Why is the modern value of Hubble's constant so different from today's measured value? At first I thought it was because the Hubble constant is related to density via: ρ_c = 3H^2/(8*π*G) but in the past 2011 - 1921 = 90 years, I don't think the density could have changed that much since it's a negligible amount of time in cosmological scales. So this means, it boils down to measurement improvements but what specifically? I would REALLY appreciate it if someone could explain and elaborate on this for me! Thanks in advance!