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

by Keijo
Tags: annihilation, antimatter, bang, field, gravitational, matter
 P: 9 I posted this question in the general physics section and the physics part has been answered somewhat. However the cosmological components remain un-answered. Would someone be kind enough to comment on those aspects please? Here is the question; Matter and Antimatter, I have always wondered what happens with regard to the gravitational field when matter and antimatter annihilate each other. Consensus among physicists is that matter and antimatter behave the same as far as gravitational potential is concerned, i.e., both will attract matter and antimatter, so that a particle of antimatter would fall toward the earth and not fly off to space. According to the currently accepted cosmological theory, essentially equal amounts of matter and antimatter were initially created in the big bang. Matter and antimatter promptly annihilated each other but there was an imbalance in favor of matter by about one part in a billion. This imbalance is what was left and now constitutes all the matter in our universe. The radiation that resulted from the annihilation is now dissipated with the expanding universe, and is now seen as the cosmic microwave background (CMB) radiation. But what about the gravity? In the Big Bang, was there briefly the gravitational field of a billion additional universes before the annihilation took place? The possibilities seem to be: 1. No there was not. This means that after annihilation there was also no residual gravitational field, in which case, contrary to what we currently believe, maybe matter and antimatter do in fact have opposite gravitational potentials. 2. Yes there was. This would mean that after the annihilation the naked gravitational field was radiated away as gravity waves. Maybe those gravitational waves, fields/potentials are still out there somewhere and can be detected or measured similar to the CMB If 2 is correct, the question is, what would this residue look like today and how would we go about detecting it? Thanks and cheers, Keijo
 HW Helper Sci Advisor P: 4,739 "billion additional universes" is crap. What you are asking that if there was an imbalance in particle - antiparticle annihilation in the primordial what happened to the gravitation field. The thing is that energy is conserved in those interactions, consider e.g. a process from the GUT theory of Georgi-Glashow, which can make a particle over-abundance possible, will still prevail energy - and gravity is about energy, no gravity "is lost" in those processes.
 P: 9 Malawi. I don't think you understood the question. I never said gravity would be lost. In that small period of time when matter and antimatter were both present the total particles that existed were approximately 9 orders of magnitude greater than they are today. Therefore if matter and antimatter behave the same gravitationally, the total gravitational potential was 9 orders of magnitude greater than what would be produced by the remaining matter in the universe today. Where is the error in that logic?
HW Helper
P: 4,739

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

the energy content is the same/constant, so same gravity "content"
 P: 9 Thanks Malawi. That's what I believe and it takes me to where I'm heading. Since the gravity content remains, is this content taken into account by cosmologists when calculating the expansion rate of the universe? Also where is it? I ask because I have seen no reference to it, in my readings.
 HW Helper Sci Advisor P: 4,739 one uses the photon density and baryon density, so yes, I suppose so.
 P: 9 "I suppose so" is not a convincing reply. Can you be more specific?
 HW Helper Sci Advisor P: 4,739 one counts the photons, photons came from particle and antiparticle annihilation, and one counts baryons (the ammount of particles which was left over) Recall that baryons in astrophysics also includes the leptons (the electron). Then we can also calculate the neutrino density parameter. So, since one includes the number density of photons, one has covered what was left over in primordial particle and antiparticle annihilations. But since I am not cosmologist but more of a particle physicsts, I propose that someone more educated in this area (Kurdt, or marcus) have the last word. Can you perhaps also say what status your "reading" has, what have you read / study?
 P: 428 does the dark stuff ,dark matter and dark energy come out of the matter/anti matter going poof or directly from the BIG BANG itself or somewhere else
 Sci Advisor P: 4,596 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.

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