What is the Hubble tension?

A: There is a major disagreement among physicists about how fast the universe is expanding. Astronomers use a value called the Hubble constant to measure it. However, the two main ways to calculate this number have produced different results.

In the first method, astronomers use the cosmic distance ladder. They observe nearby objects like pulsating stars and exploding supernovae to calculate how fast they are moving away from the earth. These measurements are said to be local, and they point to an expansion rate of around 73 km/s per second per megaparsec.