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?
A follow-up with the numbers I could check, since this measurement keeps moving.
- Local ladder: SH0ES is still usually cited at H0 = 73.04 ± 1.04 km/s/Mpc (Riess et al. 2022). I could not find a newer SH0ES headline value, so check the team's latest paper if you need the current one.
- Early universe: Planck gives 67.37 ± 0.54.
- DESI: a data-driven analysis of the DESI 2024 BAO data gives H0 = 68.4 (+1.0/−0.8) km/s/Mpc. Those authors report this is still about 4.3σ away from SH0ES.
So BAO-based values keep landing near the CMB value, not the local one. That points the dispute at the distance ladder or at new physics, and so far no one has shown which. Newer 2026 preprints, such as a DESI peculiar-velocity calibration, are still being digested, and I haven't verified their headline numbers.
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