Wikipedia:Reference desk/Archives/Science/2023 July 9

= July 9 =

Gravitational repulsion of antimatter as explanation for matter dominance
According to Gravitational interaction of antimatter, some theories suggest that antimatter and normal matter repel each other gravitationally. If this were true, and the Big Bang or the events shortly thereafter happened by random chance to produce clusters of matter and antimatter, would it be possible for these clusters to have repelled each other to the point where they'd be outside each other's horizons of observability by now? Could this explain the preponderance of matter over antimatter in the current observable universe without the need for symmetry-breaking production of individual particles? Beyond measuring the gravitational force between matter and antimatter, what else might be done to test this hypothesis? Neon Merlin  01:58, 9 July 2023 (UTC)


 * In an initially random mix of equal amounts of matter and antimatter, most particles will soon have been annihilated by interacting with their anti-particles. By pure chance, in some places there will have been an excess of either matter and antimatter, leaving local concentrations of just matter or just antimatter. It should be possible to run a simulation on the evolution of a universe with a random distribution of (minute) clumps of two kinds under possibly repulsive gravitation. Just like clumps of normal matter tend to cluster together, the same should happen for the clumps of antimatter, eventually forming whole anti-galaxies. The gravitational force should cancel out over longer distances, just like for the electrostatic force. Unless we start with a universe which is largely void, with its particles concentrated in a bounded region, considerations of symmetry suggest we will not see a strong separation in such a simulation: clumps of antimatter trying to scoot out have no place to hide, since the conditions are the same everywhere, and their local absence will be filled by antimatter coming in from elsewhere. I found one recent paper about such a simulation: "Hubble law and acceleration curve emerges in a repulsive matter-anti matter galaxies simulations" [sic]. It is behind a paywall; the abstract does not suggest the simulations performed by the researchers indicated the possibility of emerging dominance.  --Lambiam 09:08, 9 July 2023 (UTC)
 * If you read down to te end you'll see that experiments with antihydrogen have shown that it respnds to gravity the same ashydrogen. There may be some difference which hasn't been measured yet but there is no repulsion. NadVolum (talk) 10:15, 9 July 2023 (UTC)
 * If you read the cited source, you'll see that they found no such thing. Using $$F$$ to represent the ratio between gravitational mass and inertial mass, the study found that, at a statistical significance level of 5% and under the assumption of the absence of systematic errors, $$-12<F<75.$$ The authors caution that the lower bound is not warranted, since the systematic effects are not very well characterized for small $$|F|,$$ suggesting instead the bracket $$-65<F<75.$$ (For the standard hydrogen atom, $$F=1.000...\,.$$) --Lambiam 16:31, 9 July 2023 (UTC)
 * Sorry I assumed they'd have the result from in there where as a side effect of their main measurent they come to the conclusion there is at most something like a 2% difference in the effect of gravity on antihydrogen. NadVolum (talk) 19:44, 9 July 2023 (UTC)