Non-uniqueness in quasar absorption models and implications for measurements of the fine structure constant

High resolution spectra of quasar absorption systems provide the best constraints on temporal or spatial changes of fundamental constants in the early Universe. An important systematic that has never before been quantified concerns model non-uniqueness. The absorption structure is generally complicated, comprising many blended lines. This characteristic means any given system can be fitted equally well by many slightly different models, each having a different value of alpha, the fine structure constant. We use AI Monte Carlo modelling to quantify non-uniqueness. Extensive supercomputer calculations are reported, revealing new systematic effects that guide future analyses: (i) Whilst higher signal to noise and improved spectral resolution produces a smaller statistical uncertainty for alpha, model non-uniqueness adds a significant additional uncertainty. (ii) Non-uniqueness depends on the line broadening mechanism used. We show that modelling the spectral data using turbulent line broadening results in far greater non-uniqueness, hence this should no longer be done. Instead, for varying alpha studies, it is important to use the more physically appropriate compound broadening. (iii) We have studied two absorption systems in detail. Generalising thus requires caution. Nevertheless, if non-uniqueness is present in all or most quasar absorption systems, it seems unavoidable that attempts to determine the existence (or non-existence) of spacetime variations of fundamental constants is best approached using a statistical sample.

Automated summary

High resolution spectra of quasar absorption systems offer valuable constraints on fundamental constant variations in the early Universe. However, model non-uniqueness poses a significant challenge, which can be quantified using AI Monte Carlo modeling. Factors such as turbulent line broadening in spectral data modeling can lead to greater non-uniqueness, emphasizing the importance of using physically appropriate compound broadening methods for accurate results. Therefore, statistical sampling may be the best approach to studying spacetime variations of fundamental constants in quasar absorption systems.