Effect of bound dineutrons upon big bang nucleosynthesis

We have examined the effects of a bound dineutron ²n upon big bang nucleosynthesis (BBN) as a function of its binding energy B_²n. We find a weakly bound dineutron has little impact but as B_²n increases its presence begins to alter the flow of free nucleons to helium-4. Because of this disruption, and in the absence of changes to other binding energies or fundamental constants, BBN sets a reliable upper limit of B_²n≲2.5MeV in order to maintain the agreement with the observations of the primordial helium-4 mass fraction and D/H abundance. We also consider simultaneous variations in B_²n and the deuteron binding energy B_D using a simplified BBN calculation. We demonstrate that only when B_D is very close to 1.7 MeV does the B_²n upper limit increase to 3.5 MeV, a value set by incompatibility of an observed primordial A=2 abundance with the decay of deuterons.