Robustness of baryon acoustic oscillation constraints for early-Universe
  modifications to $\Lambda$CDM
  Baryon acoustic oscillations (BAO) provide a robust standard ruler, and can
be used to constrain the expansion history of the Universe at low redshift.
Standard BAO analyses return a model-independent measurement of the expansion
rate and the comoving angular diameter distance as function of redshift,
normalized by the sound horizon at radiation drag. However, this methodology
relies on anisotropic distance distortions of a fixed, pre-computed template
(obtained in a given fiducial cosmology) in order to fit the observations.
Therefore, it may be possible that extensions to the consensus $\Lambda$CDM add
contributions to the BAO feature that cannot be captured by the template
fitting. We perform mock BAO fits to power spectra computed assuming
cosmological models which modify the growth of perturbations prior to
recombination in order to test the robustness of the standard BAO analysis. We
find no significant bias in the BAO analysis for the models under study
($\Lambda$CDM with a free effective number of relativistic species, early dark
energy, and a model with interactions between neutrinos and a fraction of the
dark matter), even for cases which do not provide a good fit to \textit{Planck}
measurements of the cosmic microwave background power spectra. This result
supports the use of the standard BAO analysis and its measurements to perform
cosmological parameter inference and to constrain exotic models. In addition,
we provide a methodology to reproduce our study for different models and
surveys, as well as discuss different options to handle eventual biases in the
BAO measurements.
