  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, delta-m_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 delta-m_np yet still gives useful results so that any
variation in Lambda_QCD may be constrained via the corresponding shifts in B_D
and delta-m_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
remaining required nuclear input so this method introduces a systematic error
into the calculation and we find a degeneracy between B_D and delta-m_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 -> He4 + n, would yield constraints upon any change in
B_D and delta-m_np at the 10% level.
