The rate of expansion of the universe is called the Hubble constant, named after astronomer Edwin Hubble, who, with Milton Harmarson, discovered that galaxies are receding from Earth at a speed proportional to their distance. So speed = Hubble constant × distance. Rearranging this equation gives Hubble constant = speed × distance, so the Hubble constant is expressed in kilometers per second (km/s) per megaparsec (Mpc), where one megaparsec is 3.26 million light-years. >
So what is it? How big is the universe? In 1929, Hubble obtained a value of about 500 km/s/Mpc. By 1960, accurate measurements had reduced this speed to 100 km/s/Mpc, but the astronomical community was divided into two camps, those favoring 100 km/s/Mpc and those favoring a similar 50 km/s/Mpc. Resolving this issue became a major task of the Hubble Space Telescope (HST). It took only a few years. Recent accurate measurements of the cosmic microwave background give a value of about 68 km/s/Mpc, while measurements using observations from the HST still give a value of about 72 km/s/Mpc. This disagreement is called the “Harber tension” and has not yet been resolved.
