The universe is expanding. That’s something astronomers have agreed on for a long time. Edwin Hubble, an American astronomer, was the first to discover that light from distant galaxies was shifted toward red frequencies by the time it reached us – which meant that the source of the light was moving away from us. The Hubble constant, which expresses how quickly the universe is expanding, was named in Hubble’s honor. It has a value of approximately (more on this later) 70 kilometers per second per megaparsec. For example, if an object is one million parsecs (3.26 million light-years) farther away from us than a second object, then it is moving 70 kilometers per second faster away from us than the second object.
This constant can be measured in different ways. Space telescopes, such as Hubble or Gaia, for example, measure the brightness of certain variable stars, for which the relationship between change in luminosity and luminosity is known. In this way, the distance to galaxies containing these so-called standard candles can be calculated and the velocity at which these galaxies are moving away from us can be determined.