# Big Bang - Gravitational field of antimatter + matter before annihilation

 Sci Advisor P: 4,782 Okay, so how this all works is that in the very early universe, everything was at extraordinarily high temperatures. When you have temperatures significantly above the mass-energy of the particles in question, then gravitationally, everything acts like radiation, which scales with the expansion as $$1/a^4$$. For non-interacting particles moving near the speed of light, this is relatively easy to understand: as the universe expands, the total number of particles remains the same, so they become less dense by a factor of $$1/a^3$$. But there's an additional effect: as they move, they catch up with stuff that was moving away from them. This means that relative to the local density, fast-moving particles tend to slow down. When you work through the calculations carefully, for a particle moving very close to the speed of light, this factor decreases their energy with the expansion by another factor of $$1/a$$, so that the total scaling is the way it is with radiation: $$1/a^4$$. Now, for interacting particles, there's another effect: if two particles strike one another with a kinetic energy much larger than their mass energy, as will happen often if the temperature is larger than their mass, then they will often produce new particle/anti-particle pairs, converting kinetic energy to mass-energy. When the temperature is high, these particle/anti-particle production events will be matched by annihilation events, and it will behave gravitationally in the exact same way as the non-interacting relativistic stuff above. So, what happens when the universe cools? Well, at some point, the temperature drops below the point where new particle/anti-particle pairs are produced in number, and the pairs just all start annihilating. In this case is that the matter density in this stuff (whatever it is) gets converted into lighter particles. For electrons/positrons, for instance, the only lighter particles are neutrinos and photons. But since the neutrino interactions are largely turned off at this point, the electrons nearly all annihilate to become photons. So, one way to look at it is that when the sort of particle you're talking about becomes non-relativistic as the temperatures drop much below their mass-energy, then the energy in those particles gets dumped into other radiation fields. Unless, that is, there is an imbalance in the matter/anti-matter abundance, in which case they dump as much energy as they can into radiation, then just sit around and continue to cool afterward.