BBN and Lambda_QCD

Abstract

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.