Cosmological measurements from the CMB and BAO are insensitive to the tail probability in the assumed likelihood

When fitting cosmological models to data, a Bayesian framework is commonly used, requiring assumptions on the form of the likelihood and model prior. In light of current tensions between different data, it is interesting to investigate the robustness of cosmological measurements to statistical assumptions about the likelihood distribution from which the data was drawn. We consider the impact of changes to the likelihood caused by uncertainties due to the finite number of mock catalogs used to estimate the covariance matrix, leading to the replacement of the standard Gaussian likelihood with a multivariate t-distribution. These changes to the likelihood have a negligible impact on recent cosmic microwave background (CMB) lensing and baryon acoustic oscillation (BAO) measurements. We then extend our analysis to perform a sensitivity test on the Gaussian likelihoods typically adopted, considering how increasing the size of the tails of the likelihood (again using a t-distribution) affects cosmological inferences. For an open ΛCDM model constrained by BAO alone, we find that increasing the weight in the tails shifts and broadens the resulting posterior on the parameters, with a ∼0.2-0.4σ effect on Ω_Λ and Ω_ k. In contrast, the CMB temperature and polarization constraints in ΛCDM showed less than 0.03σ changes in the parameters, except for {τ, ln(10^10A_ s), σ_8, S_8, σ_8Ω_ m^0.25, z_ re, 10^9A_ se^-2τ} which shifted by around 0.1-0.2σ. If we use solely ℓ < 30 data, the amplitude A_ s e^-2τ varies in the posterior mean by 0.7σ and the error bars increase by 6 measurements, that uncertainties in the shape and tails of the likelihood do not contribute to current tensions.