Example of degenerate likelihood. The true value of the parameters is marked with the green cross. Left panel: Completely degenerate case ν = ν_ref, in which RN and DM cannot be distinguished. The dashed line shows the degenerate maximum likelihood behaving as y ∝ 1 − x in log-scale. Right panel: Same likelihood with ν ≠ ν_ref. The degenerate tails are less pronounced and the maximum likelihood lies around the true value of parameters.