So, what exactly is the Hubble constant?
The Hubble constant is the slope of the graph you get when, at some particular moment in time, you plot the velocity of distant objects versus the distance to them: it’s in the expression . The Hubble constant has units of velocity over distance, which works out to units of inverse time: it’s the reciprocal of the age the universe! Note that the Hubble “constant” is not constant in time: it’s called “constant” because it’s the same everywhere in the universe (and it doesn’t change much on the timescale of our lives). But is always a measure of the age of universe.
