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?