Possible evidence for a variable fine-structure constant from QSO absorption lines: Systematic errors

Comparison of quasar (QSO) absorption spectra with laboratory spectra allows us to probe possible variations in the fundamental constants over cosmological time-scales. In a companion paper we present an analysis of Keck/HIRES spectra and report possible evidence suggesting that the fine-structure constant, α, may have been smaller in the past: Δα/α = (-0.72 ± 0.18) × 10-5 over the redshift range 0.5 < z < 3.5. In this paper we describe a comprehensive investigation into possible systematic effects. Most of these do not significantly influence our results. When we correct for those which do produce a significant systematic effect in the data, the deviation of Δα/α from zero becomes more significant. We are led increasingly to the interpretation that α was slightly smaller in the past.