Our observable Universe is an enormous place, with some two trillion galaxies strewn across the abyss of space for tens of billions of light-years in all directions. Ever since the 1920s, when we first unambiguously demonstrated that those galaxies were well beyond the extent of the Milky Way by accurately measuring the distances to them, one fact leaped out at us: the farther away a galaxy is, on average, the more severely shifted towards the red, long-wavelength part of the spectrum its light will be.
This relationship, between redshift and distance, looks like a straight line when we first plot it out: the farther away you look, the greater the distant object’s redshift is, in direct proportion to one another. If you measure the slope of that line, you get a value, colloquially known as the Hubble constant. But it isn’t actually a constant at all, as it changes over time. Here’s the science behind why.