Surprise! The Hubble Constant Changes Over Time
“If astronomers were more careful about their words, they would have called H the Hubble parameter, rather than the Hubble constant, since it changes over time. But for generations, the only distances we could measure were close enough that H appeared to be constant, and we’ve never updated this. Instead, we have to be careful to note that H is a function of time, and only today — where we call it H0 — is it a constant. In reality, the Hubble parameter changes over time, and it’s only a constant everywhere in space. Yet if we lived far enough in the future, we’d see that H stops changing entirely. As careful as we can be to make the distinction between what’s actually constant and what changes now, in the far future, dark energy ensures there will be no difference at all.”
The farther away you look in space, the more redshifted the light from the object you’re viewing appears. This implies that the Universe is expanding, and the rate of expansion, known as the Hubble constant, is something we’ve strived to measure for a very long time. While there’s a minor controversy over just what that expansion rate is today, whether it’s 67 or 73 km/s/Mpc, it’s perhaps surprising to learn that the expansion rate has changed over time. That means that the Hubble constant isn’t actually a constant at all! So why do we call it that? Because we’re evaluating how the Universe is expanding today. It’s constant everywhere in space, but has actually dropped. Interestingly enough, it will now asymptote to a finite, unchanging value. Many billions of years in the future, it will truly become a constant.