Uncertainty on the expected Ω_GW spectrum from BBHs due to uncertainty on the merger rate evolution parameters. Left: 95% confidence levels on the projected Ω_GW spectrum including uncertainty on the PLPP mass model and assuming a MD merger rate model with different levels of uncertainty. The hatched outline reports previous results published in Abbott et al. (2023a). Right: zoomed-in comparison at low frequencies of the uncertainty on the Ω_GW spectrum when varying R₀ vs. R₀ and γ, reporting average spectral indices referred to these two contours.