Big bang nucleosynthesis and Lambda(QCD)

Big bang nucleosynthesis (BBN) has increasingly become the tool of choice for investigating the permitted variation of fundamental constants during the earliest epochs of the Universe. Here we present a BBN calculation that has been modified to permit changes in the QCD scale, Lambda(QCD). The primary effects of changing the QCD scale upon BBN are through the deuteron binding energy B-D and the neutron-proton mass difference deltam(np), which both play crucial roles in determining the primordial abundances. In this paper we show how a simplified BBN calculation allows us to restrict the nuclear data we need to just B-D and deltam(np) yet still gives useful results so that any variation in Lambda(QCD) may be constrained via the corresponding shifts in B-D and deltam(np) by using the current estimates of the primordial deuterium abundance and helium mass fraction. The simplification predicts the helium-4 and deuterium abundances to within 1% and 50%, respectively, when compared with the results of a standard BBN code. But Lambda(QCD) also affects much of the remaining required nuclear input so this method introduces a systematic error into the calculation and we find a degeneracy between B-D and deltam(np). We show how increased understanding of the relationship of the pion mass and/or B-D to other nuclear parameters, such as the binding energy of tritium and the cross section of T+D-->He-4+n, would yield constraints upon any change in B-D and deltam(np) at the 10% level.