 photon that is creating 2-pairs and in the early universe we can have 2 photons so that there's no need a nucleus or there's something else?

That is a possible process, but there were also other particles around. It doesn't have to be a nucleus, a quark in a quark gluon plasma does the job as well, for example.

Pair production from two photons has been observed in colliders, by the way.

 

[mathman]

Pair production in the early universe was essentially photon-photon interaction. (Note mfb comment).

 

[Arman777]

I see now, thanks all