Ever since Hubble first discovered the relationship between a galaxy’s distance and its motion away from us, astrophysicists have raced to measure exactly how fast the Universe is expanding.
As time moves forward, the fabric of space itself stretches and the distances between gravitationally unbound objects increases, which means everyone should see the Universe expanding at the same rate.
Whether the Big Bang occurred, how old our Universe is, and whether it will recollapse or expand forever. How far away is it? If you can measure the individual stars within it, and you know how stars work, you can infer a distance to those galaxies. If you can measure a supernova within it, and you know how supernovae work, same deal: you get a distance.
If the Universe has less matter and more dark energy than we presently think, the numbers on the ‘leftover relic’ method could increase to line up with the higher values.
Understanding exactly how quickly the Universe expands is a vital ingredient in the recipe for making sense of where everything came from, how it got to be this way, and where it’s headed.