The short answer: about 67 to 73 kilometres per second for every megaparsec (about 3.26 million light-years) of distance. That number is the Hubble constant, and it describes the expansion rate at the present day.
The figure depends on how you measure it. Two main methods disagree by a few percent, and that gap is one of the most discussed open problems in cosmology:
- Early-universe method: Fitting the cosmic microwave background, the Planck Collaboration's 2018 results (published in 2020) give about 67.4 km/s/Mpc, assuming standard cosmology.
- Local method: The SH0ES team, led by Adam Riess, uses Cepheid variable stars and supernovae to build a distance ladder. Their 2022 paper gives about 73 km/s/Mpc.
Neither result is a measurement error that has been found. Researchers are checking the distance ladder, the calibration, and whether new physics is involved. Other teams, including DESI's baryon acoustic oscillation work, land in between or lean toward the early-universe value, depending on how their data is combined. Results from the James Webb Space Telescope are also being used to test the Cepheid calibration.
So the expansion is well established. Its exact rate is not yet settled to better than a few percent. Has anyone followed the latest distance-ladder or DESI papers?
