From Wikipedia: "Positron emission or beta plus decay (β+ decay) is a type of beta decay in which a proton is converted, via the weak force, to a neutron, releasing a positron (the antimatter counterpart of an electron) and a neutrino. Isotopes which undergo this decay and thereby emit positrons include carbon-11, potassium-40, nitrogen-13, oxygen-15, fluorine-18, and iodine-121." Carbon-11 is used in positron emission topography. Positrons are massive; therefore, β+ decay either disproves that mass is conserved or generates something with negative mass. How is the concept of negative mass anything other than non-sense? If the universe is not massive enough to stop cosmological expansion and cosmological expansion could make parts or all of the universe impractical to occupy, could β+ decay be used to control cosmological expansion?